Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 09 2024 16:14:15 AWUS01 KWNH 091614 FFGMPD LAZ000-TXZ000-092200- Mesoscale Precipitation Discussion 1145 NWS Weather Prediction Center College Park MD 1112 AM EST Sat Nov 09 2024 Areas affected...southwestern and central LA and surrounding portions of Upper TX Coast Concerning...Heavy rainfall...Flash flooding likely Valid 091600Z - 092200Z Summary...Deep tropical moisture will support very high rainfall rates of 2-3"/hr with 6-hr totals of 3-6"+. Isolated to scattered instances of flash flooding are likely (and may locally be significant). Discussion...A stalling cold front is combining with deep layer tropical moisture transport from the southeast (in association with Tropical Storm Rafael) to result in significant moisture flux convergence into western LA and surrounding portions of the Upper TX coast. Precipitable water levels are indicated to be as high as 2.2 inches (per CIRA advected layered precipitable water analysis and RAP mesoanalysis), which is well above the 90th percentile (per experimental ALPW sfc-850mb and 850-700mb percentiles and LCH sounding climatology) and more typical of late August to early September. Although instability is somewhat limited (500-1000 J/kg of ML CAPE), it remains sufficient for convection and is unlikely to be eroded with steady low to mid level moisture transport coinciding with peak diurnal heating. Likewise, deep layer shear of 20-30 kts will be supportive of organizing updrafts, while 10-15 kts of deep layer flow will keep storm motions near a crawl (particularly so closer to the coast). All in all, a concerning mesoscale setup that will be supportive of locally significant flash flooding. Updated CAM guidance (12z HREF suite) is increasingly supportive of significant rainfall totals over the next 6 hours (HREF 40-km neighborhood probabilities for 5" exceedance, near the 10-yr ARI, threshold being near 15%), suggesting the potential for localized amounts of 3-6" (with rainfall rates of 2-3" due to the deep tropical moisture and slow storm motions). With much of this same area already having received 3-6" over the prior 6 hours (mainly areas west of Alexandria, as well as near Winnie, TX), the associated 4.0" 6-hr FFGs are likely underrepresentative of the flash flood threat. Given these antecedent conditons AND the possibility for rainfall amounts to overperform (given the ARW2 solution of localized 6"+ totals), isolated to scattered instances of flash flooding are considered likely, and locally this flash flooding may be significant. The threat for significant flash flooding is also to likely extend beyond the next 6 hours, so a subsequent MPD is expected (with the threat likely becoming even more significant later this evening). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_9GOqR31doEVg5fnc5xBLOKROmNKwmylL07Vg5HTjGA4LzWucYoszOgIEinx0Dto94TB= hUNxkmu2h2KiO5ZQ46z5h_E$ for graphic product... ATTN...WFO...HGX...JAN...LCH...SHV... ATTN...RFC...LMRFC...WGRFC...NWC... LAT...LON 32279211 32069174 31219173 30399213 29919256=20 29559297 29489403 29509452 30089430 30329421=20 30769397 31419340 31809308 32219254=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .