Extended Forecast Discussion NWS Weather Prediction Center College Park MD 500 PM EDT Fri Jul 23 2021 Valid 12Z Mon Jul 26 2021 - 12Z Fri Jul 30 2021 ...Triple-digit heat in the north-central U.S. forecast to moderate slightly toward the end of next week as heat intensifies over interior Pacific Northwest... ...Overview with Weather Highlights/Hazards... Model guidance continues to advertise a warm ridge dominating much of the western U.S. while multiple shortwaves will push downstream to sustain general troughing across the Northeast, where models continue to struggle with resolving the various shortwave interactions and arrive at a common and stable solution. Rain associated with a stationary front can be expected to spread eastward from the Great Lakes early to middle of next week into the Northeast later in the week but with considerable uncertainty in its timing and intensity. Meanwhile, scattered thunderstorms can be expected Monday into Tuesday ahead of a cold front from the central Appalachians to the Mid-Atlantic where locally heavy rainfall is possible. The rain is forecast to push slowly south into the Southeast by midweek and become more widely scattered. However, the National Hurricane Center is currently monitoring the potential for tropical cyclone formation east of Florida where the disturbance may intensify and drift toward the southeast U.S. coast early to middle of next week. Meanwhile, another heatwave is forecast to peak by next Tuesday across the northern Plains and the northern Great Basin where widespread high temperatures are forecast to reach well into the 100s. Gradual moderation into the 90s is forecast for the north-central U.S. toward the end of next week but it appears that the heat will once again intensify across the interior Pacific Northwest. Across the Mid-Atlantic, summer heat is forecast to return and persist while cool than normal weather will prevail over the Northeast. An upper low drifting offshore into the Pacific and the strengthening/consolidation of the Rockies-Plains upper ridge next week should act to spread moisture to the north and west out from the Southwest. Activity to extend into California, the Great Basin, and northward over the Rockies should be less intense than the mainly short term focusing Arizona event. ...Guidance/Predictability Evaluation... The most uncertain part of the forecast is across the Northeast where models continue to struggle with resolving the various shortwave interactions converging along a mean trough. A wide variety of solutions regarding the timing/intensity and whether a low pressure system will develop over the Northeast exists later next week. In view of these model spread, more weights from the ensemble means were applied than normal especially from Day 5 onward. The 06Z GFS appeared to overdevelop the low pressure wave over the northern Plains and thus the more reasonable 00Z GFS was adopted instead. On the other hand, the 06Z GEFS mean was compatible with the ensemble means from the ECMWF and CMC through Day 7. Meanwhile, the 12Z ECMWF now indicates cyclogenesis over New England late next week, which is not well supported by its ensemble mean. The WPC medium range forecast package was based on a general model compromise among the 00Z GFS/06Z GEFS, 00Z ECMWF/EC mean, and smaller contribution from the 00Z CMC/CMC mean. A greater weight toward the ensemble means was used from Day 5 onward to handle the above normal uncertainty over the Northeast. Kong Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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 Hazards: - Heavy rain across portions of the Mid-Atlantic, Mon, Jul 26. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Excessive heat across portions of the Lower Mississippi Valley and ArkLaTex, Mon-Fri, Jul 26-Jul 30. - Excessive heat across portions of the Northern High Plains, Mon-Tue, Jul 26-Jul 27. - Excessive heat across portions of the Upper and Middle Missouri Valley, Mon-Wed, Jul 26-Jul 28. - Excessive heat across portions of the Lower Missouri Valley and Middle Mississippi Valley, Wed-Thu, Jul 28-Jul 29. - Excessive heat across portions of the interior Pacific Northwest, Thu-Fri, Jul 29-Jul 30. - Much above normal temperatures across portions of the Northern Great Basin, Mon, Jul 26. - Much above normal temperatures across portions of the Northern and Central Plains and the Midwest, Mon-Thu, Jul 26-Jul 29. - Heavy rain across portions of mainland Alaska, Tue-Wed, Jul 27-Jul 28.