Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Wed Jul 20 2022 Valid 12Z Sat Jul 23 2022 - 12Z Wed Jul 27 2022 ...Dangerous heat over the Central U.S. to persist over some areas while becoming slightly less pronounced next week... ...Heat wave over the Mid-Atlantic and parts of the Northeast likely to peak this weekend... ...Heat expected to build over the Northwest next week... ...Overview... Most of the period will feature strong upper ridging that extends across most of the south-central U.S., with some day-to-day variation in where individual high centers may be. Heat associated with this ridge will be most expansive and anomalous during the weekend when locations from the central Plains through the northern half or two-thirds of the East should see highs 10-15F above normal. Flat to broadly cyclonic flow across the northern tier should support at least a couple frontal systems that should erode some of this heat from the north over the course of the period, while also producing locally heavy showers and thunderstorms, which are most likely from the Midwest into the East. Meanwhile the Northwest will see weak upper troughing give way to a strengthening ridge, beginning a stretch of heat that should extend beyond the medium range period per the Climate Prediction Center week 2 forecast. Monsoonal moisture is likely to increase early next week over the Southwest/southern Rockies and promote daily chances of showers and storms that may produce some instances of flash flooding. There is a continued signal that the pattern evolution will support greater coverage and intensity of rainfall from the weekend into next week relative to the short range time frame. ...Guidance Evaluation/Predictability Assessment... Model guidance is quite agreeable with the overall pattern of the southern tier being dominated by upper ridging and keeping the storm track over the northern tier. Individual upper highs embedded within the ridge that vary slightly between models in terms of position and strength should not have particular sensible weather differences. Some spread remains with the more active northern tier flow, however, particularly with the smaller scale shortwaves and the associated surface wave/frontal details. The 00Z UKMET becomes a fast outlier with a shortwave over south-central Canada coming in behind a James Bay upper low on Monday, and is slightly deeper with troughing into the Great Lakes as well. Most other guidance looks within reason through early-mid next week with the northern tier flow pattern, though noted that the 06Z GFS appeared better clustered with consensus and the ensemble means compared to the 00Z GFS. Guidance has also converged reasonably well on a solution for ridging across the Northwest by midweek next week, though occasional GFS runs have been somewhat more eager to extend western Canada troughing there. The WPC forecast blend was based on mainly the 00Z ECMWF, 06Z GFS, and 00Z CMC early, and then introducing and increasing the proportion of the GEFS and EC ensemble means to around 30 percent by the end of the period amid increasing but still somewhat minor model spread. This approach led to generally good continuity from the previous forecast. ...Weather/Hazards Highlights... This weekend will see the broadest coverage of hot temperatures to the east of the Rockies, with many areas from the central Plains through the Mid-Atlantic/New England seeing highs 10-15F above normal on one or both days. The Plains forecast has been consistent while guidance for the Mid-Atlantic/New England has been trending incrementally higher lately. Forecasts show a fairly broad area of 100+F highs over the central/southern Plains and possibly extending into parts of the Mississippi Valley, and areas along the East Coast could reach the mid 90s to around 100. Heat indices could be 105-110F in many areas, and very warm lows will provide little nighttime relief. This heat wave should bring scattered daily records for highs and warm lows as well. A cold front moving into the central/eastern U.S. should begin to erode the northern periphery of the heat later in the weekend and continuing southward next week, likely leading to highs moderating to 5-10F above normal over the south-central Plains/Tennessee Valley by midweek and near normal farther northward. Additionally, the combination of shortwaves aloft and surface waves/fronts will likely produce one or more episodes of showers and storms from the northern tier states into the East during the period, with some severe weather and heavy rainfall possible. Even with differences/continuity changes for some details at the surface and aloft, the general guidance signals for highest rainfall totals have been fairly consistent for the past few model cycles, showing enhanced rainfall across the Midwest and Great Lakes and east/southeast toward the East Coast. Some adjustments are still possible as important smaller scale features get better resolved. Over the West, expect persistent monsoonal moisture and various shortwave impulses to support diurnally favored showers/storms with some locally heavy rainfall over the Four Corners/Great Basin region through the period. Guidance continues to suggest that the pattern evolution will lead to greater coverage and intensity of rainfall during the weekend and next week relative to the short range period. Potential exists for some of this moisture to interact with the fronts that reach the central Plains (one during Sunday-Monday and another around Wednesday). Meanwhile there is better agreement today that upper ridging should build into the Northwest during the early-middle part of next week, albeit still with a little timing uncertainty. Some locations in the region should see highs reach up to 10-15F or so above normal and a few daily records for highs/warm lows could be possible. Tate/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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml