Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 22 2022 19:29:19 FOUS30 KWBC 221929 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 228 PM EST Thu Dec 22 2022 Day 1 Valid 16Z Thu Dec 22 2022 - 12Z Fri Dec 23 2022 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF COASTAL NORTH CAROLINA AS WELL AS THE MID-ATLANTIC INTO SOUTHERN NEW ENGLAND...=20 ....16z update... No major updates needed to the current marginal risk areas across both the Mid-Atlantic/southern New England and eastern North Carolina for the day 1 update. For North Carolina, this appears to be a low-end threat as much of the guidance keeps the heaviest rainfall offshore or along the Outer Banks, with very low HREF FFG exceedence probablities across this region. But, the meteorology described below is still valid and so it's possible there could be some issues as the event gets ongoing into this afternoon. Kept the marginal risk for that reason, but did trim off the southern edge where rainfall has ended, and also the western edge. See previous discussion for added details. Santorelli ....Previous Discussion issued at 0830 UTC...=20 Coastal North Carolina...=20 Areas of heavy rain from the South Carolina coast into the southern North Carolina coast are expected to linger as a surface low pressure system tracks northward from the Southeast coast into eastern Virginia by later this afternoon or early evening...with a narrow warm sector east of the frontal wave. Across the Coastal Plain, 850 mb winds increasing to 60 kt by evening will support precipitable water values of 1.2-1.6 inches and MUCAPE of 500-1500 J/kg. While convection should achieve some level of organization given adequate speed shear aloft, progressive cell motions and sandy soils/marsh are limiting factors for runoff concerns but brief training (or cell mergers) is possible. The probabilities for 2 inch per hour rate from the 22/00Z HREF was generally under 30 percent...however 3 inch per hour ares can not be ruled out in the short term. Only minor adjustments were needed to the previously-issued Marginal Risk area. =20=20 Appalachians/Northern Mid-Atlantic into southern New England... Deep moisture and warmer air in the lower-levels will be drawn northward and overspread the area from the northern Mid-Atlantic and the Appalachians into southern New England as surface low pressure forms and moves northward today and this evening. In doing so...it sets up the potential for heavy to excessive rainfall...especially along the deformation zone of the surface low through the evening and then across portions of New England that works in tandem with +1.5 to +2.5 standardized anomalies of precipitable water and deep layered frontogenesis to produce areas of enhanced rainfall rates and amounts. Increasing on-shore flow into southern New England (a response to a rapidly deepening low pressure system over the Great Lakes) continues the threat of excessive rainfall there into the overnight hours. Hourly rainfall rates to a maximum of ~1" are expected with local event totals of 2-4" remaining possible. Cold to frozen soil temperatures and snow cover over northeastern Pennsylvania, southern Upstate New York and far northern New Jersey could elevate the potential for problems with run-off. Bann Day 2 Valid 12Z Fri Dec 23 2022 - 12Z Sat Dec 24 2022 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF NEW ENGLAND... New England... The threat of excessive rainfall should be on-going at the start of the period given strong on-shore flow that draws deep moisture across southern and central New England near/north of a warm front extending southeastward into the western Atlantic. Farther north, p-types across the mountains of Maine are expected to start off as frozen but transition to liquid as strong low level warm air advection moves through the region. Downstream of a deep-layered low approaching from the Midwest, divergence and diffluence aloft should be plentiful across the Northeast U.S. (especially as the upper level trough axis takes on a negative tilt). Rainfall rates are expected to peak around an inch in an hour along with storm totals in the 2-4 inch range but with higher totals focused from the northern half of New Hampshire into the western half of Maine. Only minor adjustments were made to the previously-issued Slight Risk considering that problems could result from rain on the current snow pack and/or frozen ground...or additional melting of snow pack by warmer dewpoints. Urbanized/tidal flooding could be a concern during high tides time and/or in areas with poor or already full drainage routes. In terms of the marginal risk, this cycle extended the area slightly west into upstate New York, and trimmed the southern edge out of the NYC metro/Long Island accounting for latest model trends. Santorelli/Bann Day 3 Valid 12Z Sat Dec 24 2022 - 12Z Sun Dec 25 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST... The approach of a front from the eastern North Pacific will usher another round of rain into the Pacific Northwest on Saturday. Models point to precipitable water values increasing to over an inch given on-shore flow at 850 mb of 35 kts to 45 kts during the day on Saturday. Rainfall amounts of 2 or 3 inches are shown by the models and deterministic WPC QPF across the Olympic peninsula...and much of that looks to be confined to the first 12- to 18-hours of the period before the area of precipitation pushes inland. Some modestly heavy amounts also extend southward into far northeast Oregon, so the previous shift marginal risk was extended slightly southward to account for that. Santorelli/Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6rPtgxgfxUrOELJ8h8FANS9nsvkWe4ZurNkk7T6uYR1N= r1LNA2uZ8MHSis4HdmE10Nusz9k9HJLu_hy6ypDepsT0IyI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6rPtgxgfxUrOELJ8h8FANS9nsvkWe4ZurNkk7T6uYR1N= r1LNA2uZ8MHSis4HdmE10Nusz9k9HJLu_hy6ypDeGj-v2XY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6rPtgxgfxUrOELJ8h8FANS9nsvkWe4ZurNkk7T6uYR1N= r1LNA2uZ8MHSis4HdmE10Nusz9k9HJLu_hy6ypDeHqxWrBs$=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) .