Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sun Jul 17 2022 Valid 12Z Wed Jul 20 2022 - 12Z Sun Jul 24 2022 ...Dangerous heat to persist over much of the Central U.S. midweek through the weekend... ...Overview... A strong upper ridge centered over the Four Corners region is expected to shift toward the Mid-MS and Deep South by the weekend, which will keep the persistent hot and humid weather over the central/southern Plains in place through the period. Southern areas will see the best potential for readings to approach or break daily records for highs and very warm nighttime lows. Meanwhile the West should moderate after midweek with the approach of a Pacific trough toward the West Coast and then the eastward shift of the ridge. Flow around the ridge will carry shortwaves that will feed into the northeastern U.S. mean trough which may trend a little flatter/broader by the weekend, producing episodes of showers and thunderstorms from the northern tier into the East. Monsoonal moisture over the Southwest/southern Rockies will promote daily chances of showers and storms, some of which could lead to flash flooding. The pattern adjustment late in the period may support an increase of this activity. ...Guidance Evaluation/Predictability Assessment... This cycle of model guidance continued to show fairly high agreement and predictability in the large scale pattern and its evolution with some of the major synoptic features including the Four Corners ridge, Upper Great Lakes into eastern Canada system. The largest model differences remain focused on the western/northwestern U.S. energy coming in and its progression through the northern Rockies and northern Plains, which the GFS appears to be fast and aggressive with showing. Meanwhile, the CMC is on the slower side of the model spread, at least through day 4 then begins to progress the energy eastward. A solution toward consensus, similar to the ECMWF, was preferred though acknowledging the lower confidence in the details/timing with the specific shortwaves coming through the flow and how this translates across the northern tier. By next weekend the question marks over West Coast trough details and character of the eastward-shifting upper ridge combine to affect the forecast of how much southerly flow aloft will develop to enhance the monsoonal moisture over the West. The forecast blend was composed of near equal weights of the 00Z ECMWF/UKMET/CMC and 06Z GFS for days 3-5 followed by near 50 percent of ECENS/GEFS means and the latest deterministic ECMWF and GFS. This accounted for the increasing model spread later in the forecast spread while maintaining continuity. ...Weather/Hazards Highlights... The hot and humid weather will be focused on the central U.S. where daily highs are expected to be between 5 and 15 degrees above normal with many areas seeing consecutive days above 100 for highs. Little nighttime relief is expected with very warm lows in the upper 70s to lower 80s. This heatwave will likely bring scattered records, most likely over the southern Plains to lower/mid MS Valley. The northern extent of heat each day will depend on lower-confidence frontal/wave details but some potential for above normal temperatures to extend into the northern tier of the central/eastern U.S. mid to late week. Over the West, monsoonal moisture and associated diurnally favored showers/storms with locally heavy rainfall may keep parts of the Four Corners/Great Basin region close to normal for highs through the period. The eastward shift of upper ridging late in the period may lead to an increase of moisture/rainfall next weekend. Other parts of the West inland from the Pacific Coast should start the period up to 5-10F above normal for highs but then decline more toward normal as an upper low/shortwave eject from the Pacific. Locally heavy rainfall will be possible with a fairly vigorous system forecast to track from the Upper Great Lakes through eastern Canada/Northeast U.S. mid-late week. Uncertain specifics of energy ejecting from the Pacific, as well as separate features within southern Canada/northern U.S. flow, may play a role in convective details along a trailing front that should affect northern areas after midweek. The Mid-Atlantic and Northeast will be on the very warm to hot side, with some locations possibly seeing more consecutive days of plus 5-10F anomalies for highs than previously experienced so far this summer. The front reaching the East Coast by Friday could lead to a brief and slight dip in temperatures before it weakens during the weekend. Persistent moisture will allow for multiple episodes of showers/thunderstorms over the Southeast/Florida and central Gulf Coast region. Temperatures should be near normal over those areas. Rausch/Taylor 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