Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Sun Sep 27 2020 Valid 12Z Wed Sep 30 2020 - 12Z Sun Oct 04 2020 ...Hot and dry conditions return for the West... ...Overview and Guidance/Predictability Assessment... Models and ensembles continue to show upper level flow settling into a highly amplified pattern featuring a western U.S./Canada ridge and east-central trough. This configuration will support very warm/hot and dry conditions west of the Rockies and below normal temperatures over much of the central/eastern states. Wavy fronts ahead of the upper trough may produce areas of significant rainfall over the East, especially with a well-defined Mid-Atlantic to New England wave Wed into Thu. The large scale/amplified nature of the pattern and good guidance continuity provide above average to high predictability and confidence for the mean flow at least into Sat--with two strong North Pacific systems tracking toward or near the Alaska Peninsula mid-late week likely helping to maintain the strength of the western North American upper ridge. By next Sun guidance begins to diverge regarding possible weakening of the ridge as the upstream mean trough begins to drift a bit eastward and ejecting impulses possibly filter into/through the ridge. Models are gradually trying to converge with respect to energy that appears likely to form a compact upper low over the Southeast by the start of the period early Wed followed by rapid northeastward ejection. There is still timing spread within the increasingly confident general evolution though. The 00Z/12Z CMC runs are on the fast side of the spread so that model had the lowest weight in the forecast. Other models vary with the 12Z/06Z GFS and 12Z ECMWF both adjusting a bit slower than their 00Z runs. The 12Z UKMET is near the 00Z ECMWF. Sheared nature of shortwave support lowers predictability of details for frontal waviness along the East Coast around Fri-Sat and for the wave dropping into the central U.S. next weekend. Guidance preferences led to the latest WPC medium range product suite primarily reflecting the 00Z-06Z GFS and 00Z ECMWF/UKMET from day 3 Wed into early day 5 Fri followed by increasing 06Z GEFS/00Z ECMWF ensemble mean input, reaching nearly half total weight by day 7 Sun. ...Weather/Hazard Highlights... The West will see increasing fire danger once again this week as the amplified upper ridge over the region brings a return of hot and dry conditions. Expect temperatures to reach 5-15 degrees F above normal (isolated higher values) from California/Arizona/Nevada through the Pacific Northwest. Some record-breaking values will be possible, mainly over parts of California but perhaps into western Oregon/Washington as well. By next weekend modest weakening of the upper ridge could allow for some moderation of the heat close to the West Coast. Parts of the Mid-Atlantic into the Northeast may see moderate to heavy rain/storms Wed into early Thu as a vigorous wave lifts up an East Coast front. A trailing front with one or more weak waves, reaching the East Coast late in the week, may provide another focus for rainfall over the Mid-Atlantic/Northeast along with unsettled maritime conditions. Rain with this front should be lighter due to less moisture being available than with the first system. Both fronts will tend to decelerate and possibly stall near/south of the southern Florida Peninsula and Keys where periods of rain are possible through the period. Meanwhile cold air aloft will provide instability for scattered showers across the Midwest/Great Lakes/Appalachians under the upper trough, including some Great Lake enhancement. The wave expected to drop into the central U.S. next weekend may produce an area of light to locally moderate precipitation from the Plains into Great Lakes and Ohio Valley. Cold air surges will support temperatures of 5-15 degrees F below normal for much of the central to eastern U.S. this week. The greatest coverage of double-digit anomalies should be during Thu-Sat. In this pattern there remains a chance for a first frost/freeze of the season for the Northern Plains and Upper Mississippi Valley by later in the week. Rausch/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