Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 17 2023 00:03:27 FOUS30 KWBC 170003 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 802 PM EDT Sun Apr 16 2023 Day 1 Valid 01Z Mon Apr 17 2023 - 12Z Mon Apr 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE ATLANTIC COAST OF SOUTHEAST FLORIDA... ....Great Lakes...Ohio Valley...Central Appalachians... 0100 UTC Update -- Have pared the western portion of the Marginal Risk area based on the observational and mesoanalysis trends. Latest GOES East water vapor imagery shows the back edge of the warm conveyor belt and TROWAL ('limiting streamline') having cleared southeast WI and much of southern Lower MI. Later this evening and overnight, will expect the limited FF threat to diminish further as the surface front and 1-1.25" PW axis continue to push farther downstream. Previous discussion.. 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 snow melt 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. Across the OH Valley and central Appalachians...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... 0100 UTC Update -- Mesoscale-enhanced vort lobe migrating through the eastern GOMEX will set the stage for enhanced upper convergence within an already weakly difluent upper flow pattern across S Florida. This as diurnally-driven convection along the central and Atlantic coastal areas of S Florida provide a little extra focus/low-level convergence per outflows/merging cells as the convection from the Gulf moves onshore. Even on the downward trend following sunset, mixed-layer CAPEs are still quite healthy - i.e. 2000-2500 J/Kg. This along with PWs between 1.7-1.9" in a pre-convective environment will foster rainfall rates of 2+ inches/hr, with peak rates potentially 3+ inches/hr. Based on the latest observational and mesoanalysis trends, have expanded both the Slight and Marginal Risk areas slightly, more closely matching the elevated exceedance probabilities per the 18Z HREF. For further details, please refer to the latest mesoscale precipitation discussion or MPD #194, which expires at 170500Z. Previous discussion.. 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. Hurley/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!53T3FZZ7DOoYCVW9UBVDJA6FGm33CU6jIk4vTJtawgSH= _W93Zq7yGY99epPpd1neg_HpLF2ohVBud9DD14_KBPJr39o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!53T3FZZ7DOoYCVW9UBVDJA6FGm33CU6jIk4vTJtawgSH= _W93Zq7yGY99epPpd1neg_HpLF2ohVBud9DD14_Ksl_EDkw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!53T3FZZ7DOoYCVW9UBVDJA6FGm33CU6jIk4vTJtawgSH= _W93Zq7yGY99epPpd1neg_HpLF2ohVBud9DD14_KMm8j3Xk$=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) .