Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Sat Aug 01 2020 Valid 12Z Tue Aug 04 2020 - 12Z Sat Aug 08 2020 ...Hurricane Isaias to Threaten the U.S. East Coast into next week... ...Overview and Guidance/Predictability Assessment... Latest guidance advertises a gradual transition from fairly amplified flow toward a more typical summertime pattern by the end of next week or the weekend. Based on the 03Z NHC advisory Hurricane Isaias is expected to weaken to tropical storm status by the start of the forecast period early Tue and continue lifting rapidly northeastward from near the Mid-Atlantic coast--between the initially amplified eastern North American upper trough and western Atlantic upper ridge. There continues to be uncertainty over exactly how Isaias and the upper trough/embedded closed low may interact. With time the upper low will lift out and the overall trough will become more shallow. Upstream, a ridge aloft will settle over the southern Rockies/Plains while somewhat more progressive Pacific flow will bring a couple features around the ridge's far northern periphery. Meanwhile an upper trough should develop near the California coast, most likely to be deepest around Thu-Fri. Slower GFS trends over the past day, and furthered in the new 00Z run, are helping to narrow the spread for Isaias compared to previous runs. However 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 forecast that is now early in the extended time frame. Consult National Hurricane Center advisories and accompanying products for the latest information regarding Isaias. There is still a fair amount of spread for the specifics of Pacific-origin features rounding the Rockies/Plains ridge. Solutions for leading energy already inland at the start of the period early Tue vary between a track near the Canadian border or far northern Plains and filtering into the lingering mean trough downstream versus the energy being diverted into Canada. New 00Z guidance seems to be tilting support more to the northern tier U.S. option. Small scale of this energy leads to low predictability. Timing differences develop for the second feature arriving late in the week, with some dependence on amplitude/speed of upstream flow. Latest GFS/CMC runs have tended to be on the faster side of the spread as this second shortwave ejects into Canada with ensemble means favoring a compromise solution or slightly slower toward the 12Z ECMWF. Both the 12Z UKMET and CMC differed from consensus for some aspects from the forecast so the initial blend used the latest GFS/ECMWF and their respective ensemble means, leaning more to the models early and slightly more to the means late. Manual adjustments yielded the 03Z NHC advisory track for Isaias. ...Weather/Hazard Highlights... Expect Isaias to track northeastward from near the Mid-Atlantic Tue into Wed, bringing a period of heavy rain and strong/gusty winds. In conjunction with the increasing wave heights, the moon phase will be full by 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. 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 parts of the Mid-Atlantic and Southeast into the late week/weekend. The pattern will also support a fairly wet period over the Florida Peninsula. Low-predictability energy aloft that may drift across the far northern tier could produce episodes of showers/thunderstorms from the Northern Plains into Midwest. A front initially draped over the High Plains may provide a focus for activity as well. Scattered diurnal convection will be possible over and near the southern Rockies. The amplified pattern in place at the start of the period will promote well below normal temperatures over much of the Plains/Mississippi Valley especially on Tue-Wed with some minus 10-15F anomalies 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 the upper ridge settling over the region. On the other hand most of California should see below normal highs (by up to 5-10F or so) with the upper trough nearby. Rausch/Campbell 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