1. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
AFWA/XOGM

2. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Description
Classification
Tropical vs. Subtropical ?
Climatology
Imagery Examples
Intensity Analysis Procedures

3. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Description
Known by several different terms
Kona (Hawaii)
Hybrid System (Gray 1968)
Semitropical (Spiegler 1972)
Subtropical cyclones (Simpson 1952)

4. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Description (continued)
Definition:
Low level manifestation of a cut-off low
(from Glossary of Meteorology)
Can exhibit both extratropical and tropical characteristics.
CAN transform into full tropical cyclones - even hurricane intensity!
We are required to do fixes on these systems!

5. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Definition (Glossary of Meteorology, 2nd Edition)

6. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Classification
Three Different variants:
Cold Low
Frontal Wave
Circulation that develops east of low latitude troughs, but is not a frontal wave or low (has no true fronts - no air mass discontinuity)

7. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Classification
Tropical vs. Subtropical ?
How do you distinguish between tropical & subtropical cyclones based on METSAT imagery?
Other clues?

8. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Classification (continued)
Frontal Bands have a typical frontal cloud structure. Look to baroclininc waves that become cut off from the regular flow.
Cold Lows have a circular cloud pattern with limited convection near center
Non-frontal Circulations (found east of troughs) have an amorphous cloud structure

9. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Classification (continued - Other clues)

10. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Classification (continued)
Look for clues in the upper level pattern
There should be a cut off low at 500 mb!
See examples of flow pattern

11. Cut -off low
500 mb heights
NOGAPS Analysis

12. Cut -off low

13. Cut -off low

16. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Climatology
Past Tracks
Points of origin/cyclogenesis
Type of system
NOTE: Atlantic and North Pacific only.

17. Climatology: Subtropical Cyclones

18. Climatology: Subtropical Cyclones

19. Climatology: Subtropical Cyclones
A: Cold Low
B-1: Frontal Wave
B-2: LLCC east of
U/L trough

20. Climatology: Subtropical Cyclones
Hawaii

21. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
IMAGE EXAMPLES
Azores
Western/Central Pacific

22. Subtropical Cyclones
Azores

23. Subtropical Cyclones
Azores

24. Subtropical Cyclones
Azores

25. Subtropical Cyclones
Azores

26. Subtropical Cyclones
Azores

27. Subtropical Cyclones
Central Pacific

28. Subtropical Cyclones
Central Pacific

29. Subtropical Cyclones
Central Pacific

30. Subtropical Cyclones
Central Pacific

31. Subtropical Cyclones
Central Pacific

32. Subtropical Cyclones
Central Pacific

33. Subtropical Cyclones
Central Pacific

34. Subtropical Cyclones
Central Pacific

35. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Intensity Analysis Procedures
Hebert / Poteat (H/P) Technique
Developed by National Hurricane Center
Study done in
Better verification than Dvorak for these systems
Completed July old technique
Reference:
NOAA Technical Memorandum NWS SR-83

36. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique
“ST” numbers (instead of “T” numbers) are used to describe intensity
Data T Equivalent first
Then the Pattern T
No constraints

37. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
ST 1.5 (25-30 knots max sustained winds)
Low Level Circulation Center is displaced from poorly organized convection (not necessarily dense convection) by at least 0.5 degrees latitude (30 nm) but no more than 2 degrees (120 nm)
For Cold Lows, convection MAY NOT be connected to other systems and a small area (< 3 degrees latitude or 180 nm) of deep layer convection exists near the center.
If the system meets the requirements of either of the above statements, then the intensity is ST 1.5

38. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
ST 2.5 (35-40 knots max sustained winds)
Low Level Circulation Center is displaced from increased deep layer, organized convection (not necessarily dense convection) by at least 0.5 degrees latitude (30 nm) but no more than 2 degrees (120 nm). The cloud pattern should exhibit a more markedly curved spiral band pattern (as a rough guide, it should wrap at least 0.5 divisions on the Log10 spiral).
The outer convective band should be displaced 5 to 10 degrees latitude ( nm) east of the the LLCC and possibly another exists 2 to 4 degrees ( nm) west through north of the LLCC.
If the system meets the requirements of either of the above statements, then the intensity is ST 2.5

39. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
ST 3.0 (45-50 knots max sustained winds)
Same criteria as with ST 2.5, except the system should exhibit greater SBC and better organized convection than the previous day. Overcast may be dense.
There is evidence of banding near the LLCC (less than 1 degree latitude - 60 nm - from LLCC).
If the system meets the requirements of either of the above statements, then the intensity is ST 3.0

40. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
ST 3.5 (55-60 knots max sustained winds)
Deep layer convection (frequently with dense overcast) is evident in band(s) 1 to 3 degrees latitude ( nm) from LLCC (NO CENTRAL DENSE OVERCAST!).
The outer, eastern convective band (5 to 10 degrees latitude from the LLCC) is weaker than the day before, but a new band may be forming to the west.
For systems moving rapidly eastward, there may be only a dense overcast ( > 3 degrees latitude) about 2 to 4 degrees east of the LLCC
If the system meets the requirements of any of the above statements, then the intensity is ST 3.5

41. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
NOTE for ST 3.0 and 3.5 ONLY:
If the forward speed of the system exceeds 20 knots, the excess speed should be added to the maximum wind speed obtained by the criteria for ST 3.0/3.5
Example: ST3.0 storm is moving 26 knots - ST3.0 becomes ST3.5

42. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
NOTE ST 3.5 ONLY:
SSM/I GALE WIND RADII BULLETINS MUST BE COMPLETED FOR SYSTEMS THAT ATTAIN AN INTENSITY OF ST3.5

43. Hebert / Poteat (H/P) Technique Pattern T

44. Cloud Patterns in Subtropical Cyclones / Hybrid Systems
Hebert / Poteat (H/P) Technique (..continued..)
Constraints:
There are no documented constraints on the H/P technique
No restrictions on initial classification
Can start the storm at ST2.5 without ‘breaking constraints’

45. Questions? Questions? Developed by Paul McCrone, Chief Forecaster
Air Force Weather Agency, Global Weather Division
METSAT Applications Branch (Office: XOGM)
- DSN