Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Jan 24 2024 09:26:00 FOUS30 KWBC 240848 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 347 AM EST Wed Jan 24 2024 Day 1 Valid 12Z Wed Jan 24 2024 - 12Z Thu Jan 25 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR EAST TEXAS AND MUCH OF LOUISIANA THROUGH SOUTHWESTERN AND CENTRAL MISSISSIPPI... ....Western and Central Gulf Coast to the Southern Appalachians... An upper trough over the Southwest will make meaningful progress eastward today, as a 40-50 kt low level jet out of the Gulf races northward through the lower Mississippi River Valley this morning. This digging longwave trough will support the right entrance region of a 120 kt southerly jet centered over central TX and OK. The area east of it, where the western side of the Moderate Risk is located, will be highlighted for the greatest atmospheric lift in this setup (along with maximized low-level moisture transport and available tropospheric moisture). Training thunderstorms are already ongoing along and near an outflow boundary from eastern TX through southwest and central LA, but should begin to translate eastward with the slow progression of the front. These storms will continue to be capable of 1-2"/hr rainfall rates, and may locally increase to as high as 2-3"/hr where storm segments stall and/or training occurs. The bulk of the expected 24-hr precipitation (as much as 4-8" locally across the Moderate Risk area) will occur with the first (and primary) round of precipitation this morning into the afternoon, but after a lull in the evening return flow (and the lingering frontal boundary) may allow for resurgence of convection along the Upper TX Coast into southwest LA. This could locally exacerbate flooding to be more significant across southwestern portions of the Moderate area (and even farther west into the Upper TX Coast). When added to rainfall that occurred on prior days, the overall result will likely be numerous rapid onset urban, small stream, creek, and river flooding from eastern TX through much of LA and southwest MS. Farther northeast into the Mid-South and southern Appalachians, instability will be much more limited (generally less than 200 J/kg of MU CAPE) which will limit rainfall rates (and resulting totals) significantly. Even still, northeast MS into west-central and northwest AL could see localized totals of 2-3" in relatively short order (in as little as a couple hours), and this could result in scattered instances of flash flooding. Churchill Day 2 Valid 12Z Thu Jan 25 2024 - 12Z Fri Jan 26 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST THROUGH THE SOUTHERN APPALACHIANS... Much of the risk for excessive rainfall on Thursday is dependent on the evolution of convection today across the western and central Gulf Coast, given that a secondary round of heavy rainfall is expected to still be ongoing across far east TX into southern LA into early Thursday morning. Although the intensity of convection will likely be considerably weaker with the second round (as the low-level jet diminishes to 20-30 kts with less, but still anomalous, tropospheric moisture content), much of the heavy rainfall will still be occurring over soils that are forecast to have already have received 3-6"+ of rainfall. As a result, subsequent targeted upgrades to Moderate Risk are certainly possible, and the region most favored for these upgrades is from southern LA eastward into southern MS (and perhaps even extending into southern AL). As of now, additional localized totals of 1-3" are expected across this region, with additional totals of 1-2" more likely farther northern through much of AL, northern GA, and into the southern Appalachians. Scattered instances of flash flooding are possible across the entirety of this domain (which encompasses the entire Slight Risk area) given that forecast totals today will range from 1-3" locally. The most sensitive terrain in the vicinity of the southern Appalachians (East TN, northern GA, and surrounding portions of western NC/SC) are the most likely to realize scattered instances of longer duration flash flooding (due to a lack of instability capping rainfall rates at 1"/hr). Churchill Day 3 Valid 12Z Fri Jan 26 2024 - 12Z Sat Jan 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST AND SOUTHEAST... ....Central Gulf Coast and Southeast... A frontal boundary will continue to linger across the Southeast well into Friday, allowing for yet another day of locally heavy rainfall across much of the same areas for a third day. While there is not a particularly strong signal for organized convection or very heavy rainfall, there is general consensus among the global guidance for additional localized totals of 1-3". This additional rainfall may occur over areas that are forecast to receive 2-8" of rainfall on Days 1 and 2 (with forecast 4-8" amounts confined to much of LA and southern MS), so any subsequent upgrades are highly dependent on how the prior forecast pans out. As of now, localized totals of 2-3" are most favored to occur (per GEFS and ECENS guidance) across southeastern LA into southern MS/AL and the FL Panhandle, and the ERO reflects this with a Slight Risk area. A relatively large Marginal Risk area bounds the Slight, given the uncertainty in the exact placement of the front on Friday and Friday night. ....Oklahoma surrounding portions of southeast Kansas... A cut-off low emerging from the southern Rockies in association with a potent shortwave trough is expected to result in locally heavy rainfall on Friday across portions of the Southern Plains. The GEFS guidance is a bit stronger than the ECENS at this range, suggesting that localized totals may approach the 2-4" range. While still relatively uncertain (given that most global guidance suggests localized totals of 1-2"), an inherited Marginal Risk area was maintained for this cycle. ....Pacific Northwest... An atmospheric river is forecast to spread inland across the Pacific Northwest coast on Friday, likely resulting in 1-3" localized totals along the coastal areas from northwest CA into OR/WA. While the highest totals are most likely in the CA/OR border region, the inherited Marginal Risk areas were expanded a bit to account for the uncertainty and spread in the global guidance at this range. Churchill $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .