Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 14 2023 20:15:01 FOUS30 KWBC 142014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 314 PM EST Tue Nov 14 2023 Day 1 Valid 16Z Tue Nov 14 2023 - 12Z Wed Nov 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN AND SOUTHERN FLORIDA & SOUTHEAST LOUISIANA... East-Central and South Florida... Well to the north of a front located across portions of southeast Cuba and the northwest Caribbean, a coastal front is apparent between east-central FL and the Middle FL Keys. ML CAPE to the east of the feature has been rising over the past several hours, presently 500-1000 J/kg. Bands of showers with possible embedded thunderstorms are trying to manifest within the low-level cloud streets streaming in off the Atlantic; effective bulk shear of 25-40 kts is helping to organize the morning convection. VAD wind profiles show east flow at 15-20 kts ushering in the instability from the warm Gulf Stream, which is a quite a bit higher and out of a different quarter than the mean 850-400 hPa flow (southwest at 5 kts). The flow aloft directly opposes the low-level flow, which could hold up the progression of activity with heavy rainfall from progressing too far inland. Showers and thunderstorms will likely try to shift southward while hemming in the instability closer to the coast with time as they chip away at the invading instability. With precipitable water values of 1.68-2.01" from the MFL/Sweetwater FL and XMR/Titusville FL soundings, moisture is sufficient for excessive rainfall issues.=20 Recent mesoscale guidance remains intriguing, with the 12z HREF showing a spot of 50%+ probabilities of 5" through Wednesday morning. With the front in the northwest Caribbean/Cuba lifting northward towards the area tonight, instability should remain across South FL for much of the period. The expected synoptic pattern should allow for potentially organized and short convective bands to move out of the FL Straits into South FL again late tonight into Wednesday morning. The potential is there for isolated flash flood issues into this afternoon and again overnight. Hourly rain totals to 2.5" with local amounts to 6" are possible within the Marginal Risk area. The Marginal Risk area was coordinated with the MFL/Sweetwater FL and MLB/Melbourne FL forecast offices earlier this morning. Southeast tip of Louisiana... The guidance is fairly unanimous -- outside the 00z UKMET -- that heavy rainfall is expected across the tip of southeast Louisiana with an occluding low moving just offshore the Mouth of the MS River, likely within its inverted trough/TROWAL. CAPE is predicted to rise into the 500-1000 J/kg range tonight into Wednesday morning by the 03z SREF mean and precipitable water values are forecast to rise towards 2". The probabilities of 3"+, 5"+, and 8"+ advertised by the 12z HREF are higher than across portions of southern and eastern FL. However, there is some chance that the guidance is making the low too convective and tracking it too close to the Mouth of the MS River and tucking it too far back into the upper level feature, which is their bias with convective lows, tropical or otherwise -- we saw this with the system back in April that ended up staying much farther offshore. With even global guidance such as the 00z ECMWF aboard amounts of 7", went ahead and added a Marginal Risk as a course of least regret, should the worst case scenario materialize. Any heavy rain related issues would be confined to towns such as Port Sulphur and Buras, as the region is composed of general wetlands and within a period of exceptional drought. Even though it has been raining as of late, rainfall has been approaching average values during the past week -- nothing heavy enough to nullify pre-existing drought conditions. The new Marginal Risk area was coordinated with the LIX/Slidell LA forecast office. Roth Day 2 Valid 12Z Wed Nov 15 2023 - 12Z Thu Nov 16 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST FLORIDA... 20z Update: We have upgraded a portion of the urban corridor in southeast FL to a MDT risk with this update. The threat of locally significant urban flooding is increasing across the area Wednesday into Wednesday night. Much of the forecast reasoning and synoptic setup is described well in the previous discussion below, and that has not changed. The QPF signal continues to come up, with 12z HREF neighborhood probabilities of 8" now at 70%. There remains some uncertainty with exactly how things evolve tomorrow, which does make this a challenging forecast. Most of the uncertainty is related to where a surface low forms and how that impacts instability fields as the day progresses. If a low develops quicker off the east coast of FL that would probably result in a decreasing instability trend over land and lead to lower rainfall rates. So while there is a chance the heaviest convection ends up setting up offshore, the more probable scenario at this time is for enough instability and coastal convergence to persist to allow for heavy training convection over land and an increased flood threat. Given the favorable synoptic setup and anomalous moisture in place, there is above average confidence in a swath of at least moderate to locally heavy rain over south FL, although for the aforementioned reason, confidence on the details and eventual magnitude of the event remain at only average to below average levels. Nonetheless, given the increasing QPF signal and favorable environment in place, the conditional threat of higher end urban flooding has increased enough to warrant the MDT risk upgrade with this update. Chenard ....Previous Discussion... Continued mid and upper-level trough evolution will preside over the Southeast US with the base of the mean trough into the central Gulf. Surface low present off the central Gulf Coast will continue to slowly propagate eastward with a stationary front situated west to east across the central Gulf into South FL and the adjacent FL Straits. A combination of increased difluent flow downstream of the mean trough and surface convergence within the proxy of the aforementioned stationary boundary will create a favorable environment for a prolonged heavy rain event across South FL, especially for areas south of Melbourne down into the northern Keys. PWAT anomalies approaching 2-2.5 deviations above normal are consistent across all guidance, including the latest NAEFS and ECENS within the above corridor with the primary focus along the urban setting between Fort Lauderdale to Miami. Forecast bufr soundings out of KMIA indicate a rich, moist profile on Wednesday morning and afternoon with saturation deep through the profile with a skinny MLCAPE presence indicative of a textbook heavy rain threat. Area instability isn't overly prolific (~500-1000 J/kg MLCAPE), but the deep moisture flux being advected into the region with ample ascent between 1000-500mb will generate widespread heavy precipitation with rates generally settling between 1-2"/hr and low probabilities of 3"/hr possible given the latest HREF probability around 20-25% between the 18-00z hrs on Wednesday afternoon and early evening. Latest blended guidance has ramped up totals within past succession of runs with now a distinct 2-4" clustering across the southern FL Peninsula, including the Miami Metro. Ensemble bias-corrected mean QPF has jumped over 4" for portions of south FL as well with deterministic output showing local maximum of up to 8" for where the best convergence zone settles and trains overhead. Persistent east-northeast flow due to the formation of a surface low forecast southeast of FL and a sprawling surface high to the north will provide the necessary onshore component and continued low-level moisture advection across the above area. Given the higher chance of flash flood concerns within the urban corridor extending from Miami, north to West Palm Beach, have decided to upgrade the previous MRGL risk to a SLGT with an opportunity for further upgrade to a higher risk category pending the evolution of hi-res guidance and their forecast for where the zone of best convergence and heavy rainfall will occur. Kleebauer Day 3 Valid 12Z Thu Nov 16 2023 - 12Z Fri Nov 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE EASTERN FLORIDA COAST... 20z Update: No changes made to the inherited Marginal risk area over eastern FL. Still looks like the most likely scenario is that heavier convection is offshore by this time, however some guidance still lingers areas of heavy rainfall onshore into Thursday. Thus think maintaining the Marginal risk is the best course of action for now and we can continue to monitor trends. Chenard ....Previous Discussion... Surface low formation off the eastern FL coast will propagate to the north and northeast over the course of Thursday with the best moisture field and low-level ascent being pulled away from the FL Peninsula. Much of the guidance has been keen on degrading the heavy rain potential due to the synoptic evolution of the surface reflection and upper trough across Southeast US. However, there's still some solutions that hang on to the idea of a local heavy rain threat within the confines of the eastern FL coast, especially over the Space Coast region of the state. This is in conjunction with residual moisture present along with a semi-prevalent inverted trough axis that hangs near and just off the coast of FL. ECMWF and multiple ensemble members indicate this potential with the deterministic producing a prolific bullseye of 3-5" somewhere within the confines of the eastern FL coast. This is a low probability type event, but one that could occur given the complexities of a dynamic upper trough propagation and attendant surface low in vicinity of the southeast coast. The NAM is also another model that produces heavy rainfall within the D3 time frame, but its evolution is vastly different than any other guidance at this juncture so will likely discount its solution. This is a lower end MRGL risk for the time being and the period will be monitored closely for any localized impacts within the outlined areas.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-WEWrbRlmo_Oxnj-nJR2HQL1j6zhjZmslK4yV1jSxNLT= g4dexD4kP6pIinn1z5lwLYBXHtikb4CIouR-6uNgYBFSS3U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-WEWrbRlmo_Oxnj-nJR2HQL1j6zhjZmslK4yV1jSxNLT= g4dexD4kP6pIinn1z5lwLYBXHtikb4CIouR-6uNgB3hOjos$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-WEWrbRlmo_Oxnj-nJR2HQL1j6zhjZmslK4yV1jSxNLT= g4dexD4kP6pIinn1z5lwLYBXHtikb4CIouR-6uNgJpxBAKs$=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. 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