Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jul 05 2023 Valid 12Z Sat Jul 08 2023 - 12Z Wed Jul 12 2023 ...Southern tier heat expected to rebuild over the southern Rockies/Plains during the weekend and persist into next week... ...Heavy rain/runoff concerns likely to focus over portions of the south-central Plains into Tennessee/Lower Ohio Valleys and then possibly into parts of the East Saturday through at least Sunday night... ...Overview... The upper level pattern should gradually amplify from the weekend into the middle of next week, as mean troughing becomes more established over the eastern half of the country downstream from a strengthening southern Rockies upper high and ridge extending to its north/northwest. Within the mean trough that extends into central Canada, a rather deep upper low may descend at least as far south as northern Ontario. Meanwhile expect weak troughing to persist along or offshore the Northwest coast but with a lot of uncertainty for the embedded details. The forecast pattern evolution will likely lead to an active period of shower and thunderstorm activity across multiple areas east of the Rockies along with hotter than average temperatures persisting over the southern Rockies/Plains through the period and possibly expanding into the Southwest by early next week. ...Guidance Evaluation/Predictability Assessment... The models and ensembles continue to show good agreement for the overall pattern evolution but there have been ongoing shortwave detail uncertainties (having low predictability due to their smaller scale) that have been causing difficulty in resolving specifics of individual surface waves/frontal systems (and convective episodes) to the east of the Rockies. Somewhat larger scale differences become more apparent over the East by around days 4-5 Sunday-Monday as latest CMC runs show greater upper trough amplitude than other guidance while the UKMET has been on the other side of the spectrum with a slower eastward advance of height falls. Sensitivity of surface details to shortwave specifics has led to some unavoidable continuity adjustments for some surface details in recent runs. Meanwhile, model trends and continuity are steadily increasing confidence in the idea of a rather deep upper low over Canada dropping down at least as far as northern Ontario by early next week. In latest runs, the new 00Z CMC is a southern extreme as it tracks the upper low over Lake Superior by day 7 Wednesday while the 00Z GFS shows a track just a little farther north a day or two later. In recent days the operational GFS/ECMWF runs have generally been leading the ensemble means a bit overall, though with typical variability in exact upper low track and shape of the overall circulation. The upper trough near the Pacific Northwest coast should be comprised of multiple smaller scale features, including an initial Vancouver Island low, a separate compact feature that tracks in from the southwest, and then additional energy arriving from farther upstream. Based on the array of 12Z/18Z guidance available for the updated forecast, the starting blend used an operational model composite for days 4-5 Saturday-Sunday with greater emphasis on the GFS/ECMWF relative to the CMC/UKMET. The CMC diverged enough from other guidance over the East by day 5 Monday to favor excluding it from the forecast mid-late period. The forecast incorporated 30-40 percent total weight of the 18Z GEFS/12Z ECens means by days 6-7 Tuesday-Wednesday to mitigate some of the detail differences that develop farther out in time. ...Weather/Hazards Highlights... The broad cyclonic flow over the eastern half to two-thirds of the country with embedded shortwaves will promote an active period for showers and thunderstorms. During the Days 4-5 period covering Saturday through Sunday night, the most prominent focus for episodes of strong and heavy convection should extend over parts of the south-central Plains into the Tennessee/Ohio Valleys with a combination of one or more surface waves/fronts, upper shortwaves, along with favorable moisture/instability. Both days of Excessive Rainfall Outlooks depict a Slight Risk area over parts of this overall region. Model/ensemble probability guidance clusters fairly well for the central/eastern part of the Day 4 Slight Risk and most of the Day 5 Slight Risk, while on Day 4 the 00Z GFS/UKMET/CMC offer a developing cluster hinting at a band of heavy rainfall potential along a moisture/instability axis extending back through the south-central Plains. Meanwhile on Day 5, the latest GFS/ECMWF runs and their corresponding ensembles offer the best clustering for heavy rainfall potential from the northern Mid-Atlantic into parts of the Northeast with a shortwave/surface system that may provide a fairly pronounced moisture focus. The CMC/UKMET differ with their shortwave/QPF details so confidence in the specifics is a bit lower than over some areas farther west. One area that may be worth monitoring for a Marginal Risk on Day 5 may be over northern Minnesota into the Upper Great Lakes, as guidance suggests development of convection from a combination of a weakening stalled front near the area and then a stronger front dropping south from Canada. Potential east-west orientation of enhanced rainfall could be an issue, but guidance differs on the specifics and most of this area has had below average rainfall over the past couple weeks with minimal totals expected in the short range. Thus will wait for improved clustering in location/amounts before depicting an area. After early Monday, expect the potentially heavy rainfall over the Northeast to continue progressing through the region while south-central U.S. convection may drop toward the eastern half of the Gulf Coast. The front dropping into the northern tier by Monday should push southward with time and may serve as a focus for showers/storms through midweek. Most of the West should remain dry through the period. The strengthening southern Rockies/High Plains ridge, which may drift a bit west with time, will likely expand coverage of hazardous heat across the southern tier during the forecast period. Expect the greatest temperature anomalies of plus 10-13F for highs to be near the upper high center (western Texas and southeastern New Mexico) with broadening coverage of highs at least a few degrees above normal elsewhere, including into the Southwest by the start of next week. Likewise, expect gradually increasing coverage of maximum heat index values reaching 110F or greater over southern/eastern Texas into the Lower Mississippi Valley, while lower humidity will lead to maximum heat index values over the Southwest near or slightly below the expected air temperatures reaching up to 110-115F from late weekend onward. Low temperatures in the upper 70s to low 80s will not provide much relief overnight and could exacerbate heat stress as well as yield some daily records for warm minimum temperatures. Meanwhile parts of the northern-central Plains should see highs up to 10-15F below normal on Saturday followed by a trend toward normal readings for the first half of next week. The Interior Northwest/far northern Rockies should see highs 5-15F above normal Saturday-Monday and then a moderating trend. Much of the eastern half of the lower 48 should see near to slightly below normal readings. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw