Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 19 2022 20:26:49 FOUS30 KWBC 192026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM 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 ACROSS CENTRAL AND SOUTH FLORIDA...=20 =20 Southern stream shortwave energy will be quickly traversing the central and eastern Gulf Coast region this period and crossing the FL Peninsula as a wave of low pressure advances east along a quasi-stationary front out over the Gulf of Mexico. There should be plenty of convection out over the Gulf of Mexico early Tuesday along the aforementioned frontal axis, but the overall flat mid-level flow and lack of any substantial backing of the heights will support the main convective axis tending to stay well south of the FL Panhandle, with gradually an impact possibly for areas of the central and southern FL Peninsula later in the day where there will be ultimately be a combination of stronger low-level moisture convergence/forcing and instability. There will be some upper-level divergence in place too, especially early in the period, but this tends to weaken across the FL Peninsula Tuesday night, and that coupled with the loss of any diurnal heating near the front, plus the gradual development of surface low pressure off the northeast coast of FL does yield substantial question marks on how far south and east any organized convective threat can make it Tuesday night. One other negative component will be that the southwest low-level jet upstream over the Gulf of Mexico will be weakening in time, and is quite modest by the time it reaches the FL Peninsula. Its really only the 12Z FV3 and 12Z GEM regional that have a signal for organized convection impacting the FL Peninsula, and most of that is the central to south-central part of the FL Peninsula in close proximity to the front with some localized 2 to 4+ inch amounts advertised. The NAM conest, HRRR, ARW and ARW2 solutions are all considerably more modest and disorganized by comparison. All of the HREF guidance suggests a lack of instability north of the front and related low track leading to generally modest rainfall across the FL Panhandle and north FL. Based on the latest HREF trends, and synoptic setup, it was decided to remove the Marginal Risk from the FL Panhandle and north FL, and keeping it confined to central and south FL where much better instability/forcing, and an axis of high PWs (locally approaching or exceeding 1.75 inches) for heavier rainfall rates will be in place by comparison. The excessive rainfall threat is also really conditioned on heavy rains impacting the more sensitive urbanized locations, so this overall threat is expected to be quite isolated in nature. Orrison Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-v7yPRWNjDGvSdoSUf7FOAaFaSOsAftdkEbxfTvhgLWT= 7VXBZTzxgLAEnf0kD_PLhfhQHQMiIs1Dw2ZAMHZWMxBo5SE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-v7yPRWNjDGvSdoSUf7FOAaFaSOsAftdkEbxfTvhgLWT= 7VXBZTzxgLAEnf0kD_PLhfhQHQMiIs1Dw2ZAMHZWxGm3xq4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-v7yPRWNjDGvSdoSUf7FOAaFaSOsAftdkEbxfTvhgLWT= 7VXBZTzxgLAEnf0kD_PLhfhQHQMiIs1Dw2ZAMHZW-mgQHL8$=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) .