Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 25 2025 00:06:35 FOUS30 KWBC 250006 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 706 PM EST Fri Jan 24 2025 Day 1 Valid 01Z Sat Jan 25 2025 - 12Z Sat Jan 25 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF SOUTHERN CALIFORNIA... 2030Z Update... Very little change has been made to the previous outlook area involving southern California. The 12Z HREF guidance continues to advertise the development and deepening of a closed low which will drive moderate to locally heavy shower activity especially Saturday night and early Sunday across the Transverse Ranges, the Los Angeles Basin, and gradually portions of the Peninsular Ranges. Generally, with the heavier convective showers that materialize, some rainfall rates should approach and locally exceed a 0.25"/hour, but remain below a 0.50"/hour. On an isolated basis, should heavier sub-hourly rates approach or exceed a 0.25"/hour, there will concerns for debris flow/mudslide activity and possible flash flooding. Orrison Previous discussion... ....Southern California... A developing closed low over central California by late Saturday will foster an area of surface low pressure across the Central Valley which will then refocus offshore of the coastal ranges Saturday night and Sunday morning. This will help set the stage for weak and relatively moist onshore flow to aim into the Transverse Ranges (and eventually into the Peninsular Ranges as well). This onshore flow coupled with DPVA and steepening mid- level lapse rates will allow for the development and expansion of moderate to locally heavy shower activity, and especially by Sunday morning when some pockets of locally stronger convection may be possible. This will include areas from Santa Barbara County southeast through San Diego County, and also some western portions of San Bernadino and Riverside Counties. A somewhat stronger and wetter solution continues to be advertised by much of the latest model guidance, and generally as much as a 0.5" to 0.75" of rain is forecast by Sunday morning (with highly localized stronger orographic ascent/upslope flow facilitating isolated convective activity capable of 0.25"+/hr rainfall rates and totals of 1"+). Given the ongoing wildfire/burn scar sensitivities over the region, these rains Saturday night and Sunday morning may be heavy enough to result in some localized debris flow and related flash flooding activity. Therefore, the Marginal Risk has been maintained over portions of the Transverse Ranges and the Los Angeles Basin. Some local adjustments were made based on the addition of high-res guidance (primarily the NAM-nest) focusing on upslope regions most at risk for highly localized 1"+ totals. Churchill/Orrison Day 3 Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF COAST INTO THE LOWER MISSISSIPPI VALLEY... 2030Z Update... Only very minor tweaks were made to the Marginal Risk area across southern California as an upper-level low center continues to impact the region. Rainfall rates may be a bit higher this period for the Los Angeles Basin and the adjacent foothills of the Transverse and Peninsular Ranges. Slightly greater instability and orographic focusing of shower activity may yield some rates well into the 0.25" to 0.50"/hour range with the stronger convective elements. However, there will also be notably lower snow levels as height falls become more pronounced over the region which will tend to mitigate the concerns for runoff problems. For the more sensitive burn scar locations at lower elevations, the concerns for debris flow/mudslide activity and localized flash flooding will continue. For areas of southeast Texas and into the Lower Mississippi Valley, the models continue to indicate a wave low pressure impacting the coastal areas, with a resurgence of Gulf moisture and a modest plume of instability. There are notable uncertainties with respect to the overall mass field evolution of this system and also how much instability will be present. Overall, the guidance agrees on an axis of moderate to heavy rainfall with embedded elevated convection given the warm advection pattern, but rainfall rates may be tempered by the relative lack of instability. For now, the Marginal Risk area across the region will be maintained, but its possible that future considerations will need to be given to a potential Slight Risk upgrade. Orrison Previous discussion... ....Southern California... Localized areas of moderate to heavy shower activity look to continue into Sunday and Sunday night for upslope portions of the Transverse and Peninsular Ranges (and to a lesser extent the adjacent Los Angeles Basin and coastal areas). An additional 0.75" to 1.50" is generally forecast for much of the same areas as Day 2, and concerns are greatest for isolated convection with 0.25"+/hr rates interacting with recent burn scar areas resulting in the potential for localized flash flooding and debris flows. While the bulk of the rainfall should be beneficial for the region, interests in the burn scars (and more sensitive flash flooding areas) should stay tuned to the forecast as more high resolution model data becomes available in the next 24-48 hours to better hone in on the threat and specific areas. ....Texas Gulf Coast into Lower Mississippi Valley... As a shortwave trough departs eastward by Saturday, low-level return flow will return to the western Gulf Coast resulting in areas of moderate to heavy rainfall by Sunday morning. While areal average QPF is generally expected to be capped at 1"/6-hr period, 24-hr totals could add up to be locally as high as 3-4" (per ensemble bias corrected guidance and GEFS/ECENS probabilities for 3" exceedance of 1-5%). While instability will be rather limited, MU CAPE up to 500 J/kg could support localized embedded convection (with high-res guidance providing more insight in future cycles). For now, the inherited Marginal Risk area was maintained and adjusted based on the latest guidance (and is in line with the GEFS-driven machine learning first guess guidance, depicting a Marginal Risk area). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8Eo9o9VRfCTISih3ekYqwyFhgqKUVGt0OJviR8BNX7zI= 8tNmLu50rivvnWbOeScgX5hGzhdLIhQFJyihhXbuTpbBSn8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Eo9o9VRfCTISih3ekYqwyFhgqKUVGt0OJviR8BNX7zI= 8tNmLu50rivvnWbOeScgX5hGzhdLIhQFJyihhXbubS0jQKs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Eo9o9VRfCTISih3ekYqwyFhgqKUVGt0OJviR8BNX7zI= 8tNmLu50rivvnWbOeScgX5hGzhdLIhQFJyihhXbut8AOvLM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .