Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Fri Nov 17 2023 Valid 12Z Mon Nov 20 2023 - 12Z Fri Nov 24 2023 ...Pattern Overview... The dominant focus of next week's forecast will be a deepening storm system forecast to track from the south-central Plains through the Great Lakes and into east-central Canada, in response to digging western Canada energy that phases with an initial Plains upper low/trough. This interaction should yield a deep upper low somewhere between the Upper Midwest/Great Lakes and southern Canada by Wednesday. The upper low and trough to the south should gradually lift northward late in the week while a shortwave trough moves into the West. The windy storm system will likely produce significant precipitation over the eastern half of the country during the first half of the week, followed by an expanding area of below normal temperatures over many areas east of the Rockies with some snow possible to the lee of the Great Lakes. The West will be fairly dry but northern areas may see some light precipitation as the late week trough moves in. ...Guidance Evaluation/Predictability Assessment... Based on the 12Z/18Z guidance, the primary consideration during the first half of the period was to exclude the UKMET as its northern stream pattern from the Pacific through North America strayed to the fast side. This left an average of the 18Z GFS, 12Z ECMWF, and 12Z CMC to provide a reasonable depiction of consensus ideas for the Plains through Great Lakes storm system. This solution did yield a slightly slower trend for the portion of the trailing front crossing the southern tier to the East Coast. Among the new 00Z runs, the UKMET is still running on the progressive side in the northern stream while the GFS is a southwestern extreme with its upper low as of early Wednesday (leading to a farther south surface low). Both the 18Z and 00Z GFS runs hold onto more troughing over the Canadian Maritimes versus most other guidance by Tuesday-Wednesday, slowing frontal progression over the Northeast after early Wednesday. However differences are still within typical spread for five days out in time thus allowing for keeping the GFS as part of a compromise. Later in the period, the 00Z GFS tracks its eastern upper low farther southeast than consensus while a model/mean composite offers a good starting point for the upper trough moving into the West Thursday-Friday. A model/mean blend also mitigates detail differences that develop in some solutions for flow near the western half of the Canadian border. Thus the initial GFS/ECMWF/CMC blend transitioned to include some of the 18Z GEFS/12Z ECens means after midweek. ...Weather/Hazards Highlights... Heavy rainfall potential will increase across the south-central to eastern U.S. for the upcoming holiday week as a deepening low pressure system tracks from the south-central Plains through the Great Lakes. Also the tightening pressure gradient behind the low should produce a period of high winds across the High Plains early next week, with less extreme but still brisk to strong winds across the Plains and into the Great Lakes. Instability ahead of the system could bring severe weather potential to the central Gulf Coast around Monday, as highlighted by Storm Prediction Center outlooks. With the southern tier convective potential, the Day 4 Excessive Rainfall Outlook covering Monday-Monday night maintains a Marginal Risk area from the prior Day 5 outlook to reflect the ongoing signal for organized and possibly heavy rainfall. The main adjustment in this cycle is to account for a slightly slower frontal progression. For the new Day 5 ERO (Tuesday-Tuesday night), a Marginal Risk area has been depicted from the Mid-Atlantic southwest into Alabama as abundant moisture should stream into the southern and eastern U.S. ahead of the Mississippi Valley-Great Lakes storm/frontal system. While activity will be fairly progressive and ground conditions will start out on the dry side, localized runoff issues will be possible. Within this overall area, the Mid-Atlantic may be a little more sensitive given the combination of highest QPF signals in model/ensemble guidance and potential for impaired drainage due to leaves/etc. Precipitation will continue through New England by midweek. Snow will be possible on the far northwest periphery of the moisture shield, and potentially in New England at the start of the event. A period of lake effect snow will be possible after system passage, with duration and totals dependent on how quickly the systems lifts away. The Rockies may see lingering precipitation early in the week. Otherwise the West should be dry early-mid week. A system moving into the region late in the week may bring mostly light totals across northern areas, possibly extending into the Plains Friday. The main story for temperatures will be the increasing coverage of below normal readings from the northern-central High Plains eastward mid-late week. By that time expect locations from the High Plains into Great Lakes to see one or more days with highs 10-20F below normal. The West will trend from near or slightly below normal early in the week to moderately above normal Wednesday, and then drift back to near or slightly below normal toward the end of the week. Rausch 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw