Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Feb 02 2024 Valid 12Z Mon Feb 05 2024 - 12Z Fri Feb 09 2024 ...Atmospheric River increasingly likely to impact California with heavy to possibly significant rain and flash flooding through Monday with moisture also spreading inland across the West... ...Overview... The guidance continues to show an active weather pattern across the West as a large scale positively tilted mean trough continues to promote a strong southwesterly flow of moisture into the West through about Midweek. over a multi-day time frame. Increasing confidence for possibly significant rain/mountain snow likely to focus over parts of California into Monday. With time some of this moisture should extend into parts of the Intermountain West and Rockies as the upper trough drifts eastward and broadens. Meanwhile an upper low/strong surface system should track across Florida on Monday though most of the heaviest rainfall will be off the coast by then. Additionally, a strong upper ridge 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 latest runs of guidance maintain a high confidence for an active wet period for the West Coast/parts of the Southwest with a gradual eastward drift with time and eventual broadening later in the period. Plenty of uncertainty still in the details of individual upper features/energy dropping into the western part of the trough, especially later in the week. The GFS was more amplified with this late week, wanting a closed low over the Southwest days 6-7. Also some timing uncertainty on a shortwave ejecting from the trough, with a modestly strong surface low tracking from the Central Plains into the Midwest Wednesday-Friday. Otherwise, there is overall good agreement with the amplified ridge shifting slowly eastward with time, as well as better agreement that the upper low over the Southeast should merge with another low to the north and move more off the coast than previous model runs may have shown. The WPC blend for tonight favored the ECMWF with other deterministic guidance for the first half of the period. By day 5 and beyond, used a combination of the GFS/ECMWF with the ensemble means to help smooth out the late period detail differences. Overall, this maintained good continuity with the previous WPC forecast. ...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 another AR in the short range period working to pre-condition soils, a moderate risk remains in effect for this region on the Day 4/Monday Excessive Rainfall Outlook with only minor modifications needed to the previous issuance. The slow movement of the supporting upper trough may 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. Heavy snow in the higher terrain across much of the West is expected in this pattern, with possibly significant multi-day totals 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, though with still plenty of uncertainty in the exact amounts. 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. Florida should see a couple cool days with highs 5-10F below normal by Tuesday-Wednesday 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