Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Jun 14 2021 Valid 12Z Thu Jun 17 2021 - 12Z Mon Jun 21 2021 ...Record to dangerous heat continuing through the week across much of the Interior West... ...Extreme heat possible for the Southwest/California through the end of the week... ...Heavy rainfall threat over parts of the South by the weekend... ...Overview... During the course of the Thursday-Monday period most guidance expects the large scale pattern to transition from a strong Four Corners ridge and East Coast trough toward broadly cyclonic flow across the northern half of the lower 48. The ridge initially over the West will support a hazardous heat wave over portions of the region with a fairly broad area of temperatures approaching/exceeding daily record values through the end of the week. Gradual weakening/suppression of the western ridge should lead to some moderation of temperatures after Saturday even if readings remain above normal. Meanwhile precipitation east of the Rockies should generally increase in coverage and intensity. The National Hurricane Center continues to monitor the potential for a disturbance to develop over the western Gulf, with a general consensus suggesting that this feature could track northward and bring a heavy rainfall threat to parts of the South. Farther north a front that will progress eastward from the Plains/Upper Midwest and then stall as it awaits trailing waviness reaching the Northwest/Plains Saturday-Monday will be a focus for showers/storms over multiple days. ...Guidance/Predictability Evaluation... A general model consensus to start the period followed by a gradual trend toward an even blend of the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean provided a reasonable starting point for depicting the most common elements of guidance. The GFS continues to be a bit on the southern side of the spread for the front progressing over areas east of the Rockies in response to amplifying northern tier U.S./southern Canada flow. However the occasional tendency for convection to extend farther south and east than model forecasts would argue for holding onto some GFS concepts as part of a blend. Guidance is still in the process of figuring out the details of eastern Pacific into western North America flow. Over the past day the model consensus has trended noticeably weaker with what energy reaches the northwestern U.S./southwestern Canada late week into the weekend (a nod to recent ensemble mean forecasts that have been fairly weak due to member spread, and also plausible given initial progression of northeastern Pacific flow). Latest CMC runs provide an exception in being stronger than other guidance with ridge that builds over the East Pacific and deeper with its trough over the Northwest during the latter half of the period. By the start of next week the surface pattern over the central U.S. will be sensitive to the exact strength of the shortwave. Models still suggest the initial East Coast upper trough may separate/close off a low as it departs but the effects should remain offshore. Not a lot has changed over the past day with the forecast of possible tropical development emerging from the Bay of Campeche with a northward track over the western Gulf of Mexico. The guidance average for timing appears to have stabilized but there is still a broad range of possible track/timing. The CMC maintains its multi-day trait of being on the fast side of the envelope while UKMET runs are slowest. Latest ensemble means are on the western side of the spread but with a fair amount of spread among individual members. The manual forecast remains fairly close to yesterday's 17Z NHC-WPC coordination with day 7 Monday extrapolation leading to a position near the 18Z GFS. While confidence in the details is still quite uncertain, the overall pattern does support an increasing potential for some heavy rainfall to reach parts of the South by the weekend/early next week. ...Weather Highlights/Hazards... Expect the most extreme temperatures over the West to continue through Friday with a broad area of 10-20F above normal readings that will approach or exceed daily record highs/warm lows over much of the southern half of the region. The combination of extreme daily temperatures and duration, as well as being rather early in the season for such an event, should make this a dangerous heat wave for sensitive groups. Excessive heat warnings/watches are in place for many areas of the Southwest and into the California Central Valley next week, with temperatures climbing well into the 110s in the lower deserts with low temperatures only in the 80s and 90s. Coastal locations in Southern California will likely be spared much of the heat owing to cool ocean temperatures and enough onshore flow. By Sunday-Monday, the Southwest and most of California should moderate somewhat as the Four Corners upper high weakens and drifts southward. Areas farther north may still see highs 10F or more above normal. The Central Plains region should experience some of the heat on Thursday as well with highs 10-20F above normal. After Thursday the front over the central/eastern U.S. will bring temperatures down toward normal with perhaps a few pockets a little below normal depending on the day. The East will be on the cool side late this week before rebounding toward normal, though clouds/rain reaching parts of the South may lead to below-normal highs late in the period. Areas near the Gulf Coast should continue to monitor forecasts/National Hurricane Center discussions over the coming days for the latest information regarding any potential tropical cyclone development by the latter half of the week into the weekend. Regardless of the strength or precise track of any possible system, the northward flow of tropical moisture would bring the threat of heavy rainfall to portions of the southern tier--which would be particularly troublesome over areas that have received considerable rainfall over recent weeks. The cold front pushing eastward from the Plains/Upper Midwest will produce some rain and thunderstorms from the Midwest eastward/southeastward into the eastern U.S. Some of this activity could be locally heavy between the Upper Midwest and Ohio Valley and nearby areas but it may take into the short range time frame to resolve heavy rainfall potential more precisely. Additional rainfall should develop over the Plains/Midwest during the weekend and early next week as waviness develops on the trailing part of the stalled front. Some scattered diurnal convection is possible over the Four Corners, primarily confined to the terrain. Rausch/Fracasso 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