Extended Forecast Discussion NWS Weather Prediction Center College Park MD 456 PM EDT Tue Jun 15 2021 Valid 12Z Fri Jun 18 2021 - 12Z Tue Jun 22 2021 ...Record to dangerous heat continues this week across much of the Interior West... ...Extreme heat for the Southwest/California through the end of the week... ...Tropical system likely to bring heavy rainfall and flooding to Central Gulf Coast Friday and this weekend... ...Overview... Dangerous, record-breaking heat wave over the Southwest and Interior West is expected to persist through the period with daytime highs still reaching 10F to as much as 20F above normal for some locations. Meanwhile, downstream of the anomalously strong ridge, troughing will be favored over the Great Lakes and Northeast U.S. which should provide some drier/cooler days later this week. Then its interaction with another low pressure system and moisture drawn northward from the Gulf is likely to produce widespread showers and thunderstorms late in the weekend and early next week. Attention is also focused on the likely development of a tropical disturbance currently in the Bay of Campeche. This feature is expected to organize and strengthen as it approaches the Gulf Coast. There is increasing potential for heavy rainfall and flooding along the Gulf Coast by late Friday through this weekend. ...Guidance/Predictability Evaluation... Compared to the last model cycle, there's been a slight westward drift in the forecast track of the potential tropical system as seen by most of the 12Z deterministic runs (ECMWF/CMC/GFS/UKMET). These model solutions suggest a low center along the Texas coast by 12Z Saturday. The timing remains more elusive and there continues to be two scenarios - the ECWMF/GFS are closer together showing a slightly faster solution (12Z Sat low offshore the upper TX coast) while the CMC/UKMET remains the slower models (off the southern TX coast valid 12Z Saturday). A time trend analysis over the last several runs does suggest a slight slower trend is favored, at least through Saturday. Then as the system moves inland late in the weekend and early next week, it will likely be drawn north/northeast with the approaching trough. But given the ongoing spread and varying trends over the past few days, the updated forecast made only modest changes from continuity for strength and track (slightly west, slightly slower). Aside from the tropical system, the latest model guidance shows fairly good agreement in the synoptic pattern. The Four Corners ridge weakens/shunts to the south as mean troughing asserts itself over the central U.S., carved out by a shortwave trough that first comes through the Pacific Northwest on Friday. This area of low pressure is expected to deepens/strengthen as it ejects out in the Plains and Great Lakes and should bring with it a seasonably strong cold front across the central U.S. to Ohio Valley. There remains some model differences with its interaction on what will be a stationary/wavy boundary in place ahead of it but a general model blend was sufficient at this time. All in all, the WPC forecast for the medium range period used mostly the GFS/ECMWF with some inclusion of the CMC outside of the tropical areas. By day 5-7, heavier components of the GEFS/ECENS means were used to account for uncertainty with the various systems affecting the CONUS including the mid-latitude frontal passages as well as the potential tropical system. ...Weather Highlights/Hazards... Another record breaking and dangerous heat day is expected for Friday across much of the Southwest and Interior West where highs well into the 100s and 110s are expected for the Southwest into the California Central Valley. Further north, 90s to low 100s are forecast as far north as eastern Washington. These readings are 10F to 20F above normal. 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. While some moderation is expected this weekend and early next week, the Great Basin and portions of the Northwest will still be 10F to 15F above normal. 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 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 anomalously high moisture and likely lead to an area of heavy rainfall. 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. Several frontal passages across the Great Lakes to Ohio Valley could bring a focused area of heavy rainfall this weekend and again early next week. The first frontal system is forecast to settle across the lower Ohio Valley Saturday/Sunday and its interaction with another approaching front Monday/Tuesday could lead to a few repeating rounds of thunderstorms. Some of this could be locally heavy. Taylor/Rausch Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Fri-Wed, Jun 18-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 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 Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Fri-Tue, Jun 18-Jun 22. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Fri-Mon, Jun 18-Jun 21. 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