Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 16 2023 19:38:19 FOUS30 KWBC 161938 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 337 PM EDT Sun Apr 16 2023 Day 1 Valid 16Z Sun Apr 16 2023 - 12Z Mon Apr 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE ATLANTIC COAST OF SOUTHEAST FLORIDA... ....16Z Update... ....Southeast Florida... In coordination with the MFL/Miami, FL forecast office, a Slight Risk upgrade was introduced for this update. Flooding from the over 2 feet of rain in and around the Ft. Lauderdale area a few days ago continues this morning. The combination of the sea breeze, daytime heating causing MUCAPE to spike above 2,000 J/kg, PWATs over 1.5 inches with surface dewpoints well into the 70s, and an approaching upper level trough will result in showers and thunderstorms developing near the middle of the FL Peninsula this afternoon, and initially tracking east towards the Atlantic coast. As the storms move over the I-95 corridor, the steering flow will collapse, resulting in very slow moving/stationary storms, with some backbuilding. The backbuilding will become more prominent into the evening as the MCS over the central Gulf moves into the southwest FL Gulf Coast, and combines with the storms ongoing on the Atlantic side. The MCS will from there move the storminess east into the Atlantic, but the period of nearly stationary storms and MCS rainfall could result in several inches of rain over the urban corridor. For now, most of the guidance suggests the heaviest rainfall amounts will be north of Ft. Lauderdale, but that confidence is low, and regardless, Ft. Lauderdale itself may still see rainfall totaling 1-3 inches, which will worsen ongoing flooding. The 12Z sounding from MFL this morning shows very light and chaotic steering flow over that area already, which may further heighten the risk for heavy-rain producing storms to get stuck in one area. Elsewhere, the Marginal Risk was expanded a bit further north up the Space Coast of central FL for sea breeze convection likely to impact areas up north. However, the MCS over the Gulf is unlikely to contribute to higher rainfall totals like areas further south, but what storms do impact that area may be locally strong and capable of producing heavy rain over the urban corridor, causing an isolated flash flooding risk. ....Southern Appalachians... The Marginal Risk area over the Applachians was expanded southward to cover portions of southwest VA, far eastern TN and northwestern NC with this update. While storms that develop over this area later today are likely to be progressive, the low FFGs in that area may still be low enough that any stronger storms may produce enough rain to cause an isolated flash flooding risk. Certainly the threat is a bit higher further north in the Appalchians towards western PA. ....Michigan/Eastern Wisconsin... No changes were made with this update, periods of moderate rain are ongoing over this area, which when moving over areas with very low FFGs, particularly in the U.P. of Michigan, may result in isolated flash flooding, as the main-stem rivers in the area are already flooding due to rapid snowmelt the past few days. Thus, any rainfall added to those watersheds will have no place to drain to. Wegman ....Previous Discussion... ....Great Lakes... Waves of low pressure are expected to lift northward along an existing stationary boundary across Wisconsin today. Increasing large scale forcing for ascent is expected as the mid/upper level shortwave trough takes on a negative tilt, pivoting the surface low northward and then northwest. The strong ascent coupled with an axis of higher moisture streaming northward wrapping into the system should yield showers and embedded thunderstorms across eastern Wisconsin through the U.P. of Michigan. Guidance shows a few rounds of precipitation through the afternoon, then much colder air begins to wrap into the system with a gradual west to east changeover to mixed precipitation, at least through central Wisconsin. Far eastern Wisconsin into Michigan will remain warm enough through the period to support rain where localized totals of 1-2" will be possible. Some of this rainfall will fall on sensitive ongoing snowmelt areas, particularly for the U.P. of Michigan where flash flood guidance is particularly low and the additional rainfall could lead to or initiate new flooding. ....Ohio Valley... Strong forcing for ascent is expected over the region ahead of the negatively tilted shortwave trough over the Upper Midwest. A progressive front will sweep through the region and with peak heating and modest amounts of moisture, a line or two of stronger thunderstorms will move through with isolated 1"+/hr rain rates possible. While storm motions are likely to be quick enough to limit widespread issues and antecedent conditions are rather dry, the potential exists for a quick/intense period of rainfall over susceptible areas leading to isolated flash flooding. ....Southeast Florida... Deep convection is expected to develop across the Florida Peninsula this afternoon as a embedded shortwave trough currently over the north-central Gulf begins to advance toward the region. An increase in the low level convergence with the approach of the mid level shortwave trough combined with decent instability and moisture should yield several clusters of strong thunderstorms. Intense rain rates approaching 2"/hr will be possible with localized 3"+ totals possible based on the 00Z hi-res guidance and HREF neighborhood probabilities which for 3" approach 50 percent (20 percent for 5"). This intensity and amount of rainfall over a short period could lead to flash flooding over more susceptible/vulnerable locations like urban areas from the Treasure Coast southward through the Miami/Ft. Lauderdale metro. Taylor Day 2 Valid 12Z Mon Apr 17 2023 - 12Z Tue Apr 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF ATLANTIC COASTAL SOUTHEAST FLORIDA AND MUCH OF INTERIOR NEW ENGLAND AND FAR NORTHERN NEW YORK... ....Southeast Florida... A stationary front over southern Florida will induce an easterly flow off the Gulf Stream into the Miami-Ft. Lauderdale area of southeast Florida. While convection isn't likely to be anywhere near as robust, widespread, or as strong Monday afternoon as compared with that developing now and continuing through this evening, the easterly flow into the coast should induce localized convergence at the coastline, which may support additional convection in many of the same areas hard hit this past week and expected to pick up additional rainfall this afternoon and evening. Thus, in coordination with the MFL/Miami, FL forecast office, introduced a Marginal centered around Ft. Lauderdale for the expected thunderstorm activity. ....Interior New England & Far Northern New York... The cold front associated with the wrapped up low over Michigan will track northeastward across New England on Monday. Ahead of the front, highly anomalous moisture with PWATs exceeding 1", which is about 3 sigma above normal will move into New England on southeasterly flow off the Atlantic. While storms associated with the front will be progressive, the combination of the highly anomalous moisture, low FFGs (as low as 0.75 inches in an hour in some spots), and very strong forcing with a highly energetic negatively tilted trough should be enough to overcome the fast movement of the storms and low instability in isolated instances in the Marginal Risk area. Snowmelt will also add a little additional water to the local watersheds in northern New Hampshire up through western and northern Maine. Wegman Day 3 Valid 12Z Tue Apr 18 2023 - 12Z Wed Apr 19 2023 ....20Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8SuX0E3RwjD7UQmnTJtVnqHx-d7uoC2mWlm42hThpHlB= tzCtiq9VJcGFmnNr-W6hH4xcbjJRAvAMXXvOmV0-wAmsAMA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8SuX0E3RwjD7UQmnTJtVnqHx-d7uoC2mWlm42hThpHlB= tzCtiq9VJcGFmnNr-W6hH4xcbjJRAvAMXXvOmV0-SZ7REB8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8SuX0E3RwjD7UQmnTJtVnqHx-d7uoC2mWlm42hThpHlB= tzCtiq9VJcGFmnNr-W6hH4xcbjJRAvAMXXvOmV0-Sy3ryLw$=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) .