Extended Forecast Discussion NWS Weather Prediction Center College Park MD 510 PM EDT Wed Jun 16 2021 Valid 12Z Sat Jun 19 2021 - 12Z Wed Jun 23 2021 ...Dangerous heat continues across portions of the Interior West... ...Extreme heat for the Southwest/California into the weekend followed by a cooler trend... ...Tropical system to bring heavy rainfall and flooding to Central Gulf Coast and Southeast Friday and this weekend... ...Overview... Extreme heat continues over the Southwest U.S. and California through Saturday thanks to the persistent upper level ridging over the West. Some moderation in temperatures is expected though later in the weekend and next week as the ridging is gradually replaced with an upper trough forecast to approach the California coast. Until then, some much above normal temperatures of 10F to 20F will remain likely for the Great Basin and portions of the Northwest. Elsewhere across the CONUS, developing troughing over the central/east U.S. will parade a few cold fronts through the region and could spell some locally heavy rainfall over the course of several days. Finally, a tropical disturbance is expected to approach the west-central Gulf Coast later this week and weekend and will bring some significant rainfall totals to the area. This is likely to lead to flooding along portions of the central Gulf Coast as well as inland across portions of the Southeast. ...Guidance/Predictability Evaluation... The model guidance has improved and clustered somewhat with the track and timing of the potential tropical system current in the Bay of Campeche forecast to drift northward toward the west-central Gulf later this week. Compared to the previous forecast cycle, only minor nudges to the east were needed for the day 3 position and the forecast favored a GFS/ECMWF blend. The CMC and UKMET continued to be on the slightly slower/southwestward side of the model envelope, but that spread has lessened in the last few cycles. The system will weaken as it moves inland over Alabama this weekend then become absorbed in the mean flow over the Carolinas before moving offshore. The ECMWF still wants to hold onto a stronger low center as it lifts northeastward through the Carolinas and Mid-Atlantic but for now, in coordination with NHC, a weaker system is the preference. A blend of the latest GFS/ECMWF was the preferred approach for a consensus based forecast approach for the rest of the CONUS. The leading shortwave crossing the Great Lakes/Northeast along with associated front, plus the trailing upper trough that amplifies into the central/eastern U.S. during the first half of next week, are agreeable in concept. Shortwave energy helping to carve out the deepening central U.S. trough will spawn a deepening low pressure system Sunday into early next week. As it approaches the Great Lakes, guidance all points to a sub-1000 mb low by Monday. Its associated cold front will sweep through the central U.S. with sprawling high pressure overtaking the central U.S. by the middle of next week. Meanwhile, a West Coast trough will settle offshore California and the latest model guidance shows fairly good/agreeable support here such that a general model blend was sufficient. ...Weather Highlights/Hazards... Record breaking and dangerous heat should continue into Saturday over the Southwest, California Central Valley, and Interior West where highs well into the 100s and 110s are likely. This region should begin to see temperatures moderating a little on Sunday and to a greater extent during the first half of next week. Meanwhile areas to the north will trend hotter into early next week with some areas seeing highs reach into the 90s and possibly low 100s as far north as eastern Washington. These readings are 10F to 20F or so above normal and could produce a few isolated daily records. The combination of extreme temperatures and duration will make this a dangerous heat wave for sensitive groups. Excessive heat warnings/watches are in place for many areas of the Southwest and the California Central Valley through Saturday. For the central and eastern U.S., the combination of frontal passages, potential for scattered to numerous daily thunderstorms, and a looming tropical system will likely keep temperatures at or slightly below the mid/late June normals. The broadest area of cool readings will progress from the northern Plains into the Great Lakes/Ohio Valley Monday-Wednesday. There is increasing concern for potentially significant flooding impacts resulting from heavy rainfall associated with the tropical system across the Central Gulf Coast beginning Friday and this flooding threat then spreads inland across portions of the Southeast this weekend. Given the much above normal rainfall seen this spring contributing to high/wet soil moisture content, there is concern for some significant flooding impacts. A leading front that settles from the Plains into Mid-Atlantic this weekend followed by strengthening Plains through Upper Great Lakes low pressure that returns the first front northward and then sweeps a cold front through the central/eastern states early next week will produce multiple rounds of showers/storms. Some of this rain may be heavy. Also worth monitoring will be to what extent the moisture from the tropical system may interact with the second front as it reaches the eastern U.S. Taylor/Rausch Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sat-Wed, Jun 19-Jun 23. - Heavy rain across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sun-Mon, Jun 20-Jun 21. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding possible across portions of the Southeast and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Excessive heat across portions of the Central Great Basin, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Sat-Wed, Jun 19-Jun 23. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sat-Mon, Jun 19-Jun 21. - Excessive heat across portions of the Southern Plains, Sun-Wed, Jun 20-Jun 23. 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