Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sat Nov 04 2023 Valid 12Z Tue Nov 07 2023 - 12Z Sat Nov 11 2023 ...Overview... The upper flow pattern across the contiguous U.S. during the Tuesday-Saturday period 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. The leading system in the series will affect the Northeast U.S. and southeastern Canada Tuesday into Wednesday with mostly light precipitation and breezy conditions. Then a positively tilted shortwave emerging from the West, eventually reinforced by some Canadian flow, 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 in the West, precipitation should linger into Tuesday before a likely break around midweek. 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 latest models and ensembles generally depict the three primary systems of interest in principle, but with some important detail differences yet to be resolved. For the Northeast U.S. into Canada system Tuesday-Wednesday, latest CMC runs and the 12Z UKMET are on the slower side of the spread but the CMC catches up the remaining majority cluster by early Wednesday. With the next system, guidance clusters decently for the Plains to eastern Great Lakes wave from Wednesday into early Thursday but rapidly diverges thereafter in response to differences in northern tier U.S./southern Canada energy aloft. Solutions for the surface low range from a system occluding into Canada such as seen in the new 12Z GFS to a very suppressed wave across the northern Mid-Atlantic per the 00Z ECMWF. The 12Z GFS represents a significant change from the 00Z/06Z runs that tracked across southern New England. Ensemble means have been split as well, with the CMCens becoming northwest and occluded like the 12Z GFS, the ECens suppressed like its parent run, and the GEFS trending from an in-between 06Z run to a more occluded 12Z version. Given the spread and variability among latest guidance, prefer an intermediate solution with additional consideration of continuity that depicted a suppressed wave but not to the degree of the 00Z ECMWF. Also of note, by late in the week the average of guidance has trended a bit slower with the wavy front crossing the eastern U.S. (with the wave/s influence by shortwave energy tracking out of the West) and the updated blend reflects this idea. Over the West, a default model blend resolves minor detail differences for the leading shortwave that crosses the region Tuesday-Thursday. The next shortwave that arrives by around Friday has been more troublesome for resolving details, with a fairly wide spread for how embedded energy may be distributed as the amplifying trough moves inland. Some GFS/ECMWF runs over the past couple days or so have been suggesting that some energy in the southern part of the upper trough could pull off to form a closed low, with 00Z/12Z GFS runs that did this tending to be more pronounced in depth/slowness relative to the ECMWF runs. However this idea has tended to be a little less common among the ensembles, leading to most means maintaining a phased trough, and CMC runs have kept the shortwave open. Among the means, the 12Z/03 ECens mean showed the most hint at southern flow separation. With the 00Z/06Z guidance as a whole tilting a little more away from the southern part of the trough separating, including the 06Z GFS adjusting farther northeast with its upper low, forecast preference nudged a little in the phased direction. The 12Z GFS makes a dramatic change in favor of this idea as it carries the core of its deepest energy across the Northwest, and the 12Z ECMWF now carries a closed or nearly closed low a little more slowly across the same region. Forecast preferences led to starting with a 00Z/06Z operational model blend for the first half of the period and then rapidly transitioning toward 40-50 percent total input from the 06Z GEFS/00Z ECens means with the remainder coming from minority components of the 06Z GFS and 00Z ECMWF/CMC. ...Weather/Hazards Highlights... The western U.S. can expect precipitation to continue into Tuesday, but with lessening amounts compared to the short range period. Higher elevations like the Cascades and northern Rockies are likely to see snow. Precipitation should trend much lighter for the West on Wednesday as upper-level ridging briefly builds back in, though the central Rockies could have a period of snow. 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. Farther east, a couple low pressure and frontal systems tracking through the Midwest/Great Lakes region to Northeast should bring rounds of precipitation next week. A leading system exiting New England after Tuesday will bring mostly light totals and a brief period of breezy conditions. Amounts and north-south extent of precipitation remain uncertain with the next wave forecast to progress northeastward from the central Plains mid-late week, with some connecting surface reflection potentially extending north into Canada. After early Thursday guidance diverges on the structure of this system, increasing the uncertainty in 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. Light lake effect precipitation will be possible after this system passes. Some rain is possible farther southwest along the frontal corridor, with the best chance for higher rainfall totals late in the week over southern Texas where the available moisture will be the highest. Much of the eastern two-thirds of the country will see above average temperatures as the period begins Tuesday, with the greatest anomalies of 15-25F for highs across the southern High Plains as temperatures rise well into the 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 Tuesday-Thursday, with warmer than average temperatures across the Gulf Coast states as well. Daily record high temperatures could be set from the south-central to southeastern U.S. especially on Tuesday and 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. Rausch/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