Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Jun 28 2021 Valid 12Z Thu Jul 01 2021 - 12Z Mon Jul 05 2021 ...Historic heat wave over the Northwest becomes less intense and continues through the 4th of July weekend... ...Heavy rain is a threat along a front shifting from the Tennessee Valley to Northeast southward to near the Gulf Coast by the weekend... ...Overview... The upper high/ridge causing record-breaking heat in the short range period is forecast to shift eastward into the Northern Plains through the weekend while weakening somewhat, as an upper trough moves into British Columbia and across southern Canada. Meanwhile, an upper-level trough/low is expected to meander over the Great Lakes region and push a front slowly southward across portions of the central and eastern U.S., causing possibly heavy rain. ...Guidance/Predictability Evaluation... Model and ensemble solutions are reasonably agreeable regarding the amplified upper-level pattern described above, but some differences remain in question. With the positively tilted trough in the East, the timing/movement are in question at least partly due to the related issue of whether or not an upper low closes off. In terms of the deterministic guidance, the 00Z ECMWF came in with a low closing off only briefly on Friday in southeastern Canada, after its previous runs as well as other guidance created and kept a closed low through at least the weekend. The trough also progresses a bit more quickly in the ECMWF for this reason, while the 00Z UKMET is a bit faster compared to other solutions as well. On the other hand, 00Z and 06Z GFS runs retrograde its closed upper low westward over the Middle Mississippi/Ohio Valleys during the weekend. The 00Z CMC appeared to be a good compromise between the faster and slower solutions, and was able to be used through most of the period along with the agreeable ensemble means. Ensemble spread shows that the position/timing of the trough and thus the front pushing slowly ahead of it remain uncertain, but felt this was a good approach for now for the latter part of the period, after a mainly deterministic model blend for the beginning of the period. The general idea of upper ridging across western parts of the U.S., with the exceptions of the aforementioned shortwave moving across southern Canada and additional troughing impacting the Pacific Northwest early next week, were indicated by the majority of model guidance. ...Weather Highlights/Hazards... The historic heat wave over the Pacific Northwest is expected to be less intense by midweek as the highly anomalous upper ridge is forecast to weaken slightly and shift eastward. Nevertheless, high temperatures across the interior sections of the Northwest will remain 15 to 25 degrees above normal through the holiday weekend, and warm overnight lows should increase the heat stress to these areas. Some possible rain showers around the Northern Rockies will do little to break the growing drought in these areas. Downstream, the positively-tilted upper trough drifting slowly across the north-central U.S. will solidify and reinforce a wavy and slow moving front from the Southern Plains to the northeastern U.S. for the latter half of the week. Deep moisture streaming in from the Gulf of Mexico will continue to help organize showers and storms, significantly increasing rainfall and runoff potential along and ahead of the front with an elongated and focused zone from the Southern Rockies/Plains northeastward across the Mid-South, Ohio Valley, and the Northeast. Episodes of cell training and flooding are likely across some of these areas, and the heavy rainfall axis should slowly drop southward toward the Gulf Coast by early next week. A heat wave across the Northeast to the Mid-Atlantic during the short range period should be broken by Friday as the upper trough/low centered over the Great Lakes directs moisture into much of the East Coast ahead of a frontal boundary. It appears that the East Coast will enter a period of damp/wet weather just in time for the July 4th holiday weekend with cool daytime temperatures and warm and humid nights. Cool days and warm nights are expected for the Deep South as well where the heavy rain threat continues into early next week. In contrast, dry and hot weather will return across the Northern Plains and should continue into early next week under the weakened but still anomalous upper ridge. Tate/Kong 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