Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Sat Jun 12 2021 Valid 12Z Tue Jun 15 2021 - 12Z Sat Jun 19 2021 ...Record to dangerous heat continuing through next week across much of the Interior West and through midweek over the northern Plains... ...Extreme heat possible for the Southwest/California next week... ...Overview... Today's guidance maintains reasonable continuity for the general pattern evolution. The very strong upper ridge centered over the Four Corners states next week may weaken somewhat by Friday-Saturday. Vigorous shortwave energy brushing the Northwest and tracking around the Four Corners ridge will push a cold front across the northern half of the West and Plains, eventually reaching northern parts of the East by next weekend. This energy should re-establish eastern North American mean troughing after it kicks out a leading trough expected to persist near the East Coast into Thursday. Expect the western upper ridge to support a large area of temperatures that approach/exceed daily record values over portions the Interior West/Rockies on most days next week and extending into California during the latter half of the week--making for a hazardous heat wave event. Some of this heat will extend into the northern Plains through at least midweek as well. In the Gulf of Mexico, there is still the potential for a disturbance to form near the Bay of Campeche and move northward late in the week but also a large degree of uncertainty in its evolution. ...Guidance/Predictability Evaluation... The models and means continue to agree well for the strength and position of the Four Corners ridge/upper high through the period. Early-mid period strength of the upper ridge is remarkable with the new 00Z GFS showing 500mb height anomalies of +2.5-3 standard deviations over parts of the Rockies/High Plains and even greater than that north of the Canadian border on Tuesday. Such heights may breach record high heights for this early in the warm season. Models/means all show some weakening of the ridge by Friday-Saturday. For the initial eastern North America upper trough, most guidance has gravitated to a common solution but the 00Z CMC differs with some details. It may still take some time to resolve the finer details of surface low evolution near New England. Among 12Z/18Z guidance an operational model blend represented consensus well. Not counting the 00Z CMC, there is decent agreement in principle for the shortwave ejecting through the Northwest and around the western ridge, ultimately settling into the long-term mean trough over eastern North America. The main issue with the leading front is that the 12Z/18Z GFS runs push it farther south over the Plains versus other guidance by late in the period (leading to a dramatic difference in rainfall over the southern Rockies/High Plains). The 00Z GFS appears to have adjusted back somewhat. Meanwhile models and individual ensemble members show considerable difficulty in handling the complexities of North Pacific flow--leading some guidance to become up to 180 degrees out of phase for eastern Pacific/western North America features by late in the week/weekend. An operational model blend for the first half of the period trending to an even weight of models/means later represents consensus well for the leading shortwave, and by way of the GFS/ECMWF offsetting late in the period over the eastern Pacific/western North America the blend ends up close to the conservative mean solution while waiting for details to become more clear. Latest guidance continues to show the possibility for some degree of tropical development to emerge from the Bay of Campeche and track northward over the Gulf of Mexico. Through the 12Z/18Z cycles the ECMWF had been the steadiest with the depiction of the surface system while the GFS/CMC were trending eastward. The 00Z cycle has yielded a reversal with all three of those models trending slower and the GFS/CMC westward. Ensemble members are still sufficiently diverse to keep their means from showing much definition. Needless to say there is still considerable uncertainty. The manual forecast reflects an extrapolation of yesterday's NHC-WPC coordination, with a weak feature reaching the west-central Gulf Coast by early next Saturday. 00Z model trends would delay the possible system's progress. ...Weather Highlights/Hazards... The most extreme anomalies within the area of intense western/northern U.S. heat will be on Tuesday when areas over the northern half of the Rockies/High Plains and possibly into parts of the Great Basin may see highs 20-30F above normal. It may not be out of the question for one or more locations to set a new record high for June. The front progressing eastward from the Northwest Tuesday onward will bring a brief cooler trend across northern parts of the West into midweek and then into the northern Plains. The southern half to two-thirds of the West should see highs consistently 10-20F above normal for most of the period, including a westward expansion of the heat into California from about Wednesday onward. Northern areas should also rebound to well above normal readings late in the week as well, though with some uncertainty due to guidance spread for details of flow aloft. Morning lows should also be 10-20F above normal over a majority of the West through the period, as well as the northern Plains into Thursday. Expect numerous daily records for highs/warm lows for most of the period. 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 are in place for many areas of the Southwest next week where temperatures may climb well into the 110s in the lower deserts. The eastern trough aloft and associated surface waves/fronts will produce some areas of rain and thunderstorms of varying intensity and during mid-late week some moderately below normal temperatures (perhaps 5 to no more than 10F below normal). The southern tier may see rain from both a wavy front and typical diurnal convective activity. Portions of the Gulf Coast region could see an increase of rainfall toward the end of next week as moisture pushes northward across the Gulf. Guidance spread and variability thus far keep confidence low for determining the timing and coverage/intensity of rainfall. Monitor forecasts/National Hurricane Center discussions over the coming days for the latest information regarding any potential tropical cyclone development. The upper trough/surface front initially affecting the Pacific Northwest will bring light-moderate rain through Tuesday or Tuesday night. Toward late week this system may produce some rainfall from the eastern Plains into the Great Lakes and Northeast. 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