Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Thu Jan 10 2019 Valid 12Z Sun Jan 13 2019 - 12Z Thu Jan 17 2019 ...Overview... Progressive and amplified flow across the North Pacific will continue to bring significant storm systems to the West Coast periodically during the medium range period. A strengthening upper ridge across the northern Rockies/Great Basin will result in split flow as these Pacific systems move eastward, with energy separating into northern and southern stream components. Northern stream energy will amplify as it enters broad cyclonic upper-level flow east of the upper-level ridge, resulting in frequent shortwaves and mean upper troughing from the Upper Midwest to the Great Lakes. A partially phased shortwave will cross the the Mississippi/Ohio Valleys on Sun with development of a coastal low pressure system along the North/South Carolina coast likely, which should then track northeastward into the Atlantic Ocean. Meanwhile, progressive flow across the North Pacific will keep the West Coast active, with an amplified trough/upper low reaching California Sun-Mon, another weakening system on Wed, and yet another approaching by Thu. A trend is becoming evident among the guidance that a rather significant pattern change could begin to unfold toward the latter portion of the medium range and beyond. Model/ensemble solutions continue to trend stronger with the upper ridge across western North America by the middle of next week, while a downstream upper vortex deepens north of Hudson Bay, setting up broad north-northwesterly flow from the Arctic into central Canada. Farther south across the CONUS, the flow appears to remain progressive, originating in the North Pacific, which should limit the southward intrusion of arctic air into the CONUS for now. Nonetheless, would expect an area of arctic air to begin pooling across central Canada by the middle of next week, with the potential for incursions southward, especially into the Midwest/Great Lakes. ...Guidance Evaluation/Preferences... A multi-model blend served as a starting point for the forecast during days 3-5 (Sun-Tue), including the 06Z GFS and the 00Z GEFS, UKMET, operational ECMWF, and ECMWF ensemble mean. Models still have yet to fully resolve the specific structure of the leading shortwave energy crossing the Ohio and Tennessee Valleys on Sun as well as the possibility of a secondary amplifying shortwave on Mon, both of which play a role in the timing/track of the low pressure system offshore. In terms of the coastal low track, guidance seemed very well clustered around the ECENS mean, which is very close to the preferred WPC solution. The strength of the low has been trending weaker in the GFS, closer to the EC. Solutions were reasonably well-clustered with the amplified system approaching California during the day 3-5 period, though there are differences in the degree to which the upper-level low is cut off from the main flow. A consensus/multi-model solution was preferred here as well. During days 6-7 (Wed-Thu) uncertainties begin to quickly crop up in both the northern and southern streams. Shortwave energy crossing the Rockies on day 5 and reaching the central U.S. by day 6 has shown quite a bit of run-to-run variability among the guidance as to how amplified the feature will become. Additional shortwave energy reaching the West Coast by Wed night/Thu shows somewhat better consensus among the guidance, especially with respect to timing of the feature. Farther north, the aforementioned highly amplified flow pattern developing across Canada will open the door for southward transport of shortwave energy originating in the Arctic, which models generally do a particularly poor job handling. The ECMWF has shown a bit more consistency with the northern stream flow during this time frame relative to the GFS. Increasing weight was given to ensemble means of the NAEFS and ECENS by days 6-7 due to the uncertainty on timing and position of shortwaves within the broad scale flow, with components of the operational EC for added detail and some continuity from the WPC forecast. ...Weather Highlights/Hazards... The low pressure system moving through the Southeast U.S. and off the Eastern Seaboard Sun-Mon is expected to produce a band of wintry precipitation from the Ohio Valley to the southern Mid-Atlantic, with heavy snow possible for some areas. Gusty winds may also be possible along the Mid-Atlantic coast Sun night into Mon as the system begins to deepen offshore. The West Coast will remain active given the rapid frequency of shortwave energy and frontal systems. The heaviest precipitation (low elevation rain and mountain snow) will be focused across California, with multiple rounds of precipitation possible through the forecast period. Total precipitation amounts over the period could be heavy, with upslope flow enhancing QPF to 4 to 8 inches in the Sierra and Coastal Ranges. Above average temperatures are expected Sun-Tue across much of the Central and Northern Plains to the Upper and Middle Mississippi Valley regions, with high temperatures generally 10-15 deg F above average. Then on Wed to Thurs, Arctic air may approach the Northern Plains and Upper Midwest regions, cooling down temperatures. The Southeast and Eastern Seaboard will initially see below average temperatures in the wake of the upper trough and low pressure system, with highs 5-10 deg below average Sun-Mon, and moderating temperatures from Tue onward. Ryan/Tate 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