Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 27 2023 23:46:44 FOUS30 KWBC 272346 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 645 PM EST Wed Dec 27 2023 Day 1 Valid 01Z Thu Dec 28 2023 - 12Z Thu Dec 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID ATLANTIC AND NORTHEAST... ....01Z Update... An area of low pressure organizing off the North Carolina coast is expected to move northeast tonight, reaching near southern New England Thursday. As this low moves up the coast, an impressive and anomalous plume of higher moisture will overspread much of the coastal Mid-Atlantic to southern New England area with PWs reaching 1-1.25", which will be 2-3 standard deviations above normal for this time of year. This will be aided by 850 mb flow of 30-40 kts out of the southeast, overrunning a stationary boundary draped along the coast from eastern VA through southern New England. Aloft, an embedded vort max within the broad/sprawling closed upper low centered over the Midwest and an impressive jet streak over the Southeast will aid further in the large scale forcing for ascent. The latest hi-res guidance and 18Z HREF is in relatively good agreement for a widespread rainfall event through tomorrow morning with most areas seeing additional hourly rain totals between 0.5-0.75" and additional totals through 12Z Thursday of 1-2" with locally higher amounts of 3" most likely from portions of eastern MD, Delaware, eastern PA, and much of central/northern NJ into the NYC metro area. The 18Z HREF shows high probabilities of at least 2" (70-80 percent) with a slight signal (10-20 percent) for 3" totals through 12Z. Much of this rainfall will fall on areas that are already extremely saturated/wet from recent heavy rainfall events over the last couple of weeks. 14-day rainfall departures in the area are between 200-300 percent of normal and the latest Flash Flood Guidance is quite low in the region, particularly from eastern PA and NJ where even 1-hr FFG is likely to be met or exceeded (generally 0.5") as well as the 3-hr and 6-hr FFGs. This is likely to result in excessive runoff and at least scattered instances of flooding through the overnight period. The Slight Risk area was expanded some to account for the trends in the hi-res guidance though the greatest concern (higher end Slight Risk range) is from eastern PA through NJ and portions of southern NY where the combination of the rainfall totals, wet ground conditions, and urbanized area could result in some greater flooding concerns. Taylor Day 2 Valid 12Z Thu Dec 28 2023 - 12Z Fri Dec 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell/Kleebauer Day 3 Valid 12Z Fri Dec 29 2023 - 12Z Sat Dec 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR NORTHERN CALIFORNIA AND SOUTHWEST OREGON... ....20Z Update... Overall pattern remains steadfast from the previous forecast issuance across northern CA and southwest OR. Atmospheric river will transpire on Friday afternoon into Saturday morning before waning in intensity and shifting focus southward. NBM neighborhood probability of at least 2" is fairly robust across the northern CA coast (70+%), with the highest totals generally between Eureka and San Francisco. IVT signature is more robust with this next wave of precip impacting the area, as well as a further penetration inland, so totals this juncture should be more conducive to flooding concerns within the area outlined, but still a lower-end risk in the grand scheme. Will keep an eye on the trends for areas further south towards Monterey as some guidance has increased totals within the coastal plain south of San Jose which may necessitate an extension of the MRGL further south. Kleebauer ....Previous Discussion... The next front and surge of Pacific moisture is expected to approach the coast during this period spreading another round of rainfall over northern California and southwest Oregon. Most of the guidance depict areal averages of 1 to 3 inches along the coast, with the higher totals primarily between Eureka and San Francisco. The threat for excessive rainfall will continue to be elevated from recent rains therefore the Marginal Risk area was maintained. Minor expansions were made to the north and south. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-f1NrVh5um3UBDcp6NG9i-E2njLRamEDtPBWEk8eLz7T= XcftpI5hG1xs_Hai9Sbx83vnKcCEnBtMlcnZHZhDnppgfCY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-f1NrVh5um3UBDcp6NG9i-E2njLRamEDtPBWEk8eLz7T= XcftpI5hG1xs_Hai9Sbx83vnKcCEnBtMlcnZHZhDudgu5bg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-f1NrVh5um3UBDcp6NG9i-E2njLRamEDtPBWEk8eLz7T= XcftpI5hG1xs_Hai9Sbx83vnKcCEnBtMlcnZHZhDZg27nR4$=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) .