Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 24 2023 20:16:01 FOUS30 KWBC 242015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Fri Mar 24 2023 Day 1 Valid 16Z Fri Mar 24 2023 - 12Z Sat Mar 25 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE OZARKS TO THE OHIO VALLEY... ....16Z Update... Expanded the Moderate Risk area a bit in deference to the 12Z HREF probabilities which has signals for 2+ inch per hour rates later in the afternoon across Arkamnsas just a bit south from the placement shown by earlier runs. Overall, though, there is good consistency in that it showed redevelopment of storms capabale of producing intense rainfall along with the potential for overlap with areas that received moderate to heavy rainfall overnight.=20 Elsewhere...the outlook areas were still in good shape and only minor changes were made there. Bann ....0830Z Excessive Rainfall Discussion... Showers and thunderstorms associated with a stalled out frontal boundary are ongoing across northern portions of the risk areas this morning. The trend in all of the convection is to gradually drift southward, and that was the theme in most of the CAMs guidance as well. Thus, the official WPC QPF forecast has shifted the axis of heaviest rainfall south, to include much of central and northern AR, the bootheel of MO, northwestern TN, and western KY. This is the area highlighted by the updated Moderate Risk. Coordination of the new Moderate Risk area was done with LZK/Little Rock, AR and OHX/Nashville, TN forecast offices. With the southward shift in the guidance, the heaviest rainfall is now mercifully separated from the areas currently experiencing flash flooding, generally across northeastern OK and southwest MO. Thus, the Moderate risk area is sufficient and no upgrades to a High are expected, short of some unexpected large increase in rainfall. It is worth noting as far as river levels are concerned that the heavy rain ongoing across central MO and southern IL and IN are upstream of where the heaviest rain is expected later today, which could locally increase river flood potential as runoff from current rain moves into areas expected to get heavy rain. PWATs in the Moderate Risk area remain between 1.25 and 1.5 inches, which is 3 sigma above normal. As a surface low develops and the LLJ rapidly intensifies this evening, expect PWATs in the mid-Mississippi Valley to spike as high as 1.75 inches, which is 4 sigma above normal. This will be when the heaviest rain is expected. Fortunately, it will be associated with a progressive cold front, which will drastically limit the time these water-logged areas see rain rates potentially getting as high as 2 inches per hour. NASA Sport imagery shows soils from northeastern AR across southern TN are saturated or are nearly so, so the soils in this area are unlikely to be able to handle multiple inches of additional rainfall. The real question is how far south the training thunderstorms will get today, with the further south into TN you go, the more saturated the soils already are from recent rains. For now the southern edge of the Moderate Risk area only partially overlaps the most saturated soils following the TN and MS/AL/GA border. However, any further southward adjustment in the guidance would raise the flood risk in these areas. Since the storms will be largely tied to the stalled warm frontal boundary before the cold front sweeps all the heaviest rain to the east, expect a roughly 100 mile wide corridor of heaviest rainfall, and outside of this frontal boundary area, there will likely be a fairly steep gradient in rainfall totals. As such, the previous Slight and Marginal risk areas were trimmed, especially on the northern side across MO/IL/IN as the last of the rainfall is ongoing as of the time of this writing, with minimal additional rainfall in these areas expected once the main frontal boundary sags far enough south. Further, instability will be minimal in these areas through the day today. Eastern areas of the inherited Slight and Marginal risk were trimmed significantly with this morning update across eastern TN, eastern KY, southern OH, western WV, and far southwestern PA. The aforementioned cold front will be the primary rainmaker by the time any significant convection reaches these areas, and as mentioned above, it will be rapidly transiting east. Further, convection associated with the warm front won't be particularly heavy in these areas, as all the best forcing remains well to the west. Thus, expected rainfall is unlikely to result in anything more than isolated flash flooding, especially since soils in this area are quite dry. ....Potential Risk Category Changes With Future Updates... None expected. Wegman Day 2 Valid 12Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 3 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... There is a Marginal Risk of excessive rainfall across portions of the Southeast U.S. as moisture starts getting drawn north from the Gulf of Mexico and encounters a quasi-stationary front that sets up across the region. In response to the increasing moisture, the atmosphere should become increasingly unstable with upwards of 1000 J per kg of MUCAPE. At the same time, Corfidi vectors support at least some training. However...the lack of any upper level support should keep any convection that fires along the front rather disorganized/scattered and high flash flood guidance values should work to limit the flash flooding potential. As a result...made few changes to the on-going Marginal Risk area. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6DzbmAqbYqWPtgFeiBr6Mb1JdmFG-3RVd0trAXkxddCm= 5e09D036ffcYZHwwJOXBipdwWwZ3uMksTy3bl4by-niWPuc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6DzbmAqbYqWPtgFeiBr6Mb1JdmFG-3RVd0trAXkxddCm= 5e09D036ffcYZHwwJOXBipdwWwZ3uMksTy3bl4bytQ_j1Yg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6DzbmAqbYqWPtgFeiBr6Mb1JdmFG-3RVd0trAXkxddCm= 5e09D036ffcYZHwwJOXBipdwWwZ3uMksTy3bl4bydUXKYi8$=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) .