Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM 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 heavy rainfall at least locally across Southern California with moisture also extending into parts of the Intermountain West and Rockies. Meanwhile a strong upper ridge across the central U.S. will continue to support well above normal temperatures that could be record-setting across the central/northern Plains and Midwest. Later in the week, energy from the mean upper trough over the West will eject into the central U.S., pushing the ridge and warm temperatures slowly eastward. ...Guidance/Predictability Assessment... Model 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. The GFS/ECMWF guidance seems to be converging somewhat on showing a shortwave or weak closed low coming into the Northwest Wednesday-Thursday, while the 00Z CMC was a stronger and farther southwest outlier comparatively with the closed low. The 12Z CMC appears somewhat more in line with the other guidance, but confidence is still not high given the energy's origins from the typically uncertain northern Pacific. Model differences persist as some shortwave energy tracks into the north-central U.S. while some lingers/is reinforced in the Northwest. A surface low associated with the north-central U.S. energy shows some fairly typical spread for the Days 5-7 period but fortunately with some model convergence in the 12Z guidance. The WPC forecast for tonight was able to utilize a deterministic model blend for the first couple of days, with the blending process initially sufficient to handle the growing uncertainty out West. Though by Thursday, increasing amounts of the agreeable ensemble means were incorporated to attempt to smooth out the differences, reaching half the blend by Day 6 and just over half by Day 7. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The slow movement of the Western U.S. upper trough should produce a continued southwesterly flow of moisture into Southern California through early Tuesday while the leading edge of the moisture should push into the Southwest/Great Basin. The Day 4/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. Continued a Marginal Risk across parts of Arizona into Wednesday as moisture may linger into the early part of the period, before the better instability and moisture pushes east. Heavy snow in the higher terrain across much of the West is expected in this overall pattern as well, with possibly significant multi-day totals of multiple feet of snow for parts of the Sierra Nevada, with the largest snow amounts now in the short range. Additional rounds of precipitation are possible across the West for the latter half of the week, but much lighter than what is expected early in the week. As leading energy from the Western trough 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. 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 much of the week. Continue to expect the greatest anomalies to be over/near the Upper Midwest where morning lows and daytime highs may be as warm as 25-35F above normal through at least Thursday with increasing potential for widespread daily temperature records. In contrast, the Southwest U.S./California should see cool daytime highs through the period with some areas around 10-15F below normal on multiple 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 Tuesday-Wednesday. Temperatures across the East should trend warmer as the week progresses with the upper ridge building in, and daytime highs by Friday-Saturday could be approaching 10-20 degrees above normal for some locations, while moderating a bit closer to normal in the Midwest. Tate/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