Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Fri Sep 20 2019 Valid 12Z Mon Sep 23 2019 - 12Z Fri Sep 27 2019 ...Heavy rainfall threats in the Southwest early in the week and Midwest/parts of the Plains during portions of next week... ...Overview... An amplified pattern this weekend will briefly transition to flatter flow during the early/middle part of next week. Then toward next Fri most guidance begins to re-establish an amplified western trough/eastern ridge regime that will likely extend for a while beyond the medium range period. Upper shortwave energy/possible embedded low may interact with East Pacific tropical moisture to bring significant rainfall to parts of the Southwest. Some of this energy/moisture extending northeastward as well as a couple fronts (the latter potentially stalling over the Midwest late in the week) will also provide support for areas of heavy rainfall over portions of the central U.S. ...Guidance Evaluation/Preferences... Overall an operational model blend provided a reasonable starting point early in the period while the 18Z GFS/12Z ECMWF and their respective ensemble means provided a reasonable solution cluster later in the period--with the stronger but uncertain details of the operational runs balancing the agreeable pattern of the means. Regarding individual features, guidance still differs some for the shortwave over the Great Lakes/Midwest as of early Mon. GFS runs and ensemble means have tended to be on the open and/or progressive side of the spread as the feature reaches New England, with the favored model blend leaning more toward the idea of a more separated evolution. New 00Z runs thus far, including the GFS, provide stronger support for an upper low closing off by Tue. On the opposite side of the country there is still a lot of spread and run-to-run variability for the trough/possible closed low dropping through the Great Basin and into the Southwest/southern Rockies, then possibly ejecting northeast into the Plains. Guidance as a whole is a bit faster than in some earlier cycles but the new 00Z GEFS mean has shifted back to a farther west position. The CMC and UKMET have displayed opposite shifts in their 00Z run, the CMC slower and the UKMET faster, with the UKMET/GFS now on the faster side. This leaves the 12Z ECMWF near the middle of the spread. Guidance differences are not as pronounced for troughing that evolves over southern Canada and northern tier U.S. by midweek. By the end of next week there is better than average model/ensemble agreement toward the idea of a deepening upper trough near the West Coast, between strengthening ridges over the west-central Pacific/Alaska and southeastern U.S. Latest operational runs suggest potential for a closed low over/near the Pacific Northwest by Fri. While exact details of the operational models are uncertain 6-7 days out in time, it seems reasonable to include about half weight of their solutions to help strengthen the ensemble means somewhat. ...Weather Highlights/Threats... The primary areas of emphasis for heavy rainfall potential will be over the Southwest early in the week and over parts of the Plains/Midwest. Over the Southwest (and favored terrain over central Arizona in particular) rainfall should be enhanced by the combination of the upper trough/low dropping southeast toward and into the region as well as leading flow of moisture originating from tropical systems Lorena and Mario. Some of the moisture/energy aloft may eventually reach into the central U.S. Beyond those ingredients, an initial southern Plains front should return northeast as a warm front Mon-Wed ahead of a cold front approaching from the northern Plains. The northern part of the cold front will continue eastward but stall over the Midwest/central Plains, leading to a more persistent focus for rainfall during the latter half of the week. The Northwest may see a couple episodes of mostly light precipitation, though pockets of moderate activity could develop depending on the details of the late week upper trough. This trough may begin to support some high elevation snow over the northern Rockies/Pacific Northwest by late in the week. Meanwhile rainfall along the front reaching the East early in the week should be trending somewhat lighter and less broad compared to the short range time frame. Areas of rain could linger for a time over New England depending on specifics of the upper trough/low affecting the region. Expect the Southeast to see consistently well above normal high temperatures with coverage of plus 10F and greater anomalies expanding northward by Thu-Fri as an upper ridge strengthens over the region. Potential for some record highs will also increase in that time frame. On the other hand the West and northern Plains should be on the cool side of normal for highs during the majority of the period. Coolest days should be Mon over parts of the Interior West with the initial trough/upper low over the region and then Thu-Fri over the Northwest as deeper upper troughing digs along the West Coast. Some highs may be 10-20F below normal on Fri. Morning lows will likely be near to above normal over much of the lower 48 during the week. Warmest anomalies for mins should be over parts of the Plains/Mississippi Valley Tue-Fri and in parts of the East by Thu-Fri. 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