Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Jul 18 2022 Valid 12Z Thu Jul 21 2022 - 12Z Mon Jul 25 2022 ...Dangerous heat to persist over the Central U.S. through the start of next week... ...Overview... A strong upper ridge centered over the Southwest/Four Corners region as of early Thursday may shift its emphasis somewhat eastward by the weekend, though with increasing uncertainty over exactly where ridging will be located. This ridge will keep hot and humid weather over the central/southern Plains for a majority of the period. Some southern locations could see readings approach or break daily records for highs and very warm nighttime lows. With somewhat less extreme anomalies, areas from the Tennessee/Ohio Valleys through Mid-Atlantic/New England will experience multiple hot days as well. The West should moderate after midweek with the approach/ejection of eastern Pacific upper trough energy. This energy and other features across southern Canada and the northern tier U.S. will flow around the ridge and feed into a broad northeastern U.S. mean trough, supporting one or more waves/frontal systems and 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. Some increase of this activity will be possible during the weekend if the Four Corners ridge does indeed adjust eastward for a time. ...Guidance Evaluation/Predictability Assessment... The most agreeable and consistent large scale aspects of the forecast continue to be with the general theme of southern tier ridging and the broad mean troughing over the northeastern quadrant of the lower 48. A leading eastern U.S. cold front anchored by eastern Canada low pressure late this week also has good agreement and continuity (though with some spread/adjustments for the low itself). Other details have lower predictability/confidence. Solutions continue to struggle with the day-to-day specifics of low amplitude mean flow from the northeast Pacific across southern Canada/northern U.S., as well as for an upper low/trough forecast to eject from the eastern Pacific. For the latter, multi-day continuity seems to be pointing toward an intermediate but somewhat slower than median timing within the established spread from recent days. The 12Z model runs seemed to improve their clustering while the 18Z GFS jumped ahead. Preference was with the 12Z cluster. The new 00Z GFS reverted to a slower solution while the new 00Z ECMWF jumped ahead toward the 18Z GFS. Another forecast problem involves the details of the southern tier ridge by mid-late period. In general, most ECMWF runs and GEFS/ECMWF means have been consistently showing an eastward adjustment of the Four Corners ridge more into the south-central Plains/Mississippi Valley. GFS runs have tended to be a bit farther east while the CMC has usually kept its ridge more over the Four Corners. The 00Z UKMET also maintains the Four Corners ridge. The CMC/UKMET do support a strengthening ridge over the Southeast/Mississippi Valley as well though. Late in the period, there is a general consensus that upper ridging should build into the Northwest after passage of the late week/weekend shortwave. The 00Z GFS, which is more aggressive than other guidance with a Gulf of Alaska into western Canada upper low/trough, suppresses Northwest heights relative to other guidance by next Monday. Consensus at the start of the period and then preference for the shortwave ejecting from the Pacific led to starting the updated forecast with varying components of the 12Z GFS/ECMWF/UKMET/CMC for days 3-4 Thursday-Friday. Increasing detail uncertainties for various aspects of the forecast favored a rapid transition toward 50-60 percent total weight of the 18Z GEFS/12Z ECMWF means by days 6-7 Sunday-Monday. Guidance changes over the past couple days have led to a greater southeastward extent of a front reaching the northern Plains/Midwest late this week. Also a slower trend for the shortwave ejecting from the Pacific and flowing around the upper ridge has led to a corresponding delay with the surface wave now expected to reach the central U.S. during the weekend. ...Weather/Hazards Highlights... The most anomalous heat over the central U.S. will likely focus over the central Plains where the best potential exists for multiple days of highs 10-15F above normal. Some of the heat may also eventually drift into parts of the Tennessee Valley. Toward Sunday-Monday a cold front may suppress the heat over the north-central Plains. Typically hotter southern Plains locations will see less extreme anomalies but overall there will be a broad area with highs reaching or exceeding 100F for highs, while very warm lows in the upper 70s to low 80s will provide little nighttime relief. This heat wave will likely bring scattered records, most likely over the southern Plains to lower/mid MS Valley. 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 within a few degrees on either side of normal during through the period. The most likely scenario of an eastward shift of upper ridging after late this week would lead to an increase of moisture/rainfall next weekend and perhaps a bit of a westward shift in the overall moisture axis. However there is a lower probability potential for the Four Corners upper ridge to be more persistent, which would tend to maintain the existing late week pattern. The approach/ejection of Pacific trough energy should bring highs down to near normal levels over an increasing portion of the West by Friday-Saturday. A rebound of upper ridging over the Northwest may bring highs to 5-10F or so above normal over that region by Sunday-Monday. The fairly vigorous late week system over eastern Canada and its trailing front into the eastern U.S. may produce some locally heavy rainfall. There will likely be other areas of enhanced rainfall from the northern tier into the East during the rest of the period, but with much less confidence in specifics due to guidance difficulty in resolving the details within southern Canada/northern U.S. flow aloft, including ejecting Pacific energy that joins this stream. 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. Continues to expect near normal temperatures 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