Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EST Fri Jan 08 2021 Valid 12Z Mon Jan 11 2021 - 12Z Fri Jan 15 2021 ...Overview... Latest models and ensembles continue to advertise an upper flow evolution that features a strengthening western ridge and late week deepening of the downstream eastern trough, in contrast to the eastern Canada through northern Mexico mean trough (with multiple streams) expected to prevail early-mid week. Heaviest precipitation during the period should be over the Northwest as shortwaves/frontal systems head into and around the mean ridge aloft. Farther east, a fairly weak northern Gulf into western Atlantic system will bring moisture across the South and Mid-Atlantic early in the week and then areas of precipitation (mostly snow) should spread across the Great Lakes and vicinity by late week. Northern tier areas will experience much above normal temperatures through the week while parts of the South will see well below normal readings early in the week before returning toward normal. ...Guidance/Predictability Assessment... In general the transition toward a larger scale ridge/trough pattern late in the week should aid predictability compared to earlier in the week when various smaller-scale features continue to produce lingering uncertainty over the Gulf/western Atlantic system. Regarding this system, there is a gradual narrowing of the spread in general but still with meaningful differences for shortwave specifics and thus surface low track and northwestward extent of precipitation. UKMET runs have been on the suppressed side of the spread due to quick shearing/ejection of the compact upper low emerging from the southern Plains/Mississippi Valley though the new 12Z run has adjusted closer to other solutions aloft. The 06Z/12Z GFS show a low-confidence Midwest shortwave interaction but yielding a shortwave that is still close to consensus. Meanwhile the new 12Z CMC becomes stronger than other solutions with a shortwave crossing New England thus leading to a deeper/northern system offshore--and in contrast the new 12Z ECMWF has decided to trend more sheared in hints of some UKMET runs. Aside from the new CMC, overall trends in the past day have trimmed the northwestern part of the envelope for low track and moisture extent. Farther upstream, energy that may drop into the southern Plains and northern Mexico is also fairly uncertain in terms of strength as well as timing of possible ejection. For most of the week surface high pressure across the southern U.S./Gulf of Mexico should be strong enough for the details aloft to have minimal consequence. However an evolution that turns out to be on the extreme/outlier side of the spectrum could produce some rainfall near the Gulf. Shortwaves coming into the northwestern U.S./southern Canada early-mid week still have fairly low predictability for exact details. Relative to the time frame there is decent agreement for another shortwave to approach the Northwest by early day 7 Fri. Meanwhile late in the week an intermediate solution looks reasonable for the amplifying eastern trough aloft. Latest GFS runs are somewhat on the deep and eastern/southeastern side of the full guidance spread. Both the ECMWF/GFS have been trending more amplified over the past couple days and strength of the upstream ridge gives partial merit to the GFS scenario though historical biases recommend toning it down a bit. Recent ensemble means have been more stable. Guidance comparisons through the 06Z cycle once again favored the 00Z ECMWF/CMC and recent GFS runs (some 00Z input to balance less confident aspects of the 06Z run) for the first half of the period, followed by a trend toward eliminating the 00Z CMC and adding 40-50 percent total weight of the 06Z GEFS/00Z ECMWF means. ...Weather/Threats Highlights... The system tracking northeast from the northern Gulf early in the week should produce an area of light/moderate precipitation over the southeastern third of the lower 48. Locally heavier rainfall may be possible near the central Gulf Coast and near the coast of the Carolinas. There is still some uncertainty over precipitation coverage/totals which will take additional time to be resolved. Snow in the northwestern part of the moisture shield will be limited by lack of cold air in place with best potential to be over higher elevations and/or at night. Rain and high elevation snow over the Pacific Northwest should be heaviest during the first half of the week. At some times the guidance spread for individual shortwaves aloft tempers confidence in precise southward extent of moisture and highest totals along favored terrain of the coastal ranges and Cascades. Some moisture should eventually reach the northern Rockies. There will be a lighter trend after midweek but another shortwave/front will likely bring another episode of at least moderate precipitation to the Northwest toward the end of the week. The deepening upper trough and leading frontal system reaching the East by Thu-Fri should bring a combination of synoptic and lake effect/enhanced snow from the Great Lakes into the northern/central Appalachians. Colder air behind the front will likely bring more lake effect focus. The temperature forecast has been consistent in showing well above normal temperatures across the northern tier through the week while the South will return toward normal after a chilly start to the week. From Montana into the Upper Midwest expect most days to have lows 15-30F above normal while similar anomalies are most likely for highs around Tue-Wed. On other days highs will tend to be 10-20F above normal. Plus 10-20F anomalies for max/min readings should extend into the central Plains. Over the West the strengthening upper ridge will promote an expanding area of highs 10-15F above normal Wed-Fri. A few isolated locations could approach daily record highs during that time. On the cool side of the spectrum, areas from the southern Plains into Lower Mississippi Valley will see highs at least 10-15F below normal on Mon before trending warmer. In spite of the significant upper trough amplification late in the week, currently expect highs over the East to decline only toward normal levels by next Fri due to the Pacific origin of the air behind the cold front crossing the region. 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Mon-Tue, Jan 11-Jan 12. - Flooding occurring or imminent across portions of the Mid-Atlantic and the Pacific Northwest. - Flooding likely across portions of the Mid-Atlantic. - High winds across portions of the Central Rockies, and the Northern/Central Plains, Thu, Jan 14. 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