Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Feb 02 2024 Valid 12Z Mon Feb 05 2024 - 12Z Fri Feb 09 2024 ...Atmospheric river likely to impact California with 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 over a multi-day time frame through about midweek. Significant rain and mountain snow are 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... While model guidance is reasonably agreeable on the overall pattern, there are some important detail differences that have sensible weather impacts. As the period begins 12Z Monday there are already some model differences with the surface low and associated cold front with the AR ahead of it. GFS runs had been on the stronger side of the guidance envelope with the surface low but the 12Z guidance from other model suites are coming in stronger than their previous runs. There has also been a bit of a faster trend with the AR to come into Southern California. Regardless of the exact strength and timing variations, there is still high confidence in a significant precipitation event unfolding early next week for California with some notable precipitation inland. Then models agree that the upper trough axis will shift across the West and toward the central U.S. throughout the week. Shortwaves within the trough are much more variable, especially by midweek and beyond, with potential for energy to dig on the western side and discrepancies in how that may combine with energy already in place. GFS runs through 00Z had been closing off a low in the Southwest with the digging energy and appeared to be outliers, and the 06Z and now 12Z GFS have backed away. Other models vary considerably but without definite outliers. An emerging surface low in the Plains to Midwest shows typical levels of spread Wednesday-Friday, though ECMWF runs may be a northern outlier. Farther east, the upper and surface lows atop Florida and the vicinity on Monday seem reasonably well handled, with ridging gradually moving into the East by the latter half of the week. Given these considerations, the WPC forecast used a deterministic model blend favoring the ECMWF early in the forecast period, introducing the GEFS and EC ensemble means into the blend by Day 5 and raising their proportion to just over half by Day 7. Overall, this maintained good continuity with the previous WPC forecast, though with a slightly faster trend of QPF/ERO into southern California per the latest guidance. ...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. 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