Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM 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 could phase with an initial Plains upper low/trough. This 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. This evolution would bring a windy storm system with potential for 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 very uncertain heavy lake effect snow potential to the lee of the Great Lakes. The West will be fairly dry but northern areas may see some precipitation as the late week trough moves in. ...Guidance Evaluation/Predictability Assessment... There remains a significant amount of uncertainty surrounding the evolution of the forecast in the central-eastern part of the nation next week. Based on the 00z/06z guidance, there was sufficient agreement on eventual phasing of northern stream energy through central Canada and a southern stream low/trough in the Plains resulting in a deep low over the Great Lakes around mid to later next week. This kind of evolution would support significant Eastern U.S. precipitation potential early next week, followed by heavy lake effect snows downwind of Lakes Erie and Ontario next Thursday/Thanksgiving. The 06z GFS was an outlier showing slightly less phasing and a more negative tilt to the southern stream system into the Mid-Atlantic and development of a deepening surface low off the Northeast Coast. The WPC forecast from this morning was based on the 00z/06z guidance, utilizing good agreement the first half of the week for a purely deterministic blend and increasing usage of the ensemble means amidst increasing uncertainty in the details of the upper low/phasing and also additional short wave energy into the West. This helped maintain good continuity with the previous WPC forecast as well. The new 12z guidance (available after forecast generation time) has trended much less phased now with this system and the speed of the northern stream trough in the 12z GFS and CMC has sped up significantly, possibly not even allowing the southern stream portion to catch up. This brings a flatter pattern across the Northeast late week and decreases potential for significant lake effect snowfall across the Great Lakes next week. However, the 12z ECMWF continues to support a more phased solution but may be more in the minority now. Needless to say, the overall pattern across the central and Eastern U.S. during the upcoming Holiday week remains very uncertain but there remains some potential for rather impactful weather to affect travel leading up to and including the Thanksgiving Holiday across the Eastern U.S. and Great Lakes regions. ...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 to reflect the ongoing signal for organized and possibly heavy rainfall, despite fairly dry antecedent conditions. For the Day 5 ERO (Tuesday-Tuesday night), a Marginal Risk area has been maintained from the previous issuance 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/urbanization/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. A more phased system as described above, would result in possibly significant snowfall totals especially downwind of Lakes Erie and Ontario for Thanksgiving, but this remains highly uncertain though could be particularly impactful given the upcoming Holiday. 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. Some of the latest guidance suggests some heavy snowfall potential for parts of the northern Rockies around Thanksgiving, but still with a lot of uncertainty in the details. 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. Santorelli/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