Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 12 2023 15:24:48 FOUS30 KWBC 121524 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1023 AM EST Thu Jan 12 2023 Day 1 Valid 12Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN CALIFORNIA COAST... An amplified longwave trough approaching the West Coast will continue to provide favorable synoptic-scale lift associated with divergent flow aloft, WAA within the 850-500mb layer, and anomalous moisture within the atmospheric column. NAEFS depicts a large area of >90 climatological percentile mean specific humidity within all mandatory levels from 925-500mb from the North Coast of California on north to the Olympic Peninsula and Cascades this afternoon and into the overnight hours. The western third of Washington, in particular, also is likely to contend with >97.5 climatological percentile. This coincides well with an IVT >750 kg/m/s along the Pacific Northwest coast, which is also >99 climatological percentile. What all these situational awareness tools show is there is a deep reservoir of moisture to work with, as well as a strong current of air aloft delivering the moisture to the region. With the mean 850-300mb flow generally oriented SSW-NNE and the trough axis slowly moving east, this keeps most of the Pacific Northwest Coast and its bordering mountain ranges at greatest risk for flash flooding. The North Coast of California remains under a Slight Risk, largely due to antecedent soil moisture conditions being so saturated (NASA SPoRT-LIS shows soil moisture saturation >90 percentile into southwest Oregon). Rainfall totals here are likely to range between 1-2" (locally higher amounts possible), but it is farther north where the most anomalous moisture and best orographic enhancement will support heavier rainfall amounts. The Olympics are likely to see 2-4 inches on average with some areas potentially seeing isolated amounts >6 inches. Freezing levels are as high as 7,000' tonight, which means aside from the tallest peaks, much of the Cascades could contend with periods of rain atop some snowpack. In collaboration with SEW, went ahead and expanded the Marginal Risk to include portion of the northern Cascades. ....Ohio Valley... Ongoing rounds of strong-to-severe thunderstorms will continue to track to the north and east through the afternoon. Strong vertical ascent associated within the right-entrance region of a strengthening 250mb jet streak over the eastern Great Lakes will also feature a 50-60kt 850mb jet over the Ohio Valley. By this afternoon, IVT values ranging between 500-750 kg/m/s will set up from the TN Valley on north into the Upper OH Valley. Latest surface analysis also showed the trailing cold front still tracking through southwest KY/northwest TN as of 15Z. Initial segments of thunderstorms racing out ahead of the cold front have already triggered some areas of flooding in parts of the Ohio Valley, and with additional cells still to come along the cold front, there could be additional areas of flooding as well. One other consideration for the Marginal Risk was noting that 0-10cm soil temperatures in parts eastern OH, southwest PA, and northern WV are close to freezing. With such a cold surface, it can act as a hydrophobic surface win some instances. So despite that hourly rainfall rates may fail to surpass 1"/hr, rates of just 0.50"/hr could still lead to very localized areas of flash flooding, especially in poor drainage areas. ....Coastal New England... Farther up the East Coast, the amplifying upper trough with a lead shortwave disturbance tracking through the Mid-Atlantic Thursday night will promote strong vertical ascent in the upper levels of the atmosphere. As the surface low deepens over PA tonight, the low level jet along the East Coast will strengthen to as much as 70 kts. NAEFS shows >99 climatological percentile IVT values over eastern MA are aimed at the coastal plain of ME and foothills of southeast NH by 06Z Friday. Instability will be hard to come by, limiting top end hourly rainfall rates around 0.5"/hr in most cases. That said, frost depths in these areas range between 5-8 inches, making any moderate-to-heavy rainfall susceptible to run-off from the cold surfaces acting in a hydrophobic manner. In collaboration with GYX and CAR, introduced a Marginal Risk due to the potential for flooding from a combination of highly anomalous moisture and soil conditions acting to cause excess run-off. Mullinax Day 2 Valid 12Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST AND ADJACENT MOUNTAINS... The AR (atmospheric river) that is expected to remain largely off the coast of CA on Thursday will move back into northern CA on Friday. The 24-36 hour break (shorter the further north you go) is not expected to be enough time for there to be much drainage of prior days rains before this next round moves in. Most of the heaviest rain (1-3" of additional rainfall across the Slight Risk area and over portions of the Olympic Peninsula) will be in the early morning hours as the main AR moves in, after which the front will weaken as it moves south down the Central Valley. Behind the AR, the usual off-and-on shower activity moving directly into the coast is expected, so coastal Northern California is not expected to be completely dry Friday afternoon and overnight, but for the most part the individual showers are not expected to produce widespread flooding activity. Churchill/Wegman Day 3 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA... Unfortunately, yet another AR is expected to come ashore during the early morning hours of Saturday. This will bring an additional 2-3" of rainfall to much of the Northern CA coastline, in addition to the foothills of the Sierra Nevada. Elsewhere along much of the CA coast, 1-2" of additional rainfall is expected.. classifying as a fairly weak AR, overall. However, given the very wet antecedent conditions, much of this rainfall will translate to immediate runoff and potential flooding concerns (particularly for burn scars and poor drainage areas). The weakening trend of the AR overall should help to alleviate widespread flooding concerns, but locally upslope totals may reach as high as 3-5".. which may necessitate targeted upgrades to Moderate Risk for future updates. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-sjXRE4tkPTuYGvj8adaSQ3rI1r-MkCTPZvxeKv-IqGq= iSNflqqRKstEwCFfxX2IMEc-X_ypcKUAxZujHMVFbRXqKFU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-sjXRE4tkPTuYGvj8adaSQ3rI1r-MkCTPZvxeKv-IqGq= iSNflqqRKstEwCFfxX2IMEc-X_ypcKUAxZujHMVFsKf_Z70$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-sjXRE4tkPTuYGvj8adaSQ3rI1r-MkCTPZvxeKv-IqGq= iSNflqqRKstEwCFfxX2IMEc-X_ypcKUAxZujHMVFWGb6BXQ$=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) .