Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Jul 03 2023 Valid 12Z Thu Jul 06 2023 - 12Z Mon Jul 10 2023 ...Major southern tier heat wave to ease this week, but may start to rebuild over the southern High Plains and vicinity during the weekend... ...Overview... The most stable upper features during the period will be a southern Rockies/High Plains ridge (possibly strengthening some by Sunday or Monday) and a central Canada mean trough that will extend into at least northern parts of the lower 48. Shortwave energy on the leading side of the trough will help to push a cold front and its associated showers/thunderstorms from the Great Lakes through southern Plains eastward to the East Coast. Upstream shortwaves embedded within progressive flow between the upper ridge and trough will likely interact with the western end of the front (initially stalled and then returning as a warm front) to produce multiple central U.S. convective systems whose focus should shift gradually eastward with time. Latest guidance is suggesting that another front may drop into the central U.S. during the weekend and early next week with support from some combination of energy ejecting from the West and/or amplification of the Canadian/northern tier U.S. upper trough. Meanwhile expect a weak trough with one or more embedded upper lows to evolve along the West Coast from late this week into the weekend, with some question as to its persistence or reloading by the start of next week. ...Guidance Evaluation/Predictability Assessment... The latest guidance continues to show reasonable agreement for the main themes of the large scale pattern, but recent runs since 12Z/02 are showing more spread for important details within the mean flow by the latter half of the period. The largest scale issues arise within the central Canada into northern U.S. mean trough from the weekend into early next week. The 12Z ECMWF amplifies digging energy more than other models (though staying within the full ensemble spread by a small margin), leading to an Upper Midwest/western Great Lakes upper low by next Monday. Latest GFS runs are not as amplified over the lower 48 but bring a deep upper low farther south over Canada than other guidance that clusters over or just north of northern Hudson Bay (where the 12Z ECMWF also has a separate upper low). CMC runs thus far have been weaker with this trough. Current preference tilts somewhat more in the direction of the ensemble means that show some trough amplification by next Monday but in a more measured fashion than the GFS/ECMWF. The developing upper trough along the West Coast shows fairly good clustering into Saturday aside from typical embedded details that have lower predictability. Then it becomes ambiguous whether the embedded upper low ejects while upstream energy filters into the overall trough (which seems to be the majority scenario at the moment) or some other scenario occurs, such as longer persistence of the main upper low/combination with the upstream energy. Over the East, predictability is fairly good to start with as a primary Upper Midwest/Great Lakes shortwave pushes along a leading cold front. However it steadily decreases as details diverge for this leading energy and weak upstream shortwaves reach the East within zonal to slightly cyclonic flow. There has been some continuity in the general idea of surface waviness along/offshore the Mid-Atlantic coast, but spread and variability for specifics. This is to be expected given typical sensitivity of weak surface features to shortwave details aloft. Based on the array of latest guidance, the updated forecast started with a composite of 12Z/18Z operational models for about the first half of the forecast and then steadily increased 18Z GEFS/12Z ECMWF mean input so that the means had a total of 60 percent weight by day 7 Monday. This helped to temper the various less confident aspects of the operational models late in the period. ...Weather/Hazards Highlights... The wavy cold front progressing eastward from the Great Lakes through southern Plains Thursday onward will likely produce multiple episodes of showers and thunderstorms, with some locally heavy. The Days 4-5 Excessive Rainfall Outlooks covering the Thursday through Friday night time frame depict a broad Marginal Risk area with this activity, from the Ohio Valley south-southwest to the Gulf Coast (Day 4) and from the Northeast into the Southeast (Day 5). There is still not sufficient agreement for location and intensity of rainfall to merit any embedded Slight Risk areas at this time, but shorter-term guidance may eventually resolve better defined areas of focus. Day 4 continues to depict a Marginal Risk area over parts of the Carolinas/eastern Georgia with lingering moisture and instability along/south of a stalled front. Meanwhile the combination of a stalled High Plains/southern Plains front eventually returning as a warm front along with progressive weak shortwaves aloft will favor a multi-day pattern of central U.S. convective systems whose focus should drift eastward with time. The Day 4 outlook covering Thursday-Thursday night plans to introduce a Slight Risk area centered over/near the western two-thirds of Kansas, reflecting the best overlap of guidance signals. The Day 5 outlook will depict a broad Marginal Risk area as there is a reasonable signal for more locally heavy convection but poor agreement for location. It appears likely that eventual guidance clustering would support one or more embedded Slight Risk areas at some point. Beyond Friday night, this central U.S. convection should continue through the east-central Plains and into the Mississippi Valley while another front approaching from the north/west may also produce some showers/thunderstorms. Areas along the East Coast should also continue to see episodes of showers/storms at least into the weekend. The highest heat index values across the southern tier during the period should be focused over southern/eastern Texas with maximum daily values likely reaching 110F or higher, with slightly less extreme values extending over other parts of Texas into the central Gulf Coast region. The far Southeast/Florida may see some heat index values reach as high as 105-110F or so. Meanwhile the upper ridge over the southern Rockies/High Plains will keep that region hot through the period, with some expansion of highs 5-15F above normal by Saturday-Monday as the upper ridge strengthens a bit. Pacific Northwest highs of 10-15F above normal on Thursday will likely moderate thereafter as the upper trough develops near the coast. Multiple days of upslope flow over the northern-central High Plains should support below normal high temperatures, with coolest readings up to 10-15F below normal on Thursday. The Northeast will see very warm to hot temperatures of 5-15F above normal late this week, eroding from the west as the cold front approaches. Much of the eastern U.S. should see near to slightly below normal temperatures after frontal passage. 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, experimental 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