Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Tue Nov 19 2019 Valid 12Z Fri Nov 22 2019 - 12Z Tue Nov 26 2019 ...Pattern Overview and Guidance/Predictability Assessment... Models have continued to trend quicker over the past 24 hours with ejecting an upper low eastward as a trough across the CONUS southern tier. The ECMWF has continued to lag this trend for days, while the GFS has led the trend, and the CMC/UKMET have often been somewhere in the middle. Farther north, an active and progressive northern stream flow regime will persist from the North Pacific east across Canada and the CONUS northern tier. A vigorous shortwave is expected to cross the lower Great Lakes and Northeast on Fri, with a surface low pressure system deepening as it moves across Quebec, and the trailing cold front moving off the Easter Seaboard. Models continue to show variability on the degree of interaction between the southern stream trough as it moves east and the progressive northern stream. Faster timing of the southern stream wave should reduce the potential for interaction with the next northern stream wave, making for somewhat less phasing of streams than indicated by the past few runs of the ECMWF. For a wave of low pressure expected to develop along the trailing surface front across the lower Mississippi Valley on Fri, the result of the aforementioned interaction will be a quicker eastward progression and perhaps somewhat less threat of a significant coastal low off New England by Sun - although as with any potential phasing interactions confidence in this aspect of the forecast is quite low, particularly given that models have seemed to struggle with timing of smaller scale northern stream shortwave energy. Farther west, models continue to depict the arrival of a strong North Pacific jet to the northwestern U.S. by late Sat, which should bring a series of shortwaves and frontal systems to the region. Early on in the extended, models show consensus that an upper low should separate from this flow and drift southward off the West Coast before once again closing off near/off the southwest U.S. coast over the weekend. The ECMWF was the lone solution that takes the upper low westward, while almost all other solutions eventually start bringing the feature eastward and merge it with a trough amplifying across the western U.S. by early next week. The latter idea was favored in this forecast, as the ECMWF has recently shown a tendency to be too aggressive with taking cutoff lows southwestward (this week's system was a similar example). Given the favored solution and support from ensemble means, expect to see amplification of a fairly significant upper trough across the western U.S. by early next week, with a cold front pressing across the West and into the central U.S., and potential for development of a surface low somewhere across the Plains by the end of the forecast period (next Tue). The WPC forecast was based heavily on a blend of the 18Z GFS/12Z UKMET/12Z CMC during days 3-5 (Fri-Sun). ECENS/GEFS ensemble means comprised a majority of the forecast weighting during days 6-7 (Mon-Tue), with some continued use of the GFS/CMC. ...Weather Highlights/Threats... The front crossing the Northeast on Fri will bring a mix of rain and snow to the region, with the best potential for accumulating snows in the higher terrain. Farther south, the wave of low pressure expected to move from the lower Mississippi Valley to the Southeast Fri-Sat is expected to produce widespread showers and some thunderstorms. Locally heavy rain will be possible across these areas, but the heavy rain threat appears at least somewhat reduced compared to the past couple days, due to the trend to a faster eastward progression of the system. Across the Pacific Northwest, expect rain and mountain snow to begin by Sat with rain/mountain snow expanding in coverage across much of the interior western U.S. Sun and Mon as the upper trough amplifies. Expect chilly temperatures to overspread the central U.S. Fri and the eastern U.S. Sat-Sun, with highs 5 to 10 (and a few locations as much as 15) deg F below average. A warming trend will bring high temperatures 5 to 10 deg above average to the northern plains and Upper Midwest over the weekend. Meanwhile, as the upper trough digs into the West by early next week, expect temperatures to drop, with highs 5 to 15 deg below average across much of the interior western U.S. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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