Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Thu Jul 30 2020 Valid 12Z Sun Aug 02 2020 - 12Z Thu Aug 06 2020 ...Tropical Storm Isaias to Threaten the U.S. East Coast this weekend into early next week... ...Overview and Guidance/Predictability Assessment... There does not seem to be much reason to make more than minimal changes to the previous overnight discussion other than to offer a bit more reliance on ensemble means given tropical and longer time frame uncertainties. Established troughing off the West Coast by the start of the medium range period (Sunday) should begin to shift into the Northwest U.S. early next week. East of the Rockies, upper troughing across the Mississippi Valley/Midwest states should stick through the period as an upper level low tries to form over the Upper Great Lakes. The latest forecast track from the NHC shows Tropical Storm Isaias tracking up the Florida east coast, and then generally up the Eastern U.S. coast next week, likely eventually becoming entangled within the trough to the west. The greatest forecast concern in the medium range period continues to be Tropical Storm Isaias and its threat to the Southeast/Eastern CONUS. Models and ensembles continue to struggle with the eventual track of this system in the extended range both with respect to the timing and strength, and how much interaction the system has with the troughing to the west. Thus, forecast confidence on track and QPF concerns continue to be low and so adjustments are likely as we get closer in time. Refer to the products and advisories issued by the NHC for the latest information concerning Tropical Storm Isaias. Overall, model and ensembles guidance continues to show better than average clustering with respect to the large scale pattern through day 5, outside of Isaias. Beyond day 5, some variance in the timing/strength details in the models begin to arise with energy entering the Northwest U.S.. Accordingly, opted to use an a blend of the GEFS/ECMWF ensemble means to best mitigate these and Isaias differences. This approach maintains good continuity with the previous WPC forecast. ...Weather/Hazard Highlights... Heavy rainfall and gusty winds are likely to accompany Tropical Storm Isaias as it begins affecting parts of the Florida and the U.S. Southeast coast this weekend. As the cyclone tracks northward, models and ensembles indicate it may interact with a cold front approaching from the West. This would spread heavy to excessive rainfall northward into the Carolinas/Mid-Atlantic region early next week. Of course, exact location and QPF amounts associated with Isaias are highly dependent on its eventual track, which is subject to change based on the latest model guidance. Elsewhere, heavy rainfall across parts of the Northeast and Lower Great Lakes is possible on Sunday, with some lingering monsoonal moisture across the southern Rockies/High Plains. By early next week, showers may begin to develop across parts of the Plains states ahead of an approaching frontal system. Meanwhile, strong ridging across the Intermountain West/Great Basin/Southwest Sunday into Monday will continue to bring above normal temperatures, with daytime highs in the 90s to near 100 for many (110 to 115 across the desert Southwest). Temperatures across the Northwest/Intermountain West should moderate back towards normal by Tuesday next week as height falls move inland from the Pacific. Upper troughing anchored over the Central U.S. will keep temperatures cool through much of the period, with daytime highs 10 to 15 degrees below normal possible especially across parts of the Central Plains and Mid-Mississippi Valley. Santorelli/Schichtel 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