Extended Forecast Discussion NWS Weather Prediction Center College Park MD 433 PM EDT Tue Jul 28 2020 Valid 12Z Fri Jul 31 2020 - 12Z Tue Aug 04 2020 ...Late week heavy to excessive rainfall threat from the Mid-Mississippi Valley to Ohio Valley... ...Tropical system threat for the Caribbean then possibly the Bahamas/Southeast U.S. this weekend/early next week... ...Overview and Guidance/Predictability Assessment... Troughs over the Pacific Northwest and over the eastern third of the U.S. will be well-established throughout the extended periods. With the sub-tropical high over the Desert Southwest, mid-to-upper level ridging will develop and into the Interior West/Rockies/High Plains. A disturbance in the Caribbean is being closely monitored by the NHC and is projected to develop into a tropical storm. This cyclone, currently referred to as Nine (if materializes, named Isaias), is a low confidence forecast at this time- especially in terms of specific timing, forecast track and impacts. In general, the deterministic and ensemble means are in good agreement with the synoptic pattern through the extended periods. Previously, the better clustering for the tropical feature in the Caribbean was a combo of the GFS and CMC while the ECWMF trended more west with the track. The latest guidance has shown that the latest couple of runs of the GFS is actually trending toward the ECWMF while the CMC is very progressive and send the tropical cyclone up the Southeast/Mid-Atlantic Coast and entangles with a cold front tracking offshore the Eastern Seaboard. Given the slight wavering in the preferred track, opted to stir close to continuity. The WPC blend consisted 00/06Z GFS/00Z ECWMF/CMC/UKMET with increasing weighting of the 00Z ensemble means by the end of the extended forecast. BY incorporating more mean data with time helped account for the forecast spread. Meanwhile, upper level high pressure anchored over the Southwest U.S. and building ridging northward should support moderately anomalous summer heat across the Intermountain West/Rockies/High Plains and the Southwest. Daytime highs near 100 degrees are possible (110 to 120 across the desert Southwest) which are roughly 5 to 10 degrees above average. From the Gulf Coast to the Southeast, daytime highs should be near normal but the combined heat and humidity will result in triple digit heat indices approaching 110 in some places. Farther north, temperatures could be 10 to 15 degrees below normal into next weekend across the Central Plains/mid-Mississippi Valley within a region of substantial cloud cover and precipitation. ...Weather/Hazard Highlights... A wavy frontal boundary will be draped from the Southern/Central Plains to the the Mid-Atlantic through the weekend and into the coming week. Pooling Gulf moisture combined with this frontal boundary will provide a focus for showers and thunderstorms across this region for several days. This poses a threat for moderate to heavy or excessive rainfall from portions of the middle Mississippi Valley to the Ohio Valley. This activity may eventually lift north with the surface low towards the lower Great Lakes, with rainfall also likely south and east along the attendant cold front into the Appalachians and Mid-Atlantic regions. Additionally, rainfall associated with the potential tropical cyclone may spread across Florida, the Southeast and portions of the Mid-Atlantic region. 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 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southern Plains, and the Ohio Valley, Fri-Sat, Jul 31-Aug 1. - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Tue, Aug 4. - Heavy rain across portions of the Southeast, Sun-Tue, Aug 2-Aug 4. - Heavy rain across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun, Aug 2. - Heavy rain across portions of the Mid-Atlantic, Fri, Jul 31. - Flooding possible across portions of the Central Plains and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Central Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Fri-Tue, Jul 31-Aug 4. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Aug 1-Aug 2. 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