Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EDT Wed Jun 30 2021 Valid 12Z Sat Jul 03 2021 - 12Z Wed Jul 07 2021 ...Triple-digit heat continues over the interior Northwest while spreading into portions of the northern Plains through the 4th of July weekend... ...Threat of heavy rain lingers near the Gulf Coast through the weekend into next week... ...Overview and Guidance/Predictability Evaluation... Model and ensemble guidance continues to show the large scale pattern settling into a more typical summer regime after this weekend, with the westerlies confined to southern Canada and northern tier of the U.S., after an amplified upper trough over the East Coast states on Saturday lifts away to the northeast. Elsewhere, a seasonably strong upper ridge will prevail over the Southwest through the period and a weakness with possible embedded low should evolve over the southern Plains into northern Mexico during Monday-Wednesday. In the tropical Atlantic a leading wave poised to enter the eastern Caribbean should eventually weaken. However there appears to be increasing potential for a trailing feature now near 40W to strengthen over the coming days and track into or near the eastern Gulf of Mexico next week. The updated forecast incorporated a composite of 00Z/06Z operational models early in the period and then transitioned toward somewhat more 06Z GEFS/00Z ECMWF mean input relative to the past two ECMWF runs and 06Z GFS by day 7 Wednesday. This cycle furthered the recent model/ensemble mean trend toward faster ejection of the East Coast upper trough, with the operational model blend fitting well within the ensemble mean template. Latest consensus including the new 12Z guidance suggests an upper low should track near Maine/Nova Scotia by day 4 Sunday and into the Canadian Maritimes thereafter. Trends aloft support associated low pressure tracking farther offshore and the trailing cold front pushing farther southeast over the western Atlantic. The low amplitude and moderately progressive nature of upper flow along the U.S.-Canadian border still suggest low predictability for specifics several days out in time. The most common theme in the guidance is a leading shortwave (12Z GFS possibly becoming too amplified by Tuesday) that will bring a front across the northern tier states while a modest mean trough sets up close to the Pacific Northwest coast. One or more shortwaves emerging from this mean trough may support waviness along the northern tier front next week. The blended approach accounts for these ideas while downplaying lower-confidence details. Farther south, the model/mean blend has been fairly stable for the upper weakness/low over the southern Plains/northern Mexico. On the other hand the tropical forecast for next week relative to the wave initially near 40W has been rapidly evolving and very uncertain. At the time of forecast preparation only GFS runs and the 00Z UKMET (reaching the southeast corner of the Gulf at the end of its run) showed a defined surface feature reaching the eastern Gulf. ECMWF runs through 00Z have not shown even a hint of a surface low. Ensembles diverge considerably with the resulting means not showing any system. In the 12Z cycle the CMC has joined the GFS/UKMET in depicting a system but the ECMWF still has none. Today's NHC-WPC coordination yielded a nod toward more definition than continuity and this will be reflected in the surface forecast update sent by 22Z. ...Weather Highlights/Hazards... The historic heat wave over the Pacific Northwest will be less intense than it has been over recent days but triple-digit heat is still likely each afternoon across some interior areas through the holiday weekend and into early next week. A portion of this heat will spread into the northern Plains, continuing into next week to the south of a front that reaches the northern tier. Best potential for afternoon temperatures to reach the century mark at some locations in Montana and the Dakotas should be during the weekend. Also expect some scattered daily record highs/warm lows over the Northwest-northern Plains on Saturday-Sunday. The moderating trend next week will gradually reduce the coverage of high temperatures that are 10F or more above normal. Some rain showers over the northern Rockies may offer slight relief to the heat but they will do little to ease the growing drought concern in these areas. One or more northern tier frontal waves could help to focus areas of heavier rainfall next week but confidence in specifics is below average at this time. Farther south, some late-day monsoonal type showers and thunderstorms are likely each day over the central to southern Rockies through the medium range period. The cold front expected to reach near the Gulf Coast this weekend and then stall/dissipate may be a focus for areas of moderate to heavy rainfall. With time the heaviest rainfall should gravitate toward the western half of the Gulf Coast and nearby areas due to the upper weakness/low likely to develop over/near Texas. Meanwhile interests near the eastern Gulf should monitor the latest National Hurricane Center products for the latest information regarding the feature that could eventually affect the region by next week. Farther north, the upper trough near the East Coast this weekend should tend to promote areas of diurnally-favored rainfall, with recent faster trends for the upper trough leading to lighter amounts and less coverage. Clouds and rainfall over the southern half of the Plains will keep that region below normal for highs during the period with some minus 5-10F or so anomalies. High pressure behind the Gulf Coast front should provide moderately below normal morning lows to portions of the eastern half of the country during the weekend. Rausch/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 Hazards: - Heavy rain across portions of the Southeast and the Mid-Atlantic, Sat, Jul 3. - Heavy rain across portions of the Southern Plains and the Lower Mississippi Valley, Mon-Wed, Jul 5-Jul 7. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Ohio Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Central/Southern Plains, the Middle Mississippi Valley, and the Great Lakes. - Flooding likely across portions of the Central/Southern Plains and the Middle Mississippi Valley. - Excessive heat across portions of the Pacific Northwest and the Northern Great Basin, Sat-Mon, Jul 3-Jul 5. - Excessive heat across portions of the Northern Plains, Sat-Sun, Jul 3-Jul 4. - Much above normal temperatures across portions of California, the Pacific Northwest, the Northern Great Basin, and the Northern Rockies, Sat-Mon, Jul 3-Jul 5. - Much above normal temperatures across portions of the Northern Rockies, the Northern Plains, the Upper Mississippi Valley, and the Great Lakes, Sat-Sun, Jul 3-Jul 4. 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