Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Feb 05 2024 Valid 12Z Thu Feb 08 2024 - 12Z Mon Feb 12 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 next Sunday-Monday. The trough will keep this region quite active with rounds of lower elevation rains and mountain snow, including possibly a couple of weak atmospheric rivers into California. Precipitation is certainly forecast to be lower than in the short range though. 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 Midwest on Thursday-Friday before it becomes suppressed and shifts eastward with time. A fairly strong storm system tracking out of the Northern Plains/Upper Midwest Thursday onward 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 Western U.S. upper trough continue to arise fairly early in the period. Model/ensemble spread eventually becomes compounded by differences with a couple Pacific shortwaves that may try to pass around or through the mean ridge nearing the West Coast, one possibly approaching the Pacific Northwest/western Canada around Saturday and another by Monday. Latest GFS runs shear out the first Pacific shortwave more than other guidance while there is little coherence among guidance for the second, with the means showing a vague hint of a weakening shortwave. Downstream over the lower 48, guidance diverges for the character of energy ejecting from the West and thus varies among each other and from run to run for the surface evolution. The 00Z ECMWF-initialized machine learning models also vary for the details, highlighting the relatively low predictability of some aspects of this overall pattern, but generally favor reasonable progression of the northern stream flow and suppressed trailing frontal waviness across the southern tier in response to a moderately slow shortwave ejecting from the West but no phasing with the northern stream. This surface pattern is closest in principle to the ensemble means and 00Z ECMWF late in the period. However the 12Z CMC/ECMWF now show more phasing aloft and bring the trailing system into the Ohio Valley/Great Lakes late in the period--a scenario that has appeared in one or more prior operational model runs. Given the lower than average confidence in the forecast, the operational model blend employed early in the period (which already blends out the lowest-predictability shortwave details) transitioned toward three-fourths GEFS/ECens mean weight with lingering 00Z ECMWF input by the end of the period next Monday. ...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 Thursday-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 in the Days 4-5 Excessive Rainfall Outlooks valid from Thursday through Friday night. 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 tracking northeastward from the northern Plains should produce an area of meaningful snow to the west of the storm track this Thursday, with best potential for at least 0.25 inch liquid in the form of snow over eastern Montana through North Dakota. Winds could be strong enough to produce some blowing and drifting of the snow. The trailing cold front should focus moisture and some weak instability to produce rain across the Mid-South. Rain rates and amounts on Thursday do not look high enough to cause flooding issues given fairly progressive movement. However from Friday into the weekend, shortwave energy digging into the Southwest could help to slow down the central/southern part of the front (which may have embedded waves) for a time and lead to increased potential for training/repeating of showers and thunderstorms over some areas. There is a lot of spread for the shortwave details among the guidance, with corresponding dispersion in location/magnitude/time frame of 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. Confidence remains too low 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 late week/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. 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 Plains/Mississippi Valley/Great Lakes 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 Thursday-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. Rausch/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