Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 15 2023 08:09:16 FOUS30 KWBC 150809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 AM EST Wed Nov 15 2023 Day 1 Valid 12Z Wed Nov 15 2023 - 12Z Thu Nov 16 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST FLORIDA... Latest satellite and radar composite shows a surface low south of the central Gulf coast with a multi-wave pattern of weak convection making their way onshore across south FL. The convective pattern moving into the Peninsula is in part to a prevailing easterly boundary to mid-level wind field caused by the southern periphery of a broad surface ridge over the east coast, and a stationary boundary bisecting the southern part of FL. This will be key in the convergence pattern that will develop later this morning, carrying through the rest of the afternoon and evening allowing for persistent heavy rainfall clusters to settle over the Keys and the urbanized corridor on the southern tip of the Mainland. Model consensus has grown for a significant rainfall event to unfold within the confines of the Keys later this morning, extending north in the afternoon as a weak area of low pressure develops in-of of the FL Straits and moves northeast in time. Deterministic QPF across much of the 00z suite indicates a widespread 3-6" of rain within the aforementioned areas, but maximum potential up to 10" is within the realm of possibility. Forecast areal average soundings across south FL from much of the CAMs indicate a general 500-1000 J/kg of skinny MLCAPE and PWAT's between 2-2.2" by later tomorrow afternoon, a good 2.5-3 standard deviations above normal. This is a textbook environment for upper quartile outcomes as the PWAT field will fall within the 90-95th percentile for climo, and the primary forcing will be present in vicinity of the targeted area. The probability fields are also very robust given the overall consensus. 00z HREF neighborhood probability for 5" is a solid 90% or greater across the upper Keys into a zone stretching from Homestead up to Miami. The 8" neighborhood probability is as high as 70% within the corridor from Homestead through Key Largo, matching the NBM 99th percentile QPF forecast within that zone. There's still some deviations in where the heaviest rainfall will occur as much of the setup will come down to low propagation after genesis, as well as the speed of the surface reflection as it makes the turn up the eastern FL coast and how the instability fields will evolve in time. At this juncture, there is plentiful support in guidance for a heavy rainfall axis to initiate within the confines of the FL Keys, pivoting northward into the southern FL Peninsula. General rainfall rates will settle between 1-2"/hr given the very high probabilistic forecast from the latest hi-res ensemble, but a 15-25% risk of up to 3"/hr is on the table within the confines of Miami down into the upper Keys. This will promote a higher probability of flash flooding concerns within the urban corridor down into the FL Keys north of Marathon. A Moderate Risk was maintained from previous forecast issuance with a slight extension south to include all of Key Largo based on the recent guidance trends. Kleebauer Day 2 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... The current MRGL was expanded to the south to include the Miami to Fort Launderdale corridor given recent trends in guidance to maintain locally heavy rainfall on Thursday morning, a compounding effect on the expected D1 precipitation. This would exacerbate flooding concerns through at least 18z before threat wanes. Elsewhere to the north, a bit more consensus on the prospects of locally heavy rainfall within the confines of the Melbourne-Orlando-Jacksonville triangle in due part to the proxy of the surface low off the east coast of FL and the energy transfer from the shortwave trough migrating eastward from the Gulf. Latest blended guidance is bullish with totals of 2-4" aligning portions of the eastern FL coast with some hint of further impacts inland into central FL. A lot comes down to the transfer of energy at 5H and whether there is a smooth transition from the initial SLP in the Gulf to the low off the FL coast. Where that handoff occurs would be important in deciphering local precipitation maxima as the trailing energy over the coast would allow for localized forcing to induce an area of heavier rain that would likely train over the same area(s) until it vacates the east coast. 6-hour 5 year ARI exceedence probabilities have increased significantly between both the 12-18z and 18-00z time stamps on Thursday with upwards of 25-30% now within an area encompassing Melbourne to Orlando, as well as fixed further south along the urban corridor from Port St. Lucie down into the Miami Metro. This was one of the reasons for the general MRGL expansion, as well as a nudge further west on the edge of the risk area. There's still a bit too much variance for where exactly the heaviest rain will occur, but the consensus is growing for somewhere near the Space Coast down to Miami and out into central FL for there to be some localized flooding concerns, especially given the PWAT anomalies of 1-2 deviations above normal lingering for much of the state. Kleebauer Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-EBba5vnl9dg9_40OdfR6p0I2LeUTbyOnPM0vdv4lEcH= So6snwHW8jfMQoST9TWg_ifBbd4ml8YOHtQ_-e9MZfJLLtk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-EBba5vnl9dg9_40OdfR6p0I2LeUTbyOnPM0vdv4lEcH= So6snwHW8jfMQoST9TWg_ifBbd4ml8YOHtQ_-e9MsFGcJiU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-EBba5vnl9dg9_40OdfR6p0I2LeUTbyOnPM0vdv4lEcH= So6snwHW8jfMQoST9TWg_ifBbd4ml8YOHtQ_-e9M7u08QIc$=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) .