Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 11 2023 01:01:20 FOUS30 KWBC 110101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Sun Dec 10 2023 Day 1 Valid 01Z Mon Dec 11 2023 - 12Z Mon Dec 11 2023 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN NEW England... ....Eastern U.S... Areas of heavy rain continued to affect portions of the East Coast this evening with narrow axes producing 1+ in/hr rates over North Carolina and the Delmarva Peninsula at 00Z, out ahead of a cold front that stretched from southeastern New York into western North Carolina. Consolidation and deepening of a surface low over southern New England is expected tonight as a shortwave trough crossing the southeastern U.S. acquires a negative tilt and strengthening upper level divergence sets up over the region within the right entrance region of a potent 150-170 kt jet max translating across the Northeast. 850 mb winds will also increase overnight with speeds exceeding 70 kt over coastal locations of New England (higher offshore) before 12Z according to the latest short term model guidance, supporting precipitable water values up to about 1.5 inches over southern New England. Instability will continue to be a limiting factor with only a few hundred J/kg of MUCAPE expected which will limit peak rainfall rates into the 0.5 to 1.0 in/hr range, although localized rates over 1 in/hr will be possible with 0.5 to 1.0 inches in 30 minutes contributing to excess runoff. Training axes of heavy rain from SW to NE are expected to allow for an additional 2-4 inches from Long Island into coastal Maine, and spotty potential for 5+ inches of additional rain through 12Z. The Moderate Risk remains in effect across southern New England with a higher end Slight Risk extending from Long Island into central/eastern Maine. While the rainfall may not exceed FFG on the 1 hour scale, 2+ inches in 3 hours will be possible and the longer duration of heavy rain will likely lead to some flooding, especially for area streams and low lying/flood prone locations...including poor drainage and urban areas. Farther south across the Mid-Atlantic, the Marginal Risk was left in place given the low-end potential for narrow axes of training with 1+ in/hr rainfall rates and additional totals of 2-4 inches on a very localized basis. ....Northwest... The atmospheric river has been gradually fading during the day on Sunday with peak IVT values at 00Z in the 400-500 kg/m/s range along the southern Oregon coast. IVT will continue to weaken as a mid-level shortwave and associated cold front advance inland overnight, with lowering precipitable water and 850 mb winds. Isolated 0.25+ in/hr rainfall rates will remain possible over the southern Oregon Cascades and Coastal Ranges but additional rainfall through 12Z of 0.5 to 1.5 inches (perhaps a spot 2 inch area in far southwestern Oregon) is not expected to pose enough of a flooding threat to support a Marginal Risk, so it was removed with this update. ....Florida/Georgia... The cold front sweeping through the southeastern U.S. was analyzed over south-central GA at 00Z, forecast to continue quickly move east into the western Atlantic through early Monday morning. The deepest moisture ahead of the front was associated with a weakening line of thunderstorms to the east, over the Florida Peninsula. Therefore, the threat for heavy rainfall has ended for Georgia and northwestern Florida, and the Excessive Rainfall risk areas have been removed with this update. Otto Day 2 Valid 12Z Mon Dec 11 2023 - 12Z Tue Dec 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MAINE... ....2030Z Excessive Rainfall Discussion... Moderate to heavy rain is expected to be on-going at the start of the Day 2 period across portions of eastern New England as surface low pressure continues to deepen and moves into/across Maine.=20 Models still support the previous forecast rationale and only minor tweaks were needed. Bann ....0830 Excessive Rainfall Discussion... The event described in more detail in the day 1 discussion will be ongoing Monday morning across portions of eastern New England. By 12z Monday the low pressure should be over southern New England with it continuing to deepen as it quickly moves northeastward into ME. A very strong low level jet ahead of the system will focus a corridor of strong low level convergence and frontogenesis spreading from eastern MA into southern and central ME. Despite limited to no instability, the dynamic forcing of the system should still support embedded convective elements with rainfall rates locally exceeding 0.5"/hr. Given the fast motion of the system by this time additional rainfall after 12z Monday should not be all that extreme...with the general consensus supporting 1-2". This alone would not pose much of a flood risk...however this will be in addition to what has already fallen over the day 1 period. Given event totals expected to reach 3-5" and the potential for the localized higher rates along and ahead of the low track...we think a continuation of the Slight risk from day 1 is warranted. Thus we will upgrade the day 2 to a Slight risk from coastal NH into central ME...generally covering areas where heavy rainfall should be ongoing Monday morning and where event total rainfall has the highest probabilities of getting into the 3-5" range. Chenard Day 3 Valid 12Z Tue Dec 12 2023 - 12Z Wed Dec 13 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6sZtGxsYrVbsNiMCU8nn4qf0N3BqdbFDpbOTdvFydE6Z= rkUDJNsmyI0MmKTjaqQbaXntOriQFB0SUpBcFZZ_t2NPhH8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6sZtGxsYrVbsNiMCU8nn4qf0N3BqdbFDpbOTdvFydE6Z= rkUDJNsmyI0MmKTjaqQbaXntOriQFB0SUpBcFZZ_LwxpvJ8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6sZtGxsYrVbsNiMCU8nn4qf0N3BqdbFDpbOTdvFydE6Z= rkUDJNsmyI0MmKTjaqQbaXntOriQFB0SUpBcFZZ_RLHAcQQ$=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) .