Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 26 2022 01:03:01 FOUS30 KWBC 260102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 802 PM EST Fri Nov 25 2022 Day 1 Valid 01Z Sat Nov 26 2022 - 12Z Sat Nov 26 2022 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDDLE AND UPPER TEXAS GULF COASTS... ....0100 UTC Update... Pared the Moderate, Slight, and Marginal Risk areas a bit based on the latest observational and guidance trends. Still a bit concerned about the instability (or lack thereof), however a few pieces of high-res CAMs (FV3, ARW2, and some of the recent HRRRs) still show pockets of 3+ inches within the Moderate Risk area as the increasing S-SSE moisture transport and deep-layer instability pushes farther inland. Per the 18Z HREF exceedance probabilities, probs of 1-2+ inch/hr rainfall rates also remain highest along parts of the Middle and Upper TX coast, including much of the Houston-Galveston metro area. Hurley ....Previous Discussion... Another round of heavy rainfall is expected to develop this evening through early Saturday morning across portions of Texas, particularly the middle to upper Texas Gulf coasts, which may lead to some instances of significant flash flooding due to the repeating nature of the storms. As of early this morning, the closed upper low is beginning to move into far western Texas and is expected to slowly track east over the next 24 to 36 hours. After yesterday's rainfall event, there is a lull in activity early this morning with a stationary boundary draped offshore and generally northeasterly flow in place. The bulk of today should be relatively quiet but beginning after 21Z, but especially later tonight into early Saturday morning, another round of heavy rainfall is expected to develop. The combination of the strong upper divergence provided by the closed upper low and a jet streak positioned along the coast will provide favorable forcing for ascent across much of the outlook area. In the low levels, anomalously high moisture is expected to be drawn northward through central and eastern Texas with the latest forecast guidance showing upwards of 1.8" to 2"+ PWs over the risk area by this evening, especially along the Texas coast. With the low level jet ramping up after 00Z and upwards of 30 to 40 kts of inflow at 850 mb, deeper convection should develop and track near the mid to upper Texas coasts through early Saturday morning and the slower progression/movement of the synoptic features likely will allow for training/repeating rounds of convection at times within the plume of deeper moisture. Forecast guidance shows an axis of instability hugging the coast (upwards of 1500 J/kg at times) while lesser amounts of near 500 J/kg spread inland into portions of east-central Texas. The amount of instability and how far inland it reaches remains one of the uncertainties and could impact the intensity of the rainfall and its impacts. At this time, the thinking is that the intense rain (1-2" hourly totals with isolated 3" hourly totals possible) will likely be found along the coast as supported by the 00Z HREF probabilities. Areal averages of 2-3" appear likely with isolated amounts of 3-4" possible within the Moderate Risk area. Over the last 24 hours, portions of this area picked up 1" to locally 3"+ of rain so some of the soils are beginning to saturate with the latest NASA SPoRT soil moisture product shows saturation levels reaching 80-90+ percent for the top 40 cm. As a result, this additional rainfall falling on more susceptible locations (including highly urbanized centers) leads to an increased flash flood risk with some potential for localized/isolated instances of significant flash flooding within the Moderate Risk if the training/repeating rounds set up. Taylor Day 2 Valid 12Z Sat Nov 26 2022 - 12Z Sun Nov 27 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF LOUISIANA AND EASTERN TEXAS... ....20Z Update... A Slight Risk of excessive rainfall has been introduced with this update for eastern Texas and much of Louisiana with this update. The primary driver of this upgrade was the excessive rainfall that occurred overnight last night, which drastically decreased FFGs from Galveston Bay into southwest Louisiana, and again in and around Baton Rouge. Both areas saw anywhere from 2.5 to 4 inches of rain overnight. Since these areas are especially susceptible to flash flooding from additional rainfall now, the expected precipitation on Saturday of 1-2" broadly over this area (of course with locally higher amounts with convective elements), will be enough to induce additional flash flooding. Secondly, most of the CAMs have slowed down the departure of the conveyor belt of storms moving northward off the Gulf, such that western areas of the Slight Risk will continue seeing heavy rainfall ongoing from Day 1. With the Day 1 Moderate Risk already in place, it required an upgrade to a Slight for these areas, to go along with the locally lower FFGs. Across northeast Texas and northern Louisiana, the convective training cells are expected to move well inland from the Gulf, while still producing heavy rainfall. Soils in this area remain nearly saturated from recent rains, and are also susceptible to at least a couple rounds of heavy rain from the thunderstorm cells moving over the area. East of the Slight Risk area across MS/AL/FL Panhandle, by that point the storms will be weakening as the main forcing both weakens and races northeast. Therefore, despite the lower FFGs, especially across southern AL, think by that point Saturday afternoon/evening the storms will move through quickly enough and have weakened enough to only cause a few hours of even light rainfall, so the inherited Marginal Risk area remains unchanged. Wegman ....Previous Discussion... Locally heavy rainfall will remain possible across portions of eastern Texas, central Gulf Coast, and the Lower Mississippi River Valley Saturday into early Sunday. A closed upper low, slowly moving over Texas, is expected to lift to the northeast during the period, gradually increasing its speed while also weakening and opening up. At the start of the period, a large area of favorable forcing for ascent will be found over eastern Texas immediately downstream of the upper low while in the lower levels, southerly flow will help draw northward deeper moisture. Precipitable water values are forecast to reach or exceed 1.5" in the outlook area, which is between 2-3 standard deviations above normal. However, deeper instability is likely to remain closer to the coast with amounts up to 500 J/kg likely. This should limit the greatest intensity of the rainfall to the coastal areas despite having the deeper moisture lifting well north and the favorable forcing ahead of the progressing upper low. Rainfall amounts of 1-2" over the period are likely in the Marginal Risk area with some signal for isolated higher amounts across southern Louisiana eastward across the central Gulf Coast. Some of these locations picked up a good soaking rainfall in the last 24 hours and as a result, the soils may be a bit more sensitive to additional rainfall. However, the lack of deeper instability and increasing progressiveness of the system offsets a higher risk category at this time and should keep any flash flooding concerns for urban areas or other sensitive flood prone areas. Taylor Day 3 Valid 12Z Sun Nov 27 2022 - 12Z Mon Nov 28 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7TrogCbEvThP61ckgnym4_I8GFNfDICYWyv-z1CVpo1m= _bm9jgmkUPJV231gk79zJyOS_pYXPzoH4RWKDWkkxpMzECM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7TrogCbEvThP61ckgnym4_I8GFNfDICYWyv-z1CVpo1m= _bm9jgmkUPJV231gk79zJyOS_pYXPzoH4RWKDWkkAhL_V8k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7TrogCbEvThP61ckgnym4_I8GFNfDICYWyv-z1CVpo1m= _bm9jgmkUPJV231gk79zJyOS_pYXPzoH4RWKDWkkaSRTwxU$=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) .