Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 28 2023 19:40:59 FOUS30 KWBC 281940 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 PM EDT Tue Mar 28 2023 Day 1 Valid 16Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL CALIFORNIA COAST AND SAN FRANCISCO BAY AREA, AS WELL PARTS OF THE SOUTHEAST... ....16Z Outlook Update... A few changes were made to the ongoing Marginal and Slight Risk areas from southeastern Mississippi to southeastern Georgia this morning. These changes generally account for latest trends, which indicate that heavier rainfall (and spots of 1-2 inch/hr rainfall amounts) have now shifted eastward toward the MS/AL border eastward across southern AL. The Slight Risk was nudged slightly northward across AL to account for a slight northward trend in convective coverage noted earlier across MS. Areas of 1-2 inch/hr rainfall rates are expected to continue across these areas through the early evening as convection moves from west to east. The overall forecast philosphy is on track for this area, and the previous discussion below has additional details. Across California, no changes have been made to the Marginal and Slight Risk areas. The overall forecast is on track, with IVT values peaking at 500-600 kg/m*s still expected to continue impacting areas from San Francisco southward to near Los Angeles during the forecast period. Cook ....Previous Discussion... ....California... The Day 1 ERO across California remains on track, with only slight adjustments compared to the previous forecast. An upper level polar cutoff low begins the day Tuesday morning centered off the Oregon coast, and tracks southeastward to be centered over the northern California coast by Wednesday morning. A roughly 110 kt westerly upper level jet streak wrapping around the low will direct a plume of Pacific moisture and precipitation into California as it tracks south. PWATs are still expected to peak between 0.75-1.00", which is about +1.5-2 sigma above the climatological normal. IVT values also peak at about 500-600 kg/m*s along central California between ~ 12-18Z Tuesday. This will bring with it a brief increase in snow levels as high as 7,000 ft along coastal ranges during the day Tuesday. The attendant surface low will follow a similar track as the upper level cutoff low through this period, making it a vertically stacked low, that will result in largely unidirectional flow throughout the column south of the low. Since the vertically stacked low has polar origins, it will have plenty of cold air and a lack of plentiful Pacific moisture with it. Thus, this rainfall event is not expected to be as intense as the atmospheric rivers impacting the state in recent weeks. Nevertheless, the residual impacts from those ARs linger across coastal central California, evidenced by the high soil moisture percentiles (>80%) within the top 100 cm per latest NASA SPoRT, and the injection of scattered 3-5 inch rainfall totals per the CAMs into the coastal mountains are expected to cause or worsen flooding impacts as that rainwater flows down the mountains and swells already high rivers. Thus, the Slight Risk was maintained due to steady forecasted rainfall through the Bay area and especially the mountains north of there, while also spanning south to include the westernmost Transverse Ranges of Santa Barbara County. Further inland, slightly lower snow levels, much higher mountains, and more recovery time from the last round of atmospheric rivers were factors that led to the continuance of the Marginal Risk, as much of the heaviest precipitation is likely to fall as snow. The highly favorable antecedent conditions for flash flooding remain a significant factor in the Slight Risk area. ....Central-Eastern Gulf Coast Region and Parts of the Southeast... Compared to yesterday's Day 2 ERO, the Slight Risk was expanded a little farther east across south-central GA and northern portions of the FL Big Bend, to include Tallahassee and Valdosta. Potentially heavy rainfall could overlap areas with recent and/or ongoing rainfall during the Day 1 period. By Tuesday evening, as the main shortwave to the north traverses the Mid Atlantic Region, the surface cold front will get more of a southerly push through North Florida. Beforehand however, pre-frontal convection will capitalize on decent deep-layer instability (mixed-layer CAPEs 1000-2000 J/Kg) along with PWs ~ 1.6" to 1.8" in supporting hourly rainfall rates of 1.5" to 2+". These more scattered cells will have the potential to grow into organized linear clusters ahead of the eventual line of storms associated with the progressing cold front. This cold front should push through the entire region by about 06Z and end the threat of excessive rainfall. 00Z CAMs all show localized rainfall totals of potentially 3-4+ inches, which would pose an isolated to scattered risk for flash flooding, especially over urban areas and areas where heavy rainfall occurred the prior 48 hours. Very low FFGs reside just to the north of the Slight Risk across AL, so this area will need to be monitored in case any heavier QPF amounts shift northward. Hurley/Snell Day 2 Valid 12Z Wed Mar 29 2023 - 12Z Thu Mar 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND SOUTHERN CALIFORNIA... ....2030Z Outlook Update... The ongoing forecast is on track, with an isolated flash flood risk persisting across central and southern California through 12Z Thursday. Onshore flow and IVT will continue on the southeastern periphery of a low west of the California coast. This low will slide southeastward toward the Los Angeles area through the forecast period, with occasional moderate to heavy rainfall occurring due to 400-500 kg/m*s IVT values affecting coastal ranges. Areas of 0.25-0.5"/hr rain rates remain probable in this scenario, with localized flash flooding occurring in hydrologically sensitive/urbanized areas. See the previous discussion for more information. Cook ....Previous discussion... The atmospheric river impacting California will slide southward into Southern California early on Wednesday and orient the axis of highest atmospheric moisture content (400-500 kg/m*s) orthogonal to the mountain ranges of SoCal. Snow levels are expected to be up to 6000 ft across SoCal as well, until crashing Wednesday night and remaining below 4000 ft for much of the state during the Day 2 period. Rainfall rates may briefly peak at 0.25-0.5"/hr while relatively quick moving across the southern part of the Marginal Risk, but saturated soils could support a localized flooding/rapid runoff threat. The second phase of the Marginal Risk (mainly across central CA, but eventually sliding into the Transverse Ranges) involves the vertically stacked strong polar upper-level low sliding across the region. A secondary convergence axis wrapped around this low along with steeping lapse rates and a subtle layer of instability may contain scattered convective showers capable of containing localized heavy rainfall rates. Per the global guidance and GEM-regional, this convergence axis and areas of scattered showers starts near the central CA coast and Sacramento Valley midday until sliding down the central CA coast and into the Transverse Ranges after about 00z Thurs. Total QPF amounts during this entire time period are not that impressive alone, but when combined with the potential 2-3" amounts just prior and the already overly saturated ground, these convective showers and periods of modest rainfall could produce isolated flooding concerns. Hurley/Snell 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!60MmfREHzkMba0lCGTBBjMrKh9Tpt0hjdIS2klVTVVb1= -lDcw04pPv5f_MWYR1_kJEBSyCUKPQEhl_FqfhEYgYDW4C8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!60MmfREHzkMba0lCGTBBjMrKh9Tpt0hjdIS2klVTVVb1= -lDcw04pPv5f_MWYR1_kJEBSyCUKPQEhl_FqfhEYMkYyH38$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!60MmfREHzkMba0lCGTBBjMrKh9Tpt0hjdIS2klVTVVb1= -lDcw04pPv5f_MWYR1_kJEBSyCUKPQEhl_FqfhEYo3oc604$=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) .