Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Sat Feb 13 2021 Valid 12Z Tue Feb 16 2021 - 12Z Sat Feb 20 2021 ...Widespread record cold temperatures over the Plains and Mississippi Valley through Tuesday followed by only gradual moderation... ...Stormy pattern with wintry precipitation events from coast to coast and late week heavy rainfall potential across parts of the East... ...Overview and Guidance Evaluation... By the end of next week and weekend the models and ensembles are still advertising a flattening trend for mean flow aloft across the lower 48. This evolution would bring a drier period and more moderate temperatures to areas east of the Rockies by the end of the forecast, while primarily focusing rain/mountain snow from the northern half of the West Coast into the Rockies. In the meantime the prevailing pattern with an amplified mean trough over the central U.S. will feature one ejecting system lifting through the East on Tuesday, followed by upstream western energy digging into the Plains/northern Mexico by Thursday and then progressing through the eastern half of the country. The associated low pressure/frontal system initially pushing through the West will emerge from the Gulf and continue northeastward after midweek. With widespread record cold air in place over the Plains/Mississippi Valley as of Tuesday and only gradual moderation expected thereafter, a widespread area of wintry weather is likely to extend from the Southern Plains through the Northeast. Guidance is still in the process of resolving details for eastern U.S. low pressure at the start of the period on Tuesday. Recent trends have favored slower departure of a compact upper low over southern Canada, diminishing the potential for the stronger/westward eastern U.S. surface depiction seen in some earlier ECMWF runs. The 00Z ECMWF still has some vestige of that idea. On the other hand the GFS is on the faster side of the envelope. The combination of trends and spread favor an average of latest available models from the 00Z/06Z runs. Meanwhile the models/means continue to be split over remaining forecast details. The 00Z ECMWF/ECMWF mean continue to be faster and stronger with leading Pacific shortwave energy that comes into the West during the latter half of the week, ultimately flattening western-central U.S. flow late in the period. Other guidance suggests some degree of mean ridging ahead of a trailing consensus shortwave expected to reach the West by early day 7 Saturday. These differences result in the ECMWF/ECMWF mean being faster than other solutions for the late week eastern system, with associated differences in surface low track leading to meaningful spread for areas of heaviest rainfall and most significant winter weather impacts. Persistence of the guidance spread over the past day as well as fairly modest predictability for these details from the large scale mean flow perspective recommend maintaining an intermediate solution within the current envelope. New 12Z ECMWF trends seem to support this approach. The day 5 model consensus is good for the front reaching the Pacific Northwest on Thursday and then the model/mean blend begins to tone down the less confident aspects of guidance as the front heads inland and the next system arrives into the region by Saturday. ...Weather/Threats Highlights... The period starts with a surface low deepening as it exits the Mid-Atlantic/Northeast on Tuesday. The main threat for wintry weather by this point will be across much of the Northeast, while the heaviest rain farther south should be off the coast. Some moderate rain should linger across Florida along the trailing boundary on Tuesday. The next system moving through the West on Tuesday will spread rain and mountain snow from the Pacific Northwest/northern California southeastward through the Great Basin and the Rockies into midweek, followed by increasing coverage and intensity of precipitation over the Southern Plains/Lower Mississippi Valley into the East from Wednesday through Friday. The models continue to show a signal for heavy precipitation from the lower Mississippi Valley into much of the East. Moderate to heavy rainfall is likely from the Gulf Coast through Tennessee Valley/southern Mid-Atlantic, with a broad area of winter weather impacts on the northwest/northern side of the precipitation shield from the ArkLaTex region northeastward into the Northeast. Also to note, the Storm Prediction Center continues to mention severe weather potential within the warm sector of this system (and the preceding system), with details to be better resolved over the coming days. 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. In terms of temperatures, still anticipate a widespread area of record lows/cold highs over the southern half to two-thirds of the Plains and lower/mid Mississippi Valley through Tuesday, with temperatures generally 30-45 degrees below normal. Although the central U.S. will see moderating temperatures by mid-late week, this may involve anomalies recovering to "only" 20-35F below normal by Thursday before finally leaving just some pockets of 10-20F below normal readings by Saturday. Some of this cold air will extend in modified fashion farther eastward but likely remain west of the Appalachians. The Plains moderating trend may be sufficient to yield modestly above normal highs over parts of the Dakotas and vicinity by next Saturday. Temperatures in the West will be near to moderately below normal through midweek followed by a warming trend, especially over the Southwest where highs may reach 5-10F or so above normal by Friday-Saturday. Clouds/precipitation over the Northwest late in the week/weekend should raise morning lows a bit but keep highs slightly below normal. The extreme Southeast/Florida will be the one area of persistent warmth through early Friday, with morning lows in particular expected to be well above normal. However below normal temperatures should spread across the Sunshine State by later Friday/Saturday after passage of the front anchored by the system affecting much of the East. Rausch/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