Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sun Feb 10 2019 Valid 12Z Wed Feb 13 2019 - 12Z Sun Feb 17 2019 ...Overview... From the multi-day mean perspective the models and means are fairly stable and agreeable in showing a Rex block configuration over the east-central Pacific, with the ridge that extends into Alaska supporting a positively tilted downstream trough near the West Coast. Over time the core of negative height anomalies within this trough may drift a fraction eastward from the Pacific Northwest coast. Mean flow to the east of the trough should generally be west-southwesterly, with weaker evidence of southeastern ridging than expected in the short range time frame early this week. Within this broad scale pattern there continue to be significant uncertainties regarding phasing/stream interaction for individual features and thus corresponding sensible weather effects. However the specifics turn out, the pattern should promote periods of significant precipitation over areas of the West as well as over the north-central Plains and much of the East as deepening low pressure tracks along the strong gradient between very cold air over the northern states and warmth over the South. ...Guidance/Predictability Assessment and Preferences... At the start of the period confidence is much lower than would typically be the case for a day 3 forecast regarding trough evolution along the West Coast, with the feature composed of multiple pieces of shortwave energy. Over the past 24-36 hours the majority of guidance has made a pronounced shift toward a slower/sharper trough that recent ECMWF runs have been advertising versus the flatter and more progressive scenario which now is depicted only by the GFS. The 00Z GFS/FV3 GFS is a little closer to the ECMWF cluster than the 06Z GFS. Preference more toward the current majority would lead to stronger low pressure along the West Coast at the start of the period and a significant inflow of low latitude moisture into California. Consult the Model Diagnostics Discussion PMDHMD for updated preferences based on new 12Z guidance. The forecast of low pressure progressing through the West and then likely deepening as it tracks northeast from the central Plains will depend not only on the shortwave specifics initially near the West Coast and how the energy evolves over the central U.S., but then the degree of interaction with northern stream flow over the central/eastern states. Ultimately there is actually still decent agreement in principle on the general longitude of upper troughing but individual models/ensemble members are still quite dispersed for the surface low due to variations in energy/phasing aloft. Best consensus has the surface system reaching the Middle Mississippi Valley by day 5 Fri and near the eastern Great Lakes by day 6 Sat. The 06Z GEFS mean and 00Z FV3 GFS appear less likely with low tracks farther north than most other models/means. As the upper trough reaches the East, GFS runs and the 06Z version in particular may still be overdoing the amplitude of the upper trough given teleconnection-favored flow that favors some degree of above average heights aloft. Note that this relationship does not preclude an amplified trough that is sufficiently progressive but other guidance is somewhat closer to the favored pattern. The next system tracking southeastward toward the Pacific Northwest late this week into the weekend shows better than average clustering, with an average among models/ensemble means generally providing a reasonable starting point. Toward the end of the period late in the weekend the guidance spread increases for details of the overall mean trough, in response to differences in how upstream shortwave energy heads into the mean ridge. Differences are generally within typical error range for day 7 forecasts so an intermediate model/ensemble mean solution would be reasonable, toning down the more western northern part of the axis in the 00Z ECMWF/CMC versus sharper/more elongated southern part of the trough in the GFS runs (which creates somewhat more west-central U.S. shortwave ridging than what may be favored by teleconnections). However downstream as mentioned above the GFS is somewhat more questionable--thus tilting the overall blend somewhat more in the ECMWF/ECMWF mean direction. Finally, around midweek a consensus approach looks good for the storm system over/near New England with a triple point development taking over and tracking into or near the Canadian Maritimes. ...Weather Highlights/Hazards... Areas of the West will see significant precipitation during the period with mean troughing supporting periods of active weather. Rain and higher elevation snow may be particularly heavy over California around Wed-Thu as a potentially complex system could promote a flow of low latitude moisture into the area. This system will then spread moisture across the remainder of the Interior West and Rockies. Additional though less extreme moisture should reach the Northwest and central West Coast late week into the weekend as low pressure tracks toward the Pacific Northwest. Snow levels will likely remain quite low over the Northwest through the weekend. Expect the low pressure system affecting New England on Wed to produce wintry precipitation across much of New England with best potential for heaviest snow over portions of northern Maine. The next deepening low pressure system moving from the central U.S. toward the Great Lakes Thu onward could produce an area of wintry weather north of the low track, across portions of the Midwest/Great Lakes on Thu and perhaps portions of New England on Fri-Sat. Lower than desired confidence in the track of this second system continues to make it difficult to pinpoint exact locations and degree of winter weather impacts. Farther south this second system should produce widespread showers and thunderstorms as Gulf of Mexico moisture streams northward into the deepening surface low. Locally heavy rains will be possible from portions of the Southeast/Tennessee Valley regions to the Mid-Atlantic. The pattern will continue to favor very cold temperatures from the Pacific Northwest to the northern Rockies and northern Plains through the week. High temperatures of 15 to 30 deg F below average are likely across many of these areas through the forecast period. Anomalies could become even a little more extreme over the extreme northern Plains by next Sun. Meanwhile areas from the southern/central Plains to the Eastern Seaboard will see near to above average temperatures varying by day with the warmest temperatures ahead of the late week low pressure system, when highs may reach 10 to 15 deg above average for some areas. Anomalies for morning lows could be somewhat higher. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities 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