Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Sun Jan 26 2020 Valid 12Z Wed Jan 29 2020 - 12Z Sun Feb 02 2020 ...Overview and Guidance/Predictability Assessment... Models/ensembles continue to show consensus that upper-level ridging should build across the eastern Pacific and into the western U.S. through mid to late this week. This will keep an active North Pacific storm track directed toward the northwestern U.S. and western Canada, with a number of shortwaves/frontal systems expected to affect the region through the medium range. Spread among the guidance with these Pacific systems was near or perhaps slightly better than average, and increased gradually through time in a typical fashion. Meanwhile, broad cyclonic flow is forecast to persist across the central and eastern U.S., with shortwave energy spilling over the western ridge and undergoing some potential for amplification farther east. Model consensus is good with respect to a weakening shortwave forecast to cross the lower Mississippi Valley/Southeast on Wed (day 3), and the ECMWF/GFS solutions were comparable. A surface low still appears likely to pass across the northern Gulf of Mexico on Wed before weakening as it moves east into the Atlantic Ocean by Thu. Guidance still suggests that a period of split flow will occur east of the ridge by later in the week, with a quickly amplifying shortwave separating from the northern stream across the Rockies and south central U.S. Wed-Thu. Some degree of interaction/phasing of this system with additional northern stream energy still appears probable by late in the week. Differences in the intensity and speed of the southern stream feature, along with timing and structural differences with northern stream energy continue to introduce a period of compounding uncertainty into the forecast across the eastern third of the CONUS by the Fri-Sun (day 5-7) time period. The 12Z ECMWF was slower and deeper than the overall model/ensemble consensus with the southern stream energy, which resulted in essentially delaying the stream interaction by 24 hours until the next northern stream wave approaches. The ECMWF still showed development of a potential area of low pressure moving from the Gulf of Mexico across the Southeast U.S. and along the Eastern Seaboard, but it was quite a bit slower than the GFS, which commenced the phasing earlier and was thus faster with a deepening surface low moving north across the eastern U.S. The 18Z GFS solution was reasonably well-centered in a cluster of ensemble members from the ECENS, GEFS, and CMCE ensemble systems, while the number of members supporting the deterministic 12Z ECMWF was much less. The 00Z GFS did not make a fundamental change from the 18Z run in terms of the timing of the system, but it did shift the surface low track a bit farther west/inland across the Appalachians as opposed to the 18Z GFS track generally along the Eastern Seaboard. Given overall better consistency shown by the GFS and the fit with overall ensemble member consensus, opted to lean toward the GFS (18Z) idea with this system by the day 5-7 time period. Given the spread and variability in the guidance forecast confidence across the eastern third of the CONUS by late this week is below average. Farther west, some consensus was evident that another vigorous (but perhaps positively-tilted) shortwave and associated frontal system should approach the Pacific Northwest by next Sun. The WPC forecast was initially based on a heavily deterministic blend of the 18Z GFS/12Z ECMWF during days 3-4, with the forecast during days 5-7 based on a blend of the GFS along with the ECENS/GEFS ensemble means. ...Weather/Hazard Highlights... The Pacific Northwest will see a prolonged period of active weather throughout the extended forecast period as a series of 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 northern extent of the upper ridge builds in. Farther east, the low pressure system crossing the northern Gulf of Mexico on Wed should produce areas of showers and possibly thunderstorms across the Gulf Coast states. As the next area of low pressure develops in the Gulf of Mexico by later in the week, a zone of more significant lift and overrunning should develop, resulting in the development of a larger shield of precipitation across the Gulf Coast Thu-Thu night, spreading into the Southeast on Fri and the Mid-Atlantic/Northeast Fri night-Sun. Given the uncertainty by late in the week with the low-track and timing, precipitation type is also highly uncertain. Nonetheless, sufficient cold air may be in place north of the system to support some degree of winter weather threat. Continued to indicated low to medium probabilities on Winter Weather Outlooks across portions of the Appalachians/Mid-Atlantic/Northeast from Fri into the weekend. Temperatures are forecast to be near or above average across most of the CONUS through the medium range, with the upper-level flow not conducive to southward transport of cold air from higher latitudes. The most anomalous temperatures are expected from the northern Rockies to portions of the Northern/Central Plains and Upper Midwest, where high temperatures are forecast to range from 5 to as much as 20 deg F above average from midweek into next weekend. The western U.S. should also warm up by later in the week as the upper-level ridge expands, with highs expected to reach 10 to 20 deg above average across much of the Great Basin by Thu-Sun. 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