Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM EDT Sun Aug 02 2020 Valid 12Z Wed Aug 05 2020 - 12Z Sun Aug 09 2020 ...Isaias moving rapidly across New England Wednesday morning... ...Overview and Guidance/Predictability Assessment... Guidance continues to show similar ideas for the large scale pattern evolution but with embedded detail uncertainties. Flow between an upper low initially between the Upper Great Lakes/James Bay and a strong Atlantic ridge will direct Isaias quickly northeastward through New England Wednesday morning. Progression of the upper low will leave the eastern U.S. under somewhat weaker troughing, composed of lingering energy from midweek as well as northern tier shortwave energy that may feed into the mean trough. The trough axis should reach the East Coast next weekend and may support a frontal wave near the Mid-Atlantic. Meanwhile, an upper ridge will prevail over the southern Rockies and Plains while an upper trough will likely reach the Northwest U.S. and western Canada late this week. Upstream flow may reinforce the trough to some degree. Expect a separate and weaker mean trough to linger near California through the period. Consult National Hurricane Center advisories/products for the latest information regarding Isaias. The GFS remains the fastest global model guidance--about 250 to 300 miles ahead of its ensemble mean positions as well as the ECMWF and EC mean positions as the center of Isaias is forecast to traverse New England. A blend of the 00Z ECMWF, EC mean together with the 06Z GFS and 06Z GEFS yielded a pressure field that is very compatible with the forecast track of Isaias from the National Hurricane Center. The upper trough forecast to reach the Northwest by Thu-Fri has generally been trending stronger over recent days. By Fri the guidance has been diverging for how quickly the leading energy ejects, followed by added spread for how upstream flow feeds into the mean trough. The fast nature of the Northeast Pacific stream seems to favor a compromise approach for ejection of the leading energy, a bit faster than the slow 12Z ECMWF. By day 7 Sun most models/ensembles suggest that latest GFS runs may be a little fast to bring leading height falls into the northern Plains. The 06Z GEFS and the 00Z EC mean are quite agreeable with each other regarding this system, and so a blend of these ensemble means together with smaller portions of their deterministic solutions were used toward the end of next week. ...Weather/Hazard Highlights... Isaias could bring a brief period of locally heavy rainfall and strong gusty winds to parts of northern New England on Wed before departing into the Canadian Maritimes. Moist flow ahead of the eastern U.S. trough aloft along with a leading wavy front should produce periods of rain over some locations between the Appalachians and East Coast. There is currently a fair amount of uncertainty over precise coverage and amounts, with locations between the central/southern Appalachians and Mid-Atlantic currently expected to see highest totals. Florida should see showers/thunderstorms of varying intensity through the period. Leading northern tier energy filtering into the eastern trough aloft may produce some rainfall depending on its strength. Farther west, a couple of fronts reaching the northern Plains as well as an initial High Plains front advancing as a warm front during the period should support periods of central U.S. convection with some locally heavy activity possible. Meanwhile, expect diurnal showers/thunderstorms over and near the southern Rockies. Upper ridging will support persistent above normal temperatures over the southern High Plains with readings 6-12F or so above normal on Wednesday, slowly moderating as the week progresses. Locations on the high end of that range on Wed could challenge daily record highs. The area of below normal temperatures (generally minus 5-12F anomalies) from parts of the Plains into the Ohio Valley/Great Lakes on Wed will shrink thereafter and ultimately give way to near or slightly above normal readings by late week/weekend. Separate upper troughs will bring below normal temperatures to California through the period and the Pacific Northwest through northern Rockies after midweek, again with minus 5-12F anomalies. Northern tier flow will bring an area of moderately above normal temperatures from the Northwest on Wed through the northern Plains and then into the Great Lakes/Northeast. Rausch/Kong 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