Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu Jun 17 2021 Valid 12Z Sun Jun 20 2021 - 12Z Thu Jun 24 2021 ...Dangerous heat continues across portions of the Interior West... ...Extreme heat over the Southwest/California through the weekend and then subsiding next week... ...Tropical system likely reaching the Gulf Coast late this week to spread heavy rainfall into the Southeast by Sunday... ...Overview... The extreme heat over the Southwest and California should moderate after the weekend 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 conditions over those areas, likely peaking early in the week. 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 Bay of Campeche. 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 southern U.S. by the start of the extended period early Sunday. Expect the heavy rainfall to extend across parts of the Southeast on Sunday. ...Guidance/Predictability Evaluation... Based on data through the 18Z cycle, a blend of 12Z-18Z GFS/12Z ECMWF runs and 18Z GEFS/12Z ECMWF means (mostly operational model weight early and about half models/means by days 6-7 Wednesday-Thursday) provided the best overall starting point for the forecast. There is still a fair degree of uncertainty over the specifics of the possible tropical system that may reach into the South by the start of the period and then continue northeastward as directed by the southern fringe of the westerlies. The new 00Z GFS shifted east of its prior couple runs. The CMC and UKMET continue their recent tendency to be on the fast and slow side respectively. The ECMWF has been the deepest with the surface low but the 12Z run represented a weaker trend from some earlier runs. The upper trough amplifying east of the Rockies is handled well in principle but with persistent shortwave detail differences. The 12Z UKMET/CMC offered the possibility of slightly slower upper trough/cold front progression while their new runs appear to align better to established consensus. On the other hand the 00Z GFS becomes a little faster than the majority. Guidance continues to refine details a bit for Sunday-Monday Northern Plains/Upper Great Lakes low pressure. For the upper low/trough near the West Coast, latest GFS runs have strayed a bit north/northeast of consensus by Thursday--likely due in part to being a little fast with height falls approaching from the northwest. Flow over southwest Canada should ultimately trend toward weak troughing in response to the mean ridge over the Northeast Pacific but so far guidance has been erratic over this area. Preferred model/ensemble consensus suggests this trough could push a front into the northern High Plains by next Thursday. The new 00Z GFS compares much better than prior runs for this aspect of the forecast. ...Weather Highlights/Hazards... The record-breaking and dangerous heat continuing 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. On the other hand persistence of 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. Some daily records for highs/warm lows will 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. 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