Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Feb 11 2021 Valid 12Z Sun Feb 14 2021 - 12Z Thu Feb 18 2021 ...Widespread record cold temperatures over the Plains and Mississippi Valley into early next week followed by only gradual moderation... ...Stormy pattern will bring multiple wintry precipitation events from coast to coast... ...Overview and Guidance Evaluation... Models and ensembles maintain a very active synoptic pattern through the medium range period, with a series of systems digging into and ejecting from a long-term mean trough aloft anchored over the contiguous U.S. These systems will bring episodes of rain/mountain snow from the Northwest states southeastward (perhaps low elevation snow early in the period over the Northwest) and then a general axis of wintry weather from the southern High Plains through the Northeast. The southern part of the winter weather threat area will include sleet/freezing rain, potentially significant over some areas. Expect rain farther southeastward. In addition to the active pattern, a broad area across the Plains and Mississippi Valley should see record cold max/min temperatures Sunday-Tuesday. Some locations over the Plains could see one or more days with daytime highs of 30-45F or so below normal and morning lows only slightly less extreme. This very cold air will enhance the winter weather threats. Latest guidance generally clusters well for individual systems through the period. A blend of the 00Z ECMWF/UKMET and 06Z GFS represented the prevailing majority for the first half of the forecast and then modest inclusion of the 06Z GEFS/00Z ECMWF means accounted for typical late-period detail uncertainty. The CMC does not compare well to this majority in multiple respects. The largest-scale issue is that it is slower than other guidance with the southern Canada upper low that should begin opening up as it tracks just north of Lake Superior on Sunday. This difference leads to the CMC being much more suppressed than consensus with the system that reaches the East by day 5 Tuesday. Otherwise there is still some difference in storm structure at that time, with ECMWF/UKMET runs better developed than the GFS with a wave that tracks the west of the Appalachians before emphasis shifts to the wave farther east. Thus far ECMWF runs have been more consistent than GFS runs. Even earlier in the period the 12Z CMC leans to the northern side of the spread for the weak wave off the East Coast by the latter half of the weekend. Meanwhile the 12Z CMC looks fast compared to other guidance for the shortwave reaching the Northwest early next week, though the 00Z run did not appear as extreme. By the latter half of the period there is good agreement in principle for the western U.S. shortwave amplifying into the Plains/northern Mexico while low pressure develops to its southeast. The GFS/GEFS are characteristically a bit faster than the CMC and ECMWF/ECMWF mean. However the differences are minor compared to typical spread/errors for days 6-7 forecasts. ...Weather/Threats Highlights... Looking at individual systems in sequence, the system off the East Coast to start Sunday may bring some lingering winter weather to parts of the Northeast while a trailing wavy front may focus locally moderate/heavy rainfall over the Southeast. The next system should track from the southern Rockies into the Gulf and then into the eastern U.S. Sunday-Tuesday, accompanied by a broad area of precipitation. The greatest winter weather threats will extend across the southern Rockies, the southern half of the Plains, and continuing northeastward into the Mid-Atlantic/Northeast. Snow will fall sufficiently far into the cold air but there will be potential for areas of significant sleet/freezing rain to the south of the snow area from southern parts of the Plains northeastward. Farther south/east rain may be locally moderate to heavy. The following system will likely bring a couple waves of precipitation into the Northwest and northern California during Sunday-Monday, including the possibility of snow at fairly low elevations over some areas in the Pacific Northwest. The moisture will continue southeastward through the Great Basin and Rockies, followed by increasing coverage and intensity of precipitation over the Southern Plains/Lower Mississippi Valley into much of the East during Wednesday-Thursday. As upper ridging from the Atlantic builds toward southern Florida and the upper trough heads into the Plains/northern Mexico, combined with a more favorable surface evolution, the system may feature a stronger flow of moisture from the Gulf than the preceding system early in the week. Therefore the heavy precipitation potential will require monitoring over the coming days. As with the previous system, expect a broad area of winter weather impacts from the Southern Plains northeastward. Yet another system should bring rain/mountain snow to the Northwest after midweek. There should be potential for many daily records to be broken for both min and cold high readings over portions of the Plains and Mississippi Valley from Sunday into Tuesday. Within this area there may be one or more days with temperatures of at least 30-45F below normal. This event may not be extreme enough for all-time or monthly records, but could lead to some instances of the "coldest so late in the season" distinction. At least for parts of the Southern Plains (represented by Oklahoma City/Dallas-Ft Worth) a majority of the coldest events have been from around mid-December to mid-February. Although the central U.S. will see moderating temperatures by mid-late week, this may involve anomalies recovering to "only" 20-30F below normal. Some of this cold air will extend in modified fashion into the Ohio Valley and vicinity, and at times farther eastward. Temperatures in the West will be near to moderately below normal with some variability associated with system progression. The extreme Southeast/Florida will be the one area of persistent warmth, with lows in particular expected to be well above normal most days. Rausch 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