Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 20 2022 19:59:39 FOUS30 KWBC 201959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM EST Tue Dec 20 2022 Day 1 Valid 16Z Tue Dec 20 2022 - 12Z Wed Dec 21 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN FLORIDA... ....16Z update... Across Florida, there remains a low end threat for excessive rainfall, likely focused across urban locations within the southern half of the Peninsua. The greatest chances will occur this evening into the first half of the overnight as the shortwave mentioned in the previous discussion approaches (see below). Organized convection looks to reach the southwestern Peninsula along with a surge in favorable upper level ascent with the potential for 2-3 in/hr rainfall rates late this afternoon into the evening. Forecasts of available instability and onshore low level flow also favor portions of the southeastern Florida coast into the early overnight with localized potential for rainfall rates in excess of 2 in/hr. Farther north along the coasts of Georgia and South Carolina, the previously issued Marginal Risk was removed with this update. Model guidance from 12Z is showing any appreciable instability (100+ J/kg) remaining offshore and only 1 to perhaps 2 inches (as a max) of rainfall over the 24 hour period. 12Z HREF probabilities of 1+ in/hr are only about 5 percent along the coast of Georgia and given drier than average rainfall over the past couple of weeks, any rainfall that is received should not cause issues with runoff. Otto ....previous discussion follows... Florida Peninsula... Divergence aloft will be sponsored by the right entrance region of an upper level jet over the Southeast U.S. and difluence over the Florida peninsula and the northeast Gulf of Mexico. A blossoming area of showers and thunderstorms over the eastern Gulf of Mexico will make its way inland on Tuesday and Tuesday night/early Wednesday morning. Precipitable water values of 1.25-1.75" are expected within an increasingly saturated atmosphere. Overlapping low- to mid-level frontogenesis combined with 500-1000 J/kg of MUCAPE is expected to foster heavy rainfall, which could be as much as 2" in an hour where cells train/thunderstorm activity organizes. Various pieces of deterministic guidance show the potential for local amounts in the 2-5" range, mostly across the central and southern FL peninsula. Portions of central and southern FL have received 300-600% of their average rainfall during the=20 past week, but since this is their dry season, this amounts to spotty 2-4", which in the warm season would be an average day.=20 The Marginal Risk has been maintained for this area, with the biggest potential impacts in urban areas. Georgia and South Carolina coast... The guidance continues to show a signal for potential heavy rainfall near or just offshore the coast of GA and SC. This occurs to the north of a surface low which slowly spins up in place offshore eastern FL before eventually heading out to sea once a fast moving shortwave along the Gulf Coast escorts it away from the region. Overlapping low- to mid-level frontogenesis should aid heavy rainfall production. Model guidance showing the potential for 2-5" amounts near to just offshore as their mass fields show just enough dispersion in the low levels/near 850 hPa and instability fields to open to the door to more ashore solutions. Instability is expected to rise to 250-1000 J/kg near to just offshore, in the vicinity to the Gulf Stream, with precipitable water values also following suit, rising to 1-1.25" within an an environment with 1000-500 hPa thickness values in the low to mid 550s decameters, implying nearly complete saturation.=20 With the guidance showing heavy rainfall so close to the coast, reintroduced a Marginal Risk area in this region as a precaution.=20 Coastal lows with this sort of evolution have been known to produce heavy rainfall in this region in the past. Hourly rain totals to 1.5" should be possible within this sort of environment. As this is a marshy area for the most part, any issues -- should the heavy rainfall make it ashore -- would be confined to the urban areas within the Marginal Risk's bounds. =20=20 Roth Day 2 Valid 12Z Wed Dec 21 2022 - 12Z Thu Dec 22 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 3 Valid 12Z Thu Dec 22 2022 - 12Z Fri Dec 23 2022 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE MID-ATLANTIC INTO SOUTHERN PORTIONS OF THE NORTHEAST... Coastal North Carolina... Areas of heavy rain are expected to be ongoing from the South Carolina coast into southern North Carolina coast Thursday morning. A surface low will track northward along the Southeast coast through Thursday evening with a narrow warm sector east of the frontal wave. Across the Coastal Plain, precipitable water values of 1.2-1.6 inches and MUCAPE of 500-1500 J/kg are expected along with increasing upper level support through 00Z Friday with increasing diffluence and left exit region divergence related to a subtropical jet streak translating offshore of the Southeast. Progressive cell motions and sandy soils/marsh are limiting factors for runoff concerns but brief training of heavy rain allowing hourly rainfall totals of 2 or more inches will be possible. The latest model consensus is for 24 hour maximum rainfall of 2-4 inches centered on the North Carolina coast, but the 12Z FV3 and 06Z GEM_regional supported 6+ inches on or just offshore of the North Carolina coast. The Marginal Risk was maintained from continuity but any issues that occur will likely be confined to urban areas. =20 Appalachians/Northern Mid-Atlantic into southern New England... While minor timing and placement differences exist in the latest model guidance, there is agreement that a mid-upper level trough will strengthen and become a powerful closed low as it approaches the Ohio Valley from Thursday into Friday morning. As this occurs, increasing upper level diffluence and divergence is expected to develop over the northern Mid-Atlantic and southern New England Thursday evening and overnight. Increasing low level moisture transport will overspread the East Coast with standardized precipitable water anomalies of +1.5 to +2.5 forecast by the 12Z GFS to stretch from the Mid-Atlantic into southern New England during the forecast period. While 1+ inch precipitable water values are not forecast by the model consensus to penetrate very far inland and weak instability (less than 500 J/kg MUCAPE for the northern Mid-Atlantic into the Northeast) should limit hourly rainfall totals to the 0.5 to 1.0 inch range, 1 to 2 inch rainfall totals in a 6 hour period and 24 hour totals of 2 to 4 inches will remain possible. Cold to frozen soil temperatures and snow cover for northwestern portions of Marginal Risk area were considerations for a targeted Slight Risk, covering northeastern Pennsylvania, southern Upstate New York and northern New Jersey. However, after collaboration with the affected WFOs, there was agreement to hold off on anything higher than a Marginal Risk at this time. Consideration to adding a small Slight can be given with future forecast cycles if model output becomes more aggressive. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8PfXXAL1WVgp6FB-aSwkXhuq38ILKy1KxAx2u_ebHp-w= qzpA0xDHS5JAUpQBsory-8vWSTpO2K_BangWaR-Ts1yRoFg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8PfXXAL1WVgp6FB-aSwkXhuq38ILKy1KxAx2u_ebHp-w= qzpA0xDHS5JAUpQBsory-8vWSTpO2K_BangWaR-TWysiEto$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8PfXXAL1WVgp6FB-aSwkXhuq38ILKy1KxAx2u_ebHp-w= qzpA0xDHS5JAUpQBsory-8vWSTpO2K_BangWaR-T55n_GfA$=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. 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