Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Feb 06 2024 Valid 12Z Fri Feb 09 2024 - 12Z Tue Feb 13 2024 ...Overview... A broad reloading upper trough should hold on over the West into Saturday, before beginning to slowly shift eastward as upper ridging tries to build in near the West Coast again and a couple of strong systems push through the East Pacific. The trough will continue to keep this region quite active with rounds of lower elevation rains and mountain snow, including possibly a couple of weak atmospheric rivers into the West Coast. Meanwhile, a strong upper ridge shifting out of the Central U.S. will continue to support well above normal temperatures that could be record-setting across the parts of the Upper Great Lakes-Northeast on Friday-Saturday before it becomes suppressed and shifts eastward off the coast. A fairly strong storm system tracking out of the Great Lakes on Friday will produce some cold sector snow and brisk winds. Then additional shortwaves/energy from the mean upper trough over the West will periodically eject into the central U.S. There is significant uncertainty over how associated systems will reflect at the surface to the east of the Rockies, but the general pattern appears favorable for increasing rainfall across the Mid-South/Southeast by late week or weekend. ...Guidance/Predictability Assessment... Shortwave differences within the Wester U.S. upper trough and how the trough evolves and eventually pushes eastward continues to present the most uncertainty during the medium range period. As early as Friday-Saturday, there are notable differences in shortwave energy that digs southward along the West Coast and how that becomes absorbed into the mean trough. Models have shown better agreement for a much more amplified (with a possible brief closed low) over the Southwest this coming weekend, but as that southern stream energy moves into the south-central Plains and possibly interacts with northern stream energy dropping out of Central Canada presents significant uncertainty in the pattern evolution across the Midwest/East early next week. Through the new 00z run tonight, the GFS shows distinct separation between the two systems, with the northern stream system out running the southern stream and a strong low pressure system that tracks across the Southeast and eventually off the Mid-Atlantic Coast. Prior runs of the ECMWF and CMC on the other hand showed more interaction in the two streams resulting in a sharp shortwave/amplfied trough and strong surface low that tracks northward into the Midwest and off the Northeast Coast at the same time. However, the new 00z runs (available after WPC forecast generation) tonight came more in line with the GFS scenario. Still too early to suggest this may be a trend, but it will need to be watched as it would have sensible weather impacts. Both the GEFS and ECENS ensembles show a lot of uncertainty, with the resulting mean forecast quite weak and washed out. The WPC late period forecast used a blend of the ensemble means, with some attempts to add a little system definition from yesterdays 12z ECMWF. This did result in noticeable changes to previous WPC continuity, with the depiction of a more defined wave/system given the better model support. Either way, a very low confidence forecast across the Eastern third of the country early next week with plenty of time for model swings and variability. Elsewhere, there is a lot of uncertainty as well with potential for a couple of weaker shortwaves to impact the West Coast Sunday-Tuesday, but questions arise with how much these are able to disrupt/get through the overall mean ridge that tries to build back in over the West. A blend of the ensemble means helped to smooth out these differences and was a good starting point for the forecast out West late period. ...Weather/Hazards Highlights... General broad troughing over the Southwest should keep the Western U.S. periodically active with widespread lower elevation rain/mountain snows into next weekend. Best snow potential should be over the favored terrain of the Four Corners states. Additional rounds of rain and mountain snows could come into California through Friday, though with very low confidence on exact amounts and locations of heaviest precipitation. Given extreme rainfall across much of California recently, the area is more sensitive than usual to additional rainfall, but the fact that any digging dynamics aloft should take an along-coast or inland path should reduce the heavy precipitation threat. Guidance spread for shortwave details aloft and resulting QPF keeps confidence too low to depict any risk areas on the Day 4 Excessive Rainfall Outlook valid on Friday. Precipitation may clear out of much of the Intermountain West by Saturday, but one or more progressive systems could bring episodes of light to moderate totals to the Pacific Northwest. Fairly strong low pressure will be tracking out of the Great Lakes on Friday, though with still some lingering snow and gusty wind threat on the backside of the system. The trailing cold front should focus moisture and some weak instability to produce rain across the Mid-South, especially into the 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 Tennessee Valley into Lower Mississippi Valley and Southeast. This overall area has a mix of antecedent soil moisture conditions. Despite the presence of anamolous moisture and some instability, confidence remains too low though to introduce any risk areas in the Days 4-5 EROs at this time, but again the general pattern may ultimately support a heavy rainfall event sometime in the weekend time frame. Meanwhile, the far northern fringe of the southern tier 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. 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 which will gradually shift from the Great Lakes/Midwest toward the East Coast. Departures from normal should eventually become somewhat less extreme with time. The Upper Midwest/Great Lakes should be warmest versus normal on Friday with anomalies up to plus 30-35F for morning lows and plus 20-30F for daytime highs, leading to widespread daily temperature records. By the weekend expect the East to see lows of 15-30F above normal and highs 10-20F above normal while the central U.S. trends closer to normal. 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. 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