Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 23 2022 20:32:07 FOUS30 KWBC 232032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 PM EST Fri Dec 23 2022 Day 1 Valid 16Z Fri Dec 23 2022 - 12Z Sat Dec 24 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF INTERIOR NEW ENGLAND... ....16Z Update... For this afternoon's ERO update, the Slight Risk across New England was expanded towards the southwest to include much of interior New England to the northern Hudson Valley of eastern New York. Widespread minor river flooding is ongoing across much of this region. Temperatures warming to close to 60 degrees in eastern MA, with widespread 50s from northern NH southwest to the Hudson Valley is contributing to rapidly melting snowpack across this area. Meanwhile, a strong low moving across the Great Lakes is drawing plentiful Atlantic moisture northward into New England. PWATs around 3/4 inch are 2.5 standard deviations above normal. This is resulting in periodic squalls of heavy rain. With the rivers already full, the additional rainfall and snowmelt through this afternoon is enhancing the chances for flash flooding, with FFGs under 1.5 inches in one hour from the Hudson Valley east across MA and southern VT & NH. Conversely, the Marginal risk across much of northern ME was trimmed southward with this update. Much of the area is in the 20s at present with an E to ENE wind direction predominant. This is greatly limiting how much warmer air is able to intrude inland to that area, as the freezing line roughly cuts the state in half from north to south. While temperatures may warm up a bit in the new Marginal Risk area this afternoon, far northern Maine is unlikely to break the freezing mark for very long if at all, eliminating any chances for flooding there. As the well-advertised Arctic air mass moves in from the southwest this afternoon and evening, rapidly falling temperatures and the end to organized precipitation will quickly end the flash flooding threat as well. Expect trimming of the risk areas from the west with future updates as the cold air moves in. Wegman ....Previous Discussion... The threat of excessive rainfall continues over parts of interior New England to the coast of Maine today as a plume of moisture, supported by onshore flow and a strong low level jet, enters the region. A deep upper-level low will continue pushing it's surface reflection and powerful attendant cold front through the Ohio Valley and East Coast today. A coastal low and attendant quasi-stationary front will lift north this afternoon, advecting with it a plume of anomalous moisture into portions of the Northeast. The merging of the two systems will allow for the enhancement of precipitation, especially over the terrain of New Hampshire and Maine where 50kt+ low level jet winds will support rain rates of around 0.5in/hr. Precipitation should start as snow early this morning before quickly transitioning into a steady rainfall today and wrapping up as snow tonight. The slight risk from yesterday's issuance was adjusted by cutting back the southern tip into northern New Hampshire to account for heightened FFGs and the latest 00z guidance suite, which suggest a more northerly solution with the axis of greatest QPF. There are concerns of melting snow causing additional flash flooding concerns. The heaviest rainfall should come to an end by this evening. Kebede Day 2 Valid 12Z Sat Dec 24 2022 - 12Z Sun Dec 25 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL MOUNTAINS AND CASCADES OF SOUTHERN WASHINGTON AND OREGON... ....2030Z Update... The inherited Marginal risk area has been expanded to include the southern Washington and Oregon Cascades. PWATs around 1 inch, which are 2.5 standard deviations above normal will move into the Marginal Risk area Christmas Day. High snow levels will result in all but the highest peaks getting generally 1-3 inches of rain. The rainfall combined with snowmelt should result in isolated instances of flash flooding in the Marginal Risk area. More rainfall is expected in the coastal ranges and especially the Olympics, where 2-4 inches of rain are expected, with isolated higher amounts. Thus, the Marginal Risk area is maintained. Wegman ....Previous Discussion... The arrival of an East Pacific front will advect a relatively weak plume of moisture into the Pacific Northwest on Saturday. Rain rates of around a quarter of an inch with higher values over the Olympics may support some isolated flash flooding risk. Thus, the highest qpf is likely to accumulate over the terrain, particularly the Olympic Mountains where between 2-3 inches and locally higher amounts are possible. Kebede Day 3 Valid 12Z Sun Dec 25 2022 - 12Z Mon Dec 26 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE OLYMPIC MOUNTAINS AND PENINSULA... ....2030Z Update... In contrast to Day 2, the inherited Marginal Risk area was trimmed to only include the Olympic Mountains and Peninsula, where an additional 1-3 inches of rain are expected. PWATs will remain around 1 inch for much of the day, though the greatest forcing further north will limit the atmosphere's effectiveness at wringing out that moisture as rain except in the Marginal Risk area. This rain when added to the Day 2 rainfall could cause additional isolated flash flooding. Meanwhile further south, expected rainfall amounts of around 1 inch through early Tuesday morning will not be enough to cause a significant flash flood risk. Heavy rain will redevelop in abundance further south along the CA/OR border on Tuesday. Wegman ....Previous Discussion... A new, more powerful low pressure system will enter the Gulf of Alaska on Sunday, sending a potent cold front directly into the Pacific Northwest. This cold front will be the focus for additional heavy rainfall over portions of the coastal Pacific Northwest, much like Saturday. Thus the heaviest precipitation is expected to occur over the higher elevations, especially the Olympic Mountains where the greatest chance for isolated flash flooding may occur. This marginal risk will extend south into northwestern Oregon where up to a quarter inch rates are possible. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!67UwCdrsAes0U45oaEPEgZkvlXPo8-vwPRS8qyall_b_= mwDyUp5yWOqGw9GocnFrD9yHPc6OxChCq1Uos193Cfyq1bk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!67UwCdrsAes0U45oaEPEgZkvlXPo8-vwPRS8qyall_b_= mwDyUp5yWOqGw9GocnFrD9yHPc6OxChCq1Uos193FQErMbQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!67UwCdrsAes0U45oaEPEgZkvlXPo8-vwPRS8qyall_b_= mwDyUp5yWOqGw9GocnFrD9yHPc6OxChCq1Uos193hmMDMQc$=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) .