FLOW CHART TO EVALUATE WET MICROBURST AND LARGE POTENTIAL FROM PULSE OR MULTICELLULAR

TED FUNK SOO, WFO Louisville

Consider these parameters to make informed warning decisions for wet microburst/hail producing pulse or multicell . Understand the pre– 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 ▼ 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 freezing level (0 ºC line)? b) still aloft, c) descending rapidly. ▼ and melting of ice should enhance downdraft. ▼ ▼ ▼ ▼ ▼ ▼ 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 / 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.