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