Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 21 2022 20:30:34 FOUS30 KWBC 212030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 PM EST Wed Dec 21 2022 Day 1 Valid 16Z Wed Dec 21 2022 - 12Z Thu Dec 22 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Otto Day 2 Valid 12Z Thu Dec 22 2022 - 12Z Fri Dec 23 2022 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF MIDDLE AND UPPER EASTERN SEABOARD...=20 =20 Coastal North Carolina...=20 Areas of heavy rain are expected to be ongoing from the South Carolina coast into the southern North Carolina coast at the start of the period Thursday morning. A surface low will track northward from the Southeast coast into eastern Virginia through Thursday 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 (highest instability along coast) combined with increasing upper level diffluence and divergence related to a subtropical jet streak translating offshore of the Southeast. Some degree of 850-700 mb frontogenesis will also be a factor, translating northward along the North Carolina coast. 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) of thunderstorms would allow hourly rainfall totals to rise to 1-2" (12Z HREF probabilities up to 30 percent for 2 in/hr and cannot rule out 3 in/hr if there is a short term hangup of cell progression). While the heaviest rainfall may fall just offshore, storm totals of 2-4" appear reasonable with the threat for heavy rain likely ending beyond 00Z Friday. Only minor adjustments to the Marginal Risk were made for this update and thinking continues to be that any issues will likely be confined to urban areas.=20 =20=20 Appalachians/Northern Mid-Atlantic into southern New England...=20 An upper level trough will strengthen and become a powerful closed low aloft as it approaches the Ohio Valley from Thursday into Friday morning. As this occurs, increasing upper level diffluence/divergence is expected to develop over the northern Mid-Atlantic and southern New England Thursday evening and overnight, ahead of a 150-170 kt speed max near 250 mb at the base of the trough. Increasing warmth and moisture flux will overspread the East Coast with standardized precipitable water anomalies of +1.5 to +2.5 forecast by the 12Z NAM/GFS to stretch from the Mid-Atlantic into southern New England during the forecast period. Deep layered frontogenesis is forecast over the Mid-Atlantic but with better definition in the 700-500 mb layer, oriented roughly WNW to ESE through ~03Z. Given steering flow from SSW to NNE, expect laterally oriented waves of heavy rain to move northward through the northern Mid-Atlantic region from early Thursday morning through the early overnight. =20 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 and greatest from central Virginia to the northern Mid-Atlantic coast), strong forcing for ascent should still support areas of heavy rain. Hourly rainfall totals to a maximum of ~1" are expected with local amounts of 2-4" remaining possible. The highest 12Z HREF probabilities of 3+ inches in 24 hours exist from the Poconos into the New Jersey Highlands and Catskill Mountains with values of 40 to 60 percent. Cold to frozen soil temperatures and snow cover over northeastern Pennsylvania, southern Upstate New York and far northern New Jersey could lead to a small area of Slight Risk-type impacts. The size of the region this would cover is considered too small to be depicted in a probabilistic outlook.=20 =20 Otto/Roth Day 3 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... An anomalous moisture plume (East Coast atmospheric river with standardized precipitable water values of +2 to +3) will be oriented into southern New England at the start of the period with areas of heavy rain ongoing from southern to 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. Expectations are for areas of heavy rain with rainfall rates peaking near 1 inch in an hour along with storm totals in the 2-5 inch range but with higher totals focused from the northern half of New Hampshire into the western half of Maine. Weak MUCAPE (generally less than 500 J/kg) should limit rainfall intensity but 6 hour totals of 1-2 inches appear likely in a few spots. The rainfall is expected to occur within an area of snow cover with liquid equivalents of 1-4" (CoCoRaHS and NOHRSC). Not helping matters is the increasing dew points, with the 12Z model consensus hinting that coastal New Hampshire/Maine flirts with values close to 50F, which along with the relatively warm rain would also aid melting. The combination is expected to lead to runoff problems consistent with a Slight Risk and only minor adjustments were made to the previous outlook for this update. Otto/Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9M3Ih8S5ByylMd4mtJXrVV3lNM63KdniWb0Ut6lSX1o0= 9vuhWz5le4Uy9TIVYsix2WmWI_P4uD9CkQ1PibJg_hei3-Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9M3Ih8S5ByylMd4mtJXrVV3lNM63KdniWb0Ut6lSX1o0= 9vuhWz5le4Uy9TIVYsix2WmWI_P4uD9CkQ1PibJgunDmfY0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9M3Ih8S5ByylMd4mtJXrVV3lNM63KdniWb0Ut6lSX1o0= 9vuhWz5le4Uy9TIVYsix2WmWI_P4uD9CkQ1PibJgM3dx2zQ$=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) .