Hail Forecasting

1. Hail Forecasting Jeffry Evans SPC

2. Hail Basics In order to grow hail, you need: • Embryo • Liquid Water • Cold Temperatures • Time within updraft The most destructive hailstorms are supercells!

3. Hail Basics In order to grow hail, you need: • Embryo • Liquid Water • Cold Temperatures • Time within updraft The most destructive hailstorms are supercells!

4. 3. Cold temperatures • Aloft • Lapse Rate • Instability • Increased residence time within sub-freezing • Lower Troposphere • WBZ • Freezing levels • Evaporation beneath cloud base

5. 4. Time within updraft • Smaller stones fall slower & have more melting area. • Supercell? • 0-6 km shear > 35-40 kt favors rotating updrafts. • Well demonstrated that rotating updrafts enhance buoyancy. • Perturbation pressure gradient/Vertical accelerations • Estimates of 50% of updraft velocity • As storms near a “steady state”, updrafts and residence time for hail increase. • Updrafts intensify and “tilt” over inflow region • WER • BWER

6. Some studies/algorithms • Pino and Moore (1990) • Use soundings and a one-dimensional cloud model to estimate hail size. • Takes into account melting effects in warm, Tw • Unfortunately, provides little skill in operations. • Lack of shear term. • Brimelow et al. (2002) • Combined cloud model and hail growth model • Is capable of distinguishing between non-severe and severe-hail events.

7. Brimelow et al. (2002) • Uses a CAPE * 0-6 km shear parameter (normalized) to estimate duration of updrafts (EnergyShearIndex), based on 5 previous studies of supercell updrafts: ESI values >5 are assumed to have no entrainment into the updraft. Once ESI reaches 5, no further benefits to hail growth.

8. Edwards and Thompson (1998) • WBZ, CAPE, CAPE density, and other thermodynamic parameters alone showlittle skill in forecasting hail size.

10. Edwards and Thompson (1998) • WBZ, CAPE, CAPE density, and other thermodynamic parameters alone showlittle skill in forecasting hail size. • Similar results in examination of VIL and VIL density. • “…on a nationwide basis, commonly used hail predictors showed little or no skill in predicting hail size “ • Stresses need to include kinematic fields to better forecast hail size.

11. Local study at SPC • Collection of large hail sounding data base at SPC supports this (Jewell, ~400 sndgs). • Must incorporate shear for accurate hail size forecasting!

12. SPC operations • Main concern with both previous studies is limited coverage of UA soundings. • Problem addressed at SPC by using model forecast soundings (PFC).

14. SPC operations • Main concern with both previous studies is limited coverage of UA soundings. • Problem addressed at SPC by using model forecast soundings (PFC). • Modified Brimelow used at SPC since 2001 (Jewell). • Uses ensemble approach (25 members), changing T and Td up to 1o C. • Results have been favorable, especially in discriminating very large hail events from lesser size hail.

16. Actual hail: .88-1.0 inch

17. Hail Model

18. Hail Model – RUC40

19. Hail Model – NAM40X

20. Verification

21. SHIP - Sig. Hail Parm. SHIP = MUCAPE * MUMR * 7-5LR * 500T* 0-6 SHR / Constant MUMR = Mixing Ratio of Most Unstable Parcel

22. Sig. Hail Parameter Distribution of SHIP magnitude for SIG (right) and NON-SIG (left) Same as before, but 1.75” and 2.00” removed “Choose your own denominator”

23. 4.25” 2.00”

24. Verification

25. SHIP on Mesoanalysis Page Remember, SHIP values do not equal forecast hail sizes!

26. Lets look at a couple of cases:

27. WBZ/Freezing levels not particularly low

28. 4.9 0.7

29. 8 sig. hail rpts Ern NC; 3” max occurred near 21z

32. Long-lived, tornadic supercell produced swath of sig. hail with several 4.25”.

33. Things to keep in mind: • No singular parameter responsible for hail growth/production. • Many variables go into hail growth, all dependent on each other! • Mustn’t trust thermo. parameters by themselves.