Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1118 AM EDT Thu Sep 06 2018 Valid 12Z Sun Sep 09 2018 - 12Z Thu Sep 13 2018 ...Heavy rainfall likely to extend into the Ohio Valley/eastern Great Lakes/Northeast by this weekend/early next week... ...15 UTC Update... An amplified flow pattern is expected from the North Pacific across North America during the medium range, with an anomalous blocking ridge in place across the Bering Sea, downstream persistent negative height anomalous across the northwestern U.S., and a persistent upper low across northern Canada. One major question is whether this amplification will continue further downstream with an upper ridge expected to develop off the Eastern Seaboard across the North Atlantic, which will be major player in the eventual track of Hurricane Florence. Additionally, Tropical Depression Gordon (expected to be a post-tropical cyclone) by the medium range, will move northeastward from the Mid-Mississippi Valley to the Great Lakes Sun-Tue, bringing heavy rain along with it. Models showed reasonably good consensus on the medium range track of Gordon. In general, model spread in terms of the overall synoptic pattern across North America during the medium range remained a bit below average and predictability seems relatively good. The major exception to this is the eventual track of Hurricane Florence, for which models/ensembles continue to show a large spread during days 6-7. Please refer to NHC advisories for the latest information on Hurricane Florence. A multi-model deterministic blend (including the ECMWF/GFS/CMC) was used as a basis for the WPC forecast during days 3-5, with ensemble weighting (ECENS/GEFS) boosted during days 6-7. This kept the forecast close to continuity, with no major changes for this update. Ryan Previous Discussion (issued at 0643 UTC) ...Pattern Overview and Guidance/Predictability Assessment... The latest runs of guidance maintain continuity fairly well in showing pronounced trough amplification along and just offshore the West Coast. Current consensus shows this evolution will be in response to the opening up of a very strong ridge positioned over the Bering Sea at the start of the period and likely merging with a separate ridge building over the Pacific. While there is good agreement and thus fairly high confidence for the large scale pattern there are ongoing uncertainties regarding embedded details. Farther east, the post-tropical reflection of Gordon will track northeast from the middle MS Valley, behind strong eastern Canada/northern New England high pressure. As upper ridging rebuilds along/offshore the East Coast after early Sun, attention will turn to Hurricane Florence whose track could reach fairly close to the East Coast toward the end of the period Wed-Thu. Days 3-5 Sun-Tue emphasized the 18Z GFS and 12Z ECMWF/UKMET with a small minority of 18Z GEFS/12Z ECMWF mean input. An average of those three operational models provided a good intermediate track for post-tropical Gordon with only modest adjustments in continuity. Over the past day ECMWF runs have held fairly steady with track/timing while the GFS has been continuing a longer-term slower/southward trend. By day 4 Mon adjustments in the new 00Z GFS have brought its solution close to the favored intermediate track based on 12Z/18Z guidance. Meanwhile the blend yielded little change in the forecast upstream. A leading shortwave ejecting from the initially established northeast Pacific trough will bring a frontal system into the northern Plains by the start of the period Sun, followed by eventual dissipation over the Northern Tier as best dynamics lift into Canada. At the same time a low at the surface and aloft will drift toward Vancouver Island, pushing a leading cold front into the Northwest. Days 6-7 Wed-Thu steadily trended the forecast more toward the ensemble means, both to downplay lower-confidence details within the eastern Pacific/West Coast upper trough as well as to yield a solution closer to the middle of the envelope for Hurricane Florence. Within the western trough there is reasonable agreement that the upper low nearing Vancouver Island on Tue should subsequently open up with surrounding trough energy ejecting inland. Then a number of operational runs suggest another bundle of energy dropping through northwestern Canada will form another closed low reaching close to Vancouver Island. Incorporating both operational and the more open ensemble mean solutions seems reasonable given typically lower detail predictability this far out in time. Such a blend also tempers emerging differences in trough orientation near the coast toward day 7 Thu. The 18Z GFS/12Z CMC showed a sharper/neutrally tilted trough while other guidance showed somewhat more southwestward elongation. Either scenario could be plausible from a multi-day mean perspective. Ahead of the trough expect a mean frontal boundary to settle from the northern Plains southwest through California, parallel to flow aloft. As for Florence's track, confidence in any solution is rather low as model/ensemble members have been diverging considerably after about Sun. By day 7 Thu 18Z GFS/12Z ECMWF runs that bring Florence to or near the East Coast are in the western part of the overall ensemble spread. The new 00Z GFS has returned a bit to the east. ...Weather Highlights/Hazards... Expect an area of heavy rainfall to accompany the northeastward progression of post-tropical Gordon from the middle MS Valley Sun onward. Guidance is still in the process of fine-tuning the exact track and timing but the overall consensus has held fairly steady over the past day, highlighting best potential for highest totals to extend across the Ohio Valley and into the eastern Great Lakes. Rainfall amounts should decrease somewhat as the moisture continues into the Northeast. A front to the east may provide some focus for activity over the Mid-Atlantic. Locations to the southwest of the Ohio Valley, through parts of Texas, may also see locally enhanced rainfall ahead of a surface front/trough. By next Wed-Thu the track of Hurricane Florence will likely provide the greatest uncertainty for sensible weather effects near the East Coast. Even if the track remains far enough offshore to keep the region dry, some coastal areas may see brisk winds. Across Florida expect a continuation of diurnally favored convection. The evolving pattern over the eastern Pacific/western North America will bring moisture into the Pacific Northwest after Sun, with precipitation likely persisting through the end of the period. Expect highest totals to be confined to Vancouver Island with a fairly rapid decrease as one travels southward, with most measurable rainfall staying north of California. Locations from the extreme northern Rockies across the northern Plains may see at least a couple episodes of light to locally moderate precipitation as features eject from the upstream trough aloft. The cooling trend over the Northwest should bring multiple days of highs 5-10F below normal next week. Within the leading warm sector from the Southwest into the northern Plains, some northern Plains locations may see a couple days up to 10-15F above normal. Farther east, below normal daytime highs (locally 10-15F below normal) from the central-southern Plains into Mid-Atlantic/Northeast on Sun will steadily moderate thereafter. Min temperatures in particular should trend above normal over the East by Tue-Thu. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml