Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 15 2023 20:29:27 FOUS30 KWBC 152029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EST Wed Nov 15 2023 Day 1 Valid 1849Z Wed Nov 15 2023 - 12Z Thu Nov 16 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH FLORIDA... ....16 UTC Update... Overall depiction of the mesoscale forecast and surface environment featuring highly anomalous precipitable water values over South Florida and the Keys remains valid this morning. The expectation is that a surface wave will develop along a stationary boundary currently analyzed through the Florida Straights and track to the northeast. This will promote organized convective bands along and to north of the stationary boundary, oriented up the Keys and into South Florida following convergence along the coast. The overall coverage/footprint differs some amongst the CAM guidance based on the degree of organization, and the location of wave development, track, and speed will be a potential caveat in realizing some of the high rain totals possible, as a more southerly/easterly track may keep more convection east of the wave, offshore, and limit the total rainfall. However, confidence still remains high that slow and deviant storm motions will allow for at least localized but very significant rainfall totals, with highly impactful instances of urban flooding possible for South Florida. Updated 12Z CAM guidance shows very high (80-90%) HREF exceedance probabilities for 5" totals and moderate to high probabilities (50-80%) of 8" totals, aligned with the individual deterministic guidance indicating the potential for localized totals as high as 10". This rainfall may all be realized in a short timeframe later this afternoon and into the overnight hours. Rainfall rates of 2"/hr are likely and may be as high as 3"/hr where 500-1000 J/KG of MLCAPE and weak shear could support mesocyclone development in some cells. The Slight Risk has also been expanded southwest to include all of the Keys given the strong support for the high 3"/hr rates and a concern that elevated tides may limit drainage. ....19 UTC Update... A Marginal Risk was also introduced for portions of southern California given recent storm trends and concerns for sensitive terrain areas/burn scars. Precipitable water values have already climbed above 1 inch along the coast and should retain at least this value through the evening, which is over the 95th climatological percentile for the region, connected to a plume of moisture advecting in from the Pacific ahead of an approaching frontal system. Rain rates may reach 0.5"/hr, and perhaps higher where marginal CAPE and shear could contribute to a few stronger thunderstorms with mesocyclones. This could lead to a few isolated instances of flooding.=20 Putnam ....Previous Discussion... 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/near 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 this 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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE EASTERN FLORIDA COAST... A Slight Risk has been introduced for portions of central Florida/the Space Coast southward through South Florida. Incoming 12Z CAM guidance has highlighted the potential for some significant heavy rainfall totals with thunderstorms developing between a surface low moving northward just off the coast of South Florida and an upper-level shortwave approaching from the West. These storms will likely organize as convective bands where surface convergence is focused along the coast with the potential for training over the same areas increasing the threat for higher rainfall totals. Precipitable water values will not be quite as high as during the day 1 period, and begin drying further south, but should still remain around 1-2 standard deviations above the mean. These high values along with some modest ML CAPE should contribute to rain rates approaching and potentially exceeding 2"/hr. In addition, the trends in the CAMS have become concerningly somewhat similar to the prior Day 1 period in terms of both deterministic amounts and exceedance probabilities. The 12Z HREF shows some moderate exceedance probabilities (30-40%) for 5" totals with some low probabilities (20-30%) of 8" totals. The deterministic guidance also shows notable 3-5" totals with some isolated higher amounts of 7-10". As noted, it is likely that the higher threat will be further north towards central Florida/the Space Coast than south, as the track of the surface low off the coast north of South Florida should bring an end to storms here early, perhaps even prior to the start of the forecast period. However, there is enough of a signal in the guidance that storms may linger into the morning, particularly from Palm Beach County northward, to have a Slight Risk in place. Even a couple inches of additional rainfall could easily exacerbate any ongoing flooding if totals of 5-10" are realized during the prior Day 1 period. Putnam Day 3 Valid 12Z Fri Nov 17 2023 - 12Z Sat Nov 18 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Putnam Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!42FpITk3UD-WvV2QJo4KGRDT_Kx9BM4CcLTNlcHF-K_R= SulRR9sQ1X5gRqPuPtKR9la1k6HRcTdhXqiNtTQNkPqz5JU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!42FpITk3UD-WvV2QJo4KGRDT_Kx9BM4CcLTNlcHF-K_R= SulRR9sQ1X5gRqPuPtKR9la1k6HRcTdhXqiNtTQNSC9ePO4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!42FpITk3UD-WvV2QJo4KGRDT_Kx9BM4CcLTNlcHF-K_R= SulRR9sQ1X5gRqPuPtKR9la1k6HRcTdhXqiNtTQN9dAwnH8$=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) .