Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 25 2023 08:28:12 FOUS30 KWBC 250828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Sat Mar 25 2023 Day 1 Valid 12Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Southeast... A stalled out frontal boundary will be draped across the Southeast from northern Louisiana into the Carolinas Sunday. This front will act as a focus for convection across this region. However, other than the surface front itself, there's very little if any additional forcing that will work to organize and strengthen any convection that develops along the front. Thus, expect pulse and multicellular clusters to be the dominant convective modes. With the up to 1.500 J/kg of MUCAPE and PWATs to 1.5 inches the likelihood of locally heavy rainfall is moderate across this region. However, while the storms will be capable of producing heavy rainfall, their disorganization will keep the coverage of heavy rainfall rather low. Flash flood guidance is exceptionally high across this area, owing to the recent dry weather. NASA Sport shows soil moisture to average about 10-20% saturated, especially from central AL eastward. Any storms would need to produce 2.5-3.0 inches per hour in order to exceed that guidance, and more than that for 3 and 6 hour FFGs. This is highly unlikely to occur, due to both storm motion, disorganization, and unfavorable antecedent conditions. If several rounds of storms happened to go over any one area, one flash flooding event isn't entirely out of the question, but it was determined that this scenario is highly unlikely, and would fall in the 0-5% sub-Marginal risk. Thus, despite the heavy rainfall threat, it's highly unlikely that heavy rainfall will translate to flash flooding. In coordination with BMX/Birmingham, AL, MOB/Mobile, AL, GSP/Greenville-Spartanburg, SC, CHS/Charleston, SC, and ILM/Wilmington, NC forecast offices, all unanimously agreed that the lack of forcing and very high FFGs will all but certainly eliminate the flash flooding threat in this area. Thus, the inherited Marginal Risk area was downgraded to a risk-free map with this morning's update. Wegman Day 3 Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Northern California... The inherited Marginal risk area for coastal northern California north of the Bay Area, and portions of the northern Sacramento Valley was downgraded to a risk-free map with this morning's update. A vertically stacked low will drift off the coast of Oregon on Monday. The upper levels will be supportive of occasionally heavy rain, especially Monday night, as a 90 kt jet streak rounds the base of a highly positively-tilted upper level trough as it slowly moves south down the coast. This trough will bring with it a renewed push of cold air. Flow orthogonal to the coastal mountains will support locally heavier upslope precipitation as the trough, low, and its attendant surface cold front move south down the coast. With this synoptic setup in place, there are 2 primary factors that were considered in the ERO risk downgrade. First, the aforementioned cold air associated with the trough will hold snow levels to an average 1,500 to 2,500 ft above sea level range. Thus, a substantial fraction of the heaviest upslope precipitation into the mountains will fall as snow. Of course, any snowfall will not contribute to any flash flooding concerns. Thus, any potential flooding would be confined to the immediate coast and the lowlands. Further, the upper level pattern bringing cold Pacific moisture from the Gulf of Alaska into the Pacific Northwest, as opposed to tropical moisture, means there will be far less overall moisture available to fall out as precipitation. PWATs only very briefly exceed 1/2 inch with the heaviest precipitation Monday night. Secondly are the antecedent conditions. Of the entirety of the state of California, the area that has recovered the most from recent atmospheric river events has been the northern California coast. Soil moisture has fallen below 70% in this area, showing that much of the rainfall from recent weeks has had time to drain. Thus, FFGs in this area have recovered such that the chances this storm can produce enough rain to exceed those values are very low, especially considering the low snow levels converting most of the expected precipitation into higher elevation snow. In coordination with the MFR/Medford, OR and MTR/Monterey, CA forecast offices, all agreed the low snow levels will keep the flash flooding threat minimal. ....Southeast... Just as on Day 2/Sunday, the same stalled out frontal boundary draped from northern Louisiana into South Carolina will still be in place on Monday. The same scenario plays out again on Monday, where the weak frontal boundary is the only forcing for convection, despite a juicy airmass featuring PWATs above 1.5 inches and MUCAPE along the Gulf Coast getting up to 1,600 J/kg. So, in rinse-and-repeat fashion, thunderstorms are likely to develop again Monday afternoon, perhaps a little further west along the boundary, favoring the Louisiana-to-Alabama portion of the front. High FFGs will once again be the dominant factor preventing flash flooding, as the storms perhaps form around the front, but have little ability to consolidate. The strongest storms may cause briefly heavy rain, but those storms will be isolated enough that the likelihood of resultant flash flooding will be near zero once again. There is somewhat less confidence in the lack of flash flooding threat on Monday, primarily because the rainfall on Sunday will have possibly lowered FFGs in many of the same areas likely to see additional thunderstorm activity again on Monday. While the broad 5 state corridor from LA to SC is favored, where within those states those FFGs will be lowered will only really be known after Sunday's round of storms has taken its course. Thus, future forecast updates will need to evaluate how much overlap Monday's forecasted storms compare with Sunday's storms, of course taking into account just how much modification the FFGs have undergone after Sunday. Despite the potential for a reintroduction of the Marginal Risk at a later time, in coordination with BMX/Birmingham, AL, MOB/Mobile, AL, GSP/Greenville-Spartanburg, SC, CHS/Charleston, SC, and ILM/Wilmington, NC forecast offices, the Marginal Risk was also eliminated with this morning's update. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_N00IPD4O1-qTloH0fmbAQnXj2SE_le9YFWpSR8VKstK= OWSwuam5DungukoR1G6UQ_aX7mJnstlxLfNgoDuyAmlzxqw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_N00IPD4O1-qTloH0fmbAQnXj2SE_le9YFWpSR8VKstK= OWSwuam5DungukoR1G6UQ_aX7mJnstlxLfNgoDuy_q0gp4Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_N00IPD4O1-qTloH0fmbAQnXj2SE_le9YFWpSR8VKstK= OWSwuam5DungukoR1G6UQ_aX7mJnstlxLfNgoDuyjyEz05w$=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) .