Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Jan 22 2024 09:10:00 FOUS30 KWBC 220832 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 AM EST Mon Jan 22 2024 Day 1 Valid 12Z Mon Jan 22 2024 - 12Z Tue Jan 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND FROM THE UPPER TEXAS COAST THROUGH WESTERN LOUISIANA AND THE ARK-LA-TEX... ....Western U.S.... Unsettled weather is expected to continue for another day as a series of shortwaves continue to impact the region. The shortwave approaching California early this morning will continue to amplify, directing deeper moisture further south into southern California by late morning and continuing into the afternoon. Guidance shows PWs increasing to 2-3 standard deviations above normal along an axis of southwesterly flow ahead of the approaching wave. This moisture (along with left-exit region upper jet dynamics) is expected to help support some locally heavy amounts (1-2") along the coast into the coastal ranges. These rains are expected to fall onto soils already moistened by yesterday's rainfall, contributing to potential runoff concerns. As the trough continues to amplify and move inland, rain will spread across the lower Colorado Basin into southern Arizona. While expected to be beneficial for most areas, this is likely to be the second of back-to-back days of widespread precipitation impacting the area, which may raise localized runoff concerns. Meanwhile, ongoing precipitation early in the period may contribute to some additional runoff concerns along the Sierra foothills and along the central coast and valleys before drier conditions spread across the region late Monday into Tuesday. Further north, persistent onshore flow and energy aloft may contribute to additional locally heavy amounts (up to 1") along the far northern California and southwestern Oregon coasts into Tuesday. ....Upper Texas Coast and Eastern Texas into western Louisiana and Ark-La-Tex... A mid-to-upper level shortwave moving east of the southern Rockies this morning is expected to assume a negative-tilt as it moves east across the southern Plains today. Strong upper forcing, interacting with deep moisture (PWs 1.25-1.5 inches) centered along a 50+ kt low level jet will support the development of heavy rainfall moving east across eastern Texas into portions of western Louisiana and into the Ark-La-Tex. In addition to the strong forcing and ample moisture, increasing instability (MU CAPE of 500-1000 J/kg in the vicinity of the Upper TX Coast) is expected to raise the threat for heavy rainfall rates, leading to localized excessive runoff. The 00Z CAMs present a strong signal for amounts of 2-4" occurring over the region, with the best chance for 3-4" amounts in the vicinity of the greater Houston metro area (per HREF 40-km neighborhood probabilities for 3" exceedance of 40-80%). Churchill/Pereira Day 2 Valid 12Z Tue Jan 23 2024 - 12Z Wed Jan 24 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS INTO THE MID-SOUTH... ....Eastern Texas to the Mid-South... A brief window of relatively drier conditions is expected early in the period as the shortwave impacting the region on Monday lifts to the northeast through the mid Mississippi Valley on Tuesday morning. However, precipitation is expected quickly redevelop back to the west as upstream energy continues to amplify an upper trough moving across the Southwest. Amplifying southerly flow will support a resurgence of deeper moisture across eastern Texas and the lower Mississippi Valley. PWs over 1.5 inches (2.5 standard deviations above normal) will begin to extend north across eastern Texas and the lower Mississippi Valley late Tuesday into early Wednesday. This moisture (along with ample instability and large-scale ascent) will support the development of heavy rainfall across the region. Many models also present a noteworthy signal for southwest to northeast training beginning to develop along a slow-moving frontal boundary, supporting an axis of heavy accumulations over the region (most favored from northeast TX through the Ark-La-Tex). There is still some uncertainty regarding how much convection will develop along the immediate coast and limit moisture advection and the development of heavy rainfall farther north (most drastically depicted by the CMCreg), but most models now agree with the farther inland axis of QPF. This uncertainty has allowed the Slight Risk area to remain rather broad for this forecast iteration, as the only two 00z CAMs that go through the full Day 2 period (the FV3 and NAM-nest) depict a secondary axis of heavy rainfall and training farther southeast across southern LA into southwestern MS (in addition to the primary aforementioned axis). Depending on how heavy rainfall plays out on Day 1, there may be consideration for a targeted Moderate Risk upgrade with future updates (most favored across northeast TX into the Ark-La-Tex). ....Southeastern Arizona... An amplified upper trough along with anomalous moisture will continue to support precipitation across the region early in the period. While widespread heavy amounts are not expected, some additional locally heavy amounts (1"+) may produce some isolated runoff concerns early in the period. Churchill/Pereira Day 3 Valid 12Z Wed Jan 24 2024 - 12Z Thu Jan 25 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST TO THE MID-SOUTH... The aforementioned upper trough over the southwest U.S. will finally make meaningful progress eastward on Wednesday, as embedded shortwaves eject northeastward into the large scale ridge dominating the eastern CONUS. PWs will be near the climatological max (1.5-1.7") across the region of interest, as substantial moisture transport is directed from the western Gulf northeastward. This will result in multiple rounds of heavy rainfall across portions of the central Gulf Coast and into the Mid-South, which global models generally agree upon. In fact, there has been considerable agreement among the majority of the models with regard to intensity with minor east-west differences in the placement of a heavy rainfall axis. The WPC QPF now calls for an axis of 3-6" amounts, and that axis is currently located from southwest and northeast LA northeastward through north/central MS into northwest AL and adjacent portions of West/Middle TN. The GFS, GEFS, and CMC indicate nearly this exact same placement, while the UKMET and NAM are a bit farther southeast. The bigger discrepancy in guidance, however, is with the ECMWF and ECENS, which are much less intense with the gradient and displaced to the northwest (while keeping much of the more intense convection relegated to the central Gulf Coast itself). Given this rather drastic difference in solutions from one of the major model suites, the categorical outlook was held at the inherited Slight Risk (though towards the upper-end of the probabilities) until better agreement among the global suites can be established. Should the GFS/GEFS solution hold (with the ECMWF/ECENS coming into agreement), then a Moderate Risk upgrade would be needed in subsequent updates. Churchill $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .