Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Feb 07 2024 Valid 12Z Sat Feb 10 2024 - 12Z Wed Feb 14 2024 ...Overview... An upper shortwave/possible compact closed low over the Southwest should mostly hold through the region before beginning to shift eastward with time across the Southern tier of the U.S. as upper ridging tries to build back over the Northwest. This will initially bring some mountain snows to parts of the Four Corners region, but the overall general pattern continues to favor increasing rainfall potential downstream across the Mid-South/Southeast this weekend, and possible eventual lifting of a deepening East Coast to western Atlantic cyclone. Strong upper ridging ahead of this system will support much above normal temperatures as well across the East through Sunday, while period energy intrusions into the Western ridge brings generally light precipitation to parts of the Pacific Northwest. By the middle of next week, the flow across the CONUS should turn more zonal bringing calm and mostly dry conditions to much of the country. ...Guidance/Predictability Assessment... There remains good agreement on the overall pattern evolution across the CONUS during the period, but plenty of uncertainty in the details and timing of individual systems. The system of greatest interest, with the most sensible weather implications, is the potent shortwave exiting the Southwest into the Southern U.S. this weekend/early next week. Compared to prior runs, there has been some narrowing of the forecast spread for this system, but still some key differences in the timing of this system through the Southeast Monday-Tuesday. Yesterday's 12z ECMWF was a clear very slow outlier compared to the rest of the deterministic guidance/ensemble means, but it's new 00z run tonight came in better line with consensus. Some run to run variability, but the GFS has actually been the most consistent with this system the past day or so. Even with the newest 00z runs tonight, there remains uncertainty in the timing, strength, and exact track of this system, but overall there seems to be more agreement which bolsters forecast confidence somewhat. There was enough agreement for this weekend to use a purely deterministic model blend for the WPC forecast for tonight, but did use a non-ECMWF blend for the later periods with the ensemble means. This maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... General broad troughing over the Southwest should keep the Western U.S. periodically active with widespread lower elevation rain/mountain snows through Saturday with the best snow potential should be over the favored terrain of the Four Corners states. Precipitation may clear out of much of the Intermountain West by Sunday, but one or more progressive systems could bring episodes of light to moderate totals to the Pacific Northwest. A trailing cold front should focus moisture and some instability to produce rain across the Mid-South this weekend aided by shortwave energy out of the Southwest. This may increase potential for training/repeat showers and thunderstorms over some areas, but with still a lot of spread in the details and corresponding location/magnitude/timing of the heaviest rainfall. Ensembles suggest the primary region that requires monitoring extends from the Lower Mississippi Valley into the Tennessee Valley/Southeast/Gulf Coast where soil moisture values are above normal in some places. There was enough consensus in the guidance for the introduction of a marginal risk across the general region on both the Day 4/Saturday and Day 5/Sunday Excessive Rainfall Outlooks. Meanwhile, the far northern fringe of the moisture shield could produce some snow from the weekend into early next week, but highly uncertain and dependent on the exact track of the very uncertain low pressure system out of the south-central states. There is with the past guidance cycle some trend toward low development up and off the East Coast into Monday and Tuesday, albeit mainly contingent on still uncertain stream phasings. This type of scenario does offer more potential for lifting/wrapping rains with focus up the Mid-Atlantic and some window for snow for marginally cooler areas from the Appalachians through the interior Northeast. The upper ridge progressing from the eastern U.S. into the western Atlantic late this week will support a broad area of unseasonably warm temperatures across the Ohio Valley/East Coast. Departures from normal should eventually become somewhat less extreme with time but daytime highs on Saturday could be 10-20+ above normal, with some potential for daily temperature records. In contrast, the Southwest U.S./California should be persistently cool, especially for daytime highs that may be up to 10-15F below normal into the weekend. This region should begin to see a moderating trend by Sunday or Monday. Clouds/precipitation over the West should keep most morning lows within a few degrees on either side of normal but a few localized cooler spots are possible. By Tuesday-Wednesday, much of the CONUS should be near or within a few degrees of normal as more zonal flow evolves. 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