Extended Forecast Discussion NWS Weather Prediction Center College Park MD 511 PM EDT Thu Jun 17 2021 Valid 12Z Sun Jun 20 2021 - 12Z Thu Jun 24 2021 ...Extreme heat over the Southwest/California through the weekend and then subsiding next week while afternoon heat well up into the 90s expected for interior Pacific Northwest early next week... ...Tropical system will likely spread heavy rainfall inland from the Deep South to the Southeast later this weekend into Monday... ...Overview... The extreme heat currently over the Southwest and California is showing signs of moderation by early next week as an upper ridge over the region gives way to a trough (likely with an embedded low) that settles just off the West Coast. However enough ridging will persist over parts of the Great Basin/Northwest to support very warm to hot afternoons over those areas. A broad mean trough will prevail farther east with the greatest depth/amplitude likely to be over the east-central U.S. during the early-mid portion of next week. This trough and associated surface system/fronts will support areas of active weather with potential for heavy rainfall. Meanwhile, the National Hurricane Center continues to monitor a broad area of low pressure over the southwestern Gulf of Mexico. During the short-range time frame this feature should start to organize while tracking northward and bring heavy rain into the Gulf Coast region, reaching an inland position over the Deep South by the start of the extended period early Sunday. Expect the heavy rainfall to move across the interior Southeast and the Carolinas on Monday. ...Guidance/Predictability Evaluation... Global model solutions and ensembles this morning are actually in fairly good agreement with one another through the medium-range period. The system of interest is the potential for tropical cyclone formation over the western Gulf of Mexico starting from the short-term period. There remains quite a bit of uncertainty regarding the structure of this system including possible convective feedback as shown by the GFS while the ECMWF flips back-and-forth between focusing more on the broad circulation of the system and convective feedback similar to the GFS. In any event, once the system moves inland, there is rather good model agreement on taking the system across the Deep South on Sunday before recurving it across the Carolinas on Monday. The ECMWF is the most aggressive in terms of keeping a well-defined tropical low into the Carolinas on Monday and then off the Mid-Atlantic on Tuesday. This scenario is considered an outside chance at this point. Behind the tropical system, models show good agreement on the progression of the cold front through the eastern two-thirds of the country into midweek next week with the typically slightly faster GFS/GEFS solutions. In addition, there continues to be a general model tendency to phase/merge the northern stream secondary cool air reinforcement from Canada behind an intensifying low pressure system moving across the Great Lakes into Canada early next week. The WPC medium-range forecast package was derived from a composite blend of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, together with some contributions from the 00Z CMC and CMC mean. Increasing weights toward the ensemble means were incorporated for Days 6-7 to handle the model uncertainty. ...Weather Highlights/Hazards... The record-breaking and dangerous heat forecast to continue into the weekend over the Southwest and California Central Valley should begin to moderate a little on Sunday and then subside more dramatically thereafter with high temperatures trending close to normal by Tuesday-Thursday. In fact, some light snow is even possible across the highest elevations of northern Wyoming and southern Montana early on Monday behind a cold front after Sheridan, WY experienced all-time record high temperature of 107 degrees at a couple of days ago! On the other hand, the persistent upper ridging will support well above normal temperatures from parts of the Great Basin through the Northwest. The most extreme anomalies over the Northwest should be early in the week when parts of Oregon and Washington could see highs up to 20-25F above normal, where afternoon high temperatures well up into the 90s to near 100 degrees are forecast early next week. Some daily records for highs/warm lows will also be possible. By next Thursday highs of 10-15F above normal should be confined to the Interior Northwest and northern Rockies. The amplifying upper trough to the east early-mid week will produce an area of below normal temperatures progressing from the Plains into the east-central U.S., moderating some as it reaches the East Coast states. Best potential for at least a day of temperatures 10F or more below normal will be from the Dakotas into the central High Plains and eastward into the Great Lakes. Southern U.S. heavy rainfall and flooding concerns associated with the tropical system expected to reach the Gulf Coast early in the weekend should extend into the Southeast by Sunday. Portions of the South that have seen well above normal rainfall in recent weeks will be particularly sensitive to the additional rainfall from this system. The amplifying upper trough over the central/east-central U.S. and leading low pressure/frontal system should spread a broad area of showers and thunderstorms across the eastern half of the country. Some of the rain may be heavy from the Upper Midwest into the Great Lakes as well as the Ohio Valley or Tennessee Valley into the southern Plains. The Storm Prediction Center is also monitoring the potential for organized severe threats given the vigorous nature of the system. Thus far guidance is maintaining some separation between the tropical moisture over the south and the front that heads into the East by next Tuesday. Farther west, areas along eastern slopes of the northern/central Rockies may see a period of organized rainfall Sunday-Sunday night with the passage of a cold front and the shortwave energy aloft feeding into the overall trough to the east. Kong/Rausch 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