Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 12 2023 00:31:06 FOUS30 KWBC 120030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 730 PM EST Wed Jan 11 2023 Day 1 Valid 01Z Thu Jan 12 2023 - 12Z Thu Jan 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... Locally moderate rainfall (exceeding 0.5"/hr) will remain possible for the next several hours across portions of northern California with upwards of an additional 1" to locally 2" possible through early Thursday morning. The focus for this atmospheric river will begin to turn northward and more parallel to the coast after about 03Z in response to an amplifying mid/upper pattern (and inland ridging building over the Intermountain West). This will slow and shift the axis of heaviest rainfall generally northward, giving areas from the Bay Area southward a much needed break. Through about 06Z, rainfall along the approaching warm front will still pose an excessive rainfall risk where the Slight Risk was generally unchanged from the earlier issuance. The additional rainfall falling on top of already saturated soils could initiate new flooding concerns easily and/or exacerbate ongoing impacts. Taylor Day 2 Valid 12Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... ....2030Z Update... ....West Coast... Few changes were made with this afternoon's update. The model trends have shifted slightly further north and west with the plume of heaviest rainfall through the day Thursday and Thursday night. Thus, the Slight and Marginal Risk areas in northern CA were trimmed a little bit out of inland areas and towards the coast with this update, as a little bit less rainfall expected in these areas has reduced the threat just a little bit. That said, all rain that falls in the Slight and Marginal Risk areas of CA will be falling on fully saturated soils, so all of it is expected to run off, contributing to continued elevated flash flooding concerns, especially over burn scars, slot canyons, and more urbanized areas. With that said, the same north and westward shift with the current round of rain means additional rainfall is expected further north into coastal OR and WA, especially into the Olympic Mountains. The atmospheric river (AR) aligning more parallel to the coast should mean fewer extremely high measurements in the coastal mountains, as those totals are more likely when the AR is aligned more orthogonal to the coast. The Olympic Mountains however are aligned to soak up more rainfall on a nearly due southerly flow, so multiple inches of rain are possible. Fortunately it's been less wet than areas further south into CA, so widespread flash flooding issues are not expected at this time. ....Ohio Valley... A Marginal Risk area was considered with this afternoon's update across portions of the mid-Ohio Valley. An early round of thunderstorms is possible Thursday morning, which will then be followed by potentially multiple rounds of weaker showers, per the latest CAMs guidance. Unlike in CA, the antecedent conditions here are bone dry, with soil moisture in these areas among the driest in the country. None of the guidance shows even localized daily totals exceeding 2 inches, even if that rain does fall in a much shorter period of time. Thus, while the threat for flash flooding is more than zero, for now it was evaluated to be less than the 5% needed for a Marginal Risk area. In coordination with the ILN/Wilmington, OH forecast office, it was noted that some areas of south-central OH and northeast KY are still wet from the latest flooding rain event way back on January 3rd, so higher rainfall totals in these areas may necessitate consideration of a Marginal Risk in these areas. Wegman ....Previous Discussion... By Thursday morning, the aforementioned stalling warm front is beginning to make slow progress (again) eastward towards the Northern California coastline. This may result in an additional 1-3" along a relatively narrow portion of the coastline, as the AR will be increasingly oriented due north-south with the slow eastward translation of the efficient moisture transport. This introduces a fair amount of uncertainty (compared to Day 1), with model consensus depicting the heaviest QPF just offshore (as high as 3-5"). The Slight Risk was maintained for a narrow portion of the Northern California coastline where locally totals may exceed 3", as the expectation is that coast will continue to be grazed by the strong AR (with unidealized upslope flow, given the parallel orientation to the coastline). Given uncertainty is higher than usual, subsequent updates my necessitate an upgrade (to Moderate Risk) OR a downgrade (to Marginal Risk), depending on the trends of the CAMs (as more HREF members come into full focus with the afternoon update). In the meantime, a more narrow Slight and Marginal Risk were maintained for this update for Northern CA and far southwestern OR (with a Marginal Risk for the Olympic Peninsula of WA as well with 2-5" expected totals, falling mostly as rainfall, over the course of 24 hours). Churchill Day 3 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... ....2030Z Update... The Marginal Risk into Northern California was upgraded to a Slight Risk with this update. This was largely due to continued increases in forecast rainfall into this area, along with very favorable antecedent conditions for flash flooding concerns. 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 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 dry Friday afternoon and overnight, but for the most part the individual showers are not expected to produce widespread flooding activity, but certainly will delay draining of the morning's rain. Unfortunately, yet another AR is expected for the early morning hours of Saturday, just beyond the Day 3 time frame. Any increase in forward speed of the Saturday morning round of rain may require additional risk increases with future updates. Wegman ....Previous Discussion... Increased uncertainty continues into Friday morning, as much of the day may end up being a break between atmospheric river events. The aforementioned strong AR should be weakening and shifting inland during the morning hours, followed by a midday lull before the next (likely weaker) AR begins to approach from farther offshore. The Marginal Risk areas mainly reflect the lingering effects of the former (strong, but weakening) AR, and subsequent targeted upgrades to Slight may well be necessary down the line. The current expectation is for an additional 1-3" of rainfall, mostly during the morning hours on Friday (including much of the CA coast, upslope portions of the Sierra Nevada, the Olympic Peninsula of WA, and far southwestern OR). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5NUZyqeLa4BRR2_tjEd3VdYtqRcRqsjNd54tEBstatdU= w4mzgJ951uJ7BPvh7005iH23AxM487SSikr_p3oxqymdliM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5NUZyqeLa4BRR2_tjEd3VdYtqRcRqsjNd54tEBstatdU= w4mzgJ951uJ7BPvh7005iH23AxM487SSikr_p3oxhsN1fUU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5NUZyqeLa4BRR2_tjEd3VdYtqRcRqsjNd54tEBstatdU= w4mzgJ951uJ7BPvh7005iH23AxM487SSikr_p3oxrwwFN9U$=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) .