Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 New England 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 will support at least a couple frontal systems that should erode some of this heat from the north over the course of the period. Expect these fronts and embedded waves to produce episodes of 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 may extend beyond the medium range period per the Climate Prediction Center week 2 forecast. Monsoonal moisture over the Southwest/southern Rockies will 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... Compared to 24 hours ago, the guidance appears to be making some progress in resolving details within what has generally been a more agreeable and consistent large scale evolution, but with some spread persisting. There are still issues with specifics of northern tier flow from the start of the period, affecting wave/frontal details. Among 12Z/18Z solutions the 18Z GFS seemed less appealing than most due to its shortwave amplitude around Saturday-early Sunday (favorably corrected in the new 00Z run), while 12Z UKMET specifics became more questionable toward the end of its run late weekend/early Monday (still questionably sharp with its Great Lakes shortwave by Monday in the new 00Z run). Upstream there is still a moderate amount of spread for energy likely to amplify over central Canada into the northern tier U.S. as upper ridging builds into the northeastern Pacific and western Canada. The new 00Z UKMET becomes a fast extreme with its system by late Monday. An average among latest models and ensemble means continues to provide the most steady solution for the upper trough and associated surface system/front. Some GFS runs have been somewhat more eager to extend western Canada troughing into the Northwest U.S. but do eventually build a decent amount of ridging into the Northwest as reflected by recent consensus. The latest forecast started with a 12Z/18Z model blend for the first part of the period to reflect the most common elements of guidance while downplaying the less confident details from individual solutions. The blend transitioned to about half models/half ensemble means by days 6-7 Tuesday-Wednesday to yield the desired compromise for the central Canada into northern U.S. system at that time and otherwise account for increasing detail uncertainty. ...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. There will be 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. Very warm lows will provide little nighttime relief. This heat wave should bring scattered records for highs and warm lows. A cold front moving into the central/eastern U.S. will 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. 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 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 over the past day, from the Midwest and Lower Great Lakes 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. 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