Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EST Sun Nov 05 2023 Valid 12Z Wed Nov 08 2023 - 12Z Sun Nov 12 2023 ...Overview... The upper flow pattern across the contiguous U.S. during the latter part of this week into next weekend looks to be fairly zonal and progressive from the multi-day mean perspective, but multiple embedded shortwaves will be impactful in spinning up low pressure/frontal systems. As the leading system in the series pulls away from the Northeast on Wednesday, the next system of concern is a positively tilted shortwave emerging from the West, eventually reinforced by some Canadian flow, that should progress toward the eastern U.S. with associated low pressure pushing a trailing cold front southeastward across much of the country as the week progresses. This front will gradually suppress what starts as a widespread area of above average temperatures. Meanwhile, the West looks to have a likely break in precipitation around midweek, aside from snow in the central Rockies. But an approaching Pacific shortwave and frontal system should increase rain/snow chances late next week with the amplifying upper trough moving inland by the weekend. ...Guidance Evaluation/Predictability Assessment... The 12/18Z model guidance showed some notable differences in the details with the shortwave over the west-central U.S. even as the medium range period begins Wednesday. The 18Z GFS in particular appears to be an outlier in its handling of the energy, and as the shortwave progresses east, the surface low associated with it does not align well with other guidance that shows more of a trend toward split low pressure (one low north of Lake Superior that is trending stronger, and one atop Lake Huron or so). The 12Z GFS seemed more reasonable, so utilized it in combination with the 12Z ECMWF/CMC/UKMET for Wednesday-Thursday rather than the 18Z run. Southern and northern stream energy look to combine around Friday for Great Lakes troughing, but the 12Z GFS and now the 00Z GFS do seem to be on the shallow side with that troughing compared to other guidance and the ensemble means. Meanwhile the 12Z and new 00Z CMC were on the deep side, with somewhere around the 12Z ECMWF and its ensemble mean a good compromise for that feature. Upstream, eastern Pacific energy has been rather troublesome for resolving details by Thursday-Friday as some energy/troughing enters the West. Models overall over the past day have trended more toward a phased trough coming into the West on the faster side. But the 12Z ECMWF had a closed low that pushes into the Northwest more slowly, with a ridge to the east Friday-Saturday that ended up out of phase with other guidance, as the other models had a faster progression of the trough basically where the ECMWF ridge was. The EC ensemble mean reflected a slower solution of the trough/ridge as well, while the 18Z GEFS mean was rather fast and the 12Z GEFS mean in between. Ensemble members show a large amount of spread. The out of phase solutions made it tough to blend models, and this problem may persist into the incoming 00Z model suite. Confidence is low in the timing of these features and thus the pattern in general. The WPC forecast was based on a blend of the 12Z deterministic models early on, but quickly transitioned to favor the GEFS and EC ensemble means as the period progressed to over half by Day 6 and well over half on Day 7, considering the increasing spread. Expect the forecast to change with decreasing lead time, perhaps even more than usual given the high uncertainty and low confidence. ...Weather/Hazards Highlights... As a notable shortwave and its surface low reflection track east through the Plains Wednesday into 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. Overall this may be a somewhat colder system, with precipitation in the form of snow possible in areas near the Canadian border and at least the higher elevations of the Interior Northeast, perhaps even into lower elevations of northern New England. There is low confidence in snow levels and amounts at this point though. This forecast shows a trend toward snow probabilities lingering longer into Friday. Light lake effect precipitation will be possible after this system passes. Some rain is possible farther southwest along the frontal corridor, potentially 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 planned in the Day 5/Thursday 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, that could be narrowed with time as models converge. By late week, any remaining heavy rainfall should be across southern Texas ahead of the front clearing out the highest moisture. Farther west, the central Rockies and vicinity could see some notable snow around Wednesday-Thursday, but the rest of the West should be dry on average as upper-level ridging briefly builds back in. Chances for at least light precipitation may increase again late week for the Pacific Northwest, but with uncertainty in timing and amounts. Issues with the character of the supporting upper trough moving into the West also lower confidence in determining the southeastward extent of precipitation. Much of the eastern two-thirds of the country will see above average temperatures as the period begins Wednesday, with the greatest anomalies of 15-25F for highs stretching from the southern Plains into the Middle Mississippi Valley as temperatures rise well into the 70s and 80s. Expect a corridor of lows that are 20-25F above normal to extend across parts of the Mississippi Valley into the Ohio/Tennessee Valleys on Wednesday-Thursday, with warmer than average temperatures across the Gulf Coast states as well. Daily record high maximum and minimum temperatures could be set from the south-central U.S. into the Tennessee and Ohio Valleys especially Wednesday. Cold fronts tracking south and east should gradually reduce the area of above normal temperatures across the CONUS with time, cooling temperatures across the northern tier, then the central U.S. to Ohio Valley, and finally across the Southeast by Friday-Saturday. Behind these fronts, below normal temperatures should be most prevalent over the western U.S. (where cool readings should persist most of the period), spreading into the south-central U.S. and toward the Ohio Valley, Great Lakes, and the Northeast late week. 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