Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 AM EST Mon Nov 06 2023 Valid 12Z Thu Nov 09 2023 - 12Z Mon Nov 13 2023 ...Overview... The upper flow pattern across the contiguous U.S. during the latter part of this week into early next week looks to be progressive with multiple embedded shortwaves that will be impactful in spinning up low pressure/frontal systems. The first system of concern is a positively tilted shortwave over the Four Corners Thursday combining with Canadian flow to produce troughing across the Midwest/Great Lakes and Northeast late week. Eastward progression of the upper trough and associated wavy front during the latter half of the week will gradually suppress what starts as a widespread area of above average temperatures, while bringing precipitation of varying intensity to parts of the southern and eastern U.S. Upstream, an approaching Pacific shortwave and frontal system should increase rain/snow chances late this week with the amplifying upper trough moving inland by the weekend, followed by a larger scale eastern Pacific upper trough that may generate additional precipitation over the Pacific Northwest during the weekend. ...Guidance Evaluation/Predictability Assessment... Latest model and ensemble runs continue to show considerable spread for multiple parts of the forecast. With the eastern system, the 12Z UKMET seemed off initially with the Four Corners shortwave and with the Canadian energy, and was not favored. GFS runs over the past day or so have usually been on the farther north side with the associated surface low, including the 18Z GFS, but the 12Z run did cluster better with the non-NCEP guidance. Took a compromise approach with a blend of the 12Z GFS/ECMWF/CMC guidance along with the ensemble means for this feature. Overall this system has perhaps more uncertainty than normal for a days 3-5 forecast, but not as bad as upstream. Then models continue to show shortwave troughing coming into the West around Friday, with overall a trend toward shallower/weaker energy than some guidance from a day or two ago. But the timing of the shortwave tracking inland around Saturday has varied quite a bit, to the point of having some models out of phase with each other and making it difficult to blend guidance. The ECMWF has been the slowest in moving the shortwave east, but with support from the ECMWF mean. Interestingly the ECMWF machine learning models did not have as slow of a shortwave, and clustered better with the GFS/CMC and their means. So leaned more toward the GFS/GEFS mean/CMC handling of the shortwave timing, and it seemed similar to the previous forecast. In terms of the newer 00Z guidance, the GFS has slowed a bit compared to the previous runs (though not to the point of the ECMWF runs), and is stronger with the energy, even closing off a low temporarily on Saturday. The 12Z and now 00Z ECMWF runs, now along with the 00Z GFS, dig this stronger energy into the central U.S. Sunday and pushes considerable troughing east by Monday, a new trend for the GFS. Meanwhile farther west most models agree on a bout of ridging over the western to central CONUS. The 12Z and newer 00Z CMC show some exception though, pressing a trough into the north-central U.S. to suppress it. The below average confidence led to a forecast favoring particularly the GEFS mean through the latter part of the period with only a bit of the deterministic models (mostly the 12Z GFS) remaining. Expect forecast details to change in future issuances in this uncertain pattern. ...Weather/Hazards Highlights... As a notable shortwave and its surface low reflection track east through the Midwest/Great Lakes and Northeast through the latter part of the week, precipitation is likely but model differences lead to considerable uncertainty with the precipitation coverage, amounts, and type. Northern New England could see the best potential for snow on Thursday, but low confidence in snow levels and amounts remain. Light lake effect precipitation will be possible after this system passes. Rain of varying intensity is possible farther southwest along the frontal corridor, likely peaking Thursday in the south-central CONUS. Eastern Pacific and Gulf moisture are forecast to combine in the vicinity of the front, with the right entrance region of the jet stream overhead. This could lead to heavy rainfall causing flash flooding, so a Marginal Risk is maintained in the Day 4/Friday Excessive Rainfall Outlook across portions of the southern Plains and Lower Mississippi Valley. The southern portion of the risk area should see the highest instability, while farther north looks to have the highest risk of training convection. Plus model variability in the placement of the heaviest amounts leads us to cover a larger area at this point, which could be narrowed with time as models converge, but that has not happened yet. The rainfall axis will shift east with the front on Friday, with showers across the Southeast into the southern/central Appalachians and Mid-Atlantic. Any excessive rainfall should be limited to coastal Texas ahead of the front clearing out the highest moisture, so a Marginal Risk is planned for that region. Farther west, the south-central Rockies could see snow chances lingering into Thursday. The Northwest looks to see precipitation chances pick up again on Thursday-Friday with troughing coming through the region. Uncertainty in timing and amounts of the precipitation remains, but the Pacific Northwest into northern California and the northern Rockies could see moderate precipitation amounts, with lower elevation rain and higher elevation snow. Then another eastern Pacific trough should begin to direct moisture into the Pacific Northwest over the weekend, most likely over northern parts of the region. South-central to eastern portions of the country will remain warmer than average ahead of the frontal boundary on Thursday, with a corridor of lows that are 20-25F above normal extending from parts of the Mississippi Valley into the Tennessee and Ohio Valleys Thursday morning that could set daily records. The cold front will be on its way through those regions though, so highs may only warm to 5-10F above average there on Thursday, with greater warm anomalies for highs farther south across the Southeast to Mid-Atlantic. Warm lows Friday morning should be located over similar regions, but highs should cool to near or below normal in much of the southeastern CONUS. By the weekend, the main area remaining above average will be the Florida peninsula, with highs well into the 80s actually close to daily record highs. Elsewhere temperatures should be cooler and near normal for the weekend. A trend toward upper-level ridging by early next week across the central U.S. would lead to temperatures warming to around 10F above average across the northern half of the Plains into the Upper Midwest by next Monday. Tate 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