Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 AM EST Mon Feb 15 2021 Valid 12Z Thu Feb 18 2021 - 12Z Mon Feb 22 2021 ...Much below temperatures across the Central U.S. will continue this week but should finally moderate back towards normal by next weekend... ...Moderate to heavy precipitation expected across much of the East Thursday into Friday... ...Overview and Guidance Evaluation... An amplified pattern across the CONUS the first half of the medium range period will be replaced by a welcomed change to a flatter more zonal upper level pattern by next weekend. This pattern shift would allow for a moderation in temperatures across much of the lower 48 and less active storm systems. In the meantime though, a prevailing amplified mean trough over the Central U.S. on Thursday will shift into the Eastern U.S. by Friday. The associated low pressure/frontal system will emerge from the Gulf and race into the East. A widespread area of moderate to heavy precipitation (snow to the north, rain to the south) is likely from the central Gulf Coast into the Northeast Thursday into Friday. A series of fronts should also keep the Northwest active through much of the period. The troughing entering the East by Friday shows very good agreement in the models with only minor timing differences (UKMET slower than the consensus) so a blend of the deterministic GFS/ECMWF/CMC sufficed. The next system entering the Pacific Northwest this weekend exhibits a bit more variability in the guidance, especially as the shortwave shifts into the central U.S. early next week. The ECMWF is on the faster side of the spread, with the CMC a bit more amplified/slower. The latest run of the GFS lies somewhere in the middle, and has support from the generally agreeable (albeit a bit washed out) ensemble means. Therefore, WPC prefers a majority blend of the ensemble means beyond day 5, with smaller contributions from the GFS just to maintain some degree of system definition. This also maintains good continuity with the previous shift. ...Weather/Threats Highlights... Heavy precipitation will accompany the East system as it lifts from the Gulf into the Northeast late this week. Wintry precipitation on the northern side of the precipitation shield will be ongoing as the medium range period begins Thursday, with the greatest chance for significant snowfall Thursday into Friday from the Ohio Valley into the interior Northeast. To the south, moderate to heavy rainfall is likely across portions of the Southeast and southern Mid-Atlantic, with some ice/sleet possible in between the rain and snow areas. Of course, the rain/snow/wintry mix transition zones will continue to be refined in the coming days. Also to note, SPC continues to mention the severe weather potential, particularly for portions of the central Gulf Coast where moisture and instability parameters are expected to be most favorable. Behind this system, another couple fronts reaching the Northwest between Thursday and Saturday should bring the potential for additional focused rainfall along coastal portions of Washington/Oregon with heavy mountain snows in the Washington/Oregon Cascades and northern Rockies. Meaningful rain/high elevation snow may extend into parts of northern California as well. For temperatures, much below normal temperatures are expected to persist through this week from the southern Plains and northeastward, but finally should moderate back towards normal by early next week as a less amplified pattern settles in across the contiguous U.S.. Out West, temperatures should also trend warmer, especially over the Southwest where highs may reach 5-10F or so above normal Sunday into Monday, with above normal temperatures also possible across parts of the Northern Plains. Warmer than average temperatures across Florida/the Southeast will continue into Thursday, but below normal temperatures should eventually spread into the Sunshine State by Friday/Saturday after passage of the front anchored by the system affecting much of the East. 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