Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 PM EDT Sat Mar 14 2020 Valid 12Z Tue Mar 17 2020 - 12Z Sat Mar 21 2020 ...Heavy Rain Threat Southern Plains to Ohio/Tennessee Valleys... ...Heavy Snow Threat Rockies to North-Central Plains and Great Lakes/Interior New England... ...Guidance/Predictability Assessment... Most model and ensemble guidance is stable and similar in depicting a stable mean pattern featuring western U.S./eastern Pacific troughing downstream from a strong ridge over 140-160W longitude. At the same time an upper ridge over the Gulf of Mexico and northwestern Caribbean will likely become more expansive with time. Within the mean trough the leading system will be an upper low Tuesday near California. This low may elongate for a time as it tracks into the West. The strongest energy should cross the Rockies on Thu and then continue northeastward, supporting Central Plains through northeastern U.S. low pressure Thu-Sat. This system will produce a broad area of precipitation across the lower 48 next week--including rain/mountain snow over the Southwest into the central/north-central Rockies, a band of snow (some potentially heavy) from the north-central Plains into the northern Great Lakes/New England, along with heavy rainfall and severe weather potential in the warm sector. Upstream energy dropping south over the eastern Pacific may be in the form of another closed low approaching the California coast toward the end of the period next Sat. This feature would bring another area of moisture into parts of California. From day 3 Tue into early day 5 Thu the WPC forecast started primarily with an operational blend consisting of the compatable 00 UTC GFS/ECMWF. These seem well supported by GEFS/ECMWF ensembles. Most guidance differences were fairly modest across the eastern Pacific through most of the lower 48 during this period. There has been recent trending toward northward elongation of some of the energy rotating around the upper low near California as it drifts near the coast by Wed. Solutions are still varying among each other and from run to run with specifics of Canadian shortwave energy, leading to ongoing timing uncertainty for a cold front forecast to push into the Northern Plains starting Thu. Forecast preference by days 6-7 Fri-Sat shifted weighting 00 UTC GFS/ECMWF mostly to the more consistent and compatable GEFS/ECMWF ensemble means amid growing model forecast spread and run to run variance. In this period a main mid-upper low/trough and organized lead surface system ejects northeastward across the central and northeastern U.S. to be followed by a pronounced surge of cold Canadian air down across the central and eastern U.S. This offers a heavy rain threat from the southern Plains to the OH/TN Valleys along with a heavy snow threat from the Rockies through the n-central Plains/Upper Midwest and Great Lakes/interior New England. ...Weather/Hazard Highlights... Across the West, the upper low close to California at the start of the period early Tue should ultimately bring rain and higher elevation snow across the northern three-fourths of the West. Expect highest totals over favored terrain near the southern California coast and in central Arizona, and then into parts of the Wyoming/Colorado Rockies. Exact axis will depend on the path of the upper low center. Late this week continued progression of the upper dynamics and associated low pressure/frontal system will spread a large area of varied precipitation types to the east of the Rockies. Best snow potential exists from the north-central High Plains into the northern Great Lakes/New England. Some activity could be heavy, though possibly existing in a fairly narrow band which would lead to forecast challenges for specifics beyond a short-range time frame. Warm sector rainfall should be heavy over some areas across the Southern Plains and possibly into parts of the Tennessee/Ohio Valleys. The Storm Prediction Center is also monitoring severe weather threats with this system. Check for latest SPC outlooks/discussions for more information. The combination of this event as well as some areas of moderate to heavy rainfall focused along and north of a wavy front well ahead of the ejecting western system may lead to some flooding issues within the Southern Plains to Ohio/Tennessee Valley areas. The next system reaching the eastern Pacific may bring some rain/mountain snow to parts of California by next Sat. The southwestern states will see below normal highs for most of the period with the greatest coverage of minus 10F or greater anomalies for highs likely to be through Thu. Meanwhile progression of the system emerging from the West will bring an eastward/northward expansion of well above normal temperatures across the eastern half of the country into late week, with some morning lows 20-30F above normal and highs 5-20F above normal. Colder air behind the central Plains through Northeast system will bring highs down to 10-20F below normal over the northern half of the Plains especially on Fri followed by some moderation as the airmass pushes to the south and east into Sat. Rausch/Schichtel 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