Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 22 2024 20:31:29 FOUS30 KWBC 222031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 PM EST Mon Jan 22 2024 Day 1 Valid 16Z 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... ....16Z Update... No big changes were needed to the risk areas for today into tonight. Out west, the heavy rain continues from L.A. south through to the Mexican border, with more sporadic heavy rain further north. As the SoCal heavy rain area pushes inland, the Slight Risk area for the two metros may be able to be dropped early later this afternoon. Meanwhile, continued off-and-on shower activity into far northwestern CA and southwestern OR will continue into tonight, as well as upslope showers into much of the Sierras. The Marginal risk area for Arizona is for the rain currently in CA as it pushes east, so the event hasn't started there yet. Meanwhile into TX and AR, steady rain continues from north of Corpus Christi through into Arkansas. The southwestern portion of the Slight and Marginal risk areas was trimmed with this update as any organized steady rain has ended for the day. However, many of the Hi-Res guidance suggests late tonight there will be a resurgence of showers and thunderstorms over central Texas, which may then track to the Dallas-Fort Worth Metroplex, so the risk areas were left unchanged in that area. Given the antecedent dry conditions over all of east Texas, the focus for where the highest threat of flash flooding, corresponding to the higher end of the Slight Risk category is likely for the Houston area down to the nearby Gulf Coast, and likely does not extend to well north of Houston. The primary factor driving this shift in thinking is availability of instability, which should largely be held to areas close to the coast and of course the large area of urbanization including the Houston suburbs that locally lowers the FFG values for that area.=20 Wegman ....Previous Discussion... ....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 MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF FAR EASTERN TEXAS AND MUCH OF NORTHERN LOUISIANA... ....2030Z Update... ....Eastern Texas, Southern Arkansas, and Northern Louisiana... In coordination with SHV/Shreveport, LA and LCH/Lake Charles, LA forecast offices, a Moderate Risk area was introduced for the Day 2/Tuesday period with this update. Forecast rainfall has increased and with added support from high-resolution guidance providing a view into the storm evolution, confidence has increased for greater coverage of flash and river flooding in the area Tuesday night. A 1-2 punch of rain is expected to impact the area Tuesday and Tuesday night. The first and what is expected to be less impactful wave of moisture will draw a 40 kt southerly low-level-jet (LLJ) into eastern Texas and Louisiana. This will support widespread convection across northern Texas with less organized shower and isolated thunderstorm activity expected further east into LA/AR/MS. Through the afternoon, scattered showers and thunderstorms are expected across east Texas and northern Louisiana, whereas steadier rainfall is expected over southern Arkansas. These storms will coalesce and become much more organized Tuesday night as a secondary and stronger disturbance moves in from the southwest. Since the front forcing the rainfall will be nearly stationary, the storms are likely to train over the same areas multiple times, as the entire line drifts eastward with time. This will worsen ongoing flooding and is likely to induce new flash flooding across the Moderate Risk area. It's important to note that most of the ERO risk areas will be seeing a round of rainfall overnight tonight, especially across Texas and western Arkansas. Rainfall totals will locally exceed 2 inches. Given the very dry antecedent conditions in this area, this alone is unlikely to be concerning in most areas, save an isolated flash flooding threat. However, as this will be the first of several rounds of rain, each capable of producing several inches of rainfall. Thus, the effects of each of these rounds of rain will compound each other as they move over the same areas again and again. By the Day 2 period, expect greater amounts of rain, particularly across eastern Texas and into northern Louisiana, as local amounts to 5 inches are possible. When added to the rainfall expected overnight, expect more widespread flash flooding with locally significant flash flooding possible. These impacts are expected where the storms producing the heaviest rain are most persistent. Unfortunately that level of detail is still uncertain as to which counties are likely to see those impacts. However, those impacts are most likely in the Moderate Risk area.=20 Wegman ....Previous Discussion... ....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 MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR EAST TEXAS THROUGH NORTHWESTERN ALABAMA... ....2030Z Update... In coordination with all the forecast offices from LCH/Lake Charles, LA northeast through HUN/Huntsville, AL, a Moderate Risk area was introduced with this update. The event will be well underway at the start of the Day 3/Wednesday period at 12Z Wednesday. A 40-50 kt low level jet out of the Gulf will be racing northward up the Mississippi River Valley. A 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. Training thunderstorms stretching as far west as Houston will be ongoing over central and southwest LA northeast into western MS. These storms will be capable of 2+ inch per hour rainfall rates. When added to rainfall that occurred on Days 1 and 2, the result will be widespread urban, small stream, creek, and small river flooding, with more localized larger river flooding. Locally significant flash flooding is very possible, especially in those areas where the ongoing storms Wednesday morning are most persistent. This is likely to be across northern Louisiana, but there is enough uncertainty (and no high-resolution guidance yet this far out) that where this axis of heaviest rain sets up that the Moderate Risk area could still shift. On Wednesday afternoon through the overnight, the upper level jet will continue strengthening to 130 kt over the mid-Mississippi River Valley, as the LLJ from the Gulf continues to stream air with PWATs as high as 1.75 inches into the Southeast. This will allow the storms that could be ongoing from the morning and afternoon hours to persist northeastward into northern Mississippi, Alabama, and potentially as far north as central Tennessee. A couple factors come into play in this part of the country...there's a bit more orographic assistance, especially into Tennessee, but this will be the first significant round of rain for this area, which is still warming up from the recent cold snap. The result is despite somewhat dry soils (below average for this time of year) thawing ground, high rainfall rates, and rainfall totals that could exceed 4 inches in 24 hours will likely be enough rainfall to overwhelm many small streams, creeks, and any urbanized areas to result in widespread instances of flash flooding. The thawing ground and vegetation still in "winter mode" should allow much of the rainfall to translate to runoff instead of being absorbed as the soils act like clay when frozen. Into the southern Appalachians, remaining snow cover from recent winter storms will contribute additional runoff to the local river systems in the form of snowmelt. Thus, expansions of the Marginal and especially Slight Risk areas further east into the mountains and further northeast into the Ohio Valley are probable with future updates. Elsewhere...guidance has continued to slow as far as expected rainfall on Wednesday along the Gulf Coast and south-central Texas. Given the ongoing and expected wet weather in this area, the areas were greatly expanded westward with this update, with the Marginal risk now extending west of San Antonio, and the Slight expanded to near Victoria, TX. Wegman ....Previous Discussion... 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7O-2BDQ55qvXyA1iBeTuX6AK7nIcyy_6jgaXteSa_WvG= frJTZzlM0-kMGKNwix2nuEoWxkFW2RuXe5D7Bu18PuJHpEI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7O-2BDQ55qvXyA1iBeTuX6AK7nIcyy_6jgaXteSa_WvG= frJTZzlM0-kMGKNwix2nuEoWxkFW2RuXe5D7Bu18KPSxI8E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7O-2BDQ55qvXyA1iBeTuX6AK7nIcyy_6jgaXteSa_WvG= frJTZzlM0-kMGKNwix2nuEoWxkFW2RuXe5D7Bu181PeHkSM$=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) .