Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 13 2023 15:58:55 FOUS30 KWBC 131558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Mon Nov 13 2023 Day 1 Valid 16Z Mon Nov 13 2023 - 12Z Tue Nov 14 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. 1600 UTC Update -- Based on the latest observational trends, along with the 12Z CAMs (including the HREF exceedance probabilities), per collaboration with the WFOs along the western-central Gulf Coast, have removed the Marginal Risk in the Day 1 ERO update. Instability, even elevated, is seriously lacking north of the front per the latest SPC mesoanalysis, and is expected to remain as such as the surface low and associated fronts remain offshore. With the anafrontal rainfall largely stratiform, the 12Z HREF 25%+ probabilities of >0.5"/hr remain offshore, while probs of 1+ in/hr are even farther offshore over the Gulf where the better instability is present.=20 Hurley Day 2 Valid 12Z Tue Nov 14 2023 - 12Z Wed Nov 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN LOUISIANA TO THE FLORIDA PANHANDLE... Surface low will continue to propagate eastward along a stationary boundary centered offshore of the Central Gulf Coast. Elevated PWAT anomalies between 2-2.5 deviations above normal will remain fixed to the coastal areas of LA/MS with a shift eastward into southern AL and the FL Panhandle by later in the period. The general lack of convective potential will curb the threat of flash flooding for most as prime instability remains offshore, but urbanized areas within the southeast LA Parishes and along I-10 to the western FL Panhandle could exhibit some localized flooding concerns over the course of Tuesday afternoon and evening. With coastal flooding increasingly likely over the the far southeast Parishes in LA, the potential for flooding is highest within that corridor, including the city of New Orleans. There is some discrepancy in the forecast between ensemble guidance with the NBM maintaining a stronger signature of heavy rainfall along the central LA coastal areas on Tuesday morning and the GEFS/ECS/ENSBC all shifted a bit to the east. NBM is likely seeing a feedback from the NAM suite and some of the hi-res depictions inflating the totals for areas east of Lake Charles. Decided to refrain from too much deviation on the western flank of the MRGL risk over LA, but did nudge a bit to at least account for the potential. Forecast totals of 1-3" will be common along the Central Gulf Coast with the highest totals remaining along and south of the I-10 corridor, especially between New Orleans over to Mobile.=20 Kleebauer Day 3 Valid 12Z Wed Nov 15 2023 - 12Z Thu Nov 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE, AS WELL AS THE SOUTHEAST FLORIDA COAST... ....Gulf Coast... Surface low will begin its occlusion phase on Wednesday as the mid and upper-circulations become stacked over the Central Gulf Coast and meander to the east. The elevated PWAT signature will still be located along the Gulf Coast with the highest values translated east into the FL Panhandle and adjacent southern AL. A general consensus of heavy rainfall in a corridor between far southeast LA over towards Tallahassee with the best chance for 2" or greater falling over Mobile to just west of Apalachee Bay. Totals of 2-4" will be common over the FL Panhandle as the area sits underneath of the 7H low as it treks across Gulf Coast. Even the bias-corrected guidance that has trended more conservative in the past few days with regards to the precip evolution is signaling a local maximum over 4" southeast of Pensacola. Considering the latest guidance and consensus synoptic evolution, did not make many changes to the previous MRGL positioning, although there was some narrowing on the eastern edge given the agreement in the QPF footprint. Also expanded a touch to the north to account for some mid-level convergence signatures present over southern AL to the north of I-10 likely due to the increased 700mb frontogen being depicted on multiple global deterministic. General 1.5-2.5" maximums within that zone could be enough for localized flooding within urbanized settings, similar to expected impacts for areas further south. ....Southeast Florida... Increased convergence signature along the eastern FL coast will allow for convection to develop over the coastal plain and just offshore with steering pattern aimed directly at the southeast FL coast. Instability axis is forecast to reach peak on Wednesday night into Thursday morning with some guidance generating a small area of low pressure near and west of the Bahamas. Models that have a maturing SLP have the better QPF signatures confined to the coast with the ECMWF the most aggressive with the surface reflection and overall precip coverage. There is some discrepancy in the handling of the 5H trough to the west which allows for greater difluent axis to be present over the FL Peninsula with the ECMWF deterministic. Other guidance, including the ECENS and bias-corrected ensembles are generally more confined within the coastal plain from Vero Beach down to Miami. Since this falls within the urban corridor, flash flooding concerns will be higher given the impervious surfaces within the confines of the above area. Local maximum around 5" is possible, although currently at low probability, but the frictional convergence regime and potential for locally heavy rainfall with necessary rates for flash flooding is on the table. With coordination from both the Melbourne and Miami WFO's, have elected to add a targeted MRGL risk along the urban corridor in southern FL. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_JtBVZiME5bcq9oFCEnNP25deNlJH34a4ZPYGoNHhE11= Yxg7mJjoVK9f7lrYYL9aQaD3mvwHm3jrjb0HJygBDvhi_5A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_JtBVZiME5bcq9oFCEnNP25deNlJH34a4ZPYGoNHhE11= Yxg7mJjoVK9f7lrYYL9aQaD3mvwHm3jrjb0HJygBuncBB4o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_JtBVZiME5bcq9oFCEnNP25deNlJH34a4ZPYGoNHhE11= Yxg7mJjoVK9f7lrYYL9aQaD3mvwHm3jrjb0HJygBXZ0I4oI$=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) .