Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Jul 21 2022 Valid 12Z Sun Jul 24 2022 - 12Z Thu Jul 28 2022 ...Dangerous heat over the Central U.S. to persist over some areas into midweek... ...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... Guidance shows some transition of the large scale pattern over the course of the period. Initially expect strong ridging to stretch across most of the south-central U.S. with progressive flow to the north, along with weak troughing along the northern half of the West Coast. During next week an upper ridge building over the northeastern Pacific/western Canada should support an amplifying trough over central Canada and the northern Plains through Great Lakes. At the same time, western U.S. ridging will extend more into the Northwest while strongest southern tier ridging should gravitate toward the Southeast Coast (via potential merging of a retrograding Atlantic ridge and eastward-moving Plains/Mississippi Valley upper high). This pattern transition should reduce the coverage of intense heat from the central/southern Plains into the East after Sunday (when many areas should see highs 7-15F above normal) while establishing a multi-day period of hot weather over the Northwest (some highs 10-15F above normal) which may extend beyond next Thursday per Climate Prediction Center forecast. Two prominent surface fronts will likely focus areas of locally heavy showers and storms over parts of the central/eastern U.S. and support the moderating temperature trend east of the Rockies. After a relative lull in activity during the short range period, guidance continues to show a period of enhanced monsoonal moisture from about Sunday-Monday onward, promoting daily chances of showers and storms that may produce some instances of flash flooding. ...Guidance Evaluation/Predictability Assessment... In general the guidance seems to be continuing on its path toward better overall agreement, though with some continued differences for low-predictability shortwave details within northern U.S./southern Canada flow during the first half of the period. The new 00Z UKMET appears to be less comparable to other guidance with a northern tier shortwave and resulting flatter flow reaching the Great Lakes/Northeast. A model/mean consensus approach maintains a fairly stable solution for the developing central Canada into U.S. upper trough and its leading surface low pressure/frontal system, with no solutions through 18Z extreme enough to exclude from a compromise. The new 00Z ECMWF has made some significant detail changes from its prior run, decreasing confidence a little. Overall the majority cluster looks good for upper ridging that persists over the southern tier and expands over the West. The updated forecast incorporating the 12Z/18Z guidance started with an operational model blend early (more GFS/ECMWF than UKMET/CMC) and then transitioned to 40 percent total 18Z GEFS/12Z ECMWF means by late in the period, with various GFS/ECMWF/CMC weights making up the other 60 percent. ...Weather/Hazards Highlights... Broad coverage of hot temperatures from the southern half of the Plains through the East will extend through Sunday, with many areas from the central Plains through the Mid-Atlantic/New England seeing highs 10-15F above normal (perhaps a bit lower in the Ohio Valley). Expect a fairly broad area of 100+F highs over the southern half of the 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 erode the heat from the north by Monday-Tuesday but not reach far enough south to lower temperatures much over and near the southern half of the Plains. The next front that reaches the northern Plains early next week should have a stronger southward push, bringing moderately below normal highs to the northern-central Plains Tuesday onward and possibly near-normal readings farther south by Thursday. Both fronts will focus episodes of showers and storms, with some severe weather and heavy rainfall possible. The leading front heading into the East may initially favor highest rainfall totals from the eastern Great Lakes/Ohio Valley into the Northeast/Mid-Atlantic but could also focus repeat activity as far west as the Plains as it stalls before lifting back as a warm front. The second front should produce northern Plains convection that extends south/east with time. Over the West, persistent monsoonal moisture and various shortwave impulses will support diurnally favored showers/storms with some locally heavy rainfall over the Four Corners/Great Basin region from Sunday through at least Thursday. Guidance continues to suggest that the pattern evolution will support a more active period with greater coverage and intensity of rainfall versus the short range time frame. Potential still exists for some of this moisture to interact with central Plains fronts (one during Sunday/early next week and the second arriving around midweek) to enhance rainfall totals. Upper ridging that builds over the Northwest next week will bring multiple days of hot weather to that region, with some locations likely to see highs reach up to 10-15F or so above normal. Some daily records for highs/warm lows will be possible. Rausch/Tate 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