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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