Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 03 2024 18:53:03 FOUS30 KWBC 031852 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 151 PM EST Sat Feb 03 2024 Day 1 Valid 16Z Sat Feb 03 2024 - 12Z Sun Feb 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL CALIFORNIA COAST RANGES AND ACROSS SOUTHERN LOUISIANA AND FAR SOUTHERN MISSISSIPPI... ....Central Gulf Coast into the Lower Mississippi Valley... A surface low that recently emerged into the western Gulf of Mexico and its related inverted trough to its north and warm front to its east are expected to be the main drivers for heavy rainfall along the Gulf Coast. Well inland, a TROWAL is expected to enhance moderate to heavy rainfall across portions of OK/KS within a moisture regime near the 99th percentile for this time of the year, per standardized anomalies. While the KS/OK convection should pivot slightly in place around a pair of mid- to upper-level lows, convection should remain fairly progressive across the Gulf Coast as the low moves to the east, continuing to be a detriment to very heavy totals. This continues to be illustrated in the latest HREF hourly probabilities for .50" and 1"/hr amounts. Left the Slight Risk area across southeast LA and southern MS which stand the best chance for raising MU CAPE to 500-1000 J/kg and by extension heavier rainfall. We continue to believe the slight risk is a low end slight for southeast LA and southern MS, outside of any urban issues. Changes made to the previous outlook area were to shrink the Slight Risk area with some dropping of the western sides of the risk areas due to the eastward progression of moderate to heavy rainfall. ....Central California... The beginning of the next atmospheric river event for central to southern California will arrive Saturday night into early Sunday with IVT values of 600+ kg/m/s impacting coastal central California. HREF neighborhood probabilities for 3"+ amounts ending at 1200 UTC Sun are high in the max onshore flow/IVT area, due to both 50-75 kt 850 hPa inflow into the region and MU CAPE occasionally approaching 500 J/kg, which could allow hourly rain totals to approach 1" early Sunday morning/right at the end of the forecast period. Some expansion was made to the east into the Sierra Nevada to account for the onset of heavy rainfall mostly due to low- to mid-level frontogenesis moving in from the west and some snow melt and a little bit of contraction made along the western sections of the Transverse Ranges, using 1"+ in 24 hours as a guide to keep the threat area most consistent with continuity. For the entire next AR event, the risk is much greater than depicted Saturday into early Sunday morning across this region, but the timing is such that only a slight risk is warranted through 1200 UTC Sunday, with much heavier precip and excessive rainfall risks thereafter. Roth/Oravec Day 2 Valid 12Z Sun Feb 04 2024 - 12Z Mon Feb 05 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Central to Southern California... The latest models are in good agreement on the evolution of a strengthening cyclone off the central California coast early Sunday as this low pushes northward towards the northern California coast. The strong IVT values that begin to impact the central California coast late day 1, ~1000 kg/m/s, will persist into central California coast range, pushing inland into the northern portions of the San Joaquin Valley and Sacramento Valley and southward into the Transverse Range. Strong inflow and available instability across the area will allow for 1" an hour rain totals, which would be especially problematic where there are burn scars. The probabilities of 8"+ in 24 hours are high enough to introduce a High Risk for portions of the southern CA Transverse ranges. General 3 to 5" areal average amounts likely in both the Transverse and Central California Coast Ranges, with maximum totals just over 10" possible in areas of most persistent training and upslope flow. Modest expansions were made to the Slight and Moderate Risk areas. For portions of eastern CA near the NV border, including Inyo County/Death Valley, inflow from the Gulf of CA is expected to be more or less unimpeded with 850 hPa winds 40+ kts out of the southeast, setting up a moderate to heavy rain event in the desert. Probabilities of 3"+ in 24 hours is high, particularly in nearby topography where some of the precipitation would be in the form of snow at elevation. The signal was strong/broad enough to lead to some expansion of the Marginal Risk area in that area. ....Southeast, eastern Gulf Coast and Florida... An extremely anomalous cold low pushes east-southeastward through the eastern Gulf of Mexico towards the central and southern FL peninsula. Strong southerly to southeasterly low level flow will press across the eastern Gulf coast and the Southeast near its cold front and within the system's comma head. This will support a broad region of moderate to heavy precip totals across the Southeast and adjacent northeast Gulf Coast. Heaviest totals likely where this precip area pivots and low level east southeast to easterly flow persists over the same region the longest. Model consensus is for this to be from the Lower coast of South Carolina, across south Georgia , North Florida into southern Alabama and far southern Mississippi. Observed precip amounts have been below average over the past few weeks across this region, warranting just a marginal risk at the moment. The best area of instability and potential for organized convection is across Southwest to South Florida late Sunday afternoon/evening.=20 The southern peninsula Marginal Risk has been shifted from southeast FL to southwest FL (where the best onshore flow is expected) and is very dependent on the old occluded, barotropic cyclone moving southeast through the Gulf of Mexico becoming convective in nature. A first batch of heavy rainfall is expected with the front very early on, before the barotropic low approaches, with two possible instances of heavy rainfall on either end of the 24 hour period (Sunday morning and early Monday morning). The barotropic convective low is expected to be driven by 500 hPa temperatures approaching -25C moving over sea surface temperatures of 23C, which leads to a strongly unstable atmosphere. The 1000-500 hPa thickness pattern suggests that the low would be moving more east than south, if convective.=20 Precipitable water values rise back above 1" due to the convection within a 1000-500 hPa thickness regime near 5520 meters, impying complete saturation. Overall, local amounts of 3" are anticipated with hourly rain totals potentially getting to 1.5" with each instance of heavy rainfall Sunday morning and early Monday morning. There are questions about how convective the low would be on final approach, and guidance isn't extremely enthusiastic with its precipitation forecast, hence limiting the risk at Marginal. The threat is conditional on convection/sufficient instability hanging on near the low's center at FL landfall, which isn't a given. Roth 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!9eB3DF9Y5O-MnSyxWR9eOX-yllcROOSWP9C5oAc8cyAQ= RZ0TVZRtno6ZgTiXiNJ47myHpb1TPo23Mjt6bmUwdjOtqL0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9eB3DF9Y5O-MnSyxWR9eOX-yllcROOSWP9C5oAc8cyAQ= RZ0TVZRtno6ZgTiXiNJ47myHpb1TPo23Mjt6bmUwnOQGQXQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9eB3DF9Y5O-MnSyxWR9eOX-yllcROOSWP9C5oAc8cyAQ= RZ0TVZRtno6ZgTiXiNJ47myHpb1TPo23Mjt6bmUw3czvCz0$=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) .