Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sun Sep 23 2018 Valid 12Z Wed Sep 26 2018 - 12Z Sun Sep 30 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Expect the amplified pattern forecast to evolve over North America and adjacent oceans during the short range time frame to continue at least into the first half of the medium range, through late this week. Most guidance indicates that mean flow could flatten somewhat at mid-latitudes next weekend though. Over the past 12-24 hours there are noticeable trends suggesting increased potential that the strong northeastern Pacific upper ridge building into Alaska and the Arctic could close off a high center over or near the Alaska mainland. Resulting elongation of cyclonic flow over/offshore British Columbia as well as the approach of undercutting east-central Pacific trough/closed low energy would encourage inland progression and rapid weakening of a leading upper low drifting toward California through Fri. At the same time another strong upper ridge initially covering the western Atlantic and parts of the Gulf of Mexico should gradually transition its emphasis to the Gulf and southern U.S. This pattern evolution should yield a strengthening temperature contrast between very chilly air settling over the northern Plains region and warmth persisting over portions of the South and West. From day 3 Wed into day 5 Fri an operational model consensus continues to provide a good approach (and with minimal change in continuity) to resolve existing detail/timing differences for features of interest--namely a leading front heading into the East/South around midweek, Canadian low pressure whose trailing front should reach the Great Lakes/Plains by early Fri, and upper low approaching California. Then by days 6-7 Sat-Sun forecast confidence in specifics rapidly decreases as individual model/ensemble solutions diverge from each other and earlier runs within the complex evolution that may occur over/near the eastern Pacific. The ultimate timing of the upper low nearing the West Coast as of early Sat will be very sensitive to details of flow just to the north, which will be determined by exactly how the Alaska ridge evolves. Details of upstream Pacific undercutting flow may play a role as well toward the end of the period. With current signals leaning toward closure of an Alaska upper high, at least there is better agreement in principle on ejection of the upper low into the West--in contrast to some earlier GFS/GEFS runs that had left it stranded well offshore through the weekend. Faster 06Z GEFS trend toward the 00Z ECMWF mean reinforces the current majority cluster. Adjusting the forecast toward 40-60 percent ensemble input by next weekend while keeping modest weight of some operational guidance reflects latest trends for the large scale evolution while downplaying uncertain specifics. In spite of the ongoing shifts in some eastern Pacific details aloft, the surface forecast over much of the contiguous U.S. maintains decent consistency. ...Sensible Weather Highlights... The best potential for highest rainfall totals during the period extends from the southern tier states and perhaps into the southern Mid-Atlantic, associated with a leading front that will likely stall before dissipating and then another front reaching the area late in the week. Some convection along the central-northern latitudes of the leading front may be strong to severe, so check SPC outlooks for latest details. The clipper-like system that may track close to the Canadian border for a time will bring an area of generally light-moderate precipitation across extreme northern areas late in the week. Next weekend the large scale pattern evolution will likely promote the establishment of an increasingly sharp frontal boundary from the Great Lakes into central Plains and Northwest. Along and north of this front there is the possibility for enhanced precipitation but with fairly low confidence in exact timing/amounts. Low level upslope flow may aid activity along the eastern slopes of the northern Rockies with some snow possible over northern/western Montana. Farther east there are some signals for a period of locally moderate-heavy rainfall over parts of the upper Mississippi Valley/Midwest/Great Lakes. Expect chilly air to settle over the northern Plains and vicinity with highs dropping to 10-20F below normal by Thu and 15-25 below normal Fri-Sun. On the other hand the warm sector ahead of the initial eastern cold front on Wed will contain very warm min temps in particular with plus 10-20F anomalies. After Wed, morning lows over eastern/southern areas should be a little less extreme but still well above normal. Temperatures over the southern Plains will moderate after a cool day Wed. A majority of the West will likely see readings 5-10F or so above normal through late week, though with some cooling expected next weekend as the eastern Pacific upper low ejects inland and the upper pattern flattens. 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