Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Dec 13 2023 09:02:00 FOUS30 KWBC 130809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 AM EST Wed Dec 13 2023 Day 1 Valid 12Z Wed Dec 13 2023 - 12Z Thu Dec 14 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST FLORIDA... Latest surface analysis shows a stout surface ridge in place east of the Mississippi with a stationary boundary focused across the northern and western Caribbean. These two surface features and the advection of mid-level energy out of the Caribbean will be the main drivers for increased convective heavy rainfall across south FL this morning through the afternoon and early evening hours. Models are now consistent with a deep moisture advection pattern encompassing all of the southern FL Peninsula and adjacent FL Keys with a prevailing northeast boundary layer flow creating a recipe for persistent frictional convergence along the southeast FL coast, as well as blooming convection over the southern reaches of the Everglades. 00z HREF has upped the ante for local maximum potential within the 3-5" zone with neighborhood probabilities for each total (3" and 5") jumping to 70% and 45% respectively within the confines of Miami-Dade county. WV satellite shows some hints of the mid-level impulse navigating through Cuba with an expected northward progression through the FL Straits, eventually making its presence known sometimes after 12z today. Locally heavy rainfall with rates between 1-2"/hr are increasingly likely given the proposed airmass with PWATs running closer to 2", a solid 2.5-3 standard deviations above normal by early morning, carrying through the daytime hours. Some hi-res deterministic are showing some gaudy solutions with local maxima approaching 6-8" where convective risks train over the same areas given the persistent NE steering pattern. CIPS analog packages also give credence to some of these solutions with events like 12/17/2009 now showing up within the top analogs. The Southeast U.S evolution given the surface pattern across the eastern US makes sense, and that event concluded with multiple flash flood concerns within the Miami metro area which is the primary focus for today's risk given the urbanization factors. With the increasing threat of localized heavy rainfall over the Miami urban corridor, have upgraded to a Slight Risk to account for the latest trends. Kleebauer Day 2 Valid 12Z Thu Dec 14 2023 - 12Z Fri Dec 15 2023 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST FLORIDA... ....Southeast Florida... Prevailing east to northeast flow will maintain the wet period that translates off D1. Heavy rainfall potential will be more isolated comparatively, but the additional rains expected on top of what occurred the prior 24 hrs will keep the threat for flooding elevated within the urban corridor extending from Fort Lauderdale to Miami. Hi-res ensemble guidance still paints a neighborhood probability of 20% for at least 3" across the above area where precip totals the prior period may push upwards of 5". The compounding issue and focus for convection off the Atlantic is the primary concern within the D2 time frame. With coordination from the Miami WFO, have elected to maintain the SLGT risk from previous issuance given the above threat. ....Southern High Plains... A broad, closed low over NM will propagate eastward with large scale ascent maximized downstream over the southern high plains. Backed flow within the boundary layer indicates a strong low-level push of moisture up from the Gulf with PWAT anomalies soaring to 2-3 standard deviations above normal for much of the Texas Panhandle over into southwestern OK. Upstream vorticity maxima advecting eastward will aid in the development of a large swath of precipitation with embedded elevated convection as signaled by forecast soundings within the west TX domain. Precip totals of 1.5-2.5" are being forecast east of the Caprock over into north TX and the parts of the Red River basin to the west of Wichita Falls. Localized flooding is possible within that corridor where heavy rain and thunderstorms move overhead, especially with poor drainage within some of the smaller towns located east of I-27, as well as the copious amount of low-water crossings over the plains. The MRGL risk was maintained for the areas north of I-10 with enough consensus to trim the southern edge from previous forecast issuance. Kleebauer Day 3 Valid 12Z Fri Dec 15 2023 - 12Z Sat Dec 16 2023 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS SOUTH FLORIDA AND EAST TEXAS... ....South Florida... Onshore flow pattern will continue across south FL with an increase in regional moisture ahead of a longwave trough axis digging towards the Mississippi Valley. A steady stream of showers and isolated thunderstorms will create an ongoing risk of locally heavy rainfall over areas that will have seen a fair amount of rain prior to the period. QPF forecast between 1-1.5" additional is looking increasingly likely across the southeast FL coast with increasing ascent and clusters of storms possible over the Keys for the latter half of the forecast period. Analogs are trending towards a more aggressive setup within the time frame beginning late Thursday and beyond, so this will be a period of transition with flooding prospects only increasing through time. Any forward increase in the evolution of the upstream pattern could cause the precip totals to rise. ....East Texas... Our deep upper trough will translate eastward through the southern plains with sights on the Mississippi Valley by the end of the forecast period. A pool of anomalously moist air will meet the continued large scale ascent pattern will meet within the difluent sector of our mean trough. A solid 1-2" of precip over the course of Thursday into Friday morning will transpire from the Gulf coast of TX up into the Arklatex. Convective chances will be best down near the Gulf coast leading to isolated and scattered thunderstorm possibilities for areas along and south of I-20 down into the Houston metro area. Models are still a bit jumpy on the placement of the heaviest rainfall, but ensembles, including the NBM peg at least a decent opportunity for someone to receive a decent amount of rainfall within the confines of the best ascent ahead of our upper trough. Urban flooding will be the primary concern, especially for the DFW and Houston metros where the urban areas are notoriously lower for FFG exceedance markers. Kleebauer $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .