Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Mar 14 2020 Valid 12Z Tue Mar 17 2020 - 12Z Sat Mar 21 2020 ...Heavy rain threat over the Southern Plains and possibly into the Ohio/Tennessee Valleys next week... ...Snow in the Rockies may spread through the North-Central Plains and into the northern Great Lakes/New England later next week... ...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 initially 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 updated forecast started with an operational blend consisting of the 12Z ECMWF/18Z GFS/12Z UKMET/12Z CMC in order from most to least weight. Most guidance differences were fairly modest across the eastern Pacific through most of the lower 48. 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 by Thu. This spread and variability have continued into the 00Z cycle of guidance. Forecast preference by days 6-7 Fri-Sat shifted to an even model/ensemble mean blend incorporating the 12Z and prior 00Z ECMWF runs along with the 18Z GEFS/12Z ECMWF means. Over the past couple days GFS runs have tended to be on the fast side with the upstream energy dropping into the eastern Pacific, with some runs even bringing yet another shortwave toward the Pacific Northwest by next Sat. In the past day the GEFS/ECMWF means have similarly adjusted more north-south with the overall trough to reflect the closed low potential seen in ECMWF/CMC and some GFS runs, with the ECMWF/CMC and GEFS/ECMWF means offering the best cluster in general. Meanwhile the leading upper low should cross over or near Colorado on Thu and then open up beyond the Plains as it comes under increasing influence from southern Canada/northern tier U.S. flow. Thus far ECMWF runs and the GEFS/ECMWF means have provided the most stable depiction of surface low pressure. The 18Z GFS was close to this cluster while the 00Z GFS trends south as it nears the East Coast. There is still a decent potential for future adjustment of this system over the eastern half of the country given the dependence on interaction with northern stream flow. Operational model differences with this flow were the main consideration for including both ECMWF runs during this time frame. ...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 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