Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 05 2023 08:30:56 FOUS30 KWBC 050830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Fri May 05 2023 Day 1 Valid 12Z Fri May 05 2023 - 12Z Sat May 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE MID-SOUTH AND TENNESSEE VALLEY AND THE CENTRAL/NORTHERN INTERMOUNTAIN WEST... ....Mid-south and Tennessee Valley into the Southern Appalachians and Parts of the Southeast... There is still uncertainty in the exact track of where the highest QPF will set up although the trends noted in the yesterdays Day 2 ERD still seem to be holding out with lighter amounts being forecast farther south than in previous runs. With the downward trend continuing in QPF...the Marginal risk area over the Tennessee Valley was removed and refocused more along an instability axis extending from portions of Arkansas southward to the Gulf coast from southeast Louisiana eastward to roughly the Alabama/Florida panhandle line. The models were struggling with what appear to be convective feedback that showed little run to run consistency in placement. Even so...there was enough of a signal for the 05/00Z HREF to depict a sub 15 percent risk of 2 inch per hour rainfall accumulation by early evening in coastal Mississippi and adjacent Louisiana. Extended the Marginal a bit farther north than QPF suggested based on expectations of where on-going rainfall at the start of the Day 1 period may provide some priming for later rainfall. By later in the evening...models tend to show decreasing coverage of rainfall as mid-level ridging sets in.=20 ....Central and Northern Intermountain West to Adjacent Western High Plains,.. Upper level pattern changes very little across these areas through late tonight with broadly difluent upper level flow east of a deep-layered trough axis (though the favorable upper level forcing does shift a bit northeastward into western-central MT). The favorable forcing and weak deep-layer instability (relatively steep mid-level lapse rates) along a ribbon of above-average PW will favor isolated hourly rainfall rates of 0.50-1.00". These rates, along with the recent snowmelt and possibility of occurring over burn scar areas, could foster localized short-term runoff issues. As a result, the Marginal Risk that was inherited from yesterday's Day 3 ERO continues in today's Day 2 ERO. Given an uptick in the deterministic QPF and increasing risk of 2 inches of accumulation in 3 hours from the Black Hills into portions of northern Nebraska...introduced a Marginal Risk but avoided expanding too far into the driest parts of the states. Bann Day 2 Valid 12Z Sat May 06 2023 - 12Z Sun May 07 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7yIyHHkzoNW948mbhGaNmhh4vkLwQjAw7Nw5vo3t8ev4= UQ4h8sXZm9WnpwesB9Yk1i6C4-8rm92jDPqOXKeWx7asprE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7yIyHHkzoNW948mbhGaNmhh4vkLwQjAw7Nw5vo3t8ev4= UQ4h8sXZm9WnpwesB9Yk1i6C4-8rm92jDPqOXKeWnuF2at4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7yIyHHkzoNW948mbhGaNmhh4vkLwQjAw7Nw5vo3t8ev4= UQ4h8sXZm9WnpwesB9Yk1i6C4-8rm92jDPqOXKeWT7i8Ur8$=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) .