Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 and eventually reinforced by some Canadian flow. 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. Meanwhile, the West looks to have a break in precipitation around midweek, aside from snow in the central Rockies. Then 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 details of the forecast. These include ongoing issues with low pressure/fronts reaching the Northeast by Thursday (relating to details of within the supporting upper trough), differences for what becomes of shortwave energy entering the West by Friday along with energy on the back side of the downstream trough, and progression of the leading side of eastern Pacific troughing that begins to affect the Northwest next weekend. Models still vary considerably for the Thursday system over the Northeast. The GFS/GEFS generally stray on their own with a farther north track of eastern Great Lakes low pressure after early Thursday, pushing a defined frontal boundary through New England in contrast to most other guidance that takes a more suppressed track. The 00Z ECMWF looks more aggressive than other guidance in pushing shortwave energy through an upper ridge crossing New England, but the 12Z UKMET is not far behind in that regard and both end up with very similar surface forecasts that are on the most suppressed side of the envelope. CMC/CMCens runs have generally been between these extremes. Preference remains close to a compromise approach while awaiting better clustering. Small scale nature of individual impulses tracking out of the West keeps predictability low for specifics of waves along the trailing cold front. Farther west, most guidance has been recently trending more diffuse with the upper trough moving into the West. Latest GFS runs have been a bit on the fast side with eastward progression as the trough arrives into the Northwest. As the trough continues across the West, there is some timing dependence on flow on the western side of the downstream trough. The GFS remains on the fast side as it is most eager to eject the trough to the east, while the 00Z ECMWF strays slow due to holding upper troughing over the northern Plains/Upper Midwest longer than supported by other guidance. The new 12Z ECMWF has trended away from the extreme aspects of the 00Z version. The next upper trough reaching the eastern Pacific is a little faster in the 00Z ECMWF/ECens mean compared to most other solutions by varying degrees. The GEFS mean had been slowest by a narrow margin but the new 12Z CMC has now claimed that role. Meanwhile the new 12Z ECMWF has raised West Coast heights noticeably by next Sunday. The updated forecast started with a composite of 00Z/06Z operational runs early in the period to yield the desired intermediate solution within current spread, while transitioning to a model/ensemble mean mix (including the GEFS/ECens/CMCens) by next weekend to account for steadily decreasing confidence in details. ...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. Light lake effect precipitation will be possible after this system passes. Rain of varying intensity 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 maintained 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. That has not yet happened in the new 12Z runs so the outlook area is kept unchanged for now. 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. Current consensus showing a fairly weak/diffuse supporting upper trough moving into the West suggests that most precipitation should be confined to areas from the central/northern West Coast into the northern Rockies. Then another eastern Pacific trough should begin to direct moisture into the Pacific Northwest next weekend, most likely over northern parts of the region. 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, the coolest anomalies should be for daytime highs over the southern half of the High Plains late in the week with some readings up to 10-15F below normal. Otherwise highs should drop to near or slightly below normal. Ahead of the system reaching the Northeast by Thursday, New England should be on the chilly side early in the period as well. Much of the West will see modestly below normal highs mid-late week before rebounding to within a few degrees on either side of normal next weekend. 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