Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu Nov 02 2023 Valid 12Z Sun Nov 05 2023 - 12Z Thu Nov 09 2023 ...Synoptic Overview... A general quasi-zonal flow across the continental U.S. is expected going into the weekend, with broad cyclonic flow aloft over southern Canada and a shortwave trough quickly traversing the southeastern U.S. on Sunday. A shortwave trough that develops across the Plains by Monday will spur surface cyclogenesis across the Upper Midwest and Great Lakes region, and then the low reaches New England on Tuesday. Unsettled conditions are expected to continue across the Pacific Northwest with additional rounds of rainfall and mountain snow for the Northern Rockies. A Canadian surface high will continue to govern the overall weather pattern across the Northern Plains with cold weather continuing. ...Guidance Evaluation/Predictability Assessment... The early medium range period features a progressive, mostly zonal flow regime with a series of shortwaves traversing the CONUS, the timing of which impacts expected frontal system movement and sensible weather during the timeframe. An initial shortwave over the Mississippi Valley Sunday should quickly drop southeast over the Atlantic as high pressure remains in place at the surface, with the most recent runs of the GFS (00Z/06Z) moving towards the ECMWF with this timing/placement/movement. The next upstream wave develops off the lee of the Rockies and passes over the Plains, with cyclogenesis helping to deepen surface low pressure and better organize a quasi-stationary frontal system over the Midwest. Similar to the prior wave the ECMWF has been more consistent run-to-run with the timing and strength of this wave, and the most recent 00Z/06Z GFS runs once again better match the 00Z ECMWF after variable solutions from the previous days guidance, particularly the 18Z GFS which had depicted additional energy dropping southward from central Canada that would disrupt the expected pattern over the CONUS. The UKMET is also in relatively good agreement, while the CMC remains a weaker outlier. Predictability decreases between the GFS/ECMWF by the middle of the period turning back upstream over the Plains, as recent solutions have been varying phasing between weak mean ridging and yet another shortwave. Another piece of upper-level energy also drops southward from a deeper upper-level trough lingering over the northeastern Pacific towards the West Coast during this timeframe and features a similar lack of run-to-run consistency, particularly within the GFS but also with the ECMWF. Earlier runs of the GFS are faster than the ECMWF as this energy translates inland over the West and towards the Plains at the end of the period. Fortunately the most recent 00Z ECMWF and 00Z/06Z runs as well as the ECens and GEFS means are more in phase with the wave progression and offer a reasonable composite solution that avoids averaging out too many details. The updated WPC forecast blend begins with a combination of the 00Z ECMWF, 06Z GFS, and 00Z UKMET, with no contribution from the CMC given the outlier nature of its handling of the initial series of shortwaves. A portion of the prior WPC forecast is also kept to avoid any more drastic forecast changes given the higher variability with shortwaves in the pattern. A contribution from the means is increased to about half the blend by the end of the period to offer a centered compromise on increasing differences in the deterministic solutions. The updated forecast results in a bit more northward track of the frontal system developing in the Midwest as it shifts towards the Interior Northeast/New England, with the frontal boundary also lingering further north as it stretches back to the southwest through the Ohio Valley and eventually Texas. ...Weather/Hazards Highlights... Most of the continental U.S. should not see any significant rainfall hazards through this coming weekend. Similar to the past few days, the only region where there will be a risk of excessive rainfall leading to potential flooding concerns will be in the coastal ranges of the Pacific Northwest with the enhanced onshore flow that will continue into the weekend. A Marginal Risk of Excessive Rainfall has been maintained on Day 4 (Sunday) from northwestern California to the Oregon Cascades and coastal ranges as forecast QPF peaks during this timeframe. Locally heavier rainfall on the west facing slopes may just be enough to cause some minor flooding concerns, particularly for urban areas and burn scars, and especially as rainfall from the prior days brings wetter antecedent conditions. Looking ahead to Day 5 (Monday) an abatement of the onshore flow and associated rainfall is expected, and therefore no risk areas are planned. Some areas of heavier snow look possible for higher elevations of the Cascades Sunday-Monday, and for the Northern Rockies by Monday as well as precipitation chances spread further inland. Expect precipitation from the Pacific Northwest into the Rockies to generally trend lighter by the middle of next week as upper level ridging briefly builds back in. Across the eastern half of the country, a shortwave/associated surface frontal system is expected to bring precipitation chances from the Great Lakes to the Northeast states through Tuesday. There has been a northward trend in the guidance with respect to the track of the system keeping the more moderate amounts along the Upper Great Lakes and Interior Northeast/northern New England. Locations near the Canadian border may see some snow or mixed precipitation during the period while rain will be the prevailing precipitation type elsewhere. Confidence remains low in additional rain chances lingering beyond Tuesday from the Ohio Valley into the Mid-Atlantic and Northeast due to increasing guidance differences. Temperatures are expected to be pleasantly mild for this time of year across much of the Central U.S. on Sunday with highs generally running 10 to 15 degrees above early November averages. This will likely remain the case going into early next week as well, shifting from the Central Plains into the Southern Plains and eventually the Southeast. Areas in the West and Northeast will likely have a cooling trend going into the middle to end of next week, with highs running 5 to 10 degrees below average. Putnam/Hamrick 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