Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM 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... The overall synoptic pattern will be amplified, with a trough edging toward the California Coast, ridge in place over the Southwest, trough over the east-central U.S. and a ridge over the Southeast in the wake of Hurricane Isaias - the eastern half of the CONUS/Atlantic will become less amplified by the late periods. 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. With the 15Z NHC issuance, the closest model guidance to the track was a compromise of the 12Z NAM/GFS/ECWMF prior to the medium points; with mainly heavy GFS to NAM ratio Monday-Wednesday period. This maintained most of the heavy QPF offshore, however a swath of 2 to 4 with locally higher from the southern Mid-Atlantic to southern New England prior to Isaias going Post-Tropical. Refer to National Hurricane Center advisories and accompanying products for the latest information regarding Hurricane Isaias. The blend chosen was to strike a balance between the evolution of the tropical and the rest of the larger scale forecast comprised of 12Z CMC/GFS/ECWMF/UKMET with reducing of the UKMET and GFS by Wednesday (when Isaias is mostly weakened/post-tropical) with increasing of the ensemble means Thursday and Friday. ...Weather/Hazard Highlights... Per the latest NHC track, Hurricane Isaias skirts the east coast of Florida and tracks up the Eastern Seaboard. Heavy rain and strong, gusty winds are expected to accompany Isaias along its path. In conjunction with the Hurricane increasing wave heights, the moon phase will be full by the 3rd; which will increase the daily high tide, potentially increasing the risk for coastal flooding or storm surge. The highest QPF looks to occur from the Outer Banks to near Long Island where areal averages of 2 to 4 inches (locally higher amounts possible) as Isaias tracks northward. It is expected to become extra-tropical and entangles with a cold front advances through the Northeast and offshore to the Canadian Maritimes. 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. Portions of the Midwest and the Appalachians may get some rain early on then transition to a drier airmass by the mid/late periods. Rain, possibly heavy at times, will continue to possible across Florida by the end of the week as the front slows/stalls across the South/Southeast. A stationary front draped across the High Plains will begin to move through the Northern and Central Plains toward the end of the extended forecast and will provide a focus for showers and thunderstorms across parts of the central U.S., where lingering Monsoonal moisture will be pooled. Cooler than normal temperatures can be expected across a vast portion of the east-central U.S. over the next several days as the trough amplifies. Locations in the Central Plains and Mississippi Valley could see afternoon highs averaging 10 to 15 degrees cooler than typical for early August. Additionally, parts of the West Coast will begin to cool 5 to 10 degrees by mid/late week as the upper-level trough nears the California Coast. The ridge over the Southwest will help temperatures to climb 5 to 10 degrees above average. 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. 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