Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Sat Sep 26 2020 Valid 12Z Tue Sep 29 2020 - 12Z Sat Oct 03 2020 ...Overview and Guidance/Predictability Assessment... Latest models and ensemble means 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 to the west of the Rockies and below normal temperatures over much of the central/eastern states. A wavy front ahead of the upper trough may produce areas of significant rainfall over the East through midweek. The large scale/amplified nature of the pattern provides above average to high predictability for the mean flow. In particular a series of strong North Pacific systems tracking toward or near the Alaska Peninsula will likely help to maintain the western upper ridge through at least the end of the period. Regardless of how high the confidence is in the large scale forecast, there are typical embedded uncertainties that reduce confidence for some details within and ahead of the mean trough aloft. There is considerable spread and run-to-run variability for initial middle/lower Mississippi Valley upper trough energy which may separate for a time before rapidly ejecting northeastward ahead of upstream flow. ECMWF runs have been consistent with a slower/closed solution while the 00Z GFS/UKMET reflected a similar idea. However the 06Z GFS/GEFS mean trended faster and more phased, reflective of the 00Z CMC. New 12Z guidance continues to show mixed messages with the GFS/GEFS mean fairly progressive and the UKMET adjusting in that direction, but the CMC trending back to a compromise idea. The ECMWF trend a bit faster but is still on the slower/closed side of the spectrum. Issues with this shortwave energy temper confidence in specifics for the wavy surface front near the East Coast Tue-Thu. There is decent clustering for a midweek system crossing Ontario with a trailing front that will cross the central/eastern states, albeit with some persistent timing spread. Low predictability shortwave specifics will determine potential surface wave details near the East Coast by next Fri-Sat. Meanwhile over the past day the ensemble means have started to reflect recent GFS/ECMWF runs that bring a weak wave from Canada into the central U.S. by next Sat, supported by shortwave energy that rounds the western ridge during the latter half of the week. The 12Z CMC has trended closer to this cluster after a slower/northward depiction in the prior run. The full array of guidance over the East during the first half of the period supported an operational model blend somewhat closer to the 00Z ECMWF/GFS/UKMET in principle. Then the forecast transitioned to a blend of operational models/means to reflect the large scale pattern agreement while trending a little more conservative for embedded specifics. ...Weather/Hazard Highlights... The West will see increasing fire danger once again next 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 (perhaps isolated higher values) from California/Nevada through the Pacific Northwest. Some record-breaking values will be possible, especially over parts of California. Cold fronts working through the central and eastern U.S. will provide focus for showers/storms, with moderate to locally heavy rainfall possible up the Eastern Seaboard Tue/Wed. Rainfall specifics will depend on frontal wave details influenced by the degree and timing of potential upper trough separation. Cold air aloft will meanwhile provide instability for scattered showers across the Midwest/Great Lakes/Appalachians under the upper trough, including some Great Lake enhancement. Amplitude and slow eastward progression of the upper trough may also support late week frontal waves that could maintain unsettled/rainy East Coast and maritime conditions. Cold air surges should support below to much below normal temperatures of 5-15 degrees F for much of the central to eastern U.S. next week. 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 next 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