Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 21 2023 15:48:24 FOUS30 KWBC 211548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Fri Apr 21 2023 Day 1 Valid 12Z Fri Apr 21 2023 - 12Z Sat Apr 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR MUCH OF THE LOWER MISSISSIPPI AND OHIO RIVER VALLEYS... The Marginal Risk was affirmed per 12Z HREF QPF/exceedance probabilities as well as FFG with some trimming. The cold front will continue pushing through areas with Gulf-moisture content and instability, producing locally heavy rainfall particularly this afternoon. Strong southwesterly deep-layer flow will continue to be nearly parallel to the approaching frontal boundary, keeping a risk for repeating activity. Jackson Day 2 Valid 12Z Sat Apr 22 2023 - 12Z Sun Apr 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. As the front over the Midwest Friday pushes east on Saturday, a shortwave trough in the upper levels will shift the upper level trough more negatively tilted with time. This energy will continue to support a progressive front that traverses the Mid-Atlantic towards the east through the day. Instability will be limited, peaking around 1,500 J/kg over eastern NC, but barely making 300 J/kg from DC north. In NC and southeastern VA, FFGs remain high, and the guidance continues to struggle on where any local maxima of rainfall will set up, with most favoring a mostly uniform area of 1.25 to 1.75 inches of rain from NC through NY. Since the upper levels will still be moving, favoring storms that will be moving along at a decent clip, the threat for flash flooding is very low all up and down the mid-Atlantic. In coordination with the AKQ/Wakefield, VA and MHX/Morehead City, NC forecast offices, the Marginal Risk area for those regions was downgraded to a risk-free day nationwide on Saturday. Wegman Day 3 Valid 12Z Sun Apr 23 2023 - 12Z Mon Apr 24 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR MUCH OF SOUTH TEXAS AND NEW ENGLAND... ....New England... As the front over the Mid-Atlantic Saturday progresses east into New England on Sunday, the upper level pattern north and east of the front (ahead of it) will transition from an omega block to a Rex block by Monday morning. The blocking pattern will do as its name implies...block the front from progressing any further. Thus, expect a slowing of the eastward progression of the rainfall across New England. The inherited Slight Risk area including far southern Maine and coastal NH is an area of locally higher FFGs, and with no instability anywhere close to the coast, in coordination with GYX/Gray, ME forecast office, downgraded the Slight to a Marginal Risk, which encompasses almost all of New England, except northern/eastern Maine. The heaviest rainfall remains most likely along the NH/southern ME coast, but the aforementioned blocking pattern has favored a gradual westward shift in the axis of heaviest rainfall. While the Marginal Risk includes interior portions of New England, especially VT, the increasing vegetation in that area should be increasing the amount of rain needed to result in flash flooding. Thus, the threat is considered lower in far eastern NY and VT, and so the Marginal Risk area may be downgraded with future updates if current trends continue. Opted to see a few more model runs before making that decision. ....South Texas... Southeasterly return flow off the Gulf and a subtle shortwave in the jet are likely to combine forces to result in another round of heavy rain across portions of south TX on Monday. High FFGs are prevalent in most of the area, but areas around San Antonio were hit hard with training thunderstorms overnight last night, and as such local FFGs are much lower. With the axis of heaviest rain expected to stretch from Corpus Christi northwestward through San Antonio, expanded the Marginal Risk area towards the north and west to cover more of these recently lowered FFG areas. Of course, some recovery of the FFGs is expected with 2 days of relatively little rainfall over the area (a half inch or less of rain is expected Saturday), but they should still be lowered enough to justify inclusion into the Marginal Risk area for Sunday. While the guidance is still struggling on where the axis of heaviest rain will set up, any northward shifts and increases in rainfall totals may require a local upgrade to a Slight risk with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ukxyN9iEUA4OGzlYdTW_F0ZDvdZ4sDT8EJ-0UWrX6Is= vhdk8O5WL3h95wSNY1xiMW21o4l3EiFLNvOpZVazH3NmZIE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ukxyN9iEUA4OGzlYdTW_F0ZDvdZ4sDT8EJ-0UWrX6Is= vhdk8O5WL3h95wSNY1xiMW21o4l3EiFLNvOpZVazmr_QwnI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ukxyN9iEUA4OGzlYdTW_F0ZDvdZ4sDT8EJ-0UWrX6Is= vhdk8O5WL3h95wSNY1xiMW21o4l3EiFLNvOpZVazWLscHkE$=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) .