Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 19 2022 16:01:52 FOUS30 KWBC 191601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Mon Dec 19 2022 Day 1 Valid 16Z Mon Dec 19 2022 - 12Z Tue Dec 20 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND WESTERN GULF COAST STATES...=20 1600Z Update... Overall, there is not a big difference in the 12Z HREF suite of guidance compared to the 00Z/06Z cycle, but there are concerns that surface-based convection will not materialize at all this period and will stay offshore given that the mid-level flow is rather flat across the Gulf Coast and will likely lead to the surface low development over the northwest Gulf of Mexico staying offshore along with its attendant warm front. However, there will be certainly enough elevated instability transport along with favorable right-entrance region jet dynamics to favor a west to east stripe of moderate to occasionally heavy rainfall with embedded stronger convective cells potentially training over the same area. The main threat area for this should be across eastern TX through central LA, with some swaths of 2 to 3+ inch rainfall totals expected. The 12Z HREF guidance does hint at a relative min in precip across southern LA with the elevated convective axis just to the north, and any surface-based convection/heavy rainfall tending to stay offshore. The Marginal Risk area was extended back to the west-southwest across areas of south-central to southeast TX to account for the ongoing convection and locally heavy rainfall that has already occurred. However, we did pull the northern periphery of the Marginal Risk farther south given the flattening elevated instability profiles downwind and degree of dry air still lurking in the boundary layer over southern AR, far northern LA, and adjacent areas of central MS. Orrison Previous discussion... =20 The Marginal Risk area was largely maintained from continuity.=20 Overlapping low- to mid-level frontogenesis along with precipitable water values of 1-1.5" within an area with 1000-500 hPa thickness values in the mid to upper 550s decameters (a saturated atmosphere), convergent south to southwest low-level inflow of 25-35 kts with effective bulk shear to match, and 500-1000 J/kg of MU CAPE across portions of TX, LA, and MS is expected to lead to heavy rainfall from potentially organized convection across and inland of the western to central Gulf Coast. Hourly rain totals to 1.5" inland and closer to 2" near the coast are possible in this environment, with the HREF showing the most potential along the immediate shoreline/coastal marshes of Louisiana. The 00z HREF probabilities show a high chance of 3"+ across central LA and along the central LA coast, with a 20-25% chance of 5"+ near and west of Fort Polk LA. Narrow streaks near and inland of the Gulf coast have seen 300%+ of their average rainfall during the past week, making some areas more sensitive to additional heavy rains. However, much of where the higher amounts near the central LA/TX border and across western LA is wooded and show minimal overlap where heavy rains fell during the past week.=20 Urban areas would also be sensitive to such rainfall. While small pockets of Slight Risk-type impacts are possible, it wouldn't be a large enough area from a probability perspective for an upgrade. Roth Day 2 Valid 12Z Tue Dec 20 2022 - 12Z Wed Dec 21 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR FLORIDA...=20 =20 Upper level support of the right entrance region of an upper level jet over the Southeast U.S. and difluent flow aloft over the Florida peninsula and the northeast Gulf of Mexico will support a blossoming area of showers and thunderstorms over the northeast Gulf of Mexico that will make its way inland on Tuesday and Tuesday night/early Wednesday morning. Precipitable water values of 1.25-1.75" are expected within an increasingly saturated atmosphere. Overlapping low- to mid-level frontogenesis is expected to be the biggest factor for heavy rainfall as guidance forecasts MU CAPE to get into the 500-1000 J/kg in narrow strips, mostly in coastal areas of the FL peninsula. The combination of available instability and frontogenesis could lead to hourly rain totals up to 1.5" particularly near and south of Lake Okeechobee and along the FL peninsula coastline, should it anchor. Various pieces of deterministic guidance show the potential for local amounts in the 3-5" range but show no agreement as to where within this broad Marginal Risk area. Portions of central and southern FL have received 300-600% of their average rainfall during the past week, but since this is their dry season, this amounts to spotty 2-4", which in the warm season would be an average day.=20 The Marginal Risk has been maintained for this area, with the biggest potential impacts in urban areas along/near either coast. =20 Roth/Bann Day 3 Valid 12Z Wed Dec 21 2022 - 12Z Thu Dec 22 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 =20 Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6GZkqA4gEjxMSvKFsdUkiDzIUc7RNpkThIa3IPqLgPJ2= nqBnFxwGuH2fsnRdfxR7nxL4UF7-d6ouvv2oiblcEhoa5aU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6GZkqA4gEjxMSvKFsdUkiDzIUc7RNpkThIa3IPqLgPJ2= nqBnFxwGuH2fsnRdfxR7nxL4UF7-d6ouvv2oiblceskgQHk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6GZkqA4gEjxMSvKFsdUkiDzIUc7RNpkThIa3IPqLgPJ2= nqBnFxwGuH2fsnRdfxR7nxL4UF7-d6ouvv2oiblc8RNWlLQ$=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. 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