Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Mon Sep 23 2019 Valid 12Z Thu Sep 26 2019 - 12Z Mon Sep 30 2019 ...Tropical Storm Karen over the Caribbean... ...Rain and heavy mountain snows with much below normal temperatures over the West... ...Heatwave for the South/Southeast U.S... ...Heavy thunderstorms/rain for parts of the central U.S.... ...Pattern Overview and Guidance/Uncertainty Assessment... There is still good overall agreement 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 southeastern U.S./western Atlantic ridge at the surface and aloft may direct the system toward the Bahamas late in the period. The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the 00 UTC GFS/ECMWF/Canadian and 00 UTC GEFS/ECMWF ensemble means. This maintains good WPC continuity in a pattern with above normal flow transition predictability. ...Weather Highlights/Threats... Locations over and near the Bahamas and Florida may see increased rainfall toward the end of the period, whether just from moist easterly deep-layer flow or the potential approach of Karen. Monitor NHC forecasts for the latest information regarding Karen. Highest wintery 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. 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. Parts of the East/Southeast will see a protracted heatwave with plus 10-20F anomalies for highs and lows, with record lows also on tap from the south-central Plains then into the Midwest later period. 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 some uncertainty over timing and extent. The 00 UTC GFS/ECMWF runs do now offer a more common and less progressive solution that seem reasonable considering the initial closed nature of the system. Schichtel 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