Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EST Thu Dec 05 2019 Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019 ...Heavy rain potential for Mid-South to Northeast early next week... ...Well below normal temperatures over the northern Plains/Upper Midwest early-mid week... ...Pattern Overview and Guidance/Predictability Assessment... The main weather feature over North America through the medium range is a sprawling Arctic low that anchors over Hudson Bay and the broad cyclonic flow pattern around this deep low. Deterministic guidance quickly diverges with timing of the main trough rounding this low as it shifts southeast from the Pacific Northwest in a rather positive tilt Sunday through Monday before amplifying as it crosses the eastern CONUS Tuesday and Wednesday. A southern stream low may spin off over the southwestern US border, but the 00Z CMC has a much more cutoff solution on Day 3 which disrupts timing from then on and is thus ruled out from the model blend. The GFS has been much faster than the UKMET/ECMWF with the leading amplifying shortwave Monday into Tuesday over the Midwest. Since the 06Z GFS is even faster than the 00Z GFS, preference was given to the 00Z GFS starting on Day 4/Monday. Details on shortwaves across the CONUS widely varies in deterministic guidance by Day 6/Wednesday, so a majority of the blend is given to ensembles for Days 6/7. Even the 00Z ECENS and 00Z GEFS diverge quite a bit by Day 7, so preference was given to the 00Z ECENS which has less overall spread than the 00Z GEFS (which had less spread than the 06Z GEFS). There is decent agreement in this model preference for a western CONUS/Canadian ridge coming ashore Monday and persisting through Thursday. ...Weather Highlights/Threats... Areas of rain and higher elevation snow shift southeast with the positively tilted trough from the Intermountain West Sunday night, lingering over the southern Rockies through Monday. The building ridge then keeps The West dry through midweek except for the Pacific Northwest as shortwave energy rounds the northern part of the ridge starting Tuesday night. The precipitation forecast for low pressure developing in the lee of the central Rockies Sunday night quickly becomes uncertain due to timing and magnitude differences through the midweek. There are signals for heavy rainfall over a broad area from the Mid-South to the northern Mid-Atlantic and southern/eastern New England Monday/Monday night. Locally heavy snow is possible in the comma head of this low over the Upper Midwest Sunday night through Monday with potential Lake Superior enhancement. Lake effect snow should spread across the remainder of the Great Lakes Monday into Tuesday in the wake of the low which is particularly cold. The warm sector ahead of the this developing central Plains/Upper Midwest shunts east with temperatures 10 to 25 degrees above normal (the greatest anomaly with low temps) through Tuesday. A polar plunge is expected behind the cold front with low temperature anomalies of 15 to 20 degrees below normal over the northeastern part of the Great Plains/Upper Midwest starting Monday, peaking as much as 30 degrees below normal on Wednesday over the eastern Dakotas and MN. Keep in mind this Day 6 forecast is mainly based on ensemble means, so the details are yet to be borne out. As of now the extent of this particularly cold air is contained to the core of the Arctic low which remains centered over Hudson Bay through the medium range timeframe, so low anomalies are generally contained to the Great Lakes region. Jackson Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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