Extended Forecast Discussion NWS Weather Prediction Center College Park MD 514 PM EDT Fri Jul 24 2020 Valid 12Z Mon Jul 27 2020 - 12Z Fri Jul 31 2020 ...Tropical Storm Hanna to gain strength over the Gulf of Mexico and offer a Gulf Coast to South Texas/Mexico heavy rain and flooding threat this weekend/early next week... ...Overview... A subtropical high centered over the Southern High Plains on day 3/Mon is forecast to slowly retrograde toward the Four Corners region through the medium range period. Meanwhile, an upper low moving through central Canada should spread troughing into the Midwest/Great Lakes and then into the Northeast/Mid-Atlantic by midweek, pushing a cold front through those areas. Additionally, another upper low is expected to drop through the Pacific, bringing troughing to the Pacific Northwest after a stint of ridging early in the week. ...Weather/Hazard Highlights... What is currently Tropical Storm Hanna is forecast to make landfall in Texas this weekend and move westward. By the start of the medium range period Mon, Hanna may be a remnant low, but could continue to bring heavy rainfall especially to Mexico through the early workweek, with some rain still possible in South Texas. Refer to the latest from the National Hurricane Center on the forecast for Hanna. Additionally, tropical moisture and instability along the central Gulf Coast could lead to locally heavy rainfall there through Tue. A cold front is forecast to move progressively across the Great Lakes and Ohio Valley and then through the Northeast and Mid-Atlantic by Wed. Rain and thunderstorms are expected along and ahead of this front, and locally heavy rainfall is possible where storms organize. Farther west, the trailing part of the front is expected to stall and lift back northward a bit across the Plains and Mississippi Valley, leading to rounds of rainfall there. Current rainfall forecasts call for moderate to heavy rainfall over the Central/Southern Plains through Wed, as well as over the Central/Southern Rockies where monsoonal/tropical moisture should stream in and enhance rainfall. Then rain chances increase for the Lower Mississippi and Tennessee Valleys into the Southeast from midweek on, along and south of the front that will provide a focus for heavy rain. With upper ridging across the Northwest, temperatures around 10-15 degrees above normal are forecast Mon/Tue. These will moderate as the next upper low approaches. Ahead of the cold front, the Northeast and Mid-Atlantic can expect warm to hot temperatures through Tue as well, with another round of excessive heat concerns over portions of the Mid-Atlantic. Meanwhile, high temperatures below normal by 5-15 degrees are forecast for the Southern/Central Rockies and Plains through Wed given the rain chances. ...Guidance/Predictability Assessment... The guidance in the beginning of the period was relatively well clustered, so a deterministic model blend of the 00Z ECMWF, UKMET, and CMC and the 06Z run of the GFS was used as a starting point. While model guidance was consistent with the longwave trough/ridge/trough pattern, it differed in terms of placement and timing of shortwave energy rounding the central U.S. ridge for the latter half of the week. Increased use of the 00Z EC and 06Z GEFS ensemble mean solutions as the days progressed helped mitigate these smaller-scale, less predicable features. The 06Z GFS and GEFS mean runs were preferred over the 00Z mainly because they had better clustering with other guidance with a slower movement of the Pacific upper low/trough. Also noted that the 00Z CMC at day 5/Wed onward developed a mid-upper low in central Canada unlike other guidance, leading to a more suppressed ridge and different evolution of the eastern trough. Thus, removed the CMC from the blend at that point. Tate Hazards: - Heavy rain possible from Arkansas eastward across the interior Southeast into the Carolinas and southern Virginia, Wed-Fri, Jul 29-Jul 31. - Heavy rain across portions of the central High Plains to central/southern Rockies, as well as along the central and eastern Gulf Coast, Mon-Tue, Jul 27-Jul 28. - Flooding possible across portions of the central and southern Plains. - Flooding occurring or imminent across portions of the northern and central Plains. - Excessive heat across portions of the the Mid-Atlantic region, Mon-Tue, Jul 27-Jul 28. - Much above normal temperatures across portions of the Pacific Northwest, Mon-Tue, Jul 27-Jul 28. - Much above normal temperatures across portions of the northern Mid-Atlantic into southern New England, Mon, Jul 27. 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