Case Studies of Warm Season Cutoff Cyclone Precipitation Distribution Jessica Najuch Department of Earth and Atmospheric Sciences University at Albany,

Case Studies of Warm Season Cutoff Cyclone Precipitation

Distribution

Jessica NajuchDepartment of Earth and Atmospheric Sciences

University at Albany, State University of New York

Advisors: Lance Bosart and Dan Keyser

NWS Focal Points: Tom Wasula and Ken LaPenta

Introduction

• Forecasting heavy precipitation associated with warm season cutoff cyclones can be very challenging

• These challenges arise from physiographic features and from rapid changes in cutoff cyclone structure

• These forecasting problems are particularly difficult in the northeastern US

Motivation

• Given these forecasting problems, there needs to be more understanding of the diverse precipitation patterns associated with cutoff cyclones

• These precipitation patterns lead to many unforecasted flash floods

• Continue previous warm season cutoff cyclone research done by Matt Novak (CSTAR)

Literature Review

• Hawes, J. T. and S. J. Colucci, 1986: An Examination of 500 mb Cyclones and Anticyclones in National Meteorological Center Predication Models

• G. D. Bell, and L. F. Bosart, 1989: Climatology of Northern Hemisphere 500 mb Closed Cyclone and Anticyclone Centers

• M. J. Novak, 2002: Warm Season 500 hPa Closed Lows

• B. A. Smith, 2003: Cutoff Cyclones: A Global and Regional Climatology and Two Case Studies

Focus

• Stratify precipitation distribution relative to cutoff cyclone tracks as identified in composites developed by Matt Novak (2002)

• Map/understand cutoff cyclone precipitation characteristics in composites especially in relation to terrain

• Document mesoscale precipitation signatures in case studies representative of each of the composites

Warm Season Composite Mean Cutoff Cyclone Tracks M. Novak (2002)

Focus Continued

• Understand role of terrain/low-level jet interactions in determining the precipitation distribution in case studies representative of each of the composites

• Use composites to look for changes in orientation of the cutoff in each case

• Assess precipitation signatures in terms of shear/CAPE profiles in selected case studies

Climatology of Monthly Precipitation Distribution

• NCEP Unified Precipitation Dataset (UPD)

• 51 year dataset, daily observations from 12Z–12Z

• Each day a cutoff with precipitation passed through 34°-48°N and 60°-92°W

• June through September (1948-1998)

• Calculated daily precipitation and percentage of climatological precipitation

Outer Domain

inches/day

mm/day

inches/day

mm/day

inches/day

mm/day

inches/day

mm/day

inches/day

mm/day

% of Climo

% of Climo

% of Climo

% of Climo

% of Climo

Climatology of Monthly Tracks• Used NCEP/NCAR reanalysis dataset

• Plotted 500 hPa geopotential heights at 30 m intervals

• Tracked cutoff cyclones through a subjective hand analysis at 6 hour intervals (1980-1998)

• A cutoff cyclone was defined by one closed 500 hPa isoheight for at least 24 hours

Results of the Climatology• There is a general eastward shift of heavy precipitation due

to cutoff cyclones from June to September

• The most intense daily rainfall associated cutoff cyclones occurs in the month of August

• The highest percentage of precipitation due to warm season 500 hPa cutoff cyclones occurs in the month of June and the lowest in the month of August

• Daily precipitation associated with cutoff cyclones is most widespread along the Atlantic Coast

Cases

1. 6/30/98-7/1/98

-Great Lakes Category of a Closed Low

2. 7/3/96-7/5/96

-Hudson Bay Category of a Closed Low

Case: 6/30/98 – 7/1/98

• Great Lakes Category of a closed low

• Produced all types of significant weather, many tornadoes

• OH, WV, 6-10” of rain

• VT, NY flash floods

Key Players of this Case• Region 1: Severe weather reports due to Midwest nocturnal

convection between 00Z and 09Z on 30 June 1998

• Region 2: Severe weather reports associated with a pre-frontal trough and warm sector between 06Z on 30 June 1998 and 14Z on 01 July 1998

• Huge swath of heavy precipitation fell over NY/PA border extending to Cape Cod

• Jet-dynamics well in place but no strong baroclinic zone present

• 500 hPa trough pivots from positive tilt to a slightly negative tilt

Maximum Precipitation:Woonsocket, RI3.58 inches/~91 mm

2-day precipitation (in) ending 12Z 1 July 1998

980630/0000F00

1000 Hght (m) and 1000-500 Thickness (dam) 850 Hght (m) and Isotachs (m s-1)

500 Hght (dam) and Abs. Vorticity (x10-5 s-1) 250 Hght (dam) and Isotachs (m s-1)

1000 Hght (m) and 1000-500 Thickness (dam) 850 Hght (m) and Isotachs (m s-1)

200 Hght (dam) and Isotachs (m s-1)500 Hght (dam) and Abs. Vorticity (x10-5 s-1)

980630/1200F00

980701/0000F00

1000 Hght (m) and 1000-500 Thickness (dam) 850 Hght (m) and Isotachs (m s-1)

500 Hght (dam) and Abs. Vorticity (x10-5 s-1) 200 Hght (dam) and Isotachs (m s-1)

National Composite LVL:1

30-Jun-98 08:00:00

L – Surface Low

V – 500 hPa Vort Max

Radar Composite LVL: 10800 UTC 30 June 1998

L

V1

V2

L – Surface Low

L

V – 500 hPa Vort Max

V2

V1

Radar Composite LVL: 11300 UTC 30 June 1998

V3

National Composite LVL:1

30-Jun-98 18:30:00

L

L – Surface Low

V – 500 hPa Vort Max

V1V2

L

Radar Composite LVL: 1 1830 UTC 30 June 1998

V3

National Composite LVL:1

30-Jun-98 23:30:00

L

L – Surface Low

V – 500 hPa Vort Max

V1V2

L

Radar Composite LVL: 12330 UTC 30 June 1998

V3

V2

V1

V2

V3

V1

V2

V3

V1

V3 V2

Surface Pressure (hPa) Hand Analysis for 1200 UTC30 June 1998

── Potential Temperature (C)---- Mixing Ratio g/kg

1200 UTC 30 June 1998

── Potential Temperature (C)---- Mixing Ratio g/kg

1800 UTC 30 June 1998

http://weather.uwyo.edu

http://weather.uwyo.edu

Conclusions of First Case Study

• Antecedent convective system over the OH/TN valley provides a moisture source for large swath of precipitation over NY/PA border

• Corridor of heaviest precipitation along NY/PA border and east to southeast New England falls near 200 hPa jet-entrance region and associated 500 hPa vorticity maximum

• Heavy rain is concentrated ahead of well defined surface trough but no strong baroclinic zone is present

Conclusions of First Case Study Continued

• There is dynamical forcing as evident by strong jets and strong 700 hPa ascent

• Precipitation in northern NY and northern New England is likely driven by warm air advection as well as cyclonic vorticity advection beneath the 200 hPa jet

• Convection in eastern PA, NJ, and southeast NY, late on the 30th, occurs beneath the 200 hPa jet in conjunction with a strong low-level jet

What to Watch for When Dealing with 500 hPa Cutoff Cyclones….

• Refer to climatology to be aware of favored areas as well as amount of heavy precipitation

• Pay attention to the location, speed, and track of cutoff cyclones using real time data

• Watch for upper- and lower-level jet dynamics (exit/entrance regions) juxtaposed with vorticity maxima

• Look for surface cyclone development creating low-level flows that draw in excess moisture

558 558

558

564

570

564 570

564570

250 hPa ↓ jet

250 hPa

↓ jet

250 hPa jet →

← heavy rain

↑ ← heavy rain

V1

↑ ← heavy rain

t + Δt

tt – Δt

V2

V1

V2

V3

V2V3