Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 03 2023 00:21:09 FOUS30 KWBC 030021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 820 PM EDT Mon Oct 02 2023 Day 1 Valid 01Z Tue Oct 03 2023 - 12Z Tue Oct 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern High Plains... An upper trough propagating through the West has triggered showers and thunderstorms across the High Plains, some of which have been showing training and back building character, thus far. PWATs of 1-1.5" with increasing southerly flow are expected overnight, which will tap MU CAPE that is currently 1000-3000 J/kg. The expectation is that this activity will become forward propagating over the next few hours, which would eventually leave the tail of the convective activity as more prone to heavy rain issues as it would ultimately have the longest period with higher instability available while the more northern activity forward propagates out of the instability region. The 18z HREF indicates this as well, with its highest probabilities of 3"+ through 12z lying close to the Pecos River valley. Excessive rain areas were trimmed on the western side based on radar reflectivity and mesoscale guidance trends. ....Florida... The Marginal Risk area from the last issuance was curtailed to encompass only southernmost Florida and the surrounding Keys. A slow moving surface front is expected to continue its southward trajectory today through the Florida Straits. Any flash flood threat would be early and over any urban areas within the Marginal Risk. ....Texas Coast... Occasional heavy rain has occurred along a convergence zone within Lower Texas Coast today. Ample instability should continue, but activity has been pulse as low-level inflow has been modest which has led to occasional rearrangement of the banding as outflow boundaries shift its location around. The 18z HREF probabilities suggest the possibility of 3"+ of rainfall, initially near the Lower TX Coast but shifting more towards the Upper Texas Coast by 12z as veering low-level flow shifts the convergence zone up the coast. Any issues would be constrained to urban areas as only small pockets of the region have experienced average to above average rainfall this past week. Roth/Kebede Day 2 Valid 12Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT PLAINS AND THE GULF COAST OF TEXAS... ....21z Update... Minimal changes were needed for this updated, as guidance remains in fairly good agreement concerning a broad area of rainfall across the central portion of the country. Signals remain insufficient for a targeted Slight introduction, and any excessive rainfall capable of flash flooding is expected to be localized and difficult to pinpoint. Along the Texas coast, the Marginal Risk was expanded a bit based on the 12z HREF guidance. Conversely, the signals across the Florida Atlantic Coast did indeed continue a declining trend, so the Marginal Risk was removed for this update. Churchill ....Previous Discussion... ....Great Plains... A broad Marginal Risk of excessive rainfall is in place across the Great Plains and parts of the Upper Midwest on Tuesday as moisture streams out ahead of the deepening upper trough emerging over the High Plains. Showers and thunderstorms will focus around a cold front extending down across the Plains with the greatest chance for excessive rainfall occurring closest to the greatest instability from the Southern Plains up into northern portions of the Central Plains. Guidance has consistently signaled this, but has lacked a strong signal for especially high rain rates to overcome relatively dry soils at this time. ....Texas Coast... Another Marginal Risk area was introduced for this issuance to account for another round of convection on Tuesday along a coastal convergence zone. Operational guidance has picked up a signal for this and the similarities with today's setup of low level convergence within an unstable environment and a modest low level jet support the possibility of excessive rainfall leading to isolated flash flooding. ....Florida... Recent rainfall has made much of Florida's Atlantic Coast susceptible to flash flooding, despite the operational guidance keeping the heaviest QPF well offshore. The confluence of these two factors should support a low confidence Marginal Risk area along the coast for now. If operational QPF trends continue on a downward trend then the Marginal Risk area might be removed altogether on the next issuance. Kebede Day 3 Valid 12Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....21z Update... Confidence remains relatively high with regard to concentrated heavy rainfall on Wednesday, as WPC QPF advertises a broad area of 1-3" areal average QPF (generally encompassed by the adjusted Slight Risk area). This includes much of OK, North TX, northwest AR, southwest MO, and far southeast KS. Localized totals across this region are anticipated to range from 3-5", with the best odds of realizing these localized totals across southeast OK and adjacent portions of North TX (considered to be on the higher-end of the Slight Risk probability spectrum). Given a fair amount of spread in the global guidance (and a lack of CAM guidance through the period of interest), as well as generally dry antecedent conditions, we decided to hold off on the introduction of a Moderate Risk. Should guidance trend upward additionally and come into better agreement spatially, then a targeted Moderate Risk upgrade may be needed for subsequent updates. For now, the forecast is in good agreement with the machine learning CSU first guess guidance (depicting a high-end Slight Risk area). Churchill ....Previous Discussion... ....Southern Plains... There is reasonably high confidence in a heavy rainfall event unfolding over much of the Southern Plains of Oklahoma and north-central Texas on Wednesday. The operational models have consistently signaled 2-4" (locally higher) of 24 hour QPF for much of the current Slight Risk area centered over the Red River. The main threat of excessive rainfall will occur during the peak heating hours of the late afternoon and evening when convection is expected to initiate. The amplified upper trough swinging through the Great Plains and upper jet crossing will provide ample forcing of convection over Texas and Oklahoma. Sufficient instability of over 1000J/Kg will be in place with low level jet support to allow for higher rain rates. PWATs of 1.5-2" across the Texas Oklahoma border region will be approximately 3 standard deviations above normal. Further upgrades will be considered if confidence continues to grow. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!69FwklesQWrN1E5NlhWOhnPwN7gUUJwG8cyYWPBm-Kf4= LbKsirxGbRsYqybNkCjs5cNNXhbCr_BM8G288UU3J28ZDYg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!69FwklesQWrN1E5NlhWOhnPwN7gUUJwG8cyYWPBm-Kf4= LbKsirxGbRsYqybNkCjs5cNNXhbCr_BM8G288UU3KTHFGuI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!69FwklesQWrN1E5NlhWOhnPwN7gUUJwG8cyYWPBm-Kf4= LbKsirxGbRsYqybNkCjs5cNNXhbCr_BM8G288UU3TZ6VRv4$=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) .