Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Feb 04 2021 Valid 12Z Sun Feb 07 2021 - 12Z Thu Feb 11 2021 ...Northern Plains to Midwest Arctic outbreak eventually expanding to the south and east... ...Pattern Overview and Guidance/Predictability Assessment... Most guidance continues to show a blocky pattern over Canada with ridging in the north for most of the period and a deep upper low farther south. This mean upper low is most likely to be centered between the Great Lakes and Hudson Bay most days, possibly consisting of multiple pieces of energy/individual centers. In response to this low, broad cyclonic flow will prevail across much of the lower 48. At the same time a mean ridge over the eastern Pacific should let one or more shortwaves pass through, with these features then progressing around the southern side of the CONUS mean trough. By late in the period forecast uncertainties include how quickly the southern Canada low may begin to elongate and/or progress eastward as the northern Canada ridge weakens, as well as the strength of the eastern Pacific ridge relative to any potential shortwaves. Earlier in the period over the East the guidance has been varying with important specifics of a wave just off the East Coast on day 3 Sun as well as for another wave/frontal system that may affect the eastern states a day or so later. The dominant highlight within this pattern remains the much below normal temperatures expected over the central U.S. and southward/eastward expansion of the cold air after early next week. The updated forecast used a composite of 00Z/06Z model guidance and a small 06Z GEFS/00Z ECMWF mean component for the first half of the period followed by a moderate increase of ensemble input with the typical increase of detail uncertainties farther out in time. Already at the start of the period on Sun there is significant spread/trending for the wave expected to track off the East Coast. Up through yesterday there had been a pronounced trend toward a weaker/offshore system but overnight short-range adjustments in the GFS/GEFS mean led to a stronger and westward system. Other new guidance generally supports the overall trend but not necessarily to the extent of the GFS. Based on this, the manual forecast nudged the initial blend partially but not fully in the GFS direction. 12Z ECMWF trend would support further incorporation of the latest trends. Behind this system, guidance has followed through on signals that started showing more strongly 24 hours ago for frontal/wave development over the eastern U.S. during the first half of the week. A blended approach looks reasonable in light of spread for depth and tendency thus far for the strongest solutions to trend weaker in the next run. This evolution ultimately delays the eastward progression of the central U.S. cold air by a day or two. There is still some spread for leading diffuse shortwave energy that may filter through the East Pacific mean ridge during the first half of the week, though better clustering now exists among the GFS/UKMET/00Z ECMWF which are all weaker than the 12Z CMC. Toward Wed-Thu the ensemble means have trended stronger with the East Pacific ridge over the past day or so. This trend offers support for leaning away from solutions that bring stronger energy into the ridge, such as the CMC runs in particular and to some degree 06Z/12Z GFS (which at least keep the energy farther north). ...Weather/Threats Highlights... Anticipate terrain/upslope snow over the northern Rockies and vicinity, especially early in the period, as shortwave energy streams across the region and the potent Arctic surge becomes established. Progression of the shortwave energy within the mean trough may support some degree of wave development over the East early next week, leading to a band of potentially meaningful snow from the Midwest into the Northeast. Precipitation would likely be in the form of rain farther south. Upstream shortwave energy could generate one or more areas of lighter precipitation thereafter, with the rain-snow line sinking southeastward as the cold air expands. Farther south a lingering lead front and a secondary surge into the Southeast will pool moisture for showers/thunderstorms through this period. A Southeast coast wave developing into Sunday and tracking northeastward has continued to vary in the guidance, with latest stronger/westward trends leading to increasing potential for precipitation (including some snow) over parts of the Mid-Atlantic/Northeast. Areas to the west of the Rockies should see fairly light and scattered precipitation, but with uncertainty in specifics due to low confidence for supporting shortwave details. Depending on the details of evolution aloft, moisture may expand in coverage across the southern tier states by late next week and interact with the cold air in place to produce some areas of wintry weather in addition to rainfall closer to the Gulf Coast. Expect the most persistent core of coldest air over the central U.S. to extend from the northern half of the Plains into the Midwest and western Great Lakes. Within this area temperatures that are 15-30F below normal will be common through the period and parts of Montana may even be 30-40F below normal from late weekend into early next week. A few isolated record cold highs may be possible. Cold temperatures will also spread robustly far south and eventually to the east with time--delayed a bit from past days' forecasts due to the front/wave now expected over the East early in the week. This expansion of the cold air should bring temperatures down to 10-25F below normal from the southern Plains into parts of the Ohio Valley by mid-late week. Most areas over the South and East should be near to somewhat above normal into early next week. To the west of the Rockies, initially expect moderately above normal readings over the Great Basin/Southwest and near normal temperatures farther north. The West should see a gradual cooling trend over the course of next week, leading to below normal temperatures over the Northwest and moderation closer to normal farther south. Rausch/Schichtel 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