Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 PM EDT Sat Aug 01 2020 Valid 12Z Tue Aug 04 2020 - 12Z Sat Aug 08 2020 ...Isaias to Threaten the U.S. East Coast into Wednesday ... ...Overview and Guidance/Predictability Assessment... A closed low settles just north of the Great Lakes through the middle of next week before lifting northeast across Quebec/Newfoundland through the rest of the week. Based on the 15Z NHC advisory, Hurricane Isaias is expected to weaken to tropical storm status and be inland over NC early Tuesday and rapidly shifting northeastward along/near the Mid-Atlantic coast Tuesday then along/near the New England coast late Tuesday night into Wednesday. The main uncertainty with Isaias is its interaction with the upper trough/embedded closed low and related timing/longitudinal specifics to the track. With time the upper low will lift out and the overall trough will become more shallow. Upstream, a ridge aloft will drift east over the Rockies, reaching the Plains by late next week. An upper low/vort lobe will drift east ahead of the ridge axis over the southern Canadian Prairies/northern Great Plains through midweek and over the Great Lakes late next week. Somewhat more progressive Pacific flow will bring a low/trough east over the Gulf of Alaska and a trough over the Pac NW in the Thursday timeframe. Meanwhile a separate upper trough/possible low should develop/shift into the northern California coast Tue/Wed and persist/drift south through the rest of the week. The slowing trend in the GFS ended with the 00Z run with the 06Z and 12Z runs wavering a bit, particularly with the track through the Carolinas/Mid-Atlantic with the 12Z run seemingly settling a bit on a decent forecast in the context of latest guidance, just a little faster than the slowest 00Z GFS, and notably faster than the 00Z ECMWF which is particularly slow. This is still a sensitive forecast with respect to the upper trough/low interaction and the difficulties that some models have had in depicting the strength of Isaias continue to result in greater than desired uncertainty in specifics for a Day 3-4 forecast. Consult National Hurricane Center advisories and accompanying products for the latest information regarding Isaias. Energy of the shortwave low/vort lobe is small enough to result in low predictability. The GFS/CMC are more progressive than the slower ECMWF which is typical of model biases, so a compromise between them was made. The 00Z UKMET differed from consensus for many aspects of the forecast from consensus, so the initial blend used a compromise of the 06Z GFS/00Z CMC/00Z ECMWF (close to what the 12Z GFS later came in with) becoming more reliant on equal weighting of the agreeable 06Z GEFS mean and 00Z ECENS mean. Manual adjustments yielded the 15Z NHC advisory track for Isaias. ...Weather/Hazard Highlights... Expect Isaias to track northeastward from inland over the southern Mid-Atlantic to near the Northeastern Seaboard Tuesday night/Wednesday, bringing a period of heavy rain and strong/gusty winds. In conjunction with the increasing wave heights, the moon phase will be full by midday on the 3rd and the increase of daily high tide could potentially enhance the risk for coastal flooding or storm surge. The precise axis of heaviest rainfall between the Mid-Atlantic and Northeast will be sensitive to exact track/timing of Isaias and interaction with an approaching cold front plus the supporting upper trough/low. Consensus is improving in this heavy rain swath extending from the Mid-Atlantic to across New England Tuesday night/Wednesday. Moist flow ahead of the gradually weakening eastern trough aloft and stalling of the southern part of the eastern cold front will maintain the chance for periods of rainfall over the southern Mid-Atlantic/Carolinas into the late week/weekend. The pattern will also support a fairly wet period over the Florida Peninsula. Lower-predictability energy aloft that looks to drift across the far northern tier could produce episodes of showers/thunderstorms from the Northern Plains into Midwest through the period. Scattered diurnal convection will be possible over and near the south/central Rockies. The amplified pattern in place at the start of the period with high pressure centered over the central Plains will promote well below normal temperatures over much of the Plains/Mississippi Valley especially on Tue-Wed with some minus 10-15F anomalies for both max and min possible from the Central Plains into the Midwest. Late week/weekend temperatures over this part of the lower 48 will trend closer to normal as flow aloft returns to a more typical summer pattern. Expect the southern Rockies/High Plains to see readings 5-10F above normal most days with base of the upper ridge settling over the region. On the other hand most of California should see below normal max temps (by up to 5-10F or so) with the upper trough lingering near the coast. Jackson 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