Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 06 2023 20:18:59 FOUS30 KWBC 062018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 317 PM EST Fri Jan 06 2023 Day 1 Valid 16Z Fri Jan 06 2023 - 12Z Sat Jan 07 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A PORTION OF NORTHWEST CALIFORNIA... 16Z update... Onshore flow this morning has resulted in showers over portions of northern California and southwest Oregon and this is expected to persist into tomorrow morning. The latest CAMs are showing rainfall rates generally around 0.25 inch/hour holding steady through the Day 1 period however a couple show surges up 0.50 to 0.75 inches/hour after 03Z Jan 7th through 12Z. Most of these enhanced streaks of QPF fall within the area already identified as having a Moderate Risk for excessive rainfall but few are just outside of it. The Moderate Risk had minor expansions to account for this trend. Campbell ....Southwest Oregon into Northern California... The next round of rain into northern California and southwest Oregon is expected to begin Friday afternoon after a break in the activity Thursday night. It's a similar setup from Thursday, as a second strong low will approach southwest BC, and its trailing cold front will drape south into the coast Friday afternoon. Strengthening exit region forcing ahead of the upper level trough will allow for an uptick in low-mid moisture transport, most notably along/near the coast in northwest CA where the AR axis leads to of +1.5 to +2 standard deviations above normal along with an uptick of integrated water vapor transport (IVT) values to 400-600 kg/m/s. Per the high-res guidance, hourly rainfall rates are expected to peak near 0.50" within the Slight Risk area Friday afternoon and overnight, with the majority of high-res CAMs indicating a sliver of hourly rain rates of 0.75-1.0" within the Moderate Risk area after 00Z. Supporting the more enhanced (i.e. Moderate) Risk area over and near the Cape Mendocino area south of Eureka and into the Coastal Ranges and western Klamath Mountains will be the recent heavy rainfall and saturated soils, with between 3-5+ inches of rain over the past 48 hours. Most of the high-res CAMs show areas of 1-3+ inches of additional rainfall in the 24 hour period ending 12Z Saturday, with an area of 4-5+ inches focused within the Moderate Risk area (where the latest HREF indicates 50+ percent probabilities of 24 hour rainfall exceeding 5").=20=20 Hurley Day 2 Valid 12Z Sat Jan 07 2023 - 12Z Sun Jan 08 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA... 21Z update... The models have trended higher with the QPF as the stronger AR begins to push inland Saturday night. The general consensus has areal averages of additional 3 to 5 inches over the water logged soils, which will be a little further south than the where the heavy rainfall is expected for the current D1 period. A couple of the CAMs are depicting locals maximums of 6 to 8+ inches possible along the immediate coast of north-central California and points adjacent. This amounts are anticipated to surpass the 1/3/6 hour FFG guidance ECWMF is showing a nearly 40% probability of exceeding 3 inches of rain. Therefore, a Moderate Risk was raised for this part of the state. Recent excessive rains along with several additional inches forecast, there will likely be widespread areas of flooding and if in proximity of recent burn scars, the threat for landslides, debris flows and road damage will elevate. Campbell ....Southwest Oregon into Northern and Central California... The Day 2 ERO (Sat-Sat night) will see a continuation of the modest AR event pressing a little farther south into Central CA. The activity begins to weaken by Sat afternoon behind the initial shortwave trough, only to pick up once again in earnest Sat evening and overnight with renewed robust low-mid layer moisture transport directed toward the Central CA Coast. Current NAM CONUS Nest shows hourly rainfall rates peaking at 0.50" after 06Z Sun, which would pose a flash flood threat over several 2020-2022 burn scars west of the Sierra and south of the Shasta/Siskiyous mountains. ....Western Gulf Coast into portions of the Lower Mississippi and Tennessee Valleys... Continued with the Marginal Risk of excessive rainfall over this region, as with yesterday's Day 3 ERO. Strengthening right-entrance region upper forcing (low-mid layer frontogenesis) along with a slow moving front may be able to draw sufficient moisture flux off the western Gulf to result in a small area of training thunderstorms across this region. The models have come into better alignment with the QPF; however there area still some latitudinal differences with respect to the axis of heaviest rainfall. Considering the spread in the guidance, along with the few days that will elapse with dry conditions which will allow previous days rainfall to run through, for now have capped the ERO risk at Marginal until there is better model consensus and more confidence that FFG values will be threatened. Instability is meager as well, at least outside of the Middle and Upper TX Coast, which won't help allow any storms to develop upscale to have the possibility of producing heavy rain. Hurley Day 3 Valid 12Z Sun Jan 08 2023 - 12Z Mon Jan 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN AND CENTRAL CALIFORNIA... 21Z update... No significant changes were noted with the overall environment and synoptic pattern during this period. Although there will be a relative "lull" for a few hours as one AR wanes and the next approaches, the additional rainfall over saturated soils will keep the threat for excessive rainfall and flash flooding elevated. The Slight and Marginal Risks that were already in effect remain in good order so no adjustments were needed at this time. Campbell ....Southwest Oregon into Northern and Central California... At the start of the Day 3 period (12Z Sun), one shortwave trough will have pushed through (pcpn on the wane Sunday morning), while another, more elongated AR approaches Sunday night. Guidance shows PWs ~1.25" approaching the Central CA Coast Sunday night, along with even more anomalous low-mid layer moisture transport compared to the previous AR event(s). The 00Z GEFS in fact indicates 850-700 mb moisture flux standardized anomalies climbing to +4 along the coast by 12Z Monday. While the bulk of this particular event will be during the day Monday (current Day 4), deterministic and probabilistic QPF fields, along with analogs and ERO first guess fields using 24 hr ARI exceedance, would support continuing with the Slight Risk that was inherited from yesterday's Day 4 ERO. This especially taking into account the anomalously wet soils (above the 90th percentile in the top 1 meter layer per the latest NASA SPoRT), owing to the onslaught of recent AR events. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_wDdMNaQ1y6IX59X-fxtNLIksiDyr6PmLXpv3zVqX4ej= DmBCvGJESm7YM1RRV2ePI18r0Q9MYm149vYgubp3RnxF3_8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_wDdMNaQ1y6IX59X-fxtNLIksiDyr6PmLXpv3zVqX4ej= DmBCvGJESm7YM1RRV2ePI18r0Q9MYm149vYgubp34GPhF8o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_wDdMNaQ1y6IX59X-fxtNLIksiDyr6PmLXpv3zVqX4ej= DmBCvGJESm7YM1RRV2ePI18r0Q9MYm149vYgubp3qp3VrMs$=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. 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