Extended Forecast Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Tue Aug 02 2022 Valid 12Z Fri Aug 05 2022 - 12Z Tue Aug 09 2022 ...The Southwest Monsoon Resumes a Very Active Phase Late This Week... ...Heavy Rain Likely from the Ohio Valley Through The Northeast Late This Work Week... ...Overview... Guidance expects a fairly typical summertime pattern with the main band of westerlies across southern Canada and northern tier CONUS, though with upper ridging to the south tending to be somewhat stronger than the climatological mean. An upper low should lift very slowly northeastward offshore the West Coast. The models and ensembles agree on the large scale pattern but significant shortwave differences develop within the westerlies from fairly early in the forecast. As a result confidence decreases for surface front/wave details and in turn for associated areas of rainfall which could be heavy at times over portions of the northern half of the central/eastern U.S. Meanwhile the monsoonal pattern over the West should become more pronounced again late this week and likely remain fairly active through the weekend and perhaps into next week. ...Guidance Evaluation/Predictability Assessment... There are two primary aspects to the uncertainty within the northern tier U.S./southern Canada flow. The first is with with initial troughing over and near southwestern Canada. In the 00Z/06Z guidance there was a nearly even split between the ECMWF/CMC that took most of the trough energy bodily eastward versus the GFS/UKMET that accelerated the core of the Canadian trough onward while leaving behind a trailing shortwave that ultimately tracks along the U.S.-Canadian border. Needless to say, these different scenarios lead to significant regional differences in front/wave details. In the new 12Z cycle, the CMC has adjusted more toward the GFS cluster while the ECMWF has nudged toward holding back a little more trailing shortwave energy. The second issue arises with upstream energy originating from the Aleutians/Bering Sea. There is a general signal for a defined shortwave to progress across the western and central portions of southern Canada Sunday-Tuesday but with timing/amplitude differences affecting the progress of the leading front that may reach the northern states. GFS/GEFS mean runs have been on the flat side aloft while the past couple ECMWF runs have been on the fast and/or amplified side. The 06Z GEFS did show hints of a trend in the ECens/CMCens direction. The new 12Z ECMWF has trended noticeably flatter as the trough enters southwestern Canada but then trends toward its amplified tendency thereafter. Based on the array of guidance, the updated forecast started with an operational model composite for the first half of the period to balance the possible scenarios for northern stream shortwaves and resulting fronts/waves. Trending toward an even blend of models and ensemble means by late in the period (early next week) provided a good intermediate starting point within the full range of possibilities. ...Weather/Hazards Highlights... Expect hot temperatures over northern half of the Plains into the Midwest late this week into the weekend, eroding from the northwest as a cold front approaches. Some highs may reach 10-15F above normal and a few record highs may be possible over the High Plains. Very warm conditions should extend into the Great Lakes/Northeast through the weekend with some gradual cooling early next week. Over the West, maximum temperatures will tend to be somewhat below normal over the Great Basin/Southwest and vicinity due to persistent clouds and periods of rain. The upper shortwave initially over the Northwest should promote near to slightly below normal highs on Friday. Then the Northwest should trend more above normal as some degree of upper ridging builds. Highs could locally reach 10-12F above normal. For precipitation, the main threats for flash flooding will be associated with monsoonal moisture over the Southwest/Great Basin/Rockies as well as with fronts/waves over northern and central parts of the U.S. east of the Rockies. The combined signals for rainfall and already wet conditions over the Desert Southwest into the central Rockies/Great Basin late this week into the weekend support some Slight Risk areas in the experimental days 4-5 Excessive Rainfall Outlooks. Farther east, one area of potential focus for significant rainfall will be over the Ohio Valley and Northeast along a front that stalls and then returns north as a warm front late this week into the weekend. A trailing wavy front progressing eastward from the northern Plains may also generate some areas of heavy rainfall between the northern half of the Plains and eastern U.S. from the weekend into next week. Rausch Hazards: - Heavy rain across portions of New England, the Mid-Atlantic, the Appalachians, and the Tennessee and Ohio Valleys, Fri, Aug 5. - Heavy rain across portions of the Northern Rockies, the Central Great Basin, and eastern California, Fri, Aug 5. - Heavy rain across portions of the Upper Midwest, Fri-Sat, Aug 5-Aug 6. - Heavy rain across portions of the the Central and Southern Rockies, the Southern Great Basin, the Southwest, and southern California, Fri-Tue, Aug 5-Aug 9. - Flooding possible across portions of the Southern Appalachians, the Central Appalachians, the Ohio Valley, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Excessive heat across portions of New England, the Mid-Atlantic, and the Carolinas, Fri, Aug 5. - Excessive heat across portions of the Northern and Central Plains, Fri, Aug 5. - Excessive heat across portions of the Central Plains, the Middle Missouri Valley, and the Middle Mississippi Valley, Sat-Sun, Aug 6 Aug 7. - Excessive heat across portions of the Central and Southern Plains and the Middle and Lower Mississippi Valley, Fri-Tue, Aug 5-Aug 9. 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml