Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 03 2024 01:18:26 AWUS01 KWNH 030118 FFGMPD TXZ000-030705- Mesoscale Precipitation Discussion 0965 NWS Weather Prediction Center College Park MD 917 PM EDT Mon Sep 02 2024 Areas affected...portions of west-central and northwest TX Concerning...Heavy rainfall...Flash flooding likely Valid 030105Z - 030705Z Summary...Localized hourly rainfall totals of up to 1-2" to continue overnight, occasionally repeating/training and resulting in additional localized totals of 3-5"+. Isolated to widely scattered intances of flash flooding are considered likely, and a significant instance or two of flash flooding are possible (especially given the sensitivity of hilly terrain). Discussion...Areas of stratiform rainfall with disorganized, embedded shower and thunderstorm activity continue across much of west-central and northwest TX (including much of TX Hill Country and Big Country), in the vicinity of a quasi-stationary surface front (and just downstream of a shortwave/closed-low over West TX and adjacent portions of Mexico). While much of the earlier activity over West TX has diminished (as the front has slowly shifted south and east with overturning and destabilization), at least widely scattered convection is expected to persist into the early overnight hours, likely shifting a bit more south and east into the TX Hill Country (towards higher instability, as less overturning has occurred to the southeast with ML CAPE of 1000-1500 J/kg, limited to 100-500 J/kg currently farther north and west). With ample total tropospheric moisture content (1.8-2.2 inch PWATs, between the 90th percentile and max moving average, per DRT sounding climatology) and continued easterly low-level flow/moisture transport overnight, the 20-30 kts of effective bulk shear should continue to organize individual and multi-cell clusters, producing localized hourly totals of up to 1-2" (per MRMS estimates throughout the afternoon). Hi-res CAMs (12z and 18z runs) are a bit of a mess when comparing individual model QPF depictions through 07z, as there is relatively large spatial spread in where 1"+ amounts occur (per the 18z HREF Ensemble Agreement Scale, EAS, which is maximized at only 20% for the 1" threshold to the northwest of San Antonio, smack dab in the middle of the TX Hill Country). Despite this poor spatial agreement for 1"+ amounts, those totals are quite common throughout the various members of the ensemble, as indicated by the 40-km neighborhood probabilities for 2" exceedance (indidacted to be as high as 50-70% in that same area). Even subsequent runs of the HRRR (since 18z) have been depicting these localized high totals, despite being one of the driest members of the ensemble, as a few runs depict isolated totals of 5"+ (and which the HREF 40-km neighborhood probabilities for 5" exceedance even range from 10-30%). Given the current environment, observational trends, and the available model guidance, isolated to widely scattered instances of flash flooding are considered to be likely. This is largely because of the hydrologic sensitivity of the region, given that 6-hr Flash Flood Guidance (FFG) generally ranges from 2.0-3.0". But 1-hr FFGs are often as low as 1.0-1.5" (or even less) over the hilly terrain (with particular concerns for low water crossings). Given the potential for localized totals of 3-5"+, some significant instances of flash flooding cannot be ruled out. Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7JRVjwGI68xkzaShEh7Ck3YtJPm91rS87t7NHAbUon_SIFs5SwiELc4veprcAcFRduVE= Wo_Icse7DFxPA5qhLWEwsQY$ for graphic product... ATTN...WFO...EWX...FWD...MAF...SJT... ATTN...RFC...WGRFC...NWC... LAT...LON 32609932 32519817 31809754 30859759 30019799=20 29259853 29109943 28990043 29500128 30450158=20 31370159 32000127 32490036=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) .