Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 PM EDT Thu Jun 24 2021 Valid 12Z Sun Jun 27 2021 - 12Z Thu Jul 1 2021 ...Historic and Dangerous Heat Wave with monthly/all-time record temperatures possible over the Northwest/West... ...Heavy Rain Threat to stretch from Southern Rockies/Plains to Ohio Valley... 18Z Update: The 12Z model guidance suite maintains good overall continuity with the previous medium range WPC forecast with a blocky upper level pattern, with a large upper high/ridge remaining anchored in place over the northern Rockies, and another ridge over the western Atlantic. This leaves a quasi-stationary trough over the central Plains and Midwest along with a frontal boundary with multiple perturbations along with, fueling rounds of MCS activity that will likely produce episodes of heavy rain from western Texas to the Ohio Valley. There was a slight downward trend in magnitude of rainfall across the Midwest and Ohio Valley, whilst higher totals are favored across portions of northern Texas and into Oklahoma. The historic heatwave over the northwestern U.S. will likely challenge some monthly June records and potentially all-time record highs! The most impressive readings should be across interior portions of Oregon and Washington with some 110s possible for the lower elevations of the Columbia River Basin. The previous discussion is appended below. /Hamrick ...Guidance/Predictability Evaluation... The WPC medium range product suite was primarily derived from a composite of best clustered solutions of the 18 UTC GFS/GEFS mean, 12 UTC UKMET/ECMWF/ECENS means and the 01 UTC National Blend of Models in an overall pattern with above average predictability days 3-5 (Sunday-Tuesday). Shifted blend focus toward the still compatible ensemble means by days 6/7 amid gradually increasing forecast spread. Recent Canadian had been quicker to suppress the hot Intermountain West upper ridge and stronger/farther east with the Great Lakes/Ohio Valley trough and heavy rainfall focus. However, the 00 UTC Canadian has shifted more in line with the rest of guidance. ...Weather Highlights/Hazards... A highly anomalous upper ridge will build inland over the Northwest this weekend and the heat wave will expand over the West into next week. Temperatures will soar to 20-30 degrees above normal from Washington/Oregon to the northern Great Basin/Rockies. Numerous daily to some monthly records are likely from the Pacific Northwest and interior California to the north/central Great Basin and Northern Rockies, with temperatures peaking over 100F to 110+F for interior sections of the West. Little rainfall is expected to ease this excessive heat this forecast period in a pattern with growing drought and fire threats. Downstream, a robust upper trough will dig and settle into the north-central U.S. to solidify and reinforce a wavy and slow moving front to stretch from the south-central to northeastern U.S. this period. This will invite deep moisture return and pooling. Well organized showers and storms will significantly increase rainfall and runoff potential along and ahead of the front with an elongated and focused zone from the Southern Rockies/Plains northeastward across the Midwest/Mid-South, Ohio Valley and the Northeast. Episodes of cell training and flooding are likely across some of these areas with overall progression slowed as broadly sandwiched between the heat wave out West and a strong western Atlantic ridge to the east. Also, there is also some signal for tropical moisture to feed into southern Florida to fuel convection early-mid next week. Schichtel 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