Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Sun Jan 10 2021 Valid 12Z Wed Jan 13 2021 - 12Z Sun Jan 17 2021 ...Overview... Amplification of a mean ridge aloft near the West Coast just after midweek will promote a deepening upper trough over the eastern half of the country late this week. Surface low pressure associated with this trough may be fairly deep and track into the Upper Great Lakes and Canada, pushing a cold front into the East by Fri. The initial shortwave energy coming into the West will bring meaningful precipitation to parts of the Northwest into midweek. Then the aforementioned low/frontal system will spread an area of precipitation (more snow than rain) into the Great Lakes/Northeast and vicinity late this week into the weekend. The upper trough will lift away from the East next weekend as another Pacific shortwave coming into the West amplifies through the Rockies and central U.S., led by a front anchored by a weaker surface wave. Also most precipitation should be lighter than with the leading system. Much above normal temperatures will prevail over the northern tier states into the central Plains during the first half of the period followed by less extreme but still above normal readings over the northern Plains and New England through the weekend. Parts of the West will also be well above normal late this week. The eastern U.S. system will likely bring only modestly below normal highs. ...Guidance/Predictability Assessment... Latest models and ensembles are agreeable/consistent with the large scale pattern evolution but are showing some spread and recent refinements for specifics. There is a fairly strong consensus, including from most new 12Z guidance, suggesting the 00Z ECMWF may be somewhat fast with southern Canada low pressure early in the period. Then the past 12-24 hours of model runs have been showing a stronger signal for an upper low to close off somewhere near the Great Lakes. Recent ensemble means have been hinting at more flow separation and indeed the 12Z GEFS mean now closes off an upper low. This evolution leads to a farther south surface low track. Meanwhile there is an alternate solution cluster that has a Plains wave developing into the Great Lakes, in contrast to the southern Canada low being the primary system. This aspect of the forecast will be very sensitive to shortwave details that will take a little more time to resolve. Once the trailing front reaches the East Coast/western Atlantic, sporadic model runs of primarily the CMC and some earlier ECMWF runs have been showing potential for wave development in response to sharper shortwave energy. Amplitude of the pattern seems to favor tilting at least somewhat away from the GFS that is the most rounded with the upper trough/cleanest with its frontal passage. However given the lack of confidence in shortwave details farther westward earlier in the period, prefer to minimize depiction of a defined wave near the East Coast until there is a more coherent signal. The next Pacific shortwave is still on schedule to arrive into the West around Fri-Sat. Recent trends have generally been toward greater amplitude but the new 12Z ECMWF has reversed that somewhat. By early Sun the primary clustering of guidance says that this energy will support the best defined low pressure over the southern Plains/Rio Grande Valley. This amplification leads into a mean pattern that teleconnections support relative to the core of positive height anomalies just off the West Coast in the D+8 multi-day mean charts. For most of the first half of the period an operational model blend reflected the most common trends of latest guidance while toning down the less confident aspects of any particular solution. By the weekend moderate incorporation of the 06Z GEFS/00Z ECMWF means along with the models helped to anchor the forecast on the agreeable large scale pattern while awaiting better agreement on the specifics. ...Weather/Threats Highlights... The atmospheric river setup in the Northwest during the short range should be diminishing by the start of the medium range (Wednesday), but some moderate to locally heavy rain and mountain snow should continue to focus along the favored terrain of the coastal ranges and the Cascades. For midweek the guidance appears to be converging toward a concentrated focus over northwest California/southwest Oregon into the Oregon cascades as well as into the northern Rockies. After a brief break over the Northwest as upper ridging amplifies, expect an episode of what should be less extreme precipitation as another shortwave and frontal system pass through. Some of this moisture will then extend east/southeast through the Rockies. Meanwhile guidance is slowly getting closer together for a southern tier U.S. shortwave that may produce some areas of rainfall over the Southeast/Florida late this week. Still there is considerable spread for coverage and amounts with this activity. The deepening upper trough and leading low pressure/frontal system heading into the East after midweek should bring a combination of synoptic and lake effect/enhanced snows from the Great Lakes into the northern/central Appalachians. Precipitation should also extend into parts of the Northeast. There could be some rain in the southern part of the moisture shield along the leading front. Parts of the Great Lakes/Northeast may continue to see some snow into weekend. Any embedded frontal waves could increase precipitation above current expectation along the East Coast. Precipitation (possibly various types) may begin to expand over/east of the Plains and western Gulf Coast late in the weekend as upper troughing amplifies over the Plains/Rockies. Above normal temperatures over the northern/central Plains will be the most extreme around Wednesday-Thursday when readings should be 15-30F above normal. Morning lows could even be locally warmer at some locations near the Canadian border. A few record highs are possible over the northern Plains on Wednesday, and while morning lows the next day could also exceed records it remains to be seen whether they hold above those levels throughout the calendar day. During the rest of the period the northern Plains and New England should continue to see some plus 10-20F anomalies, especially for lows. The warming trend over the West as upper ridging strengthens will push highs to 10-15F or so above normal over some areas mid-late week. Best potential for scattered record highs will be around Thursday-Friday. Amplification of a shortwave reaching the West will suppress the warmth southward next weekend. The upper trough/front reaching the East late this week will bring highs down to near or modestly below normal levels for next weekend. It will be chilly but comparisons are to climatological averages that are near their seasonal low. 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, 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