Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 15 2023 19:28:36 FOUS30 KWBC 151928 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Mon May 15 2023 Day 1 Valid 1927Z Mon May 15 2023 - 12Z Tue May 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS TO THE EDWARDS PLATEAU AND THE MO/OK OZARKS REGION... ....1930Z Special Update... In coordination with SEW/Seattle, WA, PQR/Portland, OR, PDT/Pendleton, OR, and MFR/Medford, CA forecast offices, the ERO was updated to include a Marginal Risk for portions of the Pacific Northwest this afternoon through this evening. Abundant moisture with PWATs around 1 inch, or 2.5 sigma above normal, MUCAPE values over 1,000 J/kg in some spots and very weak steering flow has amplified the potential for strong but very slow moving storms along the various mountain ranges in the Marginal Risk area. 12Z CAMs are hinting at some of that storm development, but may be underdone given all the above favorable conditions for heavy rain. This combined with some snowmelt will support flash flooding, particularly on the various burn scars in the area where the flooding threat is greatest. Wegman ....16Z Update... ....Texas... Current ongoing convection along the Edwards Plateau remains largely stationary this morning. Meanwhile, storms approaching San Antonio and along the Rio Grande moving northwest are locally producing rainfall rates of up to 3 inches per hour. These two areas of convection are likely to merge over South-Central Texas later today, with the front to the northwest helping drive the forcing. While much of the 12Z CAMs guidance doesn't suggest much organization to the storms, they are likely to increase in coverage from the present time, and cold pools are likely to drive additional scattered convective development in the Slight Risk area. Thus, the area was shifted north and west. Meanwhile across south Texas, there are no evident sources of forcing, so convection in that area will be more isolated, and are thus more indicative of a Marginal Risk. The Slight Risk adjustment was made in coordination with EWX/New Braunfels, TX forecast office, and the Marginal Risk downgrade was coordinated with BRO/Brownsville, TX and CRP/Corpus Christi, TX forecast offices. ....Ozarks Region... The showers and thunderstorms ongoing across eastern KS and OK this morning are widely expected to begin shifting east over the next few hours while organizing into a robust, if compact mesoscale convective system (MCS). While the MCS will be moving, the potential for repeating rounds of convection is high. Atmospheric moisture in this region is approaching record territory, with PWATs to 1.5 to 1.75 inches, which is between 2.5 and 3 sigma above normal. 12Z HREF probabilities in this region include a 30% chance of exceeding a 10 year annual recurrence interval (ARI), and a 70% chance of exceeding a 2 year ARI. Neighborhood probabilities include an 80% chance of exceeding 3 inches of rain, while 5 inch probabilities are around 25-30%. The 13Z NBM also highlights this, with numerous areas of the Ozarks of southern Missouri seeing over 3 inches of rain for storm total. As the MCS causing the rain will be moving, this is likely to fall in a much shorter (~6 hour) time frame. Soils in this region are average to above average for moisture content, so it's likely much of that rain will run off. Given the rugged topography of the region, this will further increase the severity of any flash flooding. While this certainly qualifies as a high-end Slight, a potential short-fused Moderate upgrade for a portion of the Slight Risk region is possible as the convective signal becomes more clear in the coming hours. ....Gulf Coast... Some signal for afternoon shower and thunderstorm development remains along the Gulf Coast, for the entire FL Panhandle west through Baton Rouge. With several urban centers located in that corridor that would pose the highest flash flooding risk should one of those slow-moving storms move over any of those cities, the Marginal Risk was expanded west to include Mobile, Gulfport, New Orleans, and Baton Rouge. Wegman ....Previous Discussion... ....Texas... Confidence on exactly where convection will fire up across South Texas during this period remains a uncertain. Moisture and instability is expected to be in place with various boundaries/vort centers passing through that would support convective development. The recent multiday wet period has lowered local FFGs across the region- with much of the state in the 2-2.5 inch range for 3-hr FFG with values as low as 0.50 inch in the vicinity of the Houston metro. South TX appears to be the most likely location for this...both with potential convection moving in from Mexico, and with any convective vort center moving inland out of the Gulf of Mexico. These features are all pretty subtle in nature and will be hard to pin down at this lead time. The Slight Risk area was maintained at this time albeit likely on the lower end of a Slight. ....Ozarks Region... Shortwave energies are expected to traverse northward as a cold front drops south into southern Missouri/northern Arkansas. With the west-east orientation establishing across the central states, and the steady influx of moisture and instability, the convection should become more organized while increasing the rainfall efficiency and threat for flash flooding. The latest runs of the guidance continue to have some latitudinal spread but the more favorable location will be the Ozarks and into eastern Oklahoma. The Slight Risk area was maintained and the encompassing Marginal Risk area had minor reshaping to account for the latest QPF trends. ....FL Panhandle Region... The latest CAMs and global guidance show a signal for locally heavy rainfall across portions of the Florida Panhandle into far southern Georgia, with consensus of 1 to 2 inches with isolated maxes of 3+ inches. While plenty of Gulf moisture and instability will be present, the area remains in a Marginal Risk (largely for flood sensitive areas such as urban Tallahassee) as FFGs in this area are very high, so flash flooding should be isolated at best. Campbell Day 2 Valid 12Z Tue May 16 2023 - 12Z Wed May 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO AND TENNESSEE VALLEYS TO THE CENTRAL AND SOUTHERN APPALACHIANS... ....Ohio and Tennessee Valleys and the Central and Southern Appalachians... With plenty of instability (over 2,000 J/kg, and PWATs to 1.5 inches to the south of the front, there will be plenty of moisture, instability and forcing for heavy rain. Although the nature of this boundary will be more on the progressive side, there will be some upper jet support to work with by this time, with the right entrance region moving across the region. This enhanced lift should result in robust convective development over wet antecedent conditions. Most of the 3-hr FFGs from eastern Missouri to the Mid-Atlantic Coast is in the 1.0 to 2.5 inch range, with the lowest being near the areas of higher terrain. The inherited Slight Risk continues to highlight the areas were the threat for areas of excessive rainfall and flash flooding are more elevated so no adjustments were needed at this time. ....Mogollon Rim of Arizona... Recent runs of the guidance has depicted abnormally high amounts of atmospheric moisture, as high as 4 sigma above normal in this region, though there was initially no forcing to help get storms to form. However they are hinting that weal forcing will be present during this period that could be sufficient in triggering convective activity that could produce heavy rain and isolated flash flooding, especially on any old burn scars, slot canyons, and dry washes. The Marginal Risk area was maintained for this period. ....Intermountain West... A Marginal Risk area remains in effect from the Okanogan Highlands of northeast Washington east to the western Montana Hi-Line. A strong shortwave trough will skim well northeast of the area, but the combination of plenty of antecedent moisture, topography helping with forcing, and 500-1,000 J/kg of CAPE should be enough for some isolated flash flooding, especially along the mountains. The area around Great Falls, Montana is currently in the bullseye for the most rainfall, but that is subject to change. Campbell Day 3 Valid 12Z Wed May 17 2023 - 12Z Thu May 18 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. There is a non-zero threat for moderate to heavy rainfall capable of producing locally heavy rainfall across portions of the Texas and Oklahoma panhandles. The environment will be more favorable during the day 4 period than this one, but areal averages of 1 to 2 inches will very isolated maxes greater than 3 inches will be possible. Given the forecast uncertainty on the specifics, no area was raised for this period. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6I3ihLnGbE1R6aMPkIa_fzJRkm5P9FVZ2jGyNGzDtJBu= xkEANjD5Y3BwNcDk91pv9CRzlBcEURvdZqZWMLInXIZ7pHc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6I3ihLnGbE1R6aMPkIa_fzJRkm5P9FVZ2jGyNGzDtJBu= xkEANjD5Y3BwNcDk91pv9CRzlBcEURvdZqZWMLInmpzRmXo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6I3ihLnGbE1R6aMPkIa_fzJRkm5P9FVZ2jGyNGzDtJBu= xkEANjD5Y3BwNcDk91pv9CRzlBcEURvdZqZWMLInXCgn5u4$=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) .