1. Case Studies of Warm Season Cutoff Cyclone Precipitation Distribution
Jessica Najuch
Department 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
-In conjunction with Ken Lapenta

2. 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

3. 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)

4. 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

5. 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
-Which we will see in the next slide
-Even recent cutoff’s have showed difficulty in forecasts
-looking at tracks see where most intense..lowest hghts

6. Warm Season Composite Mean Cutoff Cyclone Tracks M. Novak (2002)
-Explain all 170 cases..date ext
-Most popular ..3 out of 4 is GL case
-Repeat how want to watch how precip falls along track..to the right of track..left ect

7. 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
-Changes in orientation such as tilted axis..whether the low pivots, elliptical, elongated
-can look at vort field to determine shape
-closed lows are cutoff cyclones in terms of pv
-cause where there is a closed hght contour ..virtually in all cases there will be a vort max.
-models do not capture shape and stucture of lows very well
-major differences in precip behavior relative to the cutoff structure
-worry about speeds and propagation
-where is precip relative to the shear vector

8. 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 ( )
Calculated daily precipitation and percentage of climatological precipitation

9. Outer Domain

10. inches/day
mm/day

11. inches/day
mm/day

12. inches/day
mm/day

13. inches/day
mm/day

14. inches/day
mm/day

15. % of Climo

16. % of Climo

17. % of Climo

18. % of Climo

19. % of Climo

20. 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 ( )
A cutoff cyclone was defined by one closed 500 hPa isoheight for at least 24 hours

22. 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

23. 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

24. 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

25. 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

27. Maximum Precipitation: Woonsocket, RI 3.58 inches/~91 mm
2-day precipitation (in) ending 12Z 1 July 1998

28. 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)

29. 980630/1200F00 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)

30. 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)

31. V2 L V1 Radar Composite LVL: 1 0800 UTC 30 June 1998
National Composite LVL:1
30-Jun-98 08:00:00 V2 L V1
L – Surface Low
V – 500 hPa Vort Max

32. V3 V2 L V1 Radar Composite LVL: 1 1300 UTC 30 June 1998
L – Surface Low
V – 500 hPa Vort Max

33. V3 L V2 V1 L Radar Composite LVL: 1 1830 UTC 30 June 1998
National Composite LVL:1
30-Jun-98 18:30:00 V3 L V2 L V1
L – Surface Low
V – 500 hPa Vort Max

34. V3 L V1 L V2 Radar Composite LVL: 1 2330 UTC 30 June 1998
National Composite LVL:1
30-Jun-98 23:30:00 V3 L V1 V2 L
L – Surface Low
V – 500 hPa Vort Max

38. V2 V1

39. V3 V2 V1

40. V3 V2 V1

41. V3 V2

42. Surface Pressure (hPa)
Hand Analysis for 1200 UTC
30 June 1998

43. ── Potential Temperature (C)
---- Mixing Ratio g/kg
1200 UTC 30 June 1998

44. ── Potential Temperature (C)
---- Mixing Ratio g/kg
1800 UTC 30 June 1998

47. 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

48. 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

49. 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

50. 558 564 570 250 hPa ↓ jet jet → ← heavy rain ↑ V1 V2 V3 V1 V2 V3 V2 t
t – Δt V3 V2
t + Δt