Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Jan 21 2019 Valid 12Z Thu Jan 24 2019 - 12Z Mon Jan 28 2019 ...Overview... From late this week through most of the weekend the forecast is consistent in showing a highly amplified upper pattern featuring an eastern Pacific/West Coast ridge and eastern North America trough. As in previous days the overall mean pattern appears to have fairly high predictability but embedded shortwaves that are smaller in scale will be more difficult to resolve several days out in time. D+8 mean charts suggest that the ridge may retrograde a bit late in the medium range period and beyond. Teleconnections relative to the associated positive height anomaly center favor increased potential for positively tilted troughing to develop over the West--which some guidance appears to be heading toward by early next week. At the same time another strong positive height anomaly center east of the Canadian Maritimes would favor maintaining the mean trough over eastern part of the continent. ...Guidance Evaluation/Preferences... The two features that show reasonable agreement are early in the period during Thu-Fri. These are the wavy front whose embedded low pressure consolidates/deepens as it lifts northward near the East Coast into the Canadian Maritimes, and Great Lakes low pressure with trailing strong cold front that pushes quickly south/east across the central/eastern states Thu into Fri. For the eastern system the latest GFS/ECMWF runs and their means provide the best consensus while the 12Z CMC is a bit west and the 12Z UKMET slow. In the new 00Z cycle the CMC aligns better with the majority scenario while the UKMET remains slow. Recent trends have generally been nudging the system along a little faster, suggesting even lower probability for the slow UKMET. A consensus approach also looks good for the trailing low pressure/strong cold front. Upstream features show more divergence. There is a good signal for a clipper type system to track into the northern states in response to some combination of shortwave energy rounding the mean ridge and Canadian flow. However timing differences are considerable. The past two ECMWF runs are on the opposite sides of the spectrum (12Z run fast, 00Z/20 run slow). The fast 12Z ECMWF has company in the past couple CMC runs but most other models/means are closer to the middle. Meanwhile there are indications that the southwestern part of the overall shortwave complex may sharpen as it drops through the west-central U.S. and then reaches the mean trough axis near 90W longitude during the weekend. There is a general theme of a lower surface pressures over the east-central U.S. during the upcoming weekend but considerable spread/run-run variability for the specifics. Then by day 7 Mon there is fairly high potential for low pressure to develop off the East Coast as the upper energy continues to the east/northeast. The 12Z GEFS/ECMWF means provide the best intermediate starting point given the uncertainty leading up to this system's development. The 18Z GEFS mean had a weaker surface reflection thus favoring the 12Z run. Thus far operational models have been inconsistent for strength/track of this system, ranging from the very strong/impactful 12Z ECMWF and 00Z CMC to well offshore 18Z/00Z GFS and 12Z CMC runs. Late in the period another push of height falls should drop into the central and western parts of the northern U.S., perhaps leading into a pattern that has somewhat more western U.S. troughing per teleconnections relative to the upstream ridge position. While the 12Z/18Z GFS runs are quite strong (followed by a much weaker 00Z GFS), the GFS is generally much closer to the means in principle for low pressure that may head into the Great Lakes. The early part of the forecast represents the preferred guidance consensus (more GFS/ECMWF than other operational models). The blend yields an intermediate timing for the contentious northern U.S. wave Fri-Sat. Then the forecast rapidly increases 12Z GEFS/ECMWF mean input as operational model details diverge more dramatically. The 18Z GFS is close enough to the favored pattern over the western 2/3 of the country to allow for a small minority to persist in the blend through day 7. ...Weather Highlights/Hazards... Strengthening low pressure tracking northeast from near the Mid-Atlantic will bring a period of significant precipitation to the Northeast on Thu. Best snow potential will be over interior New England. At the same time the front trailing from this system will bring rain of varying intensity to the Florida Peninsula. The Great Lakes will likely see episodes of snow through most of the period with a combination of periodic synoptic systems (with lake enhancement over some areas) as well as more simple lake effect activity. Other areas east of the Rockies may see some precipitation with low pressure/frontal passages. There is potential for more organized precipitation over parts of the eastern states by the weekend into next Mon but with low confidence in coverage/intensity. The rain-snow line should extend fairly far southward given the cold pattern becoming established over the eastern half of the country. Meanwhile at least a couple shortwaves/fronts and trailing high pressure should bring periods of snow to parts of the northern and central Rockies with most activity likely to be in the light-moderate range. Expect the core of coldest air during the period to be across the Upper Mississippi Valley/Midwest with readings of 15-25F below normal late this week into the weekend. After the Thu system brings a brief surge of warmth to the East Coast (morning lows 15-25F above normal), much of the eastern half of the country should see below normal temperatures Fri onward. Greatest coverage of highs at least 10F below normal should exist Fri-Sun. On the other hand expect above normal temperatures to persist over the West Coast states and extend into the northern Rockies and parts of the High Plains with localized anomalies exceeding plus 10F. The northern half of the West should trend cooler by next Mon. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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