Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Feb 03 2024 Valid 12Z Tue Feb 06 2024 - 12Z Sat Feb 10 2024 ...Atmospheric river winding down over California by Tuesday with moisture also spreading inland across the West... ...Overview... An active weather pattern will be in place across the West as the period begins Tuesday with a large scale positively tilted mean trough continuing to promote a strong southwesterly flow of moisture into the West through about midweek. Significant rain and mountain snows expected across California earlier in the week should be waning by Tuesday, but there should be some lingering at least locally heavy rainfall across Southern California with moisture also extending into parts of the Intermountain West and Rockies. Energy from the mean ridge over the West will eject into the Central U.S. as a strong upper ridge initially across the central U.S. will support well above normal temperatures over much of this region, drifting slowly eastward through the latter part of the week. ...Guidance/Predictability Assessment... The guidance continues to be reasonably agreeable on the overall large-scale pattern during the medium range period, but there are some notable key differences with sensible weather impacts. These differences mainly focus out West as multiple pieces of energy dig down the western side and merge with the main trough. Even as early as Wednesday, the GFS and CMC were notably stronger with energy into the Pacific Northwest, both showing a closed low over this region on Thursday/Friday with a more southward progression. The GFS has been fairly consistent in showing this evolution, just with variability in the timing/shape of the upper low. The ECMWF on the other hand has been consistent in a weaker intial shortwave midweek and eventually a more progressive amplified trough into the Central U.S. next Saturday. There is also notable, but fairly typical, differences with leading energy as it ejects out of the mean trough early week lifting into the Upper Midwest around Thursday-Friday. The ECMWF continues to be a little north outlier with the resulting surface low. The WPC forecast for tonight was able to utilize a deterministic model blend for the first couple of days, despite the growing uncertainty out West. Though by Thursday, increasing amounts of the ensemble means were incorporated to attempt to smooth out the differences. The ensemble means were relatively consistent with each other and provided a good starting point to the forecast, with some lesser amounts of the deterministic GFS/ECMWF for added system definition. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A moderate to strong atmospheric river will be ongoing as the period begins Monday with well above normal moisture supporting significant to extreme rainfall across parts of Southern California. Guidance indicates multi-day totals (day 3 into day 4) possibly near a foot for southern facing terrain near the Transverse Ranges. Especially considering the early stage of the AR this weekend will pre-condition soils, a Moderate Risk remains in effect on the Day 4/Monday Excessive Rainfall Outlook. With the southeastward movement of the AR, the risk areas expand farther into southern California compared to the short range/Sunday outlook, but certainly with some overlap between Days 3 and 4. Model trends have been slightly faster to bring heavy rain into the San Diego area by Monday and thus expanded the Moderate Risk farther south there. The slow movement of the supporting upper trough should produce a continued southwesterly flow of moisture into the region through at least Tuesday, although the leading edge of the moisture should push into the Southwest/Great Basin. The Day 5/Tuesday ERO shows a broad Marginal Risk area from Southern California into the Southwest, with smaller Slight Risks highlighted for the LA-San Diego corridor as well as across parts of west-central Arizona. Lack of instability until after the trough/front come through and limit moisture may be a limiting factor for flash flooding for the Arizona area though. Heavy snow in the higher terrain across much of the West is expected in this pattern, with possibly significant multi-day totals of multiple feet of snow for parts of the Sierra Nevada. As leading energy from the Western trough lifts ejects into the Plains, this should help spin up a moderately deep surface cyclone that tracks northeastward with time into the Upper Midwest late week. The associated cold front should help pull moisture northward to fuel precipitation, with possible snow in the north-central U.S., though with still plenty of uncertainty in the exact amounts and placement. In the Southeast, the heaviest precipitation should be moving off the Coast by Monday as a strong surface low moves across Florida, but some moisture may wrap back in across the Southeast to fuel some lingering showers and thunderstorms. The upper ridge from the southern Plains through Great Lakes will support a broad area of unseasonably warm temperatures over the central U.S. and Midwest through the period. Continue to expect the greatest anomalies to be over/near the Upper Midwest where morning lows may be as warm as 25-35F above normal and exceed daily records for multiple days, while daytime highs should be a little less extreme, up to 15-25F or so above normal. In contrast, the Southwest U.S./California should see cool daytime highs through the period with a few locations up to about 10F below normal on one or more days. Clouds/precipitation over the West should keep morning lows near to above normal until later next week. Florida should see a couple cool days with highs 5-10F below normal by Monday-Tuesday after passage of the Southeast system. Temperatures across the East should trend warmer as the week progresses with the upper ridge building in. Daytime highs by next Friday could be approaching 10-20 degrees above normal for some locations. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw