Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Sep 13 2023 Valid 12Z Sat Sep 16 2023 - 12Z Wed Sep 20 2023 ...Hurricane Lee likely to bring rain and strong winds to New England... ...Overview... Latest solutions continue to show typical spread and run-to-run variability for Hurricane Lee, slowly narrowing in on a track that should reach somewhere between eastern Maine and Nova Scotia by late Saturday. By that time the 03Z National Hurricane Center advisory shows Lee transitioning to extratropical status near the far western coast of Nova Scotia. This track would bring strong winds to New England and significant rainfall to Maine. Behind Lee, an upper trough amplifying into the eastern U.S. during Saturday-Monday should weaken toward 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. Meanwhile the guidance is advertising a significant pattern change over the western U.S. during the period, as a strong upper ridge over the region as of Saturday weakens and gives way to an amplifying trough that may also incorporate a weak upper low nearing California. This trough and leading cold front may bring some moisture into the Pacific Northwest and northern Rockies next week. Above normal temperatures over the Pacific Northwest and northern High Plains this weekend will then shift over more of the northern Plains while much of the West sees a pronounced cooling trend. ...Guidance Evaluation/Predictability Assessment... For Hurricane Lee, model runs continue their typical run-to-run wobbling for track and timing within a gradually narrowing envelope. For the updated forecast based on 12Z/18Z guidance, the 12Z/18Z GFS and GEFS mean were generally closest to the track depicted by the 03Z NHC advisory. The 12Z ECMWF/ECMWF mean were a little farther west but the ECMWF represented an eastward adjustment from its 00Z/12 run that was a western extreme, and the new 00Z ECMWF has come in close to the 12Z run. The new 00Z GFS/UKMET have adjusted a little westward. The upper trough amplifying into the East during during Saturday-Monday still exhibits some embedded low-predictability detail differences within a more agreeable mean evolution. The majority of guidance is suggesting that an embedded upper low may track over or a little north of the Upper Great Lakes, leaning away from some earlier guidance runs that indicated potential for a farther south track. Guidance agrees on weakening the trough after Monday but with continued uncertainty for details. The forecast over the West by the latter half of the period has exhibited a wide range of continuity or trends over the past couple days depending on the guidance source. Overall the ensemble means have been steadier with the current forecast of upper troughing by midweek but latest runs are showing a stronger northern stream component of flow and GEFS means have been closer to current ideas. GFS runs have varied but showed more signal for western troughing than ECMWF runs which have only very recently switched to an amplified western trough versus flat mean flow crossing Canada. While the GFS/GEFS appear to have led the trend, the 18Z GFS in particular looks excessively deep/south with its upper low. Resulting negative height anomalies seem extreme relative to teleconnections based on the strong positive anomalies forecast over or near eastern Hudson Bay in the D+8 multi-day means. The 12Z GFS and new 00Z run at least seem more reasonable with a more northward and transient upper low. The 00Z GFS could still be too deep and overdoing height falls reaching the Plains. Forecast considerations led to the starting blend consisting of a composite of 12Z/18Z models early in the period with some slight adjustments for the depiction of Lee, followed by a transition toward more ensemble input. Day 7 Wednesday incorporated 60 percent total 18Z GEFS/12Z ECens means and lingering components of the 12Z ECMWF/GFS/CMC. ...Weather/Hazards Highlights... The Day 4 Excessive Rainfall Outlook covering the Saturday-Saturday night period depicts a Slight Risk area over approximately the eastern half of Maine along with a band of Marginal Risk to the west, reflecting the potential for heavy rainfall in association with Lee. Soil conditions in Maine are on the wetter side of normal over most areas (near average over parts of the south), so expect some sensitivity to heavy rainfall. Continue to monitor National Hurricane Center products regarding the track of Hurricane Lee and potential effects on New England and the Canadian Maritimes by late week and the weekend. The Day 4 outlook continues to be clear of any other risk areas due to uncertainty in rainfall amounts/rates, but with ongoing regions of interest. These include southern Texas and the Gulf Coast (some lingering moisture and instability but inconsistent guidance signals for rainfall) along with the Tennessee Valley and surrounding areas (wide range of guidance QPFs and mixed signals for moisture anomalies over the region), plus the Florida Peninsula where a wavy front will be draped across the area should promote a wet pattern (but with drier than average soil conditions to start with and wide guidance spread for QPF). No risk areas are indicated on the Day 5 ERO (Sunday-Sunday night) but will continue to monitor the Florida Peninsula and perhaps the southern Appalachians as a cold front reaches the area. Expect the Florida Peninsula to remain in a fairly wet pattern beyond Sunday as the wavy front persists over the area, but with no clear signal yet for organized heavy rainfall. Recent trends for a deeper upper trough to reach the West next week have raised the potential precipitation totals, with best emphasis most likely to be from the Pacific Northwest into the northern Rockies. Some high elevations in the Rockies could see some snow if the upper trough is deep enough. Ahead of this trough, areas of rain could develop over parts of the Plains by around the middle of next week. Expect an area of high temperatures at least 10-15F above normal over the Northwest U.S./northern High Plains during the weekend, with a broader area of above normal lows over the West. Then an upper trough amplifying into the West should bring a pronounced cooler trend with much of the western U.S. seeing highs of 5-15F below normal by next Tuesday-Wednesday, while the initial warmth eventually moderates by midweek as it progresses across the northern Plains. The south-central Plains will start out the weekend with highs several degrees below normal followed by a warming trend over much of the southern Plains. The East should see near to somewhat below normal highs from the weekend into early next week and then near normal temperatures by the middle of next week. 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