Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Nov 13 2023 10:42:00 FOUS30 KWBC 130811 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 310 AM EST Mon Nov 13 2023 Day 1 Valid 12Z Mon Nov 13 2023 - 12Z Tue Nov 14 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TEXAS AND LOUISIANA COASTS... Surface reflection beginning to take shape over the western Gulf which has been well forecast within the deterministic guidance. Deep moisture plume with Gulf and eastern Pacific connection(s) continues to funnel northward into southern and eastern TX and will translate to the northeast over the course of the period. Primary convective focus will remain offshore, but weak elevated CAPE signatures across the TX coast on the latest hi-res bufr soundings indicates a non-zero risk of thunder over a narrow corridor extending from Corpus Christi up into the Upper Texas coast through the afternoon hours today. This correlates well within the guidance for where the heaviest rainfall is forecast, and reflected within the associated probabilities in both the latest NBM and 00z HREF. A general 1-2" with a max of 3" is forecast along the stretch from Corpus Christi up through Houston into the coastal Parishes across southwest LA. If you want to follow a general landmark for the proximity of heaviest rainfall, look for along and south of I-10 as the primary focus for the D1 MRGL where probability of exceeding 1"/6-hrs is highest at around 50-60% on the 01z NBM and 30-40% on the 00z HREF. Rates will be the deterring factor for higher than the low-end Marginal Risk, especially considering the antecedent conditions leading into the forecast rainfall which had been very dry. This will be primarily beneficial rainfall, but localized flooding within the urbanized areas along the TX/LA Gulf coast cannot be ruled out. Kleebauer 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. 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .