1. Predecessor Rainfall Events (PRE) in Tropical Cyclones - Results from a Recent Northeastern U.S. Collaborative Science, Technology, and Research (CSTAR) Project
Matthew Cote, Lance Bosart, and Daniel Keyser
State University of New York, Albany, NY
Michael L. Jurewicz, Sr.
National Weather Service, Binghamton, NY
July 10, 2008 – HPC, Camp Springs, MD

2. Outline Data Sources Definition of PRE
Motivating factors / goals for this session
Methodologies for the project
Categorize PRE / Establish climatologies for the Eastern U.S. / Atlantic Basin TC
Provide operational forecasting resources
Composites / Conceptual models
Case Study Examples
Summary

3. Data Sources WSI NOWRAD Radar Imagery HPC Surface / Radar Analyses
SPC Upper-Air / Mesoanalyses
Archived TC Tracks / Positions from TPC
NARR 32-km Datasets
NWS WES Imagery
NPVU QPE Data from NWS RFC’s

4. PRE – What are They ?
Coherent areas of heavy rainfall observed poleward of Tropical Cyclones (TC)
Distinct from the main precipitation shields of TC, or their extra-tropical remnants
Yet, still indirectly tied to TC

5. PRE Example – Frances (2004)
Main Precipitation Shield of the TC
PRE

6. Results of the Frances PRE

7. Motivation for Research
PRE can be particularly challenging phenomena for operational meteorologists
NWP models often underestimate / misplace heavy rainfall associated with PRE
Poor handling of diabatic heating transfer / upper-jet intensification
Attention is frequently diverted to different areas / times
Closer to where TC make landfall
Future time periods when the more direct impacts of TC or their remnants may be expected

8. Goals To provide NWS forecasters / operational meteorologists with:
Background Knowledge / Awareness of PRE
Forecast Tools
PRE Climatologies
Conceptual Models / Composite Charts
Case Study Examples

9. Methodology
We restricted classifications of PRE to systems that met the following criteria:
100 mm (4”) of rainfall needed to be observed within a 24-hour period
Such rainfall had to be connected with a well defined region of precipitation
Not scattered / isolated convection

10. Frequency of Occurrence
Our period of study ran from 1998 to 2006
47 PRE were identified, which were tied to a total of 21 TC
An average of about 2 PRE per PRE-producing TC (PPTC)
About 1/3 of all Atlantic Basin TC that made U.S. Landfall for this period were PPTC
A few outlier PPTC did not actually make landfall

11. PRE Statistics Separation Distance 1086 ± 482 km Median: 935 km
Agnes PRE
Separation Distance
1086 ± 482 km Median: 935 km
Bosart and Carr (1978) conceptual model of antecedent rainfall indirectly associated with TC Agnes (from 1972)

12. PRE Statistics (Continued)
Agnes PRE
Separation Distance
1086 ± 482 km Median: 935 km
Event Duration
14 ± 7 h Median: 12 h
Bosart and Carr (1978) conceptual model of antecedent rainfall indirectly associated with TC Agnes (from 1972)

13. PRE Statistics (Continued)
ROT
Separation Distance
1086 ± 482 km Median: 935 km
Event Duration
14 ± 7 h Median: 12 h
Time Lag
45 ± 29 h Median: 36 h
LOT
Bosart and Carr (1978) conceptual model of antecedent rainfall indirectly associated with TC Agnes (from 1972)

14. PRE Track-Relative Positions26 12 9

15. PRE Track-Relative Positions
Potential for excessive flooding beginning before arrival of TC rainfall 26 12 9

16. PRE Track-Relative Positions
Potential for flooding in areas not directly impacted by TC rainfall 26 12 9

17. Further Sub-Classifications
Separation by Similarity of TC Track:
Southeast Recurvatures (SR)
Highest percentage of PPTC
Atlantic Recurvatures (AR)
Most common TC Track
Central Gulf Landfalls (CG)
Lower percentage of PPTC, but high frequency PRE production within those PPTC
Other “Hybrid” TC that were harder to categorize

18. SR TC Tracks and PRE Locations
All SR PPTC Tracks; with PRE centroids (colored dots)
All SR TC Tracks

19. AR TC Tracks and PRE Locations
All AR PPTC Tracks; with PRE centroids (colored dots)
All AR TC Tracks

20. CG TC Tracks and PRE Locations
All CG PPTC Tracks; with PRE centroids (colored dots)
All CG Tracks

21. Favorable Locations for PRE
Within the Right-rear quadrant (RRQ) of an Upper-level Jet
Ahead of the Mean Long-Wave Trough Axis at Mid-levels (trough axis is west of the parent TC’s longitude)
Near or just upstream from Short-wave Ridging
Near a Low-level Front / Baroclinic Zone
On the periphery of a Tropical Moisture Plume
Near or just west of a Low-level Theta-E Ridge Axis

22. SR PPTC Composites (PRE - 12)
Trough axis
Ridge axis
θe-Ridge axis
700 mb heights (dam) and upward vertical motion (shaded, μb s-1)
925 mb heights (dam), θe (K), and mb winds (shaded, m s-1)
Center of composite TC

23. SR PPTC Composites (At Time of PRE)
Trough axis
Ridge axis
θe-Ridge axis
700 mb heights (dam) and upward vertical motion (shaded, μb s-1)
925 mb heights (dam), θe (K), and mb winds (shaded, m s-1)
Center of composite TC
Centroid of 1st composite PRE

24. SR PPTC Composites (PRE + 12)
Ridge axis
Trough axis
θe-Ridge axis
700 mb heights (dam) and upward vertical motion (shaded, μb s-1)
925 mb heights (dam), θe (K), and mb winds (shaded, m s-1)
Center of composite TC
Centroid of 1st composite PRE
Centroid of 2nd composite PRE

25. Common Detracting Elements for PRE Formation
A Zonal Flow Pattern is in place Poleward of the TC
Lack of merdional flow discourages northward return of deep tropical moisture away from the TC itself
The Long-wave Mid-level Trough Axis is already east of the TC’s Longitude
A Low-level Blocking Ridge is located north / northeast of the TC
Tends to prevent significant moisture inflow into any frontal boundaries or jet circulations that may be poleward of the TC

26. SR Null-Case Composites
700 mb heights (dam) and upward vertical motion (shaded, μb s-1)
925 mb heights (dam), θe (K), and mb winds (shaded, m s-1)
Center of composite TC

27. Case Study (TC Erin, 2007) CG Landfall PPTC
Several PRE were associated with Erin (typical of CG PPTC)
Erin’s PRE exhibited many of the “classic” synoptic-scale ingredients
Within RRQ of an upper-level jet
Deep moisture was fed northward into the PRE / pronounced theta-e ridging developed
A low-level boundary was in the vicinity

28. Track of Erin (Aug , 2007)
20/00z
19/12z
19/06z
19/00z
18/12z

29. Multiple PRE Producer (First 2 PRE)
PRE #2 – 4-8” ( mm) of rain early on 8/18/07
PRE #1 – 3-6” ( mm) of rain late on 8/17/07 (“Along-track” PRE)

30. Locally 12+” (300+ mm) of rain on the evening of 8/18/07
Erin’s 3rd PRE
Locally 10+ “
Locally 12+” (300+ mm) of rain on the evening of 8/18/07

31. Ramifications of PRE #3
12” - 15” of rain fell in 6 hours or less over parts of Southeastern MN and Southwestern WI
Record flooding
Several fatalities

32. MSLP Isobars and Mean 925-850 mb Winds
Water Vapor – 02z, 8/19/07
Significant PRE
Erin’s Moisture Plume L
TD Erin
MSLP Isobars and Mean mb Winds

33. 300 mb Analysis – 00z, 8/19/07
PRE
Jet Entrance Region

34. 850 mb Moisture Transport - 00z, 8/19/07
PRE L
TD Erin

35. Surface Analysis + Radar - 00z, 8/19/07
PRE

36. Flooding Pictures

37. Null-Case Study (TC Gabrielle, 2007)
Became a Tropical Storm over the western Atlantic, before brushing the Outer Banks of NC
Then recurved towards the east-northeast over the open Atlantic (Would be categorized as an AR TC)
No PRE were associated with this TC
Expansive ridge axis blocked advection of deeper moisture into the U.S.

38. Track of Gabrielle (Sept. 8-12, 2007)

39. 24 Hour QPE –Ending 12z, Sept. 10, 2007
Localized 1-2” (25-50 mm) rainfall amounts in a 24 hour period – Available moisture was not associated with Gabrielle

40. MSLP Isobars and Mean 925-850 mb Winds
Water Vapor – 09z, 9/09/07
Frontal Plume of Moisture…Disconnected from Gabrielle
Dry Wedge
Gabrielle
MSLP Isobars and Mean mb Winds

41. 300 mb Analysis – 12z, 9/09/07
Trough Axis
Ridge Axis L
Gabrielle

42. 850 mb Moisture Transport – 12z, 9/09/07
Axis of minimum Theta-e L
Gabrielle

43. Surface Analysis + Radar - 12z, 9/09/07
Ridge axis blocks inflow of moisture towards poleward front

44. Conceptual Model: LOT PRE (SR/AR TC)
UL Jet
LL θe-Ridge Axis
PREs
See inset
ML Streamlines
TC Rainfall
Revised and updated from Fig. 13 of Bosart and Carr (1978)
Representative TC Tracks

45. Conceptual Model (More Detailed Inset) LL Temp/ Moisture Boundary
UL Jet
LL θe-Ridge Axis
LL Temp/ Moisture Boundary
Mountain Axes
UL Jet
LL θe-Ridge Axis
PREs
PREs
Idealized LL Winds
ML Streamlines
TC Rainfall
TC Tracks

46. Summary – Forecast Challenges
NWP models are often poor with the placement / intensity of PRE
Attention is frequently diverted away from potential PRE development
PRE can impact almost any area of the CONUS

47. Summary – PRE Statistics
About 1/3 of U.S. Landfalling TC in our period of study ( ) were PPTC
LOT PRE were the most common
Typically the best synoptic enhancement
AT PRE can be the most dangerous
Double-shot of heavy rainfall
ROT PRE tended to display the highest rainfall rates
Typically slower moving PRE, with less synoptic forcing
Orography perhaps more important

48. Summary – Similarity of TC Tracks
SR TC had the highest percentage of PPTC
AR TC were the most common in our period of study
However, had a lower percentage of PPTC
CG TC had the lowest percentage of PPTC
However, CG PPTC were the most prolific PRE producers (an average of 3-4 PRE per TC)

49. Summary – Favored PRE Locations
Within the RRQ of a strengthening poleward upper-level jet streak
Downstream of a mid-level trough, which is well west of the parent TC’s longitude
Near a low-level boundary
On the northern or western fringes of a deep tropical moisture plume (evident on water vapor imagery)
Near or just west of a low-level theta-e ridge axis

50. Summary – Unfavorable Setup for PRE
A de-amplified, zonally oriented flow pattern is in place north of the TC
The main poleward mid-level trough axis is already at, or east of the TC’s longitude
A low-level blocking ridge is north / northeast of the TC

51. Future Work
Expand PRE database to include the western U.S. (Pacific Basin TC)
Add composites / conceptual models for AT and ROT PRE, and possibly other TC tracks (i.e. CG)
Develop a technique to identify / quantify PRE rainfall in TC precipitation analyses
Perform modeling studies to interrogate the role that TC have in modulating the strength of poleward jets

52. References
Atallah, E. H., and L. F. Bosart, 2003: The extratropical transition and precipitation distribution of Hurricane Floyd (1999). Mon. Wea. Rev., 131, 1063–1081.
Atallah, E., L. F. Bosart, and A. R. Aiyyer, 2007: Precipitation distribution associated with landfalling tropical cyclones over the eastern United States. Mon. Wea. Rev., 135, 2185–2206.
Bosart and F. H. Carr, 1978: A case study of excessive rainfall centered around Wellsville, New York, June Mon. Wea. Rev., 106, 348–362.
Bosart and D. B. Dean, 1991: The Agnes rainstorm of June 1972: Surface feature evolution culminating in inland storm redevelopment. Wea. and Forecasting, 6, 515–537.
Brooks, H. E., and D. J. Stensrud, 2000: Climatology of heavy rain events in the United States from hourly precipitation observations. Mon. Wea. Rev., 128, 1194–1201.
DeLuca, D. P., 2004: The distribution of precipitation over the Northeast accompanying landfalling and transitioning tropical cyclones. M.S. thesis, Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, 177 pp.
DiMego, G. J., and L. F. Bosart, 1982a: The transformation of tropical storm Agnes into an extratropical cyclone. Part I: The observed fields and vertical motion computations. Mon. Wea. Rev., 110, 385–411.
LaPenta, K. D., and Coauthors, 1995: The challenge of forecasting heavy rain and flooding throughout the eastern region of the National Weather Service. Part I: Characteristics and events. Wea. Forecasting, 10, 78–90.
Schumacher, R. S., and R. H. Johnson, 2005: Organization and environmental properties of extreme-rain-producing mesoscale convective systems. Mon. Wea. Rev., 133, 961–976.
Uccellini, L. W., and D. R. Johnson, 1979: The coupling of upper and lower tropospheric jet streaks and implications for the development of severe convective storms. Mon. Wea. Rev., 107, 682–703.
Ulbrich, C. W., and L. G. Lee, 2002: Rainfall characteristics associated with the remnants of tropical storm Helene in upstate South Carolina. Wea. Forecasting, 17, 1257–1267.

53. Any Questions ??
Thank You !!

54. WFO BGM Usage of HPC Products
Days 4-7 Gridded Output (Medium Range)
Common starting point
HPC has access to more model data / better ensembling capabilities (“Master Blender”)
Preferable to always populating with one model (GMOS grids)
Lets us focus on short-term issues

55. Usage of HPC Stuff (Shorter Range)
Model diagnostics
Will view discussions / graphics in more complicated scenarios
Especially when there’s significant model discrepancies
QPF / Excessive Rainfall
Will often use HPC QPF, or a blend of HPC and other model QPF’s in the first 24 – 48 hours
Depending on timing, may use data from a previous model cycle
Will utilize Excessive Rainfall discussions / graphics as guidance in heavy precipitation situations
Winter Weather Desk
Will typically view WWD graphics as a “reality check” against our thinking
Particularly with mixed phase events / model disagreements