Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 AM EST Fri Dec 03 2021 Valid 12Z Mon Dec 06 2021 - 12Z Fri Dec 10 2021 ...Overview... The medium range period (Monday to Friday next week) will be characterized by a broad mean trough across much of the lower 48, with periods of shortwave energy moving through the trough pushing low pressure/frontal systems across the country. Rounds of precipitation are forecast for the West, while rain is likely to spread into the southeastern quadrant of the country with perhaps some snow in northern areas, but there is still a good bit of uncertainty with specific weather impacts. ...Guidance/Predictability Assessment... The 12/18Z (yesterday) model cycle remains in fairly good agreement with the large-scale pattern, while aspects of the smaller-scale details remain quite variable. At the beginning of the period Monday, these differences include a slower upper trough over the Midwest in the 12Z UKMET compared to the slightly faster consensus of other guidance, but other than the UKMET there was reasonably agreeable timing with a Great Lakes surface low and the timing of its trailing cold front, maybe just slightly faster than the previous forecast. Another difference among guidance Monday was that the GFS runs (and many GEFS ensemble members) continue to be the strongest with a potent shortwave entering the Pacific Northwest, which as it dives south into the Intermountain West by Tuesday has implications for its speed and possible flow separation. However, at the surface the low pressure system is well clustered offshore of the Pacific Northwest on Monday. Meanwhile, southern stream energy near Baja California Monday has shown a faster trend moving eastward and getting absorbed by Tuesday compared to slower previous guidance, which is reflected in the WPC forecast. By Wednesday, recent guidance generally shows a hint of a positive tilt to the mean trough, with the exception of the 12Z CMC with its different phasing of shortwave energy that is not reflected in its new 00Z run either, so leaned away from that solution. A notable model difference by Thursday-Friday is the degree to which energy drops southward through the West as well as its timing. The GFS and CMC runs show stronger energy creating a deeper Western trough compared to the ECMWF, which shows a couple rounds of weaker shortwave energy passing through the mean trough. Confidence is low with this, given uncertainty with the shortwave energy stemming from high latitudes, and spaghetti plots of 500 mb heights show the operational GFS and CMC are within in the guidance envelope but are on the stronger end of solutions with troughing in the West. Then in the East, there is uncertainty with a surface low spinning up along the coastal Northeast late Wednesday into Thursday, with no good consensus position yet, and this will have implications for precipitation along the Eastern Seaboard. Thus the WPC forecast began the period using a blend of the 12Z/18Z GFS and 12Z ECMWF and CMC, phasing out the CMC from midweek onward and increasing proportions of the agreeable GEFS and EC ensemble means (to 60% by Day 7/Friday). This served to keep proper magnitude of features when there is good model consensus, and lessened the emphasis on individual models by the latter part of the period as uncertainty increases. ...Weather/Hazards Highlights... As a relatively strong surface low pressure system is positioned across the Great Lakes early Monday, snow is expected there while strong winds are possible in the tight pressure gradient behind the low. Conditions will be favorable for lake effect snow behind the system through Tuesday. Additionally, rain is forecast along the trailing cold front associated with the low for the Ohio Valley to south-central U.S., but amounts are likely to be tapering down by Monday. However, Gulf moisture should increase again by Tuesday into Wednesday and cause moderate to possibly locally heavy rainfall from the Lower Mississippi Valley into the Tennessee Valley and Southeast. Moisture spreading farther north into cooler air could lead to snow across the northern latitudes, with relatively higher probabilities from the Great Lakes into northern New England. A couple of shortwaves moving across the West will cause increasing precipitation chances there, with favored windward terrain seeing the highest amounts. The Pacific Northwest into much of the Intermountain West and Rockies should see precipitation Monday and clearing out by Tuesday while chances ramp up in the Southwest. Then the next round is likely for the Northwest on Wednesday and spreads into California eastward by Thursday as energy aloft drops southward. Generally higher elevation snow and lower elevation rain is forecast, though varying throughout the precipitation events and perhaps relatively low snow levels on average given periods of below normal heights aloft. Chilly temperatures are likely across the far north-central U.S. on Monday and Tuesday given snow cover from Sunday's system, with temperatures 10-20F below normal. Meanwhile the Eastern Seaboard can expect temperatures about 10-20F above normal on Monday ahead of a cold front bringing temperatures near average by Tuesday. Overall temperatures should be within about 10 degrees of normal in most places Tuesday and Wednesday, before a warmup by Thursday and Friday from the Plains eastward, with 10-15F above average temperatures. A cooldown is possible in the Northwest by the latter part of the week with the potential for troughing aloft there. Tate 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