Extended Forecast Discussion NWS Weather Prediction Center College Park MD 232 AM EDT Fri Jul 31 2020 Valid 12Z Mon Aug 03 2020 - 12Z Fri Aug 07 2020 ...Hurricane Isaias to Threaten the U.S. East Coast into early next week... ...Overview and Guidance/Predictability Assessment... Latest guidance shows a pattern that will be fairly slow to evolve. An amplified trough/ridge configuration over the eastern half of North America and western Atlantic should relax somewhat toward the end of the week. Earlier in the period Hurricane Isaias is likely to track near the East Coast, between the trough and ridge. Farther west an upper ridge centered over/near the Southwest will drift eastward with time and reach the Rockies/Plains by the end of the week. More progressive Pacific flow should bring one trough into the Northwest U.S. and western Canada early in the week. This trough should gradually weaken as it continues east/northeast. A less amplified Pacific trough should arrive around Wed-Thu. Expect separate energy to develop an upper trough with a possible embedded low near the California coast. Within the eastern North America upper trough the most prominent guidance difference arises mid-late week as the majority model/mean consensus shows an embedded upper low settling near the Upper Great Lakes for a while before ejecting late in the week, versus the 12Z/18Z GFS runs that are quicker to eject this energy. Longer term continuity seems to favor the slower closed low scenario and the GEFS mean hedges more in that direction versus the operational run so preference was toward the non-GFS solutions. Note that the new 00Z GFS has made a favorable adjustment to the slow closed low idea. The forecast of Hurricane Isaias and exact sensible weather effects along the East coast is still problematic. Overall the greater guidance spread exists for speed of northeastward progression but there is still some left-right variance, and as part of that some potential dependence on precise evolution of the upper trough to the west. Reflecting the proper strength of Isaias will be an ongoing challenge as well. Refer to National Hurricane Center advisories and accompanying products for the latest information regarding Hurricane Isaias. Within the Pacific/western North American stream, solutions stray for the leading shortwave after Mon with the ensemble means suggesting an average of recent GFS/ECMWF runs. GFS runs (especially the 18Z version) stray to the fast side with the second shortwave due to upstream energy crossing the Bering Sea/Aleutians early-mid week likewise straying faster than consensus by the latter half of the week. In order to reflect larger scale forecast preferences this forecast update started with an operational model blend early, followed by removal of the GFS and trending toward about two-thirds total GEFS/ECMWF mean weight by days 6-7 Thu-Fri. The forecast maintained modest input from the 12Z ECMWF/CMC. Manual adjustments yielded the official track for Hurricane Isaias. ...Weather/Hazard Highlights... Expect heavy rainfall and strong/gusty winds to accompany Hurricane Isaias as it likely affects areas along and near the East Coast as well as the western Atlantic during the first part of next week. Based on the track from the 03Z NHC advisory, the highest rainfall totals should extend from near the North Carolina/South Carolina coast into southern New England. There are still significant uncertainties in the forecast including possible interaction with the upper trough over the east-central U.S. and a leading wavy front that could expand the coverage of significant rainfall. Locations from the Midwest to Appalachians may see some rainfall with this wavy front during the first half of the week followed by a drier trend as high pressure builds in. The pattern should be favorable for rain of varying intensity over Florida. A front draped along the High Plains and likely to advance as a warm front later in the week may provide a focus for showers and thunderstorms at times while lingering monsoonal moisture may allow for occasional convection over the southern Rockies and vicinity. The amplified upper trough covering the east-central U.S. for a decent portion of the week will support below normal temperatures across much of the Plains/east-central U.S. Coolest anomalies in the minus 10-15F range are most likely over the Central Plains and Mid Mississippi Valley Mon-Wed. This area of cool temperatures should moderate by next Thu-Fri. The upper trough developing near the California coast may lead to some localized areas with highs 5-10F or so below normal, especially mid-late week. On the other hand highs up to 5-10F above normal will accompany the upper ridge that drifts from the Southwest into the southern Rockies/High Plains. Moist flow ahead of the eastern upper trough will lead to warm lows over the Northeast during the first half of the week but much of the East should see highs close to normal most days--a significant change from the persistent heat of July. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml