Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Nov 07 2023 Valid 12Z Fri Nov 10 2023 - 12Z Tue Nov 14 2023 ...Overview... The upper flow pattern across the contiguous U.S. late week into the weekend 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 north-central U.S. Friday combining with Canadian flow to produce troughing across parts of the central and eastern U.S. late week. Eastward progression of the upper trough and an associated wavy front will gradually suppress an area of above average temperatures across the Southeast late week, while spreading rain across the southern and eastern U.S. that could linger into early next week. Upstream, a quickly moving shortwave and frontal system or two coming through the Northwest should increase rain/snow chances there, followed by a larger scale eastern Pacific upper trough that may generate additional precipitation over the Pacific Northwest during the weekend and beyond. This eastern Pacific trough will promote upper ridging farther east into the north-central U.S. with warmer temperatures, and overall a more longwave-dominated pattern into next week. ...Guidance Evaluation/Predictability Assessment... At the start of the period Friday, there are some model differences regarding the southeastern Canada upper low and a positively tilted shortwave back into the Great Lakes/Midwest, but nothing that a multi-model blend could not handle. But more difficult is the shortwave trough coming into the Northwest Friday and tracking east through the weekend. While a small feature, the timing differences in the shortwave create complications for the model diagnostics because the models go out of phase with each other, making it difficult to blend guidance. The ECMWF and EC mean, and many EC ensemble members, are slower with the trough than the rest of the guidance and produce ridging ahead of the feature on top of where other models have the faster trough. Interestingly, the ECMWF machine learning models are faster with the trough, clustering better with the GFS suite. For this forecast for this feature, since too much blending of models was not a good option, leaned heavily toward the 18Z GFS and GEFS mean that seemed to be a good middle ground position between the slower EC runs and the faster CMC. This led to a faster trend in pushing fronts through the northern tier compared to the previous forecast, but the timing of this shortwave and the downstream effects across the northeastern U.S. early next week are still uncertain, so more changes to the forecast may be necessary in future cycles. The newer 00Z ECMWF trended faster than its previous run with this shortwave though, better clustered with the other guidance, hopefully a good sign. Farther upstream, models are indicating a split flow upper pattern where a southern stream low sits over northern Mexico early next week, while the northern stream shows ridging over the west-central U.S. possibly shifting eastward with time. This is in response to a strong trough digging over the eastern Pacific that models agree should exist early next week, but with considerable differences in the axis of the trough and possibly an embedded upper low. The 18Z and newer 00Z GFS cut off an upper low over the Pacific well west of the CONUS (around 140W), while the 12Z ECMWF indicates a phased trough pushing into the West Coast by Tuesday (ahead of other guidance and almost all ensemble members). The new 00Z ECMWF stayed quite fast in bringing the trough east compared to consensus, though a tad slower than the 12Z run. The EC ensemble mean and the GEFS mean both follow their deterministic models with the EC mean faster and the GEFS mean slower, but neither to the same extreme as the operational models. Thus a blend favoring the GEFS and EC mean (to 75 percent by Day 7) seemed to work best for the latter part of the forecast period. ...Weather/Hazards Highlights... Moisture overrunning the cold front that is likely to push through the Southeast and then stall over the northern Gulf will lead to some rain across the south-central to southeastern quadrant of the U.S. for late week into early next week. The highest rainfall totals are likely to be over southern Texas near a meandering surface low along the front, with eastern Pacific and Gulf moisture combining. On Friday, some instability could be in place especially early in the day before the front pushes through. This could lead to locally heavy rainfall rates and perhaps a nonzero chance of flash flooding. But by Saturday since the front is likely to have passed, lack of instability should mean rain rates will not be too high, so no risk area is planned for the Day 5/Saturday ERO. Into next week though, there may be more support for convection as mid-upper level energy approaches, so this will continue to be monitored. Farther north, lake effect precipitation is likely with troughing and westerly flow over the Great Lakes, likely maximized Friday. Additional upper-level energy could spread light precipitation to the Northeast early next week. Rounds of frontal systems and shortwaves pushing through the Northwest will maintain precipitation chances across the Pacific Northwest and the northern Rockies. Moderate to heavy amounts are likely to be directed into the northern parts of the Olympics and Cascades, with lower elevation rain and higher elevation snow. Southeastern portions of the country will remain warmer than average ahead of the frontal boundary on Friday, with a corridor of lows that are 10-20F above normal extending from the Lower Mississippi Valley into the Southeast to the Mid-Atlantic Friday morning before the cold front passes and limits above normal highs to the Southeast/Florida. The Florida Peninsula should be the main area above average by Saturday, with highs well into the 80s actually close to daily record highs. Elsewhere, temperatures should be near average to cooler than average through late week in the post-frontal cool high pressure. But as upper ridging builds across the north-central U.S. early next week, above normal temperatures by around 10-15F will overspread the central and northern Plains into the Midwest. Meanwhile below normal highs should linger in the south-central U.S. with lower heights overhead and clouds and rain possible, and interestingly lead to similar highs from Texas all the way north to South Dakota or so (in the upper 50s and 60s). Near to cooler than normal over the Eastern Seaboard this weekend should gradually moderate through the early part of next 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