Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 15 2023 20:10:30 FOUS30 KWBC 152010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 309 PM EST Fri Dec 15 2023 Day 1 Valid 16Z Fri Dec 15 2023 - 12Z Sat Dec 16 2023 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EAST TEXAS AND A SMALL PART OF ADJACENT LOUISIANA... 16Z update... Western portions of the boundary was trimmed eastward a little to account for lower than expected amounts from earlier in the period and that the convection is progressing a bit faster as well, thus lowering the reducing the threat for local flooding concerns. The latest guidance does support a small reshaping/expansion of the northeast part of the Marginal Risk near the tri-state border and further east into northwest Louisiana with the higher QPF now favoring far eastern Texas/western Louisiana. Campbell Showers and thunderstorms are expected later today as Gulf moisture being drawn northward from the Gulf of Mexico encounters a Pacific surface front. Trimmed some of the area from yesterday's Marginal risk where the precipitable water values failed to reach 0.8 inches...which would only be near the 75th percentile and 1.5 standardized anomalies greater than climatology...although a slight expansion was drawn to the north where the upper level flow becomes highly difluent ahead of the upper trough/mid level low during the afternoon. Low level flow is forecast to be fairly modest...so the moisture flux convergence along the surface front should not result in more than moderate to briefly heavy rainfall. The 15/00Z HREF was focused on the western portion of the Marginal Risk area while the SREF/GEFS had several membering showing 2 inch QPF contours a bit east of there. Flash flood threat is low and isolated with urbanized flooding the biggest risk over the course of Friday into Saturday morning. Bann Day 2 Valid 12Z Sat Dec 16 2023 - 12Z Sun Dec 17 2023 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF FLORIDA... 21Z update... The latest guidance depicted higher QPF moving on shore than what was in the previous WPC forecast. Most of the solution have 2 to 5 inches spread across most of central/northern Florida and north/northeast into Georgia, with a couple reaching southern South Carolina. The western bounds of the Marginal and Slight over the Gulf states remain in good order. The eastern bounds of both risk areas were expanded further up the East Coast by 2-3 tiers of counties. The highest accumulations still seem to be focused near Panama Beach/Florida bend and surrounding areas of north-central Florida. Campbell A significant uptick in QPF amounts advertised on Friday's Day 3 ERO persisted...although the spread in the model mass fields and QPF solutions were larger than usual for a Day 2 ERO. There should be some interaction between a southern stream trough approaching the area from the west and one or more northern stream shortwaves that make their way towards the Mid-Mississippi Valley by late Saturday/early Sunday...and what the implications there are for the magnitude of cyclogenesis over the Gulf of Mexico.=20 Most of the models are suggesting areal averages of 3 to 5 inches coming onshore somewhere around the Florida bend/panhandle and southeast Alabama/southern Georgia. The 15/00Z NAM still maintained the idea of a QPF maximum of more than 7 inches during the period while other guidance was not quite so bullish. Totals on the order of 3-5" will be common with heavy rainfall rates exceeding 1"/hr increasingly likely from the FL Keys all the way into parts of the Deep South. Given the complexities mentioned above...and the 24-hour probabilistic forecasts for QPF on both the ECENS/GEFS/NBM and the 15/00Z HREF through 17/00Z...broad areal coverage of 2+" of rainfall with embedded higher amounts suggests the Marginal and Slight Risk areas remain warranted...with no clear model signal at this point where the on-going outlook areas could be refined beyond some cosmetic adjustments. Bann Day 3 Valid 12Z Sun Dec 17 2023 - 12Z Mon Dec 18 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN U.S. FROM THE CAROLINAS TO THE NORTHERN MID-ATLANTIC REGION... 21Z update... The latest guidance is favoring a slightly more compressed gradient of QPF along the eastern Seaboard. The WPC forecast has a small shift eastward of the higher amounts which now are more over eastern Carolinas, eastern Virginia and Maryland. The previous WPC forecast was placed more over the central part of those states. A broad area of 2.5 to 4+ inches is expected for the eastern Carolinas with areal averages of 1 to 3 inches for points northward to southern New York. For West Coast there was a notable northward shift with most of the QPF guidance coming onshore the California Coast. As such, the Marginal Risk area that was already in effect was shifted northward to reflect this trend. Campbell Increasing coverage of moderate to heavy rainfall is expected during the period as surface low pressure deepens while moving northeastward along the eastern seaboard during the period. As it does so...strong on-shore flow with deep layered moisture transport will ensure plenty of moisture for the system to work with as shown by precipitable water values of 1.25 to 1.5 inches getting drawn into the Mid-Atlantic by the afternoon. That amount of moisture is some 3 standard deviations above climatology and places it within the 95th percentile for this time of year. The big problem is that there is a large spread in the model guidance mass fields and associated QPF fields. Think that the 15/00Z GFS is too far north/west by late Sunday night/early Monday night given other guidance being farther south and being closer to the ensemble mean. The exception was the 15/00Z NAM which appeared to amplify the mid/upper level trough too much which forces the surface low much farther south than other global models. Left part of the Slight risk area from yesterday's Day 4 ERO to cover the possibility that some rainfall lingers longer...and the Slight risk was expanded a bit northeast in the Mid Atlantic region otherwise the main changes were to shift the western periphery of the Slight and Marginal risk area a bit westward. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_G9sgkm1hmBFaBMrNR5wXq5m-cPweoIGYKFJKAMPpn4y= ZqyjAAP-uNJRkaOXvaHnYInykwBLM3vHyN9jw_3zgiMmaC4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_G9sgkm1hmBFaBMrNR5wXq5m-cPweoIGYKFJKAMPpn4y= ZqyjAAP-uNJRkaOXvaHnYInykwBLM3vHyN9jw_3zBLOwIDE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_G9sgkm1hmBFaBMrNR5wXq5m-cPweoIGYKFJKAMPpn4y= ZqyjAAP-uNJRkaOXvaHnYInykwBLM3vHyN9jw_3zmJGxRek$=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) .