Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 22 2024 20:31:00 FOUS30 KWBC 222030 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 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!_MO39j3wEyexJI0yiREmtaLxFb-xmXx5KdHRI_j0rrHy= AxN1BAO0OMcqDaAHDCcomQKRZIuNdSELDd3DEksSps6ZomI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_MO39j3wEyexJI0yiREmtaLxFb-xmXx5KdHRI_j0rrHy= AxN1BAO0OMcqDaAHDCcomQKRZIuNdSELDd3DEksSgyJFipU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_MO39j3wEyexJI0yiREmtaLxFb-xmXx5KdHRI_j0rrHy= AxN1BAO0OMcqDaAHDCcomQKRZIuNdSELDd3DEksSlSnXHmA$=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) .