Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 20 2022 16:01:06 FOUS30 KWBC 201601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM 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. Roth Day 3 Valid 12Z Thu Dec 22 2022 - 12Z Fri Dec 23 2022 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE APPALACHIANS & EASTERN SEABOARD... Coastal North Carolina... This region is expected to have the best combination of parameters -- precipitable water values of 1.25-1.5", MU CAPE of 500-1500 J/kg, overlapping low- to mid-level frontogenesis, and high enough values of 850 hPa inflow/effective bulk shear to allow for convective organization. This led to a new Marginal Risk area when compared to experimental day 4 continuity. However, the flow pattern is progressive enough that any particular spot has six hours or less of the best ingredients to produce heavy rainfall.=20 Should cells manage to merge and/or train, hourly rain totals to 2" are possible in this environment. The guidance is variable on the amounts expected, generally in the 2-5" range (with the Canadian Regional showing an 8"+ bulls eyes just offshore). The land type is generally sandy soils and marshland, which would also limit the response to any heavy rainfall. Any issues that occurred would be most likely in urban areas. =20 Portions of the Appalachians/Mid-Atlantic States... The combination of moist inflow from the Gulf of Mexico and Atlantic within the low-levels combined with a couple periods of overlapping low- to mid-level frontogensis early and late period, MU CAPE values flirting with 500 J/kg early on, and precipitable water values of 0.75-1.25" within an area of low thickness values (implying saturation) support the idea of heavy rainfall. Parts of this area should start out as frozen precipitation early before becoming all rain as the precipitation intensity picks up. The expectation is for the maximum hourly rain total around 1", which could cause issues in steeper terrain, but probably be reasonably handled in urban areas. The guidance shows a signal for local amounts of 2-4". It is unclear for how long the more convective precipitation occurs -- it appears favored both early due to the instability and then late in the period as a strong deep layer low moving in from the west with rapidly cooling temperatures aloft/declining mid-level heights could theoretically initiate a narrow frontogenetic squall line moving across the Appalachians/surrounding Piedmont very late in the period/Friday morning along the incoming cyclone's cold front. Since there appear to be two distinct windows for heavy rainfall, left the Marginal Risk area from experimental day 4 continuity generally intact. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7H3T7GxtzaKyrZiOhENZRcCoQPQuygt5J6tAHvSnwzlH= Kf7IUkE8vw5LWKz0Nmn0Oiok_Qc3pl3gYLN1SEW_5VEmzLU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7H3T7GxtzaKyrZiOhENZRcCoQPQuygt5J6tAHvSnwzlH= Kf7IUkE8vw5LWKz0Nmn0Oiok_Qc3pl3gYLN1SEW_cap3KAo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7H3T7GxtzaKyrZiOhENZRcCoQPQuygt5J6tAHvSnwzlH= Kf7IUkE8vw5LWKz0Nmn0Oiok_Qc3pl3gYLN1SEW_IOYnCG0$=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) .