Flash Flood Guidance
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Issued by NWS
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611 AWUS01 KWNH 180634 FFGMPD FLZ000-GAZ000-ALZ000-MSZ000-LAZ000-181230- Mesoscale Precipitation Discussion 0286 NWS Weather Prediction Center College Park MD 234 AM EDT Sat May 18 2024 Areas affected...Southern LA...Southern MS... Southern AL...Southwest GA...Western FL Panhandle... Concerning...Heavy rainfall...Flash flooding possible Valid 180635Z - 181230Z SUMMARY...Efficient tropical rain showers with rates over 2"/hr possible crossing wet soil conditions from recent heavy rains. Fast eastward propagation may limit very high totals, but scattered incidents of flash flooding becoming possible through daybreak. DISCUSSION...06z surface analysis depicts a weak/subtle wave in proximity to the Pearl River in S MS with a rain-cooled reinforced front extending across SE LA into the Northwest Gulf. A stationary front extends eastward across S AL into S GA; both defined by the 7-10 degree Td differences from upper 60s/low 70s north relative to the upper 70s to isolated 80s from a very warm northern Gulf of Mexico. Additionally, southeast of the front especially in the warm sector south of the surface wave, there remains some temperature/dewpoint spread with temps in the low 80s and higher instability with weakly capped MLCAPEs of 3000 J/kg along and just west of the deeper overall moisture axis that goes from the western Gulf to near Mobile Bay with total PWAT values at or above 2" before pooling along the stationary front further east into GA. Aloft, WV suite denotes a dry slot across SE TX nosing into central LA generally along the DPVA on the southeast periphery of the elongated/positive tilt shortwave trough over E TX into AR. Sufficient shortwave ridging in the subtropical jet across the Western Gulf becomes diffluent downstream into the area of concern through S MS/AL providing a favorable environment for tropical showers/thunderstorms to develop over the next few hours. Deep layer flow is fairly unidirectional from WSW to SW across LA to AL before splitting more downstream into GA, so there will be favored embedded cell motions that may support repeating/training as convection becomes more abundant with time. However, stronger eastward progression/propagation is expected and may limit duration at any given spot but localized interaction/frictional convergence in proximity to the Gulf across SE LA into S MS/S AL and western FL panhandle may allow for increased duration to support the greatest totals which may reach 3-4" though 12z. Otherwise, quick hitting 2-3" totals are possible as the cell expand/mature into S AL. Here, there is some concern that cells will start to cross the most saturated soils where FFG has not rebound and are below the 1.5"/hr and >2"/3hr values, making the potential for flash flooding more likely, though there is some greater uncertainty to the efficiency and coverage of convection being furhter from the Gulf and north of the frontal zone. South-central LA... Uncertainty is very high for potential for stronger thunderstorms to maintain. As mentioned, the front was reinforeced by rain-cooled, stable air over central LA into SW MS. Over-running thunderstorms are becoming more prolific across the NW Gulf approaching the SW LA coast; but given distance from the deep moisture/unstable air, maintaining stronger updrafts is very uncertain. However, the area has also seen prolific rainfall north of I-10/12 across the area and so rates of up to 2"/hr and spots of 2-3" may present a concern as well and so the area is included in this MPD though less confident than areas of SE LA into S MS/S AL. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BMX...FFC...JAN...LCH...LIX...MOB...TAE... ATTN...RFC...LMRFC...SERFC...NWC... LAT...LON 32708500 32408377 31598328 30788408 30258618 30018739 29458837 28918971 29149131 29409271 29779333 30409234 31658910 32478690