Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Sep 23 2019 Valid 12Z Thu Sep 26 2019 - 12Z Mon Sep 30 2019 ...Areas of heavy rain possible over parts of the central U.S.... ...Areas of rain/mountain snow and much below normal temperatures over the West... ...Expanding area of record highs in the Southeast later in the week through the weekend... ...Overview... Aside from some details there is still good agreement in principle that a very amplified western trough/eastern ridge upper pattern will become established by the weekend and extend into the first part of next week. Much below normal temperatures and areas of precipitation will accompany the western trough with the northern Rockies likely to see the highest totals of rain and higher elevation snow. Farther east a couple waves emerging from the West--along with energy from the upper low initially over the extreme Southwest--will increase potential for heavy rainfall from parts of the Midwest into the Upper Mississippi Valley/Upper Great Lakes. The eastern ridge aloft will support a broad area of much above normal temperatures over the East and into the central/southern Plains with record highs most likely over and near the Southeast. Meanwhile Tropical Storm Karen will require monitoring as the eastern U.S./western Atlantic ridge at the surface and aloft may direct the system toward the Bahamas late in the period. ...Guidance Evaluation/Preferences... Overall the full array of guidance seems to have regressed a bit compared to the past couple days in terms of some important details within the western trough. A couple of the more pronounced examples: The 12Z ECMWF abruptly changed to a scenario in which the core of upper energy reaching the Northwest by day 5 Sat quickly ejects ahead of upstream energy--which so far has little support from the ensemble means. The new 00Z GFS/GEFS mean and to some degree 18Z GFS take longer than established consensus to amplify the trough, due to differences in how flow separates to the south of the eastern Aleutians mid-late week. This issue affects downstream flow across the northern tier U.S. as well. Thus the favored early-period model blend transitioned to components of the 12Z GFS/CMC and 12Z ECMWF mean/18Z GEFS mean by the latter half of the forecast. The blend kept the forecast fairly close to continuity though reflecting a general trend of guidance away from some of the most extreme negative height anomalies previously seen over or near California. Ahead of the main trough, there is decent agreement that a leading shortwave will support low pressure that tracks across the northern Plains and northeast into Canada Thu onward. Another wave appears likely to reach the northern Plains and vicinity by next Sun-Mon with the ensemble means fairly similar to each other and CMC/GFS runs. For the initial Southwest upper low the full spread of individual ensemble members remains significantly greater than usual for being only 3-4 days out in time. The 12Z models were better clustered and similar to continuity that had been best represented by recent ECMWF runs. Aforementioned issues with northern stream flow in the latest GFS lead to slower ejection in that run. Some guidance still shows the potential for the eastern ridge to show a 594dm height contour at 500mb for a time during the weekend/early next week. Connected to question marks with specifics farther west, the 12Z ECMWF was a bit east of the guidance majority with the upper high center. Eastern U.S./western Atlantic ridging aloft and high pressure at the surface may influence Tropical Storm Karen as it moves north of 20N. The most likely scenario has a westward track by late in the period but flow complexities before that time lead to fairly high uncertainty in the forecast. Please consult the NHC forecast for the latest on Karen. ...Weather Highlights/Threats... Highest precipitation totals over the West should be over the northern Rockies and possibly extending east into the northern High Plains and south into parts of the Great Basin. Best potential for significant snow will be over higher elevations of western Montana, while snow levels could trend sufficiently low to reach some valley locations. A period of low level upslope flow may enhance precipitation as well. Other areas over the West should see lesser amounts and with greater sensitivity to specifics of the upper trough. Wind will increase over much of the interior West as well, centered over the Great Basin eastward into the central Rockies. The overall pattern and specifically two separate waves will promote heavy rainfall potential over areas from the Midwest into the Upper Mississippi Valley and Upper Great Lakes. Energy originating from the compact upper low initially over the extreme Southwest may contribute to focused activity but with considerable uncertainty over timing and extent. Locations over and near Florida may see increased rainfall toward the end of the period, whether just from moist easterly deep-layer flow or the potential approach of T.S. Karen. Monitor NHC forecasts for the latest information regarding Karen. During Fri-Mon expect multiple days with highs 10-25F below normal over parts of the West and extending into the northern Plains. The core of most extreme anomalies should cover an area from parts of Oregon and northeast California/northern Nevada into Montana. Morning lows will be less extreme due to clouds and precip but there may be pockets of min temps of at least 10F below normal. Much of the East will see plus 10-20F anomalies for both highs and lows, with very warm lows also covering the central-southern Plains. Daily record highs are likely especially over the Southeast, and at times could extend a bit northward (more so at locations with a shorter period of record) or westward depending on exact position of the upper ridge. 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