Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Sun Aug 02 2020 Valid 12Z Wed Aug 05 2020 - 12Z Sun Aug 09 2020 ...Isaias to Affect New England into Wednesday... ...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 from the New England coast on Wed. 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. 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 persist near California through the period. Consult National Hurricane Center advisories/products for the latest information regarding Isaias. Guidance remains stubborn with its differences for timing leading into the start of the extended forecast (latest GFS/CMC faster than the 12Z ECMWF, while the new 00Z ECMWF has adjusted somewhat faster) and how the system may interact with the upper low/surface system to the west. Small scale of northern tier energy filtering into what remains of the eastern mean trough aloft keeps confidence in details fairly low. GFS runs have been a bit on the slow/deep side though the 00Z CMC has come in fairly similar for Wed before trending a little weaker/faster. Compared to yesterday there is a more noticeable signal that the upper trough may support a weak frontal wave near the Mid-Atlantic late week into the weekend. The upper trough reaching 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. Also next Sun the 12Z/18Z GFS runs are a bit weaker than consensus with the California upper trough. The 00Z GFS has made a partial trend toward consensus. With the UKMET still differing a fair amount from other guidance for some aspects of the forecast, the updated blend started with an emphasis on the 18Z GFS/12Z ECMWF and lesser weight of the 12Z CMC/18Z GEFS mean/12Z ECMWF mean early. Then the forecast transitioned toward an even model/ensemble weight by the end of the period next Sun. Manual adjustment provided the 03Z NHC advisory track for Isaias which should become extratropical sometime on Wed. ...Weather/Hazard Highlights... Isaias could bring a brief period of locally heavy rainfall and strong winds to parts of northern New England on Wed before continuing 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 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. 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 5-10F or so above normal. 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 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