Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 AM EST Thu Jan 14 2021 Valid 12Z Sun Jan 17 2021 - 12Z Thu Jan 21 2021 ...Overview and Guidance/Predictability Assessment... A deep cyclone should be exiting New England by the start of the period on Sunday, with upper troughing shifting into the East. Southern stream energy across the South Sunday-Monday may induce a weak low pressure area near Texas, while upper level ridging in the West gets replaced by a shortwave entering the region by Monday. The guidance shows good agreement on this overall pattern through day 4, with only minor timing differences in various features. As the Western U.S. shortwave drops into the Southwest by day 5 (Tuesday) is when things start to get chaotic. Most of the models have been consistent for the past day in showing robust amplification of this shortwave, potentially resulting in a closed low over the Desert Southwest on Tuesday. Thereafter, models begin to diverge on whether this system continues steadily eastward into the Southern U.S. and weakens (like the ECMWF and to some extent the CMC) or lingers a few days over the Southwest (GFS). This appears to be dependent on the strength of ridging out over the Pacific and additional shortwave energy rounding the top of it into the Pacific Northwest. The ensembles tend to favor a slower progression of this system, but maybe not quite as intense as the last couple of runs of the GFS would suggest. Run to run variability on the details of this system are high and given the day 6-7 time frame, weighting towards the ensemble means on this one is preferred. The WPC forecasts were derived from a mostly deterministic blend of the GFS/ECMWF days 3-5, quickly transitioning towards the ensemble means by days 6-7. Did continue to include small contributions from the GFS/CMC late period just for some added definition. The ECMWF was just too fast with the Southwest low/energy and so was removed from the blend. This maintains good continuity with the previous WPC forecast as well. ...Weather/Threats Highlights... Low pressure exiting New England on Sunday will allow for lingering gusty winds across the Northeast, lake effect snow showers around the Great Lakes, and snow showers in the central Appalachians. The next system entering the West should bring light rain or mountain snows to parts of the Pacific Northwest and northern Rockies on Sunday, and the central/southern Rockies on Monday. Precipitation should begin to expand into the Four Corners regions and west-central Gulf Coast early next week associated with the amplified system in the Southwest. Greater moisture transport ahead of this system would increase the potential for widespread rainfall from East Texas into the Lower Mississippi Valley/Tennessee Valley Tuesday into Wednesday. Uncertainty is high in the details of this system, so specific amounts are impossible to pin down at this point, but there is increasing model signal for the potential for moderate to heavy rainfall in this region. Some snowfall is also possible in the mountains of Arizona and New Mexico. Temperatures 10 to 15 degrees above normal are possible Sunday across California/Desert Southwest and the Northern Plains but should trend cooler next week as upper troughing moves through. Elsewhere, the Southeast begins the period slightly below normal with values moving back towards normal or just slightly above normal the rest of the week. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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