Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 25 2025 19:45:28 FOUS30 KWBC 251944 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 244 PM EST Sat Jan 25 2025 Day 1 Valid 16Z 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... ....Southern California... A developing closed low over central California will foster an area of surface low pressure across the Central Valley which will then refocus offshore of the coastal ranges tonight into 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 tonight, and especially by early 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. The 12Z HREF guidance does support somewhat wetter rainfall totals locally compared to continuity across especially the San Gabriel and San Bernadino Mountains with as much as 1 to 1.5 inches of rain forecast by early Sunday morning (12Z). This will be aided by orographic ascent/upslope flow into the terrain, and there may be some spotty rainfall rates across these areas approaching and perhaps briefly exceeding a 0.25"/hour. Elsewhere, the rainfall totals going through early Sunday should be generally under 1 inch. However, given the ongoing wildfire/burn scar sensitivities over the region, and especially Los Angeles County, these rains may be heavy enough to result in some highly localized debris flow/mudslide concerns and related flash flooding activity. This will especially be the case if some heavier convective elements materialize and produce higher sub-hourly rainfall rates that can approach or exceed a 0.25"/hour. No changes have been made to the existing Marginal Risk area across the region. Orrison Day 2 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... ...20Z Update: Limited changes were necessary for either risk area during the period as general continuity in the magnitude and orientation of convection for both areas was relatively maintained. Some minor details were added/emphasized within the "Texas Gulf Coast..." sub-heading below. Kleebauer ...Previous Discussion..=20 ....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), though snow levels will be falling and that should help to mitigate runoff issues at higher elevations. An additional 0.75" to 1.50" is generally forecast for much of the same areas as Day 1, 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 potential debris flows/mudslides (driven primarily by sub-hourly rates of 0.25", if able to manifest with very limited instability). 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. Churchill ....Texas Gulf Coast into Lower Mississippi Valley... As a shortwave trough departs eastward this afternoon, low-level=20 return flow will return to the western Gulf Coast along with a wave of low pressure, resulting in areas of moderate to heavy rainfall=20 by Sunday morning. While areal average QPF is generally expected to be capped near 1" over a 3-6 hr period, 24-hr totals could add up=20 to be locally as high as 3-4". While instability will be rather=20 limited, MU CAPE up to 700 J/kg could support localized embedded=20 convection capable of intra-hour rates closer to 2"/hr which adds=20 to the threat, especially within any urbanized zones. There was a=20 better consensus on the threat within the latest CAMs suite with=20 the ECMWF still a bit further southwest with one of the primary QPF maxima compared to the other global deterministic. This is skewed=20 compared to ML output which is pointed further northeast over far=20 east TX into LA with less of an emphasis closer to Houston proper.=20 The current output is still within reason for the MRGL risk but=20 will monitor for any trends in the short term guidance that could=20 sway for a targeted higher risk (SLGT) over the urban corridor.=20 Churchill/Kleebauer Day 3 Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-r7yLU3JNzXf9rBe0sMpWQSGS_EALPaSC2Qi9rdlFeqy= RPWGMW9YrGDlfqSHmLPPAdgyTxwbEB2sJz8sm_Cnm0TcFT4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-r7yLU3JNzXf9rBe0sMpWQSGS_EALPaSC2Qi9rdlFeqy= RPWGMW9YrGDlfqSHmLPPAdgyTxwbEB2sJz8sm_Cnq0dK1fw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-r7yLU3JNzXf9rBe0sMpWQSGS_EALPaSC2Qi9rdlFeqy= RPWGMW9YrGDlfqSHmLPPAdgyTxwbEB2sJz8sm_Cnn2ZW6w4$=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) .