Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Sat Jan 25 2020 Valid 12Z Tue Jan 28 2020 - 12Z Sat Feb 01 2020 ...Overview and Guidance/Predictability Assessment... Models and ensembles show relatively good consensus that upper-level ridging should build across the eastern Pacific toward the western U.S. through the middle to end of next 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 North Pacific systems was average, and increased gradually through time as is typical. Meanwhile, broad cyclonic flow is forecast to persist across the central and eastern U.S., with shortwave energy spilling over western ridge and undergoing some potential for amplification farther east. Model consensus has improved a bit with a shortwave crossing the south central and southeastern U.S. Tue-Wed (days 3-4). This is before the broader flow amplification takes place, so this shortwave seems likely to gradually weaken as it passes through a region of relatively zonal flow overall. A surface low initially across Texas on Tue appears likely to pass across the northern Gulf of Mexico on Wed, then should quickly weaken as it moves east into the Atlantic Ocean by Thu. Models also suggest the potential for splitting flow east of the ridge by late next week, with a a shortwave separating from the northern stream across the south central U.S. Wed-Thu, and potentially phasing or interacting with the next shortwave which looks to amplify across the north central U.S. by Fri-Sat. This series of events by the middle to end of next week essentially introduces some degree of compounding uncertainty into the forecast, so forecast confidence by late next week is well below average, especially across the eastern third of the CONUS. Models/ensemble show some consensus that an area of low pressure should develop across the western Gulf of Mexico by Thu night-Fri, which should then move northeastward. That is where solutions diverge widely, however, given the aforementioned uncertainty surrounding the evolution aloft. By Sat (day 7), the range of solutions for the eventual low track includes everything from a western track through the Tennessee Valley and west of the Appalachians, to a track along/off the Eastern Seaboard (or even out to sea). Based on the current spread of models/ensembles and a heavily ensemble-based blend, the day 6-7 surface progs indicate low pressure from the Gulf of Mexico crossing the Southeast and moving along the Southeast U.S. coast through 12Z next Sat. The WPC forecast was initially based on a blend of the 18Z GFS/12Z ECMWF during days 3-5 (Tue-Thu). More weight was placed on the GFS relative to the ECMWF. The GFS low track across the northern Gulf of Mexico Tue-Wed with the leading shortwave was preferred over the more suppressed ECMWF track, based on an evaluation of clustering among ensemble members - elsewhere the solutions were comparable during that time frame. By days 6-7 (Fri-Sat), given the quickly increasing forecast uncertainty, forecast weighting was quickly shifted toward the ECENS/NAEFS ensemble means. ...Weather/Hazard Highlights... The Pacific Northwest will see a prolonged period of active weather throughout the medium range as a series of frontal systems impact the region, spaced approximately 24-36 hours apart. Each of these should produce potentially heavy rain/snow dependent on elevation, with snow levels slowly rising a bit by late next week as upper-level heights rise in response to the building ridge axis. These systems are also expected to spread potentially heavy snow inland across the northern Rockies Tue-Tue night and again by Thu-Thu night. Farther east, the low pressure system crossing the southern tier on Tue should spread scattered showers and possibly thunderstorm to portions of the Southern Plains and western Gulf Coast. As the system moves into the Gulf of Mexico on Wed, models suggest that the deepest moisture should remain south of the low track, but scattered showers may still be possible across the Gulf Coast states. The wave of low pressure expected to develop across the Gulf of Mexico by late in the week should have a zone of much more significant lift and overrunning associated with it, and should result in potentially larger area of precipitation spreading inland from the Gulf Coast to the Southeast/Mid-Atlantic and Appalachians. Given the high degree of uncertainty with the low track, precipitation-type is very uncertain at this time. Nonetheless, introduced an area of low probabilities across portions of the southern/central Appalachians and Mid-Atlantic on the day 7 winter weather outlook. In general, temperatures are forecast to be near or above average across most of the CONUS through the medium range, with the upper flow not conducive to southward transport of cold air from higher latitudes into the U.S. The most anomalous temperatures are expected from the northern Rockies to portions of the Northern Plains/Upper Midwest, where highs on Tue are expected to be 5 to 15 deg F above average through next week. Highs may approach 20 deg above average across the Northern Plains/Upper Midwest on Fri. Low temperatures may also be 15 to 25 deg above average across these areas for multiple days. The Western U.S. should also warm up by later in the week as the upper ridge expands, with highs expected to reach 10 to 20 deg above average across much of the Great Basin by Thu-Sat. 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