Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 27 2025 05:12:02 AWUS01 KWNH 270511 FFGMPD TXZ000-271100- Mesoscale Precipitation Discussion 0087 NWS Weather Prediction Center College Park MD 111 AM EDT Thu Mar 27 2025 Areas affected...Deep South Texas... Concerning...Heavy rainfall...Flash flooding possible Valid 270515Z - 271100Z SUMMARY...Highly uncertain convective environment with ample moisture, persistence of moderate to heavy rainfall may result in possible localized flash flooding potential through early morning. DISCUSSION...A very complex dynamic environment remains across much of southern Texas this evening. An elongated mid-level trough axis exists generally along 30N with multiple weaker vorticity rolls ebbing and shearing with influence of changing lower-level moisture/instability field feeding back through convective feed-back and latent heat release. WV suite denotes favorable right entrance ascent pattern with anticyclonic curved transverse banding features moving downstream into E TX/N LA; while left exit favorable divergence ascent pattern is expected to replace and further support indirect thermal circulation and back low level flow from southeast to easterly to strengthen isentropic ascent, perhaps triggering additional development as noted west of Zapata county along the western moisture gradient at 700-500mb and traced well in the CIRA LPW layers. It can be also noted the new convection is ascending along the veered sfc to 850mb and then further 850mb to 700mb from southeast to south-southwest, respectively. Additionally, RADAR and SWIR loop suggest a weak MCV from older convection just west of NW Webb county, moving east may be affording additional strengthening of the southerly turning of the winds. Overall, the more southerly flow intersects with the dying outflow/convective line across south Texas, that has laid out fairly orthogonal to the strengthening low level flow. Cell motions will be more northward, though this boundary and the effective bulk shear in the 35-40kt range is suggestive of greater cell organization for enhancing low level moisture flux into the broadening/rotating updrafts. Rapid refresh models like the RAP/HRRR have started a trend toward this convective mode, which in turn, trends to increased longevity. Overall deep layer moisture remains solid with .75-1", sfc to 850mb LPW values along the Lower Rio Grande, over-topped with .4-.6" in 850-700mb, though hints of some dry air mixing along that western gradient of dry air pressing eastward may result in some entrainment, but should also aid in steepening lapse rates for increasing some instability for deeper/stronger updrafts, though proximity to modest 700-500mb moisture allows for the total column to remain near or slightly above 1.75"; higher to the southeast nearer the Gulf source.=20=20 However, there has been some mixing/overturning from the initial convection to leave the instability field more scattered and pocketed in nature; likely resulting in some uncertainty on the persistent vigor of the updrafts. However, there are remains of 1500-2000 J/kg of CAPE mainly south of the River, that should allow for cells capable of rates over 2"/hr for an hour or so and scattered within the area of concern. Cell motions will increase potential for flanking line development/repeating with a more eastward component for the stronger/rotating cells and more north-northeastward for weaker ones. As a result the environment for intersection/repeating is going into the storm-scale interactions which are notoriously difficult to assess/forecast and so confidence is not very high in incidents of flash flooding; especially given the area of best overlap/signal of heavy rainfall is generally north and across the rural and naturally higher FFG values of Zapata,Jim Hogg, Brooks, and Kenedy counties. However, proximity of the boundary further south may be poorly resolved in the guidance nearer the urban areas north of the river, and the earlier rainfall in Starr and S Jim Hogg counties may receive an additional 2-4" locally inducing flash flooding. All considered, the risk of flash flooding remains possible through the early morning hours across Deep south Texas.=20=20 Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4rC0G-FOvh2aQdHX2I-yG6TMcHBTxX2b_Mg8p-FVIsNYVWrN_Jl-BdQzArh3JGCZL1dJ= ROIhuOotL9O3bzRCctJeKlM$ for graphic product... ATTN...WFO...BRO... ATTN...RFC...WGRFC...NWC... LAT...LON 27149894 27059845 26849767 26809716 26069705=20 25809737 25999773 25989800 26169857 26389909=20 26749934 27089943=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) .