Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Wed Mar 11 2020 Valid 12Z Sat Mar 14 2020 - 12Z Wed Mar 18 2020 ...Heavy rain threat over the Southern Plains to lower MS and TN Valleys later this week and weekend... ...Record cold likely for portions of the Northwest this weekend... ...Guidance/Predictability Assessment... A split flow pattern is expected across the CONUS during the medium range with a progressive northern stream and less progressive southern stream. Model consensus through the medium range remains relatively good at large scales, with expected increasing differences through time at smaller scales. A relatively strong low pressure system is forecast to cross eastern Canada on Sat, with the trailing frontal boundary off the Eastern Seaboard, and back across the southern U.S. where it is expected to linger for several days. One significant model trend over the past few runs has been to move this front more quickly north as a warm front by early next week, rather than keeping it stalled along the Gulf Coast as shown in previous runs. Meanwhile a shortwave, the sheared remnants of a southwestern U.S. upper low, are forecast to move eastward across the Southern/Central Plains on Sat and the Mid-Atlantic on Sun. The shortwave is expected to produce a relatively weak wave of low pressure that rides along the aforementioned frontal boundary lingering from the Southern Plains to the Southeast Sat-Sun. Farther west, a deep upper-level low is forecast to slowly move southward along the West Coast over the weekend and into early next week. Model consensus surrounding this feature has improved over the past 24 hours, with the GFS and ECMWF, as well as their respective ensemble means, quite a bit closer on timing and evolution of the feature. The apparent convergence in model/ensemble solutions has been toward the middle of the spread, and moderate speed on the progression inland. The GFS remains slightly faster than the ECMWF to break the trough east on Mon-Tue, but the ECMWF eventually catches up by Wed. Given the improved consensus overall and variability in the deterministic guidance, opted to lean heavily toward ensemble means by early next week. The WPC forecast was initially based heavily on a blend of the 12Z ECMWF/18Z GFS during days 3-4 (Sat-Sun). These solutions were comparable for all systems during that time range, and a blend appeared to reasonably represent the overall consensus. From day 5 (Mon) onward ensemble means (ECENS and GEFS) were given heavier weight in the forecast, with 70 percent ensemble means by days 6-7 (Tue-Wed). ...Weather/Hazard Highlights... The frontal boundary expected to linger from the Southern Plains to the Southeast is expected to focus showers and thunderstorms, with potentially heavy rainfall, across multiple days. Several inches of rain are possible from portions of eastern Oklahoma eastward to the Tennessee Valley. Potential heavy rains across several days could lead to flooding problems for some locations. Moisture lifting across a strong polar front in the western U.S. ahead of the incoming trough is expected to result in the potential for snowfall across portions of the Northern Plains, from Montana east into North Dakota and perhaps northwestern Minnesota Sat-Sun. The upper trough is also expected to produce widespread rain and mountain snow across much of the western U.S., with heavy snows possible across the Sierra over the weekend. A multi-day potential for snowfall is expected across much of the interior West from the Great Basin to the Rockies as the upper trough traverses the region. Strong arctic high pressure nosing down east of the Canadian Rockies will bring cold conditions to portions of the northern Rockies and northern High Plains, extending west into the Pacific Northwest through the weekend. High temperatures will range from 20 to as much as 40 deg below average for some areas, and many areas remaining well below freezing through the weekend. Both record low and record low maximum temperatures may be tied/broken especially over Washington state, despite the much larger temperatures anomalies over Montana. Cooler air should filter into much of the East Coast over the weekend as a strong polar high pressure area behind the cold front slips across New England. The Gulf Coast region, remaining south of the front at least through the weekend, should see above average temperatures and even possibly a few record highs across Florida. 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