Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 likely to bring heavy rainfall and flooding to Central Gulf Coast Friday and this weekend... ...Overview... The most extreme heat initially over the Southwest U.S. and California as of Saturday will steadily moderate thereafter as the upper ridge over the region drifts eastward and a trough with likely embedded upper low takes shape offshore California. However there will be enough upper ridging that extends over the Great Basin and Northwest to bring multiple days of highs 10-20F above normal to those regions. Within mean troughing aligned over the east-central U.S., a leading shortwave will bring a front through the Great Lakes/Northeast during the weekend. A trailing stronger upper trough will support Plains-Great Lakes-Canada low pressure whose trailing front should ultimately extend farther south over the central/eastern U.S. than the first front. These systems will support multiple areas of showers and thunderstorms. Guidance continues to suggest likely development of a tropical disturbance currently in the Bay of Campeche. This feature should organize and strengthen as it approaches the west-central Gulf Coast. There is increasing potential for heavy rainfall and flooding along the Gulf Coast by late Friday through this weekend. ...Guidance/Predictability Evaluation... Over recent days an average of GFS/ECMWF runs has tended to provide the most stable solution for the system expected to emerge from the Bay of Campeche and track toward the west-central Gulf Coast, with CMC runs initially on the fast side and then slow and the UKMET consistently slow/southwest. That said, even the GFS/ECMWF have varied some--including latest GFS runs tracking somewhat farther westward versus 24 hours ago. The new 00Z CMC/UKMET runs dramatically improve guidance clustering as of early Saturday with a position over the northwestern Gulf, poised to reach the Gulf coast near the Texas/Louisiana border. After that time the models diverge some, especially that CMC that strays to the east, but otherwise this is the best agreement that has existed so far. By early next week what is left of the system (with a wide spread between the deepening ECMWF and dissipating GFS) should continue northeastward ahead of the amplifying Plains upper trough. Elsewhere the latest GFS/ECMWF runs and their ensemble means represented the consensus evolution well. 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. Details aloft within the core of the trough over southern Canada and northern tier of the U.S. early next week have varied, thus leading to differences in specifics for Plains through Upper Great Lakes low pressure that should be strong but not extreme by historical June standards. As with the likely tropical system, new 00Z guidance offers some of the best surface low clustering seen thus far. Another shortwave rounding the Northeast Pacific mean ridge may approach the Northwest by next Wednesday. Finally, the 00Z model/ensemble runs also improve agreement for details of the trough/upper low offshore California next week. Guidance comparisons through the 18Z cycle led to the updated forecast using the 18Z GFS/12Z ECMWF and their means, with mostly operational weight early and a more even model/mean weight later in the period. Variability for the likely tropical system led to only a partial adjustment from continuity toward the latest blend. ...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 as heights aloft decline. 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. 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. The National Hurricane Center continues to advertise a high likelihood of organization and development with a tropical disturbance currently located in the Bay of Campeche. As it drifts northward, it will pull along anomalously high moisture and likely bring an area of heavy rainfall into the central Gulf Coast and nearby areas. While there still remains uncertainty in the exact location of the heaviest rainfall, the potential exists for some impactful flooding, particularly given the continued wetness and high soil moisture in place from the wet spring. 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. Rausch/Taylor 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