EASTERN NORTH PACIFIC OCEAN · 2016-07-03 · CONTENTS 1. INTRODUCTION 3 2. FREQUENCIESANDDURATIONSOFEASTROPACTROPICAL CYCLONES(1965-1974):WHOLE-AREASTATISTICS 4 2.1Introduction 4

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TECMNICAl REPORT SECTIONNAVAL POSTGRADUATE SCHOOLMONTEREY. CALIFORNIA 93940

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NAYENYPREDRSCHFAC

Technical Paper No. 7-76

THE CLIMATOLOGY AND FORECASTING OF

EASTERN NORTH PACIFIC OCEAN

TROPICAL CYCLONES

by

ROBERT J. RENARD

and

WILLIAM N. BOWMAN

JULY 1976

NAVAL ENVIRONMENTAL PREDICTION RESEARCH FACILITY

MONTEREY , CALIFORNIA 93940

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The Climatology and Forecasting ofEastern North Pacific Ocean TropicalCycl ones

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Renard and W. N. Bowman

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Naval Postgraduate School and NavalEnvironmental Prediction ResearchFacility, Monterey, CA 93940

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'9 KEY WORDS (Contlnua on reverse alda If necaaaary and Idantlty by block numbar)

TropicsCI i ma to 1 ogyHurri cane

Eastern North Pacific OceanTropical cyclone forecasting

20 ABSTRACT (Contlnua on ravaraa alda It nacaaamry and Identify by block numbar)

The eastern tropical North Pacific Ocean (EASTROPAC) is

one of the most prolific regions for troDical cycloneactivity, considering the limited area and season of occur-rence. This publication presents operationally usablestatistics on frequency, initial and terminal positions,

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20. Abstract (continued)

track, speed and duration of EASTROPAC tropical cyclones, as

derived from a recent 10-year period of adequate operationalsatellite surveillance, 1965-1974. The current status offorecasting these cyclones is highlighted by recent statisticson the accuracy of analog forecasts of the cyclone tracks.A selected bibliography and list of references is alsoincluded.

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CONTENTS

1. INTRODUCTION 3

2. FREQUENCIES AND DURATIONS OF EASTROPAC TROPICALCYCLONES (1965-1974): WHOLE-AREA STATISTICS 4

2.1 Introduction 4

2.2 Frequencies 4

2.3 Durations 6

3. DISTRIBUTION OF INITIAL AND TERMINAL POSITIONSAND FREQUENCY OF OCCURRENCE OF EASTROPAC TROPICALCYCLONES (1965-1974): ANALYSES OF AREALDISTRIBUTIONS 10

3.1 Introduction 103.2 Distribution of Initial and Terminal Positions . 103.3 Frequency of Occurrence 143.4 Formation Potential 14

4. TRACKS, DIRECTIONS AND SPEEDS OF MOVEMENT OFEASTROPAC TROPICAL CYCLONES (1965-1974) 19

4.1 Introduction 194.2 Analysis of Hurricane Tracks 194.3 Recurvature 224.4 Directions and Speeds of Movement of EASTROPAC

Tropical Storms and Hurricanes 25

5. FORECASTING EASTROPAC TROPICAL CYCLONES 68

5.1 Introduction 685.2 Forecasting Tropical Cyclone Motion by an

Analog Scheme 68

SELECTED BIBLIOGRAPHY AND REFERENCES 76

ACKNOWLEDGMENTS

The authors wish to express their appreciation to the

following: the Naval Weather Service Detachment, Asheville,

NC, for providing the historical eastern North Pacific Ocean

tropical cyclone data on tape as well as the monthly and semi-

monthly statistics of tropical cyclone movement; to NEPRF

personnel, especially Mr. Don Schertz, the drafting and editing

staff, and to Mr. Sam Brand for valuable guidance and advice;

to the technical support personnel, Department of Meteorology,

NPS, for computational and drafting efforts, especially Mr.

Michael McDermet; and to Mr. Steve Rinard, Department of

Meteorology, NPS, for the computerized forecast verifications.

Mrs. Winona Carlisle of NEPRF is thanked for typing the

manuscri pt

.

1. INTRODUCTION

The eastern tropical North Pacific Ocean (EASTROPAC)

is widely held to be a region most prolific in tropical

cyclone activity, considering the limited area and period of

occurrence (mid-May to mid-November). However, it is only

since 1965, with weather satellite observation capability

added to sporadic weather reconnaissance, that information

on numbers, stages, and tracks can be considered reasonably

complete, reliable and homogeneous. The cl imatol ogi cal

behavior of these tropical cyclones, as derived from the

10 -year period 1965-1974, is described in Sections 2, 3 and 4.

The current status of forecasting EASTROPAC tropical cyclones

is discussed in Section 5. In general, this publication is

designed to assist the operational forecaster and planner in

an area where cl imatol ogi cal estimates are especially valuable.

The EASTROPAC area, as defined for this study, encom-

passes the ocean region bounded by North America on the east

and the International Date Line (180°) on the west, although

tropical cyclone tracks may extend across the 180° line to the

western sector of the North Pacific Ocean. The south and

north boundaries are dynamically and thermodynami cal ly set by

the atmospheric and oceanic environment in which the cyclones

form and move (Gray, 1968, 1975). Hence, 5N and 35N complete

the bounds of the area considered here, although anomalous

tropical cyclone activity beyond these limits is not precluded,

as noted in Sections 3 and 4.

The source of all cyclone statistics is the EASTROPAC

tropical cyclone data bank (1949 to date) furnished to the

authors on magnetic tape by the National Weather Service

Detachment, Asheville, NC. The cl imatol ogi cal 12-hour movement

analyses in Section 4 were derived from computerized statistics

furnished by the NWSD Asheville; further quality control pro-

cessing of the data was accomplished at the Naval Environmental

Prediction Research Facility and Naval Postgraduate School,

Monterey, CA.

3

2. FRE&UENCIES AND DURATIONS OF EASTROPAC TROPICAL

CYCLONES (1965-1974): WHOLE-AREA STATISTICS

2.1 INTRODUCTION

The EASTROPAC tropical cyclone season extends from

mid-May to mid-November, although the less reliable records

of the period 1949-64 do indicate a tropical cyclone occurring

as early as 21 March (as a tropical storm, in 1951) and as*

late as 6 December (as a hurricane, in 1957). The statistics

on whole-area tropical cyclone frequency and duration

(Table 2-1) are given for all tropical cyclones; they are

listed by the three categories of tropical depression (D),

tropical storm (S) and hurricane (H), and stratified into

semimonthly and overall season intervals as well as by years.

The categories D, S and H refer to the most intense stage

attained during the life history of a tropical cyclone. All

of the life cycle (duration) of a particular tropical cyclone

has been assigned to the half- monthly period during which it

was first observed (usually, but not always, in the depression

stage ) .

2.2 FREQUENCIES

The cl imatol ogi cal expectation, derived from the period

1965-74, is 15.7 tropical cyclones a year, all categories

considered, while the frequency for named cyclones (those in

the S and H categories) is 14.5 per year. The S category is

the most common, with 7.9 occurrences per year on the average.

These figures may be compared to an average of 9.0 tropical

cyclones per year derived from the 1949-64 period, all of

which were classified as S or H. Such a figure is likely to

be a gross underestimate, due both to a lack of documentation,

EASTROPAC tropical cyclones were first named in 1960

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especially on depressions, and to generally inadequate

surveillance in the years prior to the operational-satellite

era. This comparison clearly justifies the use of only the

period from 1965 onwards for establishing EASTROPAC tropical

cy clone statistics.

The tropical cyclone season has a rather broad peak

frequency period; there is little difference in semimonthly

frequencies from 16 July-15 September although the actual

semimonthly peak, 2.45, occurs during 16-31 August, mostly

because of the high incidence of the H category (1.60 per

year) during this period. The incidence of the S category is

at a maximum during 16-31 July, causing a minor secondary

peak in overall occurrence during this period.

The frequency distributions for the individual years

show marked anomalies. All of the tropical depressions

occurred in the three years 1968, 1969 and 1974; in the years

1965-70, an average of 9.7 tropical storms and 4.8 hurricanes

occurred -- in comparison to the period 1971-74 when an

average of 5.2 tropical storms and 9.2 hurricanes occurred.

Subjectivity in the observation and documentation of EASTROPAC

tropical cyclones may account for some of the apparent varia-

bility in frequency of occurrence of the D, S and H categories

2.3 DURATIONS

The duration of a tropical cyclone has been defined to

the nearest quarter-day. However, duration statistics in

Tables 2-1 through 2-4 are given to the nearest tenth-day,

and thus should be regarded as accurate to +0.1 day. As

expected, the H category is the most long-lived, lasting an

average of 7.6 days, while the S and D categories last an

average of 4.8 and 2.9 days, respectively. The overall average

duration of a tropical cyclone is 5.8 days. The relationship

of duration to frequency is not direct, although overall the

central two-month high-frequency period also produces the

longest-lived tropical cyclones. Hurricanes occurring during

the semimonthly period 1-15 August last an average of 9.0 days.

With few exceptions, hurricanes outlast storms, which

outlast depressions, in every half-month period and in the

overall season. Hurricanes occurring in 1970 and 1972 were

particularly long-lived (9.2 and 10.2 days, on the average,

respectively). Tropical cyclones that occurred in 1972 were,

on the average, of the longest duration recorded (7.9 days).

Looking again at the two periods with unusual frequencies,

1965-70 and 1971-74, the durations of the S and H categories

tend to be longer during the period of high frequency. For

example, the average duration of the S category decreases

from 4.8 days in 1965-70 to 4.1 days in 1971-74, while for

the H category the duration value increases from 7.2 days in

1965-70 to 8.1 days in 1971-74.

The figure in Table 2-1 indicating the number of tropical

cyclone days per year, 91.6 -- comprising 3.5 D category,

37.9 S category, and 50.2 H category days per year -- is

greater than the number of calendar days with tropical cyclone

activity reported since two or more tropical cyclones occa-

sionally do exist contemporaneously. Table 2-2 stratifies

the average number of days with single (1) and multiple (2-5)

tropical cyclone occurrences by month. A 'calendar day' may

have a single or multiple occurrence, but is still counted

only once. When frequency is viewed in this manner, there is

an average of 70.7 cyclone days per year. In the period

26-27 August 1974, there were five cyclones -- four hurricanes

and one tropical storm -- coexisting in EASTR0PAC. Multiple

occurrences are most common in August; for example, four were

reported at one time in the month of August in both 1972 and

1974.

Table 2-3 gives further details on multiple occurrences

by showing the total number of days with single and multiple

occurrences by individual year. Multiple occurrences were

most frequent in 1974 and least frequent in 1969. Although it

is not shown, the existence of an EASTR0PAC tropical cyclone

was reported on every calendar date from 30 May through

8 November, even in the short time span 1965 to 1974.

Table 2-2. Average number of days with single (1)and multiple (2-5) tropical cyclone occurrences bymonth, EASTROPAC, 19 65-74.

MonthConcurrent Number of Tropical Cyclones

TotalNo. Days1 2 3 4 5

May

June

July

Augus t

September

October

November

0.9

8.8

10.1

9.8

11 .8

10.0

1 .2

0.6

4.2

6.0

2.2

0.7

0.8

1 .7

1 .2

0.4

0.1

0.2

0.9

9.4

15.1

18.1

15.3

10.7

1 .2

Total 52.6 13.7 3.7 0.5 0.2 70.7

Table 2-3. Total number of days with single (1)

and multiple (2-5) tropical cyclone occurrences byyear, EASTROPAC, 1965-74.

YearConcurrent Number of Tropica 1 Cyclones

TotalNo. Days1 2 3 4 5

1965 42.5 9.8 52.3

1966 36.0 12.0 5.5 1 .0 54.5

1967 68.2 10.8 6.8 85.8

1968 62.0 21 .0 5.2 88.2

1969 52.5 1 .0 53.5

1970 76.2 11 .8 88.0

1971 49.8 28.2 2.2 80.2

1972 39.0 10.5 10.2 .8 60.5

1973 30.5 16.5 4.2 51 .2

1974 68.8 15.5 2.8 3.5 1 .8 92.4

Ten YearTotal 525.5 137.1 36.9 5.3 1 .8 706.6

Table 2-4 provides further insight into the "duration"

problem of EASTROPAC tropical cyclones. It shows that nearly

two-thirds (61%, 2.9 days) of the life history of an average

tropical storm is spent in the storm stage, while nearly

equal amounts of time of an average hurricane are spent in

the storm stage (43%, 3.3 days) and hurricane stage (35%,

2.6 days). In the case of the S category, there are many

cases in which all or almost all of the life history was

observed in the storm stage; this would suggest that the

depression stage was very minimal, but perhaps, in reality,

not totally absent (as occurred with tropical storms Doreen

in 1965, Ilsa in 1967, and Aletta in 1974; the existence of

a depression stage was not recorded for any of these storms).

For the tropical cyclones classified as hurricanes, a storm

stage is invariably observed.

Table 2-4. Portions of the life history of the averagetropical cyclone spent in each of the three stages

:

depression, storm and hurricane; EASTROPAC, 1965-74.

Tropical Cyclone Category

Time in Stage Documented as:

Depressi on Storm Hurri cane

Depression 100%2.9 days

-- --

Storm 39%1 .9 days

61%2.9 days

--

Hurri cane 22%1 .7 days

43%3.3 days

35%2.6 days

3. DISTRIBUTION OF INITIAL AND TERMINAL POSITIONS ANDFREQUENCY OF OCCURRENCE OF EASTROPAC TROPICAL CYCLONES(1965-1974): ANALYSES OF AREAL DISTRIBUTIONS

3.1 INTRODUCTION

Areal distributions of occurrence of tropical cyclones

in the eastern North Pacific Ocean during the period 1965-74

are presented and discussed in this section. The initiation

and termination points were determined from documentation

furnished by the National Climatic Center, Asheville, NC.

Consistent with the amount, period and validity of the data

base, the position analyses are shown relative to 5° latitude-

longitude squares. A tropical cyclone was counted once only

for each occurrence in a square, regardless of the length of

time or track in that square. Such positions represent the

first and last time of detection of a tropical cyclone (usually

in the depression or tropical storm stage of intensity).

Almost certainly, such tropical circulations existed as

disturbances (i.e., areas of organized convection) for some

time both prior to the initiation times and after the termina-

tion times considered here.

3.2 DISTRIBUTION OF INITIAL AND TERMINAL POSITIONS

Figures 3-1 and 3-2 show analyses of initiation and

termination positions by means of constant percentage envelopes

of 25, 50, 75 and 95 percent. The 25% envelope may be consid-

ered to bound the smallest area containing 25% of the initial

(Figure 3-1) and final (Figure 3-2) positions. The remaining

isopleths should be considered in a similar wav.

The prime area of initiation lies between ION and 15N

from 180 to 480 miles off the west coast of the North American

continent. The two 5° latitude-longitude squares with the

maximum number of initiation positions are in the area

10

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12

10N-15N, between 95W-100W and 100W-105W, each with 25 initia-

tion positions in the 10 -year period of consideration. Nearly

50% of all tropical cyclones are first detected in the zone

10N-15N, 95W-110W, usually in the depression or tropical storm

stage of intensity. No initiation positions west of 135W have

been documented in the period of study. The most southerly

and northerly latitudes of reported initiation positions

(1965-74) are 7.7N (a tropical depression, 1974) and 20. 4N

(tropical storm Candice, 1965).

The termination area is more diffuse than the initiation

area. The 5° latitude-longitude squares with the maximum

number of reported terminal positions are 15N-20N, 100W-105W

and 20N-25N, 110W-115W, each with 13 reports, closely followed

by the square 20N-25N, 105W-110W with 12 reports. From these

squares, the terminal zone bulges north along the Baja

Peninsula and the continental Mexican coast, as well as west-

ward along a zone from 15N to 22. 5N, becoming diffuse by 135W.

Although it is beyond the area considered in this study, the

most westerly termination point reported is that of Sarah,

1967, at 151E -- some 75 degrees of longitude westward of the

most westward initiation position. Terminal positions have

not been reported at latitudes north of northern Baja in the

period 1965-74. The most southerly and the most northerly

latitudes of reported terminal positions in the Western

Hemisphere (i.e., east of longitude 180W) are 8.5N (a tropical

depression, 1974) and 31. 9 N (hurricanes Katrina, 1967 and

Hyacinth, 1972), respectively. It should be noted that

hurricane Sarah, 1967, at her last reported and most northerly

position, was located at 36. 4N in the Eastern Hemisphere.

Section 4.3 provides additional detail concerning reported

terminal positions over land.

13

3.3 FREQUENCY OF OCCURRENCE

Figure 3-3 presents an analysis of the number of 1965-74

tropical cyclones of all categories passing through or into

each 5° latitude-longitude square-. The most heavily traversed

area is between 15 and 17. 5N, from 105W to 115W. The axis of

maximum traversal relates well to the concentrated area of

initiation south of 15N and east of 105W (see Figure 3-1),

and the area of termination near latitude 15N west of 120W

(see Figure 3-2). If Figure 3-3 was modified to show the

traversal statistics for the storm and hurricane categories

only, the isoline values would be generally about one isopleth

unit less in the area east of HOW, with the major axis a few

degrees of latitude poleward in the area east of 110W. These

considerations suggest that the storm and hurricane categories

have a track further north than the depression category.

As expected, the major axis in Figure 3-3 relates quite

well to the orientation of the area! vector mean tropical

cyclone tracks (see Figure 4-23). This subject is analyzed

in greater detail in the track and speeds -of -movement statis-

tics presented in Section 4.

3.4 FORMATION POTENTIAL

The 25% isopleth shown in Figure 3-1 covers approximately

one and two-fifths 5° latitude-longitude squares. In this

area, 39 tropical cyclones were initially detected in the 10

years 1965-74 -- equivalent to 78 tropical cyclones in a

20-year period or 56 tropical cyclones per 20 years per 5°

latitude-longitude square. This is a considerably higher

value than that implied by Gray's (1975) analysis of tropical

cyclone formation potential (Figure 3-4) or the verification

of the potential with data from 1952-71 (Figure 3-5).

Further, Gray's isopleth of observed maximum initial tropical

cyclone detection in Figure 3-5 is to the northwest of that

in Figure 3-1. In general, Gray's isolines of formation

14

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potential (Figure 3-4), rather than his observed formation

(Figure 3-5), agrees best in position and orientation with the

analysis presented here (Figure 3-1).

The differences cited above may be due to Gray's use of

data for the period prior to 1965. This period is notoriously

low in the assessment of tropical cyclone activity and, due

to the inadequacies of observation, prone to a time lag in

detecting the existence of a tropical cyclone. Considering

the normal tracks in EASTROPAC, the latter problem generally

results in assigning the initial tropical cyclone position to

the west-northwest of its true location.

18

4. TRACKS, DIRECTIONS AND SPEEDS OF MOVEMENT OFEASTROPAC TROPICAL CYCLONES (1965-1974)

4.1 INTRODUCTION

This section presents an analysis of the tracks and

speeds of movement of tropical cyclones in the eastern

tropical North Pacific Ocean for the period 1965-74. In this

period, high quality satellite data allows relatively accurate

and detailed track analyses to be made. The analyses cover

the geographical area from 5N to 35N, and from the west coast

of North America westward to 180W. No attempt has been made

to follow further the very few tropical cyclones that crossed

the 180W meridian moving westward, or to incorporate these

movements into the forecasting guide. Five-degree latitude

by five-degree longitude analyses has been performed, in

consonance with the quality of the available data.

Section 4.2 examines only the hurricane portions of the

157 tropical cyclone tracks. Section 4.3 is devoted to the

tracks of all tropical cyclones which underwent recurvature.

Section 4.4 analyzes the directions and speeds of movements of

all named tropical cyclones (i.e., 145 storms and hurricanes)

that occurred during the 1965-74 period.

4.2 ANALYSIS OF HURRICANE TRACKS

During the period 1965-74, 66 hurricanes occurred.

Figures 4-1 and 4-2 show only the tracks of the hurricane

stages of tropical cyclones categorized as hurricanes. The

tracks are presented in two calendar periods: 16 July through

15 September (Figure 4-1), and 16 May through 15 July combined

with 16 September through 15 November (Figure 4-2). Thus,

the 'high season 1 (i.e., more active period) is shown in

Figure 4-1. In some cases the hurricane portion of the track

is not continuous; such a discontinuity, indicated by a

scalloped line, usually denotes the existence of an inter-

vening tropical storm stage. An example apparent in Figure

19

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4-2 is the track of an October 1967 hurricane whose hurricane-

stage track begins at 14. ON, 113. OW and terminates at 26. 8N,

111.7W, with a sub-hurricane stage (in this case, tropical

storm) from 15. IN, 114. 6W to 24. 4N, 111.2W.

Figures 4-1 and 4-2 show that tropical cyclones of

hurricane intensity in the eastern North Pacific Ocean occur

predominantly eastward of 1 30 W and that excursions westward of

this longitude are confined almost exclusively to the period

16 July through 15 September. Further, the first observed

point of occurrence of the hurricane stage is furthest west in

the 'high season' period (being generally between 105-115W).

In the remainder of the tropical cyclone season, this first-

observed point occurs east of 105W about as often as west of

this latitude.

The region between 15N and 20N eastward of 120W is the

region most heavily traversed by hurricanes, with the more

zonal ly-ori en ted tracks south of 15N and west of 120W.

4.3 RECURVATURE

Figure 4-3 shows a composite of recurvature tracks for

tropical depressions, storms and hurricanes. A recurvature

track is defined here as any track which includes a component

of movement toward the east. The first position on each of

the tracks shown in Figure 4-3 is that point at which the

tropical cyclone acquired a component toward the east. It

should be noted that the great majority of recurving situa-

tions occur north of 15 N and east of 115W. Of the 50

recurving tropical cyclones (i.e., 32% of the 10-year sample)

that occurred in the period of record 1965-74, 19 (38%) moveu

onto the west coast of North America. The prime coastal area

affected was between 17N and 27N, where 12 tropical cyclones

(24%) made landfall; three of these cyclones hit the mainland

22

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23

coast between 26N and 27N, having first crossed the lower Baja

Peninsula south of 25N. Other concentrations of landfall or

near-landfall occurred between 22N and 24N, and 1 8N and 19N.

Data in Table 4-1 indicate that 76% (38 cyclones) of the

EASTROPAC recurvature cases occur in August, September and

October. However, a recurving tropical cyclone has been

recorded in every month from May to November. Further,

recurving tropical cyclones tend to be in the tropical storm

stage at the time of recurvature in the period 1965-74. On

the average, one-quarter of the recurver's time is spent moving

with a component toward the east.

Table 4-1. Statistics on recurving tropical cyclones,

EASTROPAC, 196 5-74. Month determined by first day

of movement toward the east.

Month

May

June

July

Aug

Sep

Oct

Nov

Number andPercentage of

all Cyclonesthat Recurve

No

Year

2

5

3

8

19

11

2

%

50

62

25

09

19

58

51

91

Number of Recurving CyclonesAccording to Cyclone Stage

At Maximum Intensity

32

1

1

23

H

4*

1

5

10**

5

25

At Initial Pointof Recurvature

15 23

1

2

6

3

12

Average Per-Jrentage ofRecurvatureTi me wi thEastwardMovement

57

23

30

13

28

28

20

25

Notes: D = Depression, S = Storm, H = Hurricane.

* - Includes two cyclones with multiple recurvature tracks.

** - Includes three cyclones with multiple recurvature tracks

24

4.4 DIRECTIONS AND SPEEDS OF MOVEMENT OF EASTROPACTROPICAL STORMS AND HURRICANES

Figures 4-4 through 4-23 show 12-hour movement analyses

for 5° latitude-longitude squares for bimonthly, monthly and

annual periods. As previously noted in Section 4.1, analyses

have been made only for tropical cyclones classified as

tropical storms or hurricanes.

Each period's depiction is divided into eastern (80W to

130W) and western (130W to 180W) sections of the EASTROPAC

area on facing pages. The legend for interpreting the plotted

data (shown below) is included in the eastern section. The

data plot and corresponding interpretation for the period

1-15 October in the square 15-20N, 115-120W, is an example of

the informational content of the 12-hour tropical cyclone

center movement depictions (Figures 4-4 through 4-23).

P7 /V7

«

P8 /V8P,/V,

LEGEND

Statistics for each 5° latitude-longitude square based on 12-hourtropical storm and hurricane centermovements in the years 1965-1974.

1 p6 /v6 ( Po J

P2 V2

D/VVN/C/Y

\, D/VV: Vector mean direction, \,and D (

10' s of deg) ; and

vector mean speed, VV(in kt) , of 12-hourcyclone movements.

P5/V5 P4/V4 P3/V3

N/C/V: Number of 12-hour movecyclones occurring in

ments, N, from C individualY percent of the 10-year period.

P /v • Percentage, P, and average speed, V (in kt), of cyclonemovements toward the directions indicated by d + 22.5 deg,

as 1 (north-northeast to east-northeast) 8

(north-northwest to north-northeast); = stationary.

48/8 9/6

33/11 (T 5J

30/7.0

21/6/50

5/3

Thus, 6 (= C) hurricanes and tropical storms generated

the 21 (= N) 12-hour cyclone movements (0000 to 1200 GMT and

1200 to 0000 GMT) which terminated in this square. These 21

track segments occurred in 50% (= Y) of the years in the

10-year period, 1965-74. The 21 cases generated a vector

mean direction (= D and\) and speed (= VV) of movement

25

toward 300° at 7 kt. Thirty-three percent (= Pg

) of the

cyclone movements were in the octant bounded by the directions

west-southwest to west-northwest at a mean speed of 11 kt

(= Vg). Similarly, 48% (= P

?), 9% (= P

g) and 5% (= P|) moved

in the octant contained by the directions west-northwest to

north-northwest at 8 kt (= V-,), north-northwest to north-

northeast at 6 kt (= Vq), and north-northeast to east-northeast

at 3 kt (= V,), respectively. In addition, 5% (= PQ

) of the

storms and hurricanes sampled were stationary during the

associated 12-hour periods.

26

NOTE

Figures 4-4 through 4-23 are

presented on the following

pages. In each pair, the

western section is shown at

the top as the "a" portion

of the total plot and the

eastern section is shown at

the bottom as the "b" portion

27

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&

LEGEND

Statistics

for

each

5°

latitude-

longitude

square

based

on

12-hour

tropical

storm

and

hurricane

center

movements

in

the

years

1965-1974.

\,

D/VV:

Vector

mean

direction,*,

and

D

(

10's

of

deq)

,

and

vector

mean

speed,

VV

(in

kt),

of

1

2-hour

cyclone

movements.

ments,

N,

from

C

individual

Y

percent

of

the

10-year

period

rage

speed,

V

(in

kt).

of

cyclone

irections

indicated

by

d

+

22

5deg,

to

east-northeast)

.

.

.

8

rth-northeas

t)

;

=

stationary

si* a!" sffS N N.

ol t£" £

^ 1 «-° I** Vo. V / a« o-

? * *—— •1

N/C/Y:

Number

of

12-hour

move

cyclones

occurring

in

Pd

/V.:

Percentage,

P,

and

ave

movements

toward

the

c

as

1{north

-northeas

t

(north-nortnwes

t

to

no

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5. FORECASTING EASTROPAC TROPICAL CYCLONES

5.1 INTRODUCTION

Responsibility for forecasting tropical cyclones in

EASTROPAC rests with the National Weather Service's Eastern

Pacific Hurricane Center, Redwood City, CA (for the area east

of 140° W), and Central Pacific Hurricane Center, Honolulu, HI

(for the area 180° to 140° W) (U.S. Department of Commerce,

1976). Until recently, the main approach for forecasting

initiation, movement, intensity and dissipation of EASTROPAC

tropical cyclones has been mostly a subjective combination of

climatology (see, for example: Atkinson, 1971; Baum, 1975;

Crutcher, 1973; Crutcher and Quayle, 1974; Gray, 1975; Hansen,

1972) and persistence, with experience playing a major role

in formulating the forecast. For estimates of motion, the

recent introduction of two objective schemes (MOHATT and

EPANALOG) holds promise of improved forecast accuracy. MOHATT

is an automated statistical steering model that utilizes

diagnostic and prognostic Fleet Numerical Weather Central

model-output steering fields (Freeman, 1972; Renard and

Harding, 1975). MOHATT has not reached an operationally useful

point in EASTROPAC at this time, although it has demonstrated

skill exceeding the official forecasts in the North Atlantic

area. EPANALOG, an analog approach, is operationally ready

and has shown accuracy excelling that obtainable by existing

EASTROPAC methods; it is discussed further below.

5.2 FORECASTING TROPICAL CYCLONE MOTION BY AN ANALOG SCHEME

In 1974, Jarrell, Mauck and Renard (1975) developed an

objective analog scheme to forecast the motion of EASTROPAC

tropical cyclones. The scheme, known as EPANALOG, searches

historical tropical cyclone movement records for situations

statistically analogous to the one at hand. A composite of

68

the subsequent behavior of these selected analog cyclones is

theli used to derive the forecast of the current tropical

cycl one

.

Output includes cyclone-center position as well as 50%

probability ellipses for 24-, 48-, 72- and 96-hour forecast

intervals. Figure 5-1 illustrates a typical computerized

EPANALOG input/output message along with a plot of the output.

The forecasts have been produced operationally by Fleet

Weather Central, Pearl Harbor, Hawaii, since the beginning of

the 1 975 season .

For an area like EASTROPAC where tropical cyclones are

numerous but seasonal, and behavior is considered to be fairly

regular, an analog approach to forecasting is quite successful.

Figures 5-2, 5-3 and 5-4 depict comparative analog (EPANALOG),

official (as issued by the National Weather Service Forecast

Office, Redwood City), and persistence (linear extrapolation

of 24-hour history) forecast statistics for 1973 and 1974 by

forecast interval (24-, 48-, 72- and 96-hr), stage (depression,

storm, hurricane) and track (before and after recurvature )

.

Stage and track characteristics are specified for the verifi-

cation time of the forecast.

Except for the af ter-recurvature forecasts, EPANALOG

accuracy generally excels that of official and persistence

for all stages and track types at all forecast intervals.

The accuracy of the analog scheme is particularly evident in

the case of the hurricane stage. Here, the analog forecasts

represent more than a 20% improvement over the official

forecasts. The official forecaster did not have access to

the EPANALOG forecasts in the 2-year period summarized here.

Considering stage (Figure 5-3) the poorest forecast

results for all three types of forecasts (official, EPANALOG,

persistence) generally occur in the depression and storm

stages of the cyclone. One reason for this result is the

positive correlation of initial position accuracy and cyclone

69

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73

intensity (stage). (Note the position errors at the zero-hour

forecast interval (i.e., initial time) in Figure 5-3, espe-

cially for hurricanes versus a combination of depressions and

storms.) A closely related reason derives from the regularity

or smoothness of the track which correlates positively with the

intensity of the cyclone. The poorly forecasted recurvature

tracks are generally for depressions or storm stages. A

complication in clearly assigning reasons for stage-related

forecast behavior is the fact that the depression and storm

stages are relatively short-lived on the average (see Table

2-1), and since stage in Figure 5-3 is assigned by that

existing at verification time, the peculiarities associated

with these assigned stages are not necessarily representative

of the whole forecast interval.

Af ter-recurvature tropical cyclone tracks are notoriously

difficult to forecast in any ocean. In the eastern North

Pacific (Figure 5-4) such tracks represented only about 13% of

the 24- to 96-hour forecast cases in 1973 and 1974. Decisions

based on forecaster experience are particularly successful for

such tracks, as noted from a comparison of official and

EPANALOG accuracy.

Initiation time of the forecast is also related to

accuracy. Both for the official and EPANALOG forecasts, for

any forecast interval, accuracy is generally best for forecasts

initiated at 1800 GMT. Those starting at 0000 GMT are second

best while those started at 1200 GMT are poorest. The

* The fact that initial position errors are nearly thesame in the case of storms compared to depressions arisesfrom what is regarded as an apparent reluctance to modify 1974operational depression positions in the post-season best-trackprocessing. In 1973, the positive correlation mentioned herewas quite evident.

74

accuracy difference between best and poorest is especially

large for 24-hour forecasts (29% for official and 25% for

EPANALOG). These results relate to cyclone position accuracy,

which relates to the time of day. A combination of maximizing

conventional ship reports, satellite data and reconnaissance

reports (when available) during local daylight morning and

afternoon hours are responsible for the times associated with

best forecast accuracy.

Accuracy of forecasts in the eastern North Pacific Ocean

is comparable to that of other areas with tropical cyclones.

Regularity of life cycle compensates for scarceness of data

and the lack of objectivity in forecasting approach in the

area. Relative to the 1973 operational forecasts of the

western North Pacific and North Atlantic areas, the 1973

EPANALOG forecasts for the EASTROPAC area were found to be of

equal caliber at 24 hours and superior in relative accuracy

thereafter (Jarrell, Mauck and Renard, 1975).

75

SELECTED BIBLIOGRAPHY AND REFERENCES

Alpert, L., 1945-46: The inter-tropical convergence zone ofthe eastern Pacific region. Bull. Amer. Meteor. Soc. ;

Part I - 26, 426-432; Part II - 27, 15-29; Part III - 27,62-66.

Atkinson, G. D., 1971: Forecaster's guide to tropicalmeteorol ogy

.

Technical Rept. No. 240, Hq Air WeatherService (MAC), U.S. Air Force, Scott AFB, IL, 330 pp.

Baum, R. A., 1966: Eastern North Pacific tropical cyclones,1965. Mariners Weather Log , 10, 38-43.

Baum, R. A., 1967: Eastern North Pacific tropical cyclones,1966. Mariners Weather Log , 11, 47-51.

Baum, R. A., 1974: Eastern North Pacific hurricane seasonof 1973: Mon. Wea. Rev ., 102, 296-306.

Baum, R. A., 1975: Eastern North Pacific tropical cyclones,1974: Part 1. Mon. Wea. Rev ., 103, 301-304.

Baum, R. A., and G. E. Rasch, 1975: Digitized eastern Pacifictropical cyclone tracks . N0AA Tech. Memo, NWS WR-101,National Weather Service Western Region, Salt Lake City,UT, 198 pp.

Benkman, W. E., 1963: Tropical cyclones in the eastern NorthPacific, 1962. Mariners Weather Log , 7 , 46-49.

Crooks, R. C, 1960: Tropical cyclones in the eastern NorthPacific, 1959. Mariners Weather Log , 4, 29-32.

Crutcher, H. L., 1973: North Pacific tropical cyclone vectormean charts . Naval Weather Service Command, Washington,D.C. , 10 pp + 90 charts .

Crutcher, H. L. and R. G. Quayle, 1974: Mariner's worldwideclimatic guide to tropical storms at sea . N0AA/EDSNational Climatic Center and Naval Weather Servicepublication (NAVAIR 50-1C-61), 114 pp. plus 312 charts.(Available from N0AA, Asheville, NC)

.

DeAngells, R. M., 1967: North Pacific hurricanes: timid andtreacherous. Mariners Weather Log, 11, 193-200.

76

Denney, W. J., 1972: Eastern North Pacific tropical cyclones,1971. Mariners Weather Log , 16, 76-87.

Freeman, D. L., 1972: Forecasting the motion of northeasternPacific tropical cyclones . MS thesis (R. J . Renard

,

Ad visor), Naval Postgraduate School, Monterey, CA, 61 pp.

Fuller, J. W., 1960: Mary Barbara encounters Manzanillohurricane. Mariners Weather Log , 4, 27-29.

Gray, W. M., 1968: Global view of the origin of tropicaldisturbances and storms. Mon . Wea . Rev . , 96, 669-700.

Gray, W. M., 1975: Tropical cyclone genesis . Atmos . Sci

.

Paper No. 234, Colorado State University, Fort Collins,CO, 121 pp.

Gustafson, A. F., 1969: Eastern North Pacific tropicalcyclones, 1968. Mariners Weather Log , 13, 48-52.

Hansen, H. L., 1972: The climatology and nature of tropicalcyclones of the eastern North Pacific Ocean . MS thesis( R .

3~. Renard, Advisor), Nav al Postgraduate School,Monterey, CA, 178 pp.

Hurd, W. E., 1929: Tropical cyclones of the eastern NorthPacific Ocean. Mon . Wea . Rev . , 57, 43-49.

Jarrell, J. D., C. J. Mauck and R. J. Renard, 1975:Forecasting tropical cyclone motion over the northeasternPacific Ocean by an analog scheme. Mon . Wea . Rev . , 103,674-684.

Jauregui, E., 1967: Las ondas del este y low cicylonestropicales en Mexico (Easterly waves and tropicalcyclones in Mexico). Ingenieria Hidraulica en Mexico,21, 197-208.

Larson, R. N., 1975: Picture of the month - hurricane twinsover the eastern North Pacific Ocean. Mon . Wea . Rev . ,

103, 262-265.

McGuirrin, M., 1965: Tropical cyclones in the eastern NorthPacific, 1964. Mariners Weather Log , 9 , 42-45.

Milton, D., 1974: Some observations of global trends in

tropical cyclone frequencies. Weather , 29, 267-270.

77

Mull, M. W., 1961: Tropical cyclones in the eastern NorthPacific, 1960. Mariners Weather Log , 5 , 34-36.

Mull, M. W., 1962: Tropical cyclones in the eastern NorthPacific, 1961. Mariners Weather Log , 6 , 4 4-46.

Pou, R. L., 1973: The application of aircraft reconnaissancedata to the analysis of the structure of tropicalcyclones of the eastern North Pacific Ocean . MS thesis(R.J. Renard , Advisor) , Naval Postgraduate School ,

Monterey, CA , 38 pp.

Quinn, E. H., 1957: Tropical storms in the eastern NorthPacific, 1956. Mariners Weather Log , 1 , 2 5-26.

Quinn, E. H . , 1959: Tropical storms in the eastern NorthPacific, 1958. Mariners Weather Log , 3 , 37-39.

Ramage, C . , 1974: Monsoonal influences on the annualvariation of tropical cyclone development over the Indianand Pacific Oceans. Mon. Wea. Rev . , 102, 745-753.

Renard, R. J. and E. T. Harding, 1975: Tropical eye lonesteering trajectories (HATRACK and MOHATT) . Sec. 4 . 16,US NWS Numerical Environmental Products Manual, NAVAIR50-1G-522, Naval Weather Service Command, Washington,D.C.

Riehl, H., 1954: Tropical Meteorology . McGraw-Hill Book Co.,New York, 392 pp.

Rosendal, H. E., 1962: Eastern North Pacific tropicalcyclones, 1947-1961. Mariners Weather Log , 6, 195-201.

Sadler, J. C., 1963: TIROS observations of the summer circu-lation and weather patterns of the eastern North Pacific .

HIG Rept. No. 40, Hawaii Institute of Geophysics,Honol ul u , HI , 47 pp

.

Sadler, J. C., 1964: Tropical cyclones of the eastern NorthPacific as revealed by TIROS observations.J . Appl . Meteor . , 3, 347-366.

Serra, C. S., 1971: Hurricanes and tropical storms of thewest coast of Mexico. Mon . Wea . Rev . , 99, 302-308.

Serra, C. S., 1976: On the area covered by h urri cane-

i

nducedrainfall . Preprint vol., Conf. on Hydro-Meteorology,20-22 Apr 1976, Ft. Worth, TX, pp. 13-15.

78

Towry, S., 1975: Eastern North Pacific tropical cyclones,1974, Pare II. Mon . Wea. Rev ., 103, 550-559.

Upton, T. G., 1973: An analysis of the thermal and circulationfeatures of eastern North Pacific cyclones using aircraftreconnaissance data . MS thesis {W. J~. Renard, Advisor),Naval Postgraduate School, Monterey, CA , 52 pp.

U.S. Department of Commerce, National Oceanic and AtmosphericAdministration, 19 76: National hurricane operations plan ,

FCM 76-

1

, Federal Coordinator for Meteorological Servicesand Supporting Research. U.S. Government Printing Office,Washington , D.C. , 104 pp.

U.S. Navy, 1956: Marine climatic atlas of the world: Vol. II ,

North Pacific Ocean . NAVAIR 50-1C-529, Chief of NavalOperations, Washington, D.C., 18 pp. + 275 charts.

Wilgus, R. V., 1958: Eastern North Pacific tropical storms,1957. Mariners Weather Log , 2, 34-37.

Wilgus, R. V., 1964: Tropical cyclones in the easternNorth Pacific, 1963. Mariners Weather Log , 8 , 38-40.

79

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