Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 mid-late week will likely shift somewhat eastward by the weekend. The persistence of ridging over or near the central/southern Plains will keep hot temperatures over that region and into parts of the Mississippi Valley through the period. Southern areas will see the best potential for some daily records for highs and/or very warm 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... The latest models and ensembles continue to agree for the largest-scale aspects of the mean pattern and its general evolution, and to a reasonable degree for the dominant features mid-late week: the Four Corners ridge and the Upper Great Lakes into eastern Canada system that brings a cold front from the Midwest/Plains into the East. However significant model spread persists for how initial Pacific trough energy will eject and the specifics of any residual Pacific energy, as well as for the character of features across the northeastern Pacific through southern Canada/northern tier U.S. These issues lower confidence in frontal/wave details behind the system/front reaching the East late this week. In addition, 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 updated forecast based on 12Z/18Z guidance started with an operational model blend days 3-4 Wednesday-Thursday followed by a fairly rapid increase of the 18Z GEFS/12Z ECMWF means so that the means reached 50-60 percent total weight by next weekend. This reflects the decent clustering for primary features early and then accounts for the increasing divergence in details. Differences/changes in the southern Canada/northern tier stream continue to affect specifics of low pressure reaching eastern Canada by Friday but with less impact on the trailing front. Models still vary considerably for timing/amplitude of the feature ejecting from the eastern Pacific and tracking around the ridge, though there is modest improvement. Lately the GFS/ECMWF have been somewhat faster than the UKMET/CMC. The new 00Z UKMET has trended closer to the GFS/ECMWF but becomes quite amplified versus other guidance as the shortwave tracks inland. The new 00Z ECMWF has trended slower, highlighting the continued uncertainty. Model/ensemble spread yields low confidence in details of other southern Canada/northern tier U.S. shortwaves as well. An intermediate solution, closest to an average of the 12Z ECMWF/ECMWF mean and 18Z GEFS mean, provides good continuity for the weekend position and shape of the southern tier ridge. GFS runs have tended to center the ridge a little farther east but the 18Z and new 00Z runs are closer to preference than some earlier runs. The preferred solution would support somewhat more of an increase in western monsoonal moisture than depicted in the GFS by late in the period. ...Weather/Hazards Highlights... The central U.S. will be the focus for the most anomalously hot weather during the period, with each day likely featuring some areas with highs reaching 10-15F above normal and a fairly broad area of triple digit readings. Such anomalies should persist over the south-central Plains most of the period while locations up into the north-central Plains may see similar departures from normal Thursday-Saturday. The northern extent of heat each day will depend on lower-confidence frontal/wave details. The best potential for some daily record highs and warm lows will be over southern areas. 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