Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 AM EST Mon Jan 27 2020 Valid 12Z Thu Jan 30 2020 - 12Z Mon Feb 03 2020 ...Overview and Guidance/Predictability Assessment... Models/ensembles show upper ridging building from the East Pacific across the western U.S. through late this week, before eventually flattening some by next weekend as a strong North Pacific upper jet and shortwave dig into the Northwestern U.S. As the upper ridge builds, shortwave energy spilling over the top is forecast to dig southward and cut off across the Southwest on Thu before moving east into the Southern Plains by Fri. Additional shortwaves spilling over the more amplified ridge Fri-Sat will likely result in some form of interaction/phasing with the southern system. As is typical of such a process, models continue to struggle with this and forecast confidence remains below average. Overall, the model with the most consistency from run-to-run continues to be the GFS. It has shown some degree of variability, but not quite to the extent shown by other models (ECMWF/CMC). The GFS continues to be not quite a slow/cut off with the southern stream system by mid/late in the week, and is thus a bit quicker and more phased with the eventual low pressure that it develops in the Gulf of Mexico Fri and moves up the Eastern Seaboard over the weekend. The ECMWF has had some runs similar to the GFS, and other (more recent runs) where the southern stream system is slow enough to entirely miss the northern stream wave, resulting in a suppressed low track that moves from the Gulf of Mexico quickly off the Eastern Seaboard into the Atlantic Ocean. The 00Z model cycle tonight was the first where the system now falls within the range of the UKMET, which looks very much like the composite idea shown by the GFS over the last several runs. Finally, the GFS continues to have a significant degree of ensemble support (even from the ECENS), remaining well-centered in the ensemble spread and close to the means. By next weekend, guidance shows above average consensus on another positively-tilted shortwave approaching the western U.S., which it appears should make significant progress in breaking down the northern extent of the upper-level ridge, before undergoing some degree of amplification across the Great Basin (the specifics are a bit vague by that time, with variability among solutions). Given these considerations, the forecast for days 3-4 (Thu-Fri) was based on a blend of the 18Z GFS/12Z ECMWF, with more weight placed toward the GFS due to its preferred evolution of the cutoff southern stream system. By days 5-7 (Sat-Mon) a blend of the GFS along with the ECENS/GEFS ensemble means was used as a basis for the forecast, with increasing weight on ensemble means through the end of the forecast period. ...Weather/Hazard Highlights... The Pacific Northwest will see a prolonged period of active weather throughout the extended forecast period as a series of shortwaves/frontal systems impact the region, spaced generally 24-36 hours apart. Each of these should produce potentially heavy rain/snow dependent on elevation, with snow levels gradually rising a bit by late in the week as the ridge builds in, and then decreasing by early next week as the ridge flattens. Farther east, as low pressure develops in the Gulf of Mexico by late in the week, a zone of relatively strong lift/overrunning should develop, resulting in the development of a larger shield of precipitation across the Gulf Coast Thu night into Fri, spreading into the Southeast during the day on Fri, and potentially the Mid-Atlantic/Northeast Fri night-Sun (although the latter is still highly dependent on the specific track the surface low takes). Given the continued uncertainty surrounding this system, precipitation amounts and types remain difficult to pin down. Updated winter weather outlooks for the extended period continue to indicate low to moderate chances for significant winter precipitation from portions of the Appalachians/Mid-Atlantic to the Northeast for the Sat-Mon time period. A brief period of below average temperatures is possible across the Southern Plains on Thu beneath the cutoff southern stream system, but these temperature should quickly warm from Fri onward as the system moves east. Overall, temperatures are forecast to be near or even well above average across much of the CONUS into early next week. The largest temperature anomalies are expected across the central U.S. from Fri through the weekend, where high temperatures of 15 to 25 deg F above average are possible. The Eastern Seaboard will initially be near average, but should begin to warm by early next week in the wake of any potential East Coast low pressure system. The West will see a bit of a roller coaster with well above average temps (highs 10-20 deg above average) Thu-Sun, and then cooler temps arriving by Mon, especially in the Pacific Northwest as the upper trough digs into the region. 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