Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 PM 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 lift this front more quickly northward as a warm front by early next week, rather than keeping it stalled along the Gulf Coast. Meanwhile a shortwave (the sheared remnants of a southwestern U.S. upper low) is 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. This will be driven by extreme upper ridging over the Gulf of Alaska with 500mb heights well above average values and even likely above any observed values for this time of year over the past 40yrs or so. Model consensus surrounding the upper low 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 to slower end of the spread, and moderate speed on the progression inland. The 00Z/06Z GFSs remained slightly faster than the 00Z ECMWF to break the trough east next week but the 12Z run (available after the forecast) was closer to the ECMWF-led consensus. Given the improved consensus overall and variability in the deterministic guidance, opted to lean heavily on a deterministic blend of the 00Z/06Z guidance for Sat-Mon but then toward an ensemble mean/continuity blend by next week Tue-Wed with uncertainty in timing/shape of the upper low in the West and any sfc wave development in the East atop the strong subtropical ridge. ...Weather/Hazard Highlights... The frontal boundary expected to linger from the Southern Plains to the Southeast is expected to focus showers and some thunderstorms, with potentially heavy rainfall, across multiple days. Several inches of rain are possible from portions of eastern Oklahoma eastward to the Tennessee Valley though specifics will be refined in the short range. Potential heavy rains across several days could lead to flooding problems for some locations especially where training occurs. 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 eastward into North Dakota and perhaps northwestern Minnesota Sat-Sun. The GFS/GEFS remained more bullish than the ECMWF and other guidance. 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 one to several feet of 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. This will modify somewhat through California but still some record cold will be possible toward the Colorado River Valley. Cooler air should also filter into much of the East Coast over the weekend through early next week as a strong polar high pressure area behind the cold front slips across New England which will maintain an easterly component to the wind. 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 well into the upper 80s. Fracasso/Ryan Hazards: - Heavy snow for the southern Cascades, Sat, Mar 14. - Heavy snow across portions of the northern Plains, Sat-Sun, Mar 14-Mar 15. - Heavy snow from across the higher elevations of northern California down along the Sierra Nevada, and across northwestern Wyoming, Sat-Mon, Mar 14-Mar 16. - Heavy precipitation for portions of the higher elevations from central Nevada to southern Idaho, Sat-Sun, Mar 14-Mar 15. - Heavy precipitation for portions of the higher elevations of southern California, Arizona, Utah, and Colorado, Mon-Wed, Mar 16-Mar 18. - Heavy rain from the southern Plains eastward into the southern Appalachians as well as parts of central California, Sat-Sun, Mar 14-Mar 15. - Heavy rain across portions of extreme southern California, Mon-Tue, Mar 16-Mar 17. - Heavy rain from the central and southern Plains eastward into the Ohio Valley, Mon-Wed, Mar 16-Mar 18. - Much below normal temperatures from interior Pacific Northwest eastward across the northern Rockies into the northern Plains, Sat-Wed, Mar 14-Mar 18. 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