Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Tue Dec 24 2019 Valid 12Z Fri Dec 27 2019 - 12Z Tue Dec 31 2019 ...Overview and Guidance/Predictability Assessment... A mid/upper-level shortwave and the associated frontal system are forecast to cross the Northeast on Fri (day 3). Model consensus was good with this system and no major differences were noted. The most significant forecast issue during the medium range continues to be the progression/evolution of an upper-level low initially across the northern Baja Peninsula early Fri, which is then forecast to move east across the Southwest and toward the central U.S. by Sat. After several days of strikingly different solutions among the guidance with respect to the forward progression of the system, and resultant differences in interaction/phasing with northern stream energy across the central U.S., some trends have finally emerged that may shed some light on the eventual outcome. The ECMWF along with a number of ECENS members have increased the forward speed of the system, although not to the point of the GFS yet. The ECMWF (along with the UKMET/CMC to some degree) are still slow enough with their progression that the system misses its chance for any significant interaction with the northern stream wave, and continues to progress more slowly eastward across the lower Mississippi Valley/Southeast as a cutoff upper low. The GFS, on the other hand, has remained steadfast in its idea of the southern stream wave reaching the Southern Plains by Fri night, where it begins an interaction with a northern stream shortwave, resulting in the development of a relatively deep low pressure system across the Plains and Midwest Sat-Sun (days 4-5). This system bears quite a few similarities to a similar system several weeks ago for which the GFS significantly outperformed much of the other guidance in the medium range, the origin of which (for both) seems to be a shortwave initially over the data sparse North Pacific, south of the Aleutians. Given these factors, opted to lean heavily toward the GFS (18Z run) during days 3-5, along with a gradually increasing component of the ECENS/GEFS ensemble means as well (both of which provided a significant degree of support to an idea similar to the GFS). Following this idea, the phased system should cross the Northeast late Sun/Mon, with the cold front moving off the Eastern Seaboard by Monday morning. Models continue to show an additional trough/upper low digging southward along the West Coast Sun-Mon, with most deterministic solutions showing another cutoff low near/across southern California by Mon. Deterministic solutions are quite similar with the timing and evolution of this feature, although a look at ensemble members shows that spread is perhaps a bit higher than would be indicated by deterministic guidance alone. Given some general consensus among the guidance, a gradual trend toward heavier use of ECENS/GEFS ensemble means continued through days 6-7 (Mon-Tue). ...Weather Highlights/Threats... The upper low crossing the Southeast on Fri is expected to mountain snow to portions of the Southwest and southern Rockies. Ahead of the upper low, an overrunning setup across the Southern/Central Plains is expected to result in development of an area of precipitation, which should expand northward and eastward across the Plains and eventually the Midwest/Mississippi Valley through time as the eventual low pressure system spins up. Locally heavy rain will be possible for some areas, while in the northern portion of this precipitation shield, sufficient cold air may exist for precipitation to fall as a wintry mix/and or snow. The winter weather potential appears to be focused in a band from the southern Rockies to the central High Plains on Fri, extending into the Midwest/Upper Great Lakes by Sat. Given at least somewhat improved confidence in this scenario, boosted winter weather outlook probabilities across the Midwest/Great Lakes Sat/Sat night. Please see the medium range winter weather outlook products for more details on the winter weather threat. As the system continues east, winter weather is possible across portions of the Northeast Sun/Sun night in the cool air mass north of the warm front, with rains expected farther south along the cold front across the Appalachians, Mid-Atlantic, and Southeast. Temperatures are forecast to be well above average across the central and eastern U.S. Fri-Sun ahead of the eventual low pressure system. High temperatures of 10 to 20 deg F above average are forecast across a large area. The cold front forecast to arrive over the weekend will bring an end to the warm temperatures from west to east, with temperatures returning to near seasonal norms. Meanwhile, given persistent lower heights across the interior western U.S., highs are forecast to be 5 to 10 deg below average from the Great Basin to much of the Southwest Fri-Sun, with temperatures moderating somewhat by early next week. 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