Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Sep 14 2023 Valid 12Z Sun Sep 17 2023 - 12Z Thu Sep 21 2023 ...Hurricane Lee forecast to track away from New England after early Sunday... ...Overview... Continue to follow National Hurricane Center advisories for the latest information on Hurricane Lee, whose extratropical reflection should be to the east of Maine as of early Sunday and track away from New England thereafter, leading to a drier trend and lighter winds across the region. Behind Lee, an upper trough reaching the eastern U.S. by Sunday-Monday will gradually lift out into midweek. The front associated with this trough may produce some areas of rain with varying intensity during the weekend into early next week. A persistent wavy front should maintain a wet pattern over the Florida Peninsula for most of the period while Texas and the Gulf Coast will likely trend drier after the weekend. Over the West, guidance continues to advertise a significant pattern change after initial ridging weakens and progresses eastward, but over the past couple days there has been some variability in how next week's expected upper trough evolves. This trough and leading cold front may bring some moisture into the Pacific Northwest and northern Rockies. Above normal temperatures over the interior Northwest and northern High Plains on Sunday will shift eastward across the northern tier while most of the West sees a pronounced cooling trend. ...Guidance Evaluation/Predictability Assessment... Leading into the start of the forecast period early Sunday, models have been showing some modest east-west variability for the track of Lee in recent runs but currently they cluster well in showing a position to the east of Maine at that time. Lee will then progress away from New England during the day Sunday. For the trailing upper trough reaching the East by Sunday-Monday, recent model runs have generally trended toward a fairly open depiction as opposed to some earlier solutions that had an embedded upper low over or just north of the Great Lakes. Details are still somewhat uncertain with some room still existing for a weak/transient embedded low depending on exactly how the shortwave energy evolves. As the trough starts lifting away, the 12Z CMC showed a compact upper low dropping into the Upper Midwest early next week. Other guidance did not have this feature, so the CMC was excluded from the forecast. The new 00Z CMC has at least weakened it to a farther north shortwave but it still compares poorly to other guidance. Models and ensemble means are consistent in the general idea of an upper trough amplifying over the West next week but the operational models have been varying quite a bit for the details. 24-36 hours ago the models were suggesting that the trough reaching western North America by Monday would be what ultimately settles over the West. However runs starting with the 12Z/13 cycle heave been showing a trailing compact shortwave that ends up being the feature that digs into the region as an upstream Pacific ridge sharpens, ultimately leading to a fairly deep upper low over or near the northern Great Basin by next Thursday. GFS/ECMWF/CMC runs all show this closed low, so it seems reasonable to trend the forecast to that general idea but for now in weaker form given the detail inconsistency over recent days. Teleconnections relative to strong positive height anomalies over or near eastern Hudson Bay/James Bay by the end of the forecast period suggest that negative anomalies associated with the western trough should be fairly modest. This could be achieved either by the upper low being weaker or by being deep but progressive enough to yield modest anomalies from a multi-day mean perspective. The above considerations led to starting the updated forecast with a composite of the 12Z ECMWF/UKMET and 12Z/18Z GFS early in the period and then shifting toward 50-60 percent total input from the 18Z GEFS/12Z ECens means by days 6-7 Wednesday-Thursday. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks covering Sunday through Monday night show no risk areas for organized flash flood potential. The Florida Peninsula continues to be one region that merits monitoring though, with a fairly wet pattern expected due to persistence of a wavy front. However moisture anomalies are only moderately above normal, flash flood guidance values are high, and models do not show much clustering for any relatively heavier rain that may fall. Shorter term guidance may ultimately help to refine any areas that could at least merit a Marginal Risk. On Sunday there will be some lingering moisture and instability over southern Texas and the Gulf Coast region but no coherent signal for heavy rainfall, while a cold front pushing into the Appalachians/Southeast may produce some locally enhanced rainfall but available moisture looks to be only near climatology. The Florida Peninsula should continue to see periods of rain beyond Monday due to the wavy front, with heavier amounts still tending to be localized. The Southeast coast may be near the edge of rainfall that persists over the western Atlantic. Upper troughing expected to amplify over the West next week may bring some precipitation to at least the Pacific Northwest and northern Rockies. Differences in guidance details over recent days tempers confidence in specifics, though current solutions would suggest fairly light totals in the Pacific Northwest with somewhat higher totals possible in parts of the Rockies. High elevations in the Rockies could see some snow if the upper trough and possible embedded low are deep enough. Ahead of the western trough/low, areas of rain will likely develop over the Plains by Tuesday-Thursday. Some rainfall could be locally heavy but it will take additional time to determine which areas may see the relatively higher totals. An area of high temperatures up to 10-15F or so above normal over the interior Northwest into the High Plains on Sunday (with broader coverage of warm lows over the West) will progress eastward with time, moderating some by the time this warmth reaches the Upper Midwest/western Great Lakes by next Wednesday-Thursday. The upper trough forecast to amplify over the West next week should promote a cooling trend over the region, with a fairly broad area of highs 5-15F below normal by next Thursday. Otherwise expect most temperature anomalies to be in the single digits, including the East leaning a little below normal Sunday-Monday and the southern Plains/central Gulf Coast tending to be slightly above normal after Sunday. Rausch 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