Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 13 2022 20:06:14 FOUS30 KWBC 132006 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 305 PM EST Sun Nov 13 2022 Day 1 Valid 16Z Sun Nov 13 2022 - 12Z Mon Nov 14 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Snell/Cook Day 2 Valid 12Z Mon Nov 14 2022 - 12Z Tue Nov 15 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE UPPER TEXAS COAST AND CENTRAL GULF COAST... ....21Z Outlook Update... Southwestward expansions were made to the ongoing outlook to include areas of the middle/upper Texas Coast southwest of Houston/Galveston. 12Z models (specifically the HREF and 3km NAM) indicate a conditional risk of deep convection developing just inland southwest of Houston, with a very moist/marginally buoyant airmass supporting 1-2 inch rainfall totals in the 18-00Z timeframe. A progressive cold front should limit the duration of heavy rainfall threat, however, with the bulk of the heaviest rainfall likely remaining offshore in tandem with the better warm sector. The overall risk should remain focused on low-lying and/or urbanized areas, as it is unlikely that FFG exceedence (~3 inches/hr) will occur on a widespread basis. Farther downstream, storms along the cold front and secondarily along the warm front (located parallel but just inland to the Louisiana coastline) should result in one or two rounds of deep convection across southern Louisiana through/after 00Z, reaching the New Orleans area after 06Z or so. Deterministic QPF varies across this region, although the 12Z Euro is on the higher end of the model guidance with 2-2.5 inch totals in 3 hours at around 09Z across the region. Should CAMs or other guidance trend in that direction, a targeted Slight risk may be needed across southeastern Louisiana. The general consensus is that heavier rainfall will reach coastal Mississippi/Alabama by 12Z, and the Marginal risk was expanded slightly eastward to account for this potential. See the previous discussion below for more details. Cook ....Previous Discussion... By Monday, return flow out of the western Gulf of Mexico ahead of an approaching vigorous shortwave over the southern Plains will set the stage for potentially excessive rainfall extending from the Upper Texas Coast to southern Louisiana and potentially southern Mississippi and Alabama by the late evening hours. PWATs over 1.5" and dew points into the 60s are forecast to push just inland with the help of a slow-moving warm front and 50 kt low-level jet. This moisture source will be short-lived and struggle to push well inland as a fast moving cold front kicks the convection eastward Monday night. The best intersection of instability, moisture, and shear along the warm front is likely to be near the Upper Texas Coast and far southwestern Louisiana. Current QPF depicts widespread 1-3" rainfall amounts are possible within much of the Marginal Risk area and could reach locally higher. However, given the best instability remains right along the coast and over the northern Gulf of Mexico, there remains some notable questions regarding how far inland the heaviest rain will reach. This is especially important from a flash flood forecasting standpoint as most of these coastal regions are marshy and resilient to rainfall of this magnitude. Urban areas are most at risk given the naturally lower threshold for flash floods and ponding water over impervious surfaces. Elevated convection is likely farther north into the Lower Mississippi Valley as well, where 1" rainfall totals could be realized, but should remain under FFG. The only change to the updated outlook was to extend the Marginal Risk farther west along the Upper Texas Coast to capture the HIRES CAM guidance depiction of the early onset convection. Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4ZPSZiV7JAMyXlcozmyw_w72o_36z1v_WOZRLdB6YezE= SJVC6IYLsqGaRt0qD1DKFJxgY5ySsZeUZMbIGCYFRSVcvU0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ZPSZiV7JAMyXlcozmyw_w72o_36z1v_WOZRLdB6YezE= SJVC6IYLsqGaRt0qD1DKFJxgY5ySsZeUZMbIGCYFJUX4OGA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ZPSZiV7JAMyXlcozmyw_w72o_36z1v_WOZRLdB6YezE= SJVC6IYLsqGaRt0qD1DKFJxgY5ySsZeUZMbIGCYFxDGgMEo$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .