Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Dec 02 2021 Valid 12Z Sun Dec 05 2021 - 12Z Thu Dec 09 2021 ...Overview... Guidance shows the large scale upper pattern settling into a broad mean trough across much of the lower 48, between a strong but slowly flattening eastern Pacific ridge and another area of ridging over the mid-latitude Atlantic through the Caribbean and southern Gulf of Mexico. This pattern will support a series of surface systems and fronts that will bring areas of meaningful rain/mountain snow mainly to northern and central parts of the West and then precipitation to the eastern half of the country with snow possible over some northern areas. Although the forecast mean pattern is agreeable, details of embedded individual shortwaves become increasingly uncertain from Monday onward so confidence in some of the important sensible weather specifics is a lot lower. ...Guidance/Predictability Assessment... Aside from the new 12Z UKMET that is slow with its upper trough, guidance is slowly but steadily improving its clustering for the strengthening system tracking from the northern Plains into southeastern Canada along with the trailing front that crosses the eastern half of the country during Sunday-Monday. There is still some disagreement over exact structure of the low at times with an intermediate solution reasonable, but again minus the 12Z UKMET, consensus is now good for timing of the trailing front. Farther west, some aspects of the forecast appear to have become more divergent over the past day. Guidance continues to have difficulty in resolving the character of shortwave energy expected to be south of Alaska at the start of the period early Sunday, and in addition possibly some energy from a separate feature to the south, drop into the western U.S. over the following couple days. Among incoming guidance the 12Z GFS was on the strong/southern side of the spread with its concentrated upper dynamics and associated surface low nearing the Northwest early Monday. Interestingly the new ECMWF has adjusted a lot closer to the GFS at the surface even though the ECMWF's upper shortwave is a lot more open. From there guidance differs over how much the energy will amplify over the West, whether it may help to eject the upper low over/near Baja California, and the ultimate amplitude and timing downstream. The guidance average is waffling somewhat for how much western amplification will occur early in the week with the 00Z/06Z consensus trending a bit deeper than previous forecast. 12Z models are now increasing the potential for quicker ejection of the Baja upper low. Another bundle of North Pacific energy flowing around the eastern Pacific upper ridge should reach western North America around Wednesday with amplitude/timing differences adding to the uncertainty. Given the rapidly declining confidence in details with time, the operational model consensus employed early in the period transitioned to 60 percent total weight of the 18Z GEFS/00Z ECMWF means by day 7 Thursday to emphasize the more agreeable mean pattern. Otherwise lingering input from latest ECMWF/GFS runs provided a little enhancement where agreeable traits existed but otherwise offset to keep the blend closer to the means. ...Weather/Hazards Highlights... The strengthening storm system tracking from the northern Plains through the Great Lakes and into southeastern Canada Sunday-Monday should produce a band of meaningful snow to the north of the low track--mainly over the extreme northern Plains into Upper Great Lakes. Also the storm will likely produce a period of strong winds from the northern Plains into Great Lakes, and then after passage, a period of lake effect snow. There should be sufficient moisture along the trailing cold front to produce some locally moderate to heavy rainfall from the Ohio Valley southwestward. Relatively dry conditions leading into this event should help to temper what adverse effects could arise where rainfall happens to be more intense. While the details remain a question mark, the system pushing into the Northwest by Monday and then dropping southeastward over the West will produce an area of enhanced precipitation from the Pacific Northwest through the northern and central Rockies. Highest totals will be over favored windward terrain and the best potential for heavy snow will be over the northern Cascades and northern Rockies. The precise southward extent of precipitation over the West will depend on the uncertain amplitude and progression of dynamics aloft. Progression of this energy into the East, possible wave development, and interaction of Gulf moisture with a mean frontal boundary should lead to an increasing area of precipitation over the eastern half of the country Tuesday onward. Some heavy rainfall will be possible, with details coming into better focus when guidance eventually agrees better for important shortwave specifics. Some northern latitudes may see snow with relatively higher probabilities from the Great Lakes into New England. Upstream Pacific energy should produce one or more additional episodes of rain and higher elevation snow over portions of the West from Tuesday onward. Over a majority of the lower 48 the progressive nature of most systems will promote variable temperatures from day to day and likely average out to near or above normal. The one exception should be from the extreme northern Plains into Upper Great Lakes where snow cover in the wake of the Sunday-Monday system may lead to some readings 10-20F below normal early next week. The greatest anomalies on the warm side should be ahead of the aforementioned system, with a fairly broad area of plus 10-20F anomalies from the Plains into the East early in the period. One or more additional episodes of warmth may pass through the Southwest into the southern Plains. A few daily records for highs/warm lows may be possible over the Southwest. Rausch 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, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml