Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 16 2023 01:04:29 FOUS30 KWBC 160104 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 803 PM EST Wed Nov 15 2023 Day 1 Valid 01Z Thu Nov 16 2023 - 12Z Thu Nov 16 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH FLORIDA... A MDT risk remains over the urban corridor of southeast FL. The flash flood risk is increasing across this corridor, and will peak now into the overnight hours. Areas of urban flooding are likely to increase in coverage and significance over the next several hours within the MDT risk area. Recent obs and radar trends indicate as small area of low pressure has developed over the northern Keys and convection has increased just to the north and east of this low over far southeast FL. The 22z and 23z runs of the HRRR do not have a good handle on the structure of this rainfall, and thus its QPF placement is likely off. Instead will just need to rely on observational trends, which appear to be favoring significant rainfall over southeast FL. The low pressure had been moving pretty steadily off to the northeast, which extrapolating forward would suggest heavy convection could exit southeast FL in several hours. However most guidance indicates this low should slow its progression and possibly even stall/broaden...and it would appear that recent radar is beginning to show this slowing. If this occurs then the increased convergence and heavy rainfall over the southeast FL coast will persist into much of the overnight hours. A gradual erosion of instability suggests rainfall rates peaking 1-2"/hr, however localized embedded heavier convection could still push some spots to 3"/hr. Given the high PWs and low topped nature of this convection, it will likely continue to be warm rain dominant and thus efficient at rainfall production. While some uncertainty on exact rainfall magnitudes remain, recent observational/radar trends do seem to suggest a higher end event is indeed unfolding over far southeast FL. Would not be surprised to see additional rainfall as high as 4-10" in spots through 12z Thursday. At least localized higher end flooding appears probable as the night progresses. Elsewhere, the Marginal risk was maintained over portions of southern CA. Convective elements moving onshore will produce briefly heavy rainfall, although the quick forward speed of these cells should generally limit any flood risk. Chenard 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!9oneYuyN1OFXiOjcq1iRxrbm1xdCU3YLV2cS1erTCik5= 0Zim2YwMA-zBugZjOmEtNjfijVBRETptVc6WnqpNhIjmjz0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9oneYuyN1OFXiOjcq1iRxrbm1xdCU3YLV2cS1erTCik5= 0Zim2YwMA-zBugZjOmEtNjfijVBRETptVc6WnqpNwq1KAng$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9oneYuyN1OFXiOjcq1iRxrbm1xdCU3YLV2cS1erTCik5= 0Zim2YwMA-zBugZjOmEtNjfijVBRETptVc6WnqpNlH0kruU$=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) .