Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sat Oct 01 2022 Valid 12Z Tue Oct 04 2022 - 12Z Sat Oct 08 2022 ...Overview... A wavy pattern should be in place by the start of the medium range period on Tuesday, with a compact upper low over the East and shortwave moving through the High Plains. As the week progresses, energy diving southward across Canada combining with the latter shortwave will amplify upper troughing across the East while ridging builds across the West, with this pattern remaining generally stagnant into next weekend. Relatively quiet weather conditions are expected across the CONUS after lingering showers/gusty winds (related to what will be left of Ian) move offshore the Mid-Atlantic/Northeast Tuesday-Wednesday. Monsoonal moisture and weak energy over the Southwest could lead to showers and storms there and perhaps extending into portions of the Plains. The large-scale trough should push a potent cold front across the central and eastern U.S., with potential for some rain particularly in the Upper Great Lakes and a surface high behind the front cooling temperatures considerably. ...Guidance Evaluation/Predictability Assessment... Model guidance remains quite agreeable with the synoptic pattern through the medium range period, though they show some smaller scale differences. As the period begins Tuesday, energy associated with Ian in the form of an upper low and a surface low pressure system have fairly good agreement, with the 00Z UKMET as a slight exception as it displaced the features northeast of consensus, and the CMC held more QPF onshore than other guidance. Thus the early part of the WPC fronts/pressures forecast was based mainly on the 00Z ECMWF and 00Z/06Z GFS, along with lesser amounts of the 00Z CMC, and this blend worked well for the north-central U.S. shortwave as well. While models agree on troughing amplifying in the East during the latter half of next week, there are some minor detail differences with rounds of energy diving southward through the east side of the ridge/west side of the trough, leading to some variability in whether or not models show a closed low or two within the trough by late next week. The WPC forecast was able to maintain a majority of deterministic models even later in the forecast, but the EC and GEFS ensemble means were also incorporated by days 6-7 to tone down some of the differences. This maintained good continuity from the previous forecast. In terms of the precipitation forecast, the QPF was based on the 12Z National Blend of Models but with a wider footprint for light amounts added across the Southwest and the north-central to northeastern U.S. as the NBM typically underforecasts these low amounts. ...Weather/Hazards Highlights... The potentially slow-moving eastern U.S. upper low and associated surface low pressure system that are related to Ian's remnants may continue to produce some rainfall and gusty winds along the Mid-Atlantic and Northeast coast Tuesday-Wednesday, though models agree the heaviest rainfall should remain offshore. Elsewhere, monsoonal moisture coming into the Southwest, perhaps enhanced on Tuesday by what is currently Hurricane Orlene, will drive some diurnal showers and thunderstorms there. Locally heavy rain is possible in some locations, and some rain chances could last through late week across the Four Corners states. Meanwhile, a cold front shifting from the Northern Plains to the Upper Midwest and Great Lakes should result in some widespread showers through the region as the front moves through. Initially below normal temperatures along the East Coast should trend back towards normal by the middle of next week, while temperatures out West trend much warmer underneath a blocky upper ridge. Daytime highs over the Northwest could be 10 to 15 degrees above normal into next weekend, though the resulting temperatures peaking in the 70s and 80s (90s for parts of California) are unlikely to create any heat related hazards. Canadian high pressure spilling into the north-central U.S. and expanding eastward behind the cold front will result in much below normal temperatures shifting from the northern Plains into the Midwest and the Northeast Thursday-Saturday. Daytime highs in some locations could be 15 to 20 degrees below average with overnight temperatures dropping into the 30s for many, and even 20s for far northern areas of North Dakota/Minnesota particularly Friday morning. Tate/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml