Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Sep 22 2018 Valid 12Z Tue Sep 25 2018 - 12Z Sat Sep 29 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Latest guidance continues to agree well with the idea of an amplified pattern consisting of a Canada/continental U.S. mean trough between strong ridges over the western Atlantic-Gulf of Mexico and northeastern Pacific (eventually extending northward into Alaska and the Arctic). This pattern will likely support a series of fronts that will drop southward/southeastward from Canada and promote multiple days of below normal temperatures from the eastern slopes of the northern Rockies to the Upper Midwest. During days 3-5 Tue-Thu a blend of the 00Z GFS/ECMWF/UKMET/CMC represented consensus well for features of interest, resolving detail differences that were within typical error for the time frame involved. One front extending from eastern Canada low pressure will progress from the Great Lakes/central Plains/south-central West as of early Tue toward the East Coast while low pressure tracking out of western Canada and possibly reaching near the northern Minnesota border by Thu will anchor the next front which should extend into the central Plains/Rockies and back into the Northwest at that time. Farther east a weak western Atlantic feature at the surface and aloft may track close to the North Carolina coast before recurving to the northeast. GFS preference leaned toward the 00Z run versus the 06Z version as the latter was deeper than most other solutions for the system near the Canadian border as of Thu and on that same day more amplified than consensus for shortwave energy crossing the Gulf of Alaska. Days 6-7 Fri-Sat transitioned to a model/mean blend incorporating somewhat more 00Z ECMWF/ECMWF mean relative to the 00Z GFS/06Z GEFS mean, as solutions diverged for specifics of the Pacific/Alaska pattern with eventual effects on flow across Canada and the lower 48. In particular the differences affect how much shortwave elongation occurs over southwestern Canada and northwestern U.S. Over the past day operational GFS/ECMWF runs have essentially traded places, the GFS trending more amplified and the ECMWF trending flatter. GEFS trends reflect the operational runs in less dramatic fashion. However in contrast to the operational ECMWF, its mean has been showing a gradual trend toward slightly lower heights over this region over the past couple days. Teleconnections relative to strong positive height anomalies over the Gulf of Alaska in the D+5 through D+8 multi-day means also show potential for some modest elongated upper troughing over the Northwest. The aforementioned blend provided the best account for evidence supporting a little sharpening of shortwave energy near the Northwest but not to the extent of latest GFS solutions. It also yielded an intermediate position for the upper low off the California coast. Farther east expect the central U.S. front on Thu to continue into the western Atlantic/southeast by Sat while the next front may drop into the northern states. Southward progression of these fronts will be sensitive to details of flow aloft so there will likely be some run to run changes for this aspect of the forecast. ...Sensible Weather Highlights... The cold front progressing from the central into eastern states Tue-Wed will likely be accompanied by areas of heavy rainfall given access to moist low level flow from the Gulf. The weak feature near the central East Coast around midweek could ultimately provide a source of additional moisture for Mid-Atlantic/Northeast rainfall. The most common signal for highest rainfall totals extends from the mid-lower Mississippi Valley/Tennessee Valley into Ohio Valley with some localized potential also over the Northeast. Some convection may be strong to severe. Consult latest SPC outlooks for details on severe threats. Stalling of the front over the South will keep a lingering potential for rain over that region, with arrival of the next front (which will be drier than its predecessor upstream) also promoting areas of enhanced rainfall. Meanwhile low level upslope flow may generate some precipitation over/near the northern Rockies, especially toward the end of the period. Expect cool temperatures over the northern Plains to trend lower over the course of the period, with highs 5-15F below normal into midweek and then 10-20F below normal later in the week, possibly dropping to 20-30F below normal by Sat. On the other hand the East and West/Southwest will likely see above normal temperatures with some day-to-day variability. Greatest anomalies should be in the plus 10-20F range for morning lows from the Midwest/Great Lakes into Mid-Atlantic/Northeast early in the period but some locations over the southeast third of the U.S. may continue to see some lows at least 10F above normal Thu-Sat. Plus 5-10F anomalies should be more common over the West with only localized warmer readings. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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