Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 25 2024 15:42:50 FOUS30 KWBC 251542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1042 AM EST Wed Dec 25 2024 Day 1 Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS OF WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA... ....16z update... There are no changes in trends/analysis to change current ERO/Marginal Risk zones across the PacNW at this time... ---Prior Discussion--- There has been little change in the=20 forecasted evolution of the next atmospheric river (AR) surging=20 off the Pacific and into the Pacific Northwest today and into=20 tonight. Model guidance is keen on a surge of moisture represented by fairly elevated IVT signatures on the order of 600-800 kg/ms,=20 bringing in a wave of heavier precipitation entering coastal WA/OR by this afternoon, and spreading southward into coastal northwest CA later tonight. The latest WPC QPF calls for 2-4" localized=20 totals over 12-24 hours (and perhaps as high as 5" into the=20 Olympic Peninsula due to the added orographic enhancement). Given=20 that soil moisture anomalies (per the NASA SPoRT-LIS 0-1 m layer)=20 generally range from 70-90th percentile and are about 70-80%=20 saturated, the inherited Marginal Risk was maintained for this=20 cycle. Rainfall from prior days has resulted in rises in rivers=20 within the local watersheds of the areas anticipating the heaviest rainfall, especially within the Skokomish basin south of the=20 Olympics, as well as watersheds from southwest OR into northwest=20 CA (per USGS streamflow anomalies indicated above normal to much=20 above normal). The elevated stream flows and heavy rain will=20 enhance local flash flood prospects for anywhere in proxy to those watersheds, so the threat is a bit higher than normal compared to what is usually forecast for an AR of this strength. In addition to longer-fused flood concerns, elevated instability is forecast to reach 250+ J/Kg (MU CAPE) late in the period, which will enhance the hourly rainfall rates (increasing the threat for localized flash flooding in association with burn scars or other particularly sensitive terrain). The latest (00z) HREF depicts high probabilities (between 40-70 percent) of 0.25"+/hr rates between 06-12Z Thu (per 10-100 km Ensemble Agreement Scale neighborhood probabilities, indicating strong spatial agreement between the CAMs in association with orographic enhancement) and some chance of=20 also realizing 0.50"+/hr rates (40-km neighborhood probabilities as high as 40-60 percent). Churchill/Kleebauer/Hurley ....Southeast Louisiana... GOES-E, regional RADAR and surface observations, note weak low- level cyclonic convergence lifting northward along the SSE to N oriented warm/stationary front across south-central LA. This has locally enhanced surface heating and moisture return to enhanced some modest instability. Convergence enhanced by the confluent flow, has broken out a few thunderstorms that are breaking out across the southern parishes of St. Mary/Assumption and Terrebonne. Bulk shear values in the vicinity of the warm front support weakly rotating updrafts that further enhance moisture flux convergences supporting low-level moisture loading for some efficient rainfall production. Given .75-.9" in the sfc-850mb CIRA LPW layer and 20-30kt inflow, rates of 1.5"/hr with lower potential for locally 2"/hr rates may occur given the environment/storm mode. As such, there is potential of locally 2-3" and in the vicinity of New=20 Orleans and urban ground conditions that may be overwhelmed a bit=20 quicker than regional FFG values are. Given the small areal=20 coverage, a categorical Marginal Risk is not warranted at this=20 time, but there will be a non-zero risk of flash flooding fully=20 constricted to the urban corridors of Southeast Louisiana late this morning into the afternoon hours.=20 Gallina Day 2 Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARK-LA-TEX REGION AND SURROUNDING PORTIONS OKLAHOMA AND MISSISSIPPI... ...East Texas and Lower Mississippi Valley.. A repeat of the Christmas Eve setup (though displaced a bit north and eastward) is progged for east TX extending into the Lower Mississippi Valley as a more potent shortwave trough/cut-off low digs eastward from the Southern Plains into the Ark-La-Tex. Given highly favorable ascent aloft (right-entrance region of 90+ kt jet streak) coupled with surface cyclogenesis and an increasingly destabilizing environment ahead of the trough, convection is expected to rapidly organize and progress eastward within the expanding warm sector. Given the prior days rainfall over East TX, as well as more sensitive soils from the Ark-La-Tex northeastward into the Ozarks (per 3-hr FFGs of 1.5-2.5"), a Slight Risk was introduced (in coordination with local forecast offices) as available CAMs (00z FV3 and experimental MPAS-NSSL, 06z HRRR) suggest the potential for localized short-term (3-6 hour) totals of 2-4". This seems plausible, given the strong dynamics of the setup and sufficent tropospheric moisture for realizing 1-3"/hr rates (with precipitable water values of 1.2-1.4", which is not only well above the 90th percentile for late December, but is near average values for the peak of the wet season). As more hi-res CAMs come into range of the full event later over time, expect adjustments and refinements to the Slight Risk area. ....Pacific Northwest... A secondary IVT pulse will occur on Thursday during the prolonged AR event creating a flash flood potential extending into the D2 time frame. The multi-day precipitation forecast is upwards of 3-6" (and locally higher in the Olympics). The second pulse will actually protrude farther inland across OR, creating a risk for flooding inland (compared to the previous period). The AR will eventually slide southward with more emphasis placed on southwest OR into northern CA late in the period, leading to more heavy precip over already saturated soils. The inherited Marginal Risk was maintained, given the wet soils and fairly consistent QPF. Churchill/Kleebauer Day 3 Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS OF FAR SOUTHWEST WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA... A tertiary pulse of marginal IVT will push onshore between the southern WA coast down through northwest CA with another round of moderate to heavy rainfall signaled over the coast to just inland encompassing the coastal ranges of southwestern OR and northern CA. This is likely one of the weaker IVT pulses within the run leading into the period, however the compounding factor of significant rainfall anticipated prior to D3 allows for very wet antecedent conditions with soil moisture anomalies already breaching the 80th percentile for places up and down the OR and northern CA coasts. An additional 2-4" will lead to areas approaching 6-8+" over the span of 5 days in the anticipated impact zones lending credence to a continued MRGL risk for flash flooding. The previously inherited forecast was generally maintained with the primary concerns being increased runoff capabilities and local urban flooding for coastal towns along the OR and northern CA coasts. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd= H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC0eWzRFCw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd= H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC02XR784Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd= H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC0HudcM2o$=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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