Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 14 2023 00:17:03 FOUS30 KWBC 140016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 716 PM EST Mon Nov 13 2023 Day 1 Valid 01Z Tue Nov 14 2023 - 12Z Tue Nov 14 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann 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 WESTERN PORTIONS OF THE FLORIDA PANHANDLE... 2000 UTC Update -- Based on the latest (12Z) guidance and trends, including the full CAM suite, have pared the Marginal Risk area to areas along the Gulf Coast (including New Orleans and Mobile). Latest high-res ensemble means and PQPFs show the 24hr 2+ inch totals (and higher probabilities) mainly over the Gulf and coastal parishes in south-central to southeast LA (south of New Orleans). This is where 12Z HREF probabilities are highest (aoa 40%) of 1+ in/hr rainfall rates, particularly in the latter half of the D2 period (after 00Z Wed), while also highest (40-70%-plus) with 24 hr probs of at least 3.00". Overall a longer duration rainfall, though once again with hints of more convective contribution near the coast (esp. Louisiana) as MUCAPEs of 250-500 J/Kg get drawn in closer to coast per the latest HREF. Hurley Previous discussion below... 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 FAR SOUTHERN PORTIONS OF ALABAMA INTO THE FLORIDA PANHANDLE, AND ACROSS SOUTHEAST FLORIDA AND THE NORTHERN KEYS... 2000 UTC Update -- Again based on the latest guidance/trends, have confined the Marginal Risk closer to the Gulf Coast from east of Mobile Bay into western portions of the FL Panhandle, i.e. where the instability and corresponding uptick in high-res exceedance probabilities would be more conducive to a >5% neighborhood probability. More notably, have expanded the Marginal across southeast FL and the northern FL Keys. The synoptic setup become quite favorable for heavy rainfall across this region, as depicted with the 12Z guidance. This as the mid-upper trough becomes more amplified in the eastern Gulf, allowing for increased downstream difluence and divergence and corresponding deep-layer ascent along and north of the surface low track and associated fronts. Low-level east to southeast moisture transport into the Gold Coast is quite robust during Day 3, as are the 850 mb moisture flux anomalies per the SREF (+2 to 2.5 standard deviations above normal). However, the big drawback at this time for including a higher (i.e. Slight) risk in the D3 ERO is the spread in the guidance -- particularly with QPF and PQPF, including the ECMWF, GFS, CMC, and UKMet, likely due to the varying stages/strength of cyclogenesis from these model runs. Sufficient deep-layer instability, sufficient to support a more enhanced ERO threat, will be predicated on how well developed/amplified the system becomes during Day 3. Guidance that is more bullish with the heavier totals into portions of SE FL (including the GFS and ECMWF, albeit in different areas), indicate MUCAPEs climbing around or above 500 J/kg. Per collaboration with WFO MFL, will allow at least one more forecast cycle and (hopefully) better model consensus before revisiting the potential need for an embedded Slight Risk. Hurley Previous discussion below.. ....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!6csL4ef3ZYSx_l5W7v1p1WEKrJgadGJywyqDynDreXPQ= gw4cDyi0-NRjf3fcj0SMlIkR9RGQqIGTot8Pp91kF4sbRpk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6csL4ef3ZYSx_l5W7v1p1WEKrJgadGJywyqDynDreXPQ= gw4cDyi0-NRjf3fcj0SMlIkR9RGQqIGTot8Pp91klAFaQa8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6csL4ef3ZYSx_l5W7v1p1WEKrJgadGJywyqDynDreXPQ= gw4cDyi0-NRjf3fcj0SMlIkR9RGQqIGTot8Pp91k9a2Uwv0$=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) .