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Flow Chart to Evaluate Wet Microburst and Large Hail Potential from Pulse Or Multicellular Thunderstorms

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Flow Chart to Evaluate Wet Microburst and Large Hail Potential from Pulse Or Multicellular Thunderstorms FLOW CHART TO EVALUATE WET MICROBURST AND LARGE HAIL POTENTIAL FROM PULSE OR MULTICELLULAR THUNDERSTORMS TED FUNK SOO, WFO Louisville Consider these parameters to make informed warning decisions for wet microburst/hail producing pulse or multicell storms. Understand the pre–storm and near–storm environment, and its effects on convection, then enter the flow chart at any point to assess pertinent vari- ables. Given values are approximate and may vary geographically and from case to case. SPS = Severe Pulse Storm. UNK = Unknown. ENVIRONMENTAL PARAMETERS ▼ Evaporative Cooling and DCAPE Instability ● Is unsaturated/dry/low θe air present aloft ● Is surface-based LI ≤ -4 & CAPE ≥ 2000? Low-Level Moisture (~750-500 mb)? Is downward CAPE YES YES ● Does θe decrease with height ≥ 12-15 K? ● Are high values of low-level (DCAPE) > 900 J/kg? If so, then evapora- ►YES ►►► ►►► ● Are low-level (0-2 km) lapse rates steep, moisture present (e.g., PW > tive cooling could significantly enhance ▼ i.e., nearly dry adiabatic ( ≥ 6 K per km)? 1.4; 850 Td ≥ 12 ºC; approx)? downdraft and wind damage & hail potential. ▼ ▼ ▼ ▼ ▼ Higher NO ►► Limits updraft strength & hail potnl; weak low-level lapse rates NO ►► Lower SPS potnl but still possible. ◄◄ NO SPS potnl. limit downward momentum transfer to surface; gusty winds still possible. ▼ Vertical Wind Shear ▼ All YES answers: Height of Freezing Level / Wet Bulb Zero ● Does vertical wind shear exist in the ▼ Environment very conducive to YES ● Is freezing level (0 ºC line) < 12500 ft? YES environment? If so, then updraft rotation, severe pulse or multicell ▼ ● Is 7000 < WBZ < 12000 ft (approx)? If so, ◄◄◄ storms with wet microbursts ◄◄◄ S-R flow aloft, and deviant storm motion ◄◄◄ then hail more likely due to a shallower and/or large hail. are more likely to enhance large hail warm cloud layer than outside this range. potential. Stronger/deeper shear is best. ▼ ▼ ▼ ▼ ▼ NO ►► Large hail less likely unless significant dry air aloft; wet microburst still ◄◄ NO possible, especially if YES answers to environmental parameters. REFLECTIVITY DATA ▼ Reflectivity Distribution Boundary Intersections/Cell Mergers Reflectivity Data UNK ● Does composite reflectivity or All Tilts show ►YES ● Is the storm the result of a boundary YES ● Did the storm’s high reflectivity YES dBZ values > 60 aloft while low-level (0.5 deg) ▼ intersection or cell merger from earlier ►►► core initially form at a higher ►►► values are lower (max core is suspended ▼ storms? If so, this will enhance updraft ▼ altitude than neighboring storms ▼ aloft)? Is suspended 50 dBZ core vertically ▼ strength and microburst potential. (indicates a strong updraft)? ▼ ▼ deep (extends well above 0 ºC line)? ▼ ▼ ▼ ▼ ▼ ▼ High SPS NO High SPS NO Higher SPS potnl; NO potnl if envir. favorable; ▼ potnl. ▼ monitor closely for ▼ possible wrng. strongly SPS still possible; SPS still possible; Lower SPS potnl or pulse consider wrng. consider other variables. consider other variables. occurring or already occurred. ▼ ▼ Reflectivity Data Reflectivity vs. Height of Freezing Level Reflectivity Data ▼ ● Assuming other parameters suggest a YES ● Does the top of the 50 dBZ core approach or YES ● Are any reflectivity values ▼ SPS, choose character of storm’s max ◄◄◄ exceed 20000 ft above 0 ºC line and > 5000 ft ◄◄◄ ≥ 65 dBZ present aloft in the ◄◄◄ reflectivity core: a) already descended, ▼ above –20 ºC line? If so, then precip loading ▼ storm near or above the b) still aloft, c) descending rapidly. and melting of ice should enhance downdraft. freezing level (0 ºC line)? ▼ ▼ ▼ ▼ ▼ ▼ ▼ High SPS potnl High SPS potnl ANSWER: NO NO given favorable given favorable a) Descended: SVR threat past. envir; issue ▼ envir; issue ▼ b) Aloft: issue wrng now for lead time. wrng! Slightly lower SPS potnl but still wrng! SPS still possible; c) Descending rapidly: SVR threat possible; consider other variables. consider other variables. imminent; issue wrng at once! ▼ STORM-RELATIVE VELOCITY (SRM) ▼ Mid-Altitude Radial Convergence Updraft Rotation Storm-Top Divergence ● Is strong, deep MARC present in SRM ● Is any mid-altitude rotation present in ● Is strong storm-top divergence YES YES ►YES data (Vin+Vout) ≥ 50 kts from ~8-20 kft storm (indicates strong updraft and present in SRM data? Values ►►► ►►► ▼ (depends on storm height)? Viewing angle possible deviant motion to promote (Vout + Vin) ≥ 75 kts associated may prevent proper assessment of MARC. ▼ better large hail production at surface)? ▼ with hail ≥ 1 inch in diameter. ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ NO High SPS potnl given NO High SPS potnl given NO Higher SPS ▼ main core aloft, drier ▼ main core aloft, drier ▼ potnl but need SPS still possible, especially if bad envir air aloft, & steep LL SPS still possible; envir air aloft, & steep LL SPS still possible; to consider viewing angle; consider other variables. lapse rates; issue wrng!! consider other variables. lapse rates; issue wrng!! consider other variables. reflectivity variables. ▼ SCAN Algorithms Digital VIL / Lightning Trends ▼ OTHER PARAMETERS ►► ● Storm Cells: a) is POSH increasing and ≥ ● Is DVIL near 80 or increasing rapidly 80% AND hail size (hSize) ≥ 1.5, OR b) (intense updraft) and higher than neighbor ▼ were POSH/hSize very high but are de- YES storms? (Note: decreasing DVIL may indicate ▼ YES ◄◄◄◄◄◄◄ creasing rapidly indicating psbl descending ◄◄◄ wet microburst is imminent or occurring.) ◄◄◄ ▼ hail core below freezing level? ▼ ● Is CG lightning count higher than neighbor ▼ ● Hail Diagnostic Grids: Is Digital VIL Den- ▼ storms and increasing (indicates intense ▼ sity values ≥ 4.7? ▼ storm and possibly descending core)? ▼ ▼ High SPS potnl if envir is favorable: ▼ High SPS potnl given ▼ NO NO a) Issue wrng for minimal lead time. ▼ favorable envir and ▼ b) Issue wrng at once as SVR threat is Lower SPS potnl for hail, but still refl parameters; Lower SPS potnl than neighbor imminent or occurring; otherwise, too late. wet microburst psbl; monitor storm. issue wrng! storms, but still possible. .
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