Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 15 2023 01:08:50 FOUS30 KWBC 150107 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Thu Dec 14 2023 Day 1 Valid 01Z Fri Dec 15 2023 - 12Z Fri Dec 15 2023 THE PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE IS LESS THAN 5 PERCENT Given observational trends of less than 0.5"/hr rainfall rates and the 18z HREF guidance suggesting a less than 5% chance of 2"/3-hr exceedance through the remainder of the Day 1 period, the previous Marginal Risk was removed from the TX Panhandle and southwest OK region. The lack of a Marginal area on the experimental machine-learning first guess ERO product from CSU provided additional confidence on this decision. Farther down the coast in South TX, convection that flared up earlier has recently diminished (which is where the first guess field actually did indicate a possible Marginal Risk). Churchill Day 2 Valid 12Z Fri Dec 15 2023 - 12Z Sat Dec 16 2023 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EAST TEXAS AND NORTHWEST LOUISIANA... 21Z update... The latest guidance has an improved consensus for the highest QPF to focus over eastern Texas. There was also a broad 0.25 to 0.75 inch increase from the previous forecast as well in regards to the areal averages across this part of the region. Much of these changes were already encompassed by the Marginal Risk area that was already in effect. Very minimal reshaping of the northern and eastern bounds of the Marginal Risk was made for this forecast issuance. Campbell Stout longwave trough will traverse across the southern plains with large scale ascent shifting eastward into east TX and the Lower Mississippi Valley. Moisture field present will be applicable for producing locally heavy rainfall as the primary vorticity maxima rotating around the broad circulation will generate a period of elevated convective chances over the I-20 corridor down into areas north of the Houston metro. As you get closer to the Gulf coast, better instability presence will yield a higher risk for thunderstorms rooted at the surface with better rainfall rates possible during the second half of the period. Widespread 1-1.5" totals are forecast within both the deterministic and ensemble means, but some indications of over 2" within the better unstable environment(s) are showing up in the latest guidance with the hi-res just now peeking into the time frame. Considering just the overall synoptic pattern and ample moisture available, decided to maintain the previous MRGL risk, but not expecting a further upgrade unless we see a better convective risk unfold within the CAMs in future runs. Flash flood threat is low and isolated with urbanized flooding the biggest risk over the course of Friday into Saturday morning. Kleebauer Day 3 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... A significant uptick in the QPF amounts and placement were noted in the latest guidance for the Southeast U.S. Although some uncertainty remains on the exact locations of the QPF maximums, most of the models are suggesting areal averages of 3 to 5 inches coming onshore near the Florida bend/panhandle and north-northeast into southeast Alabama and southern Georgia with a secondary maximum focusing over the Keys and both coastlines of South Florida. There is also potential for isolated maximums to exceed 6 inches, especially for parts of northern Florida/Georgia. The Slight Risk area was expanded to cover more of northern Florida, far southeast Alabama and southwest/south-central Georgia. The Marginal Risk area was also expanded a bit along the northern periphery as well. Campbell The powerful longwave trough attributed to much of the heavy rainfall in the prior periods will continue to eject eastward, but becomes more enhanced as energy on the backside of the mean trough digs southeast into the Lower Mississippi Valley and begins to interact with a remnant shortwave over the Gulf of Mexico. A more vigorous mid-level shortwave pattern will initiate surface cyclogenesis in the central Gulf leading to much more organized precip field rooted in tropical moisture. Broad ascent pattern on the difluent side of the trough will spawn a mass of deep moist convection with backed flow over much of the FL Peninsula and adjacent central Gulf regions. 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. The complexities of the forecast come down to the overall shortwave interaction and future storm motion after initial cyclogenesis, but probabilistic forecasts for QPF on both the ECENS and GEFS, as well as the NBM indicate a wide area of potential for 2+" of precipitation with some of the deterministic pointing at QPF maximums of up to 7" given the environmental moisture availability and expected evolution of the synoptic pattern. December 2009 continues to be a prominent analog via CIPS which ended with a large areal extent of heavy precipitation over the Southeast U.S, eventually making its way north up the Atlantic seaboard. This is a textbook scenario for flash flooding across Florida and adjacent areas of the Southeast. The previous forecast maintained a fair amount of continuity, but did expand the northern edge of the SLGT risk to include more of northeast FL near Jacksonville, as well as expanding the northern edge of the MRGL over GA to reflect some trends in a further north progression of the overall QPF footprint. The western edge of the MRGL was extended to include the central Gulf coast out to eastern LA where places like New Orleans could very well see some minor flood concerns if the convective pattern holds firm from upstream. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6AEOJpx0nZ-DTJN2SgVsBdjdrbO7N-QzZqxue4AWSeVx= TYQnXMU3sUS6WE88lXwWxGGZuSefDXIV4WGbk2BtCRBu-HU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6AEOJpx0nZ-DTJN2SgVsBdjdrbO7N-QzZqxue4AWSeVx= TYQnXMU3sUS6WE88lXwWxGGZuSefDXIV4WGbk2BtKXypuHw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6AEOJpx0nZ-DTJN2SgVsBdjdrbO7N-QzZqxue4AWSeVx= TYQnXMU3sUS6WE88lXwWxGGZuSefDXIV4WGbk2Btw7CJ23o$=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) .