Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 16 2023 15:47:05 FOUS30 KWBC 161544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1044 AM EST Sat Dec 16 2023 Day 1 Valid 12Z Sat Dec 16 2023 - 12Z Sun Dec 17 2023 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF FLORIDA... 16z update: Small cosmetic changes were made to the risk areas with this update, with some northeast expansion given a tad faster progression of the axis of training showers/weak thunderstorms along and left of the strengthening surface low crossing the Big Bend of Florida later this evening into very early Monday morning. Guidance remains on track for the slightly weaker but longer duration training showers with generally 3-6" toward the Okefenokee Swamps of SE GA by 17.12z, while strengthening southeasterly fetch band off the northeast Bahamas starts to flux into SE GA increasing Hi-Res CAM totals (both in the end of the D1, as well as, start of D2 period). Further south in the warm sector, the potential for stronger thunderstorms, potentially rotating supercells, will provide stronger isallobaric wind responses and increased moisture flux into the cells allowing for the potential for higher intensity rain-rates from late evening through the early overnight period.=20 Hi-Res CAMS including HRRR 15 minute totals suggest rain-rates of 2"+/hr (15-30 minute totals of 1.5-1.75"). However, forward motion appears to be quick enough reduce duration and overall totals reducing the potential for larger areal coverage flooding to higher focused localized typically urban flooding. If training/repeating occurs the potential for much higher localized totals and potential for rapid inundation flooding would be expected; however, this is high uncertain though a model or two do suggest this potential after 00z. The greatest potential for this to unfold is generally across the southern and eastern Peninsula (including the Keys), where steering flow gradient is sharpest and the linear features can flatten to the flow increasing the overall training duration/potential. Gallina ---Prior Discussion--- Surface low pressure is expected to develop over the Gulf of Mexico today and deepen as it approaches the Southeast U.S. today with low level winds accelerating across the Florida peninsula by this evening and across portions of the Southeast U.S. later tonight. The resulting moisture transport boosts precipitable water values to 1.75 inches or greater across most of Florida east of the Big Bend region...with values exceeding 2 inches across the southern part of the peninsula. With anomalously high precipitable water values...some 2 to 3 standardized anomalies above climatology across the peninsula for this time of year...the environment should support some downpours as shown by the isolated 20 percent probabilities for 2 inches of rain in 1 hour that develops late this evening in the 16/00Z HREF runs. With the exception of the model guidance continuing to favor the area around the Big Bend region with 24-hour rainfall amounts in excess of 3 inches...model guidance was still struggling with the details about the surface low...its track...and the resulting implications for where the heaviest rainfall will fall. As a result...maintained the broad Slight risk without too many changes. Bann Day 2 Valid 12Z Sun Dec 17 2023 - 12Z Mon Dec 18 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN U.S. FROM THE CAROLINAS TO THE NORTHERN MID-ATLANTIC REGION... ....Eastern U.S... A very dynamic system will continue to ramp up over portions of the Southeast U.S. on Sunday...spreading widespread rainfall northeast along the eastern seaboard throughout much of the day. With precipitable water values increasing to 1.5 inches along the coast from the Carolinas into the Mid-Atlantic region and on-shore winds at 850 mb on the order of 60 to 75 knots developing in a tightening gradient north of the surface low...potentially heavy to excessive rainfall is expected to be embedded within the broader shield of rainfall. The spread within the model guidance has closed somewhat...although the 16/00Z NAM was given less consideration based on how it races the surface low northward so quickly and takes it on a more westerly trajectory. While it probably should not be emntire ruled out...more weight was given to the GFS/ECMWF solutions due to the support they had from the ensembles. A broad area of 2.5 to 4+ inches is expected for the eastern Carolinas...tapering to 1.5 to 3 inches in parts of the Mid-Atlantic region and southern New York. Expanded the Slight Risk area a bit westward in South Carolina where the HREF showed 3-hour QPF that exceeded 3 hour Flash Flood guidance and where there was a greater than 15 percent probability of 2 inches of rain in an hour on Sunday afternoon. Farther north...nudged the Slight Risk a bit farther to the north and east. Models show limited instability despite precipitable water values exceedin an inch along the coast from the Mid-Atlantic towards Southern New England late...but some minor urban and small stream flooding can not be ruled out. Otherwise...the changes to the risk areas were changed little from the previous ERO. ....California... Maintained a Margial Risk area for excessive rainfall...although shifted a bit northward in concert with the model QPF showing a continued northward trend. Bann Day 3 Valid 12Z Mon Dec 18 2023 - 12Z Tue Dec 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST USA... A broad shield of rainfall associated with a deep area of low pressure will continue to spread north and eastward during the period. Maintained a Slight Risk from the previous Day 4 outlook over portions of New England where combination of deep layer moisture transport and strong low level jet combine forces with the upper support of a 90 kt to 120 kt upper level jet that takes on an increasing S shape should continue to support briefly heavy rainfall rates and areas of rainfall amounts in excess of 1.5 inches. Despite precipitable water values that climb above an inch over New England....model guidance shows little instability to help enhance rates which suggests more of minor urban and small stream flooding concerns although heavy rainfall in the past week over parts of the area may result in hydrologic concerns on other tributaries or in regions of poor drainage. ....California... Rainfall becomes better organized and makes its way onshore across northern California a persistent ridge begins to break down.=20 Model guidance has greater than 50 percent chance of some northern California basins receiving more than 1.5 inches on Monday.=20 Trimmed a bit of the Marginal risk area over the Sierras where areas above 7000 ft or so should be snow...and confined the risk area where precipitable water values were low level winds transport in an airmass with precipitable water values upwards of 0.75 inches.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8UeHRVPD9K9LEYZoCmpFgFNjeXvONn2bW7NpzHUN842f= utILxTYn-cjrCzq3VoIC3QzA3us4ldy9jqU-oJAyf5bdE8s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8UeHRVPD9K9LEYZoCmpFgFNjeXvONn2bW7NpzHUN842f= utILxTYn-cjrCzq3VoIC3QzA3us4ldy9jqU-oJAy8NMB9Pg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8UeHRVPD9K9LEYZoCmpFgFNjeXvONn2bW7NpzHUN842f= utILxTYn-cjrCzq3VoIC3QzA3us4ldy9jqU-oJAyA1VlLLk$=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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