Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 PM 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 rebuild over the southern High Plains and vicinity during the weekend... ...Strong to severe storms along with a heavy rain/runoff threat to focus over the central Plains Thursday/Friday... ...Overview... It remains the case that the most stable upper features during the period will be a southern Rockies/High Plains centered 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. Guidance continues to suggest 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... Guidance solutions seem reasonably well clustered for the main themes of the mid-large scale pattern, but continue to exhibit more numerous smaller-mid scale embedded system differences over time. A composite blend of latest GFS/ECMWF/UKMET/Canadian seems to provide a good summertime depiction of the pattern and main weather focusing systems for later this week into Saturday seems overall consistent with WPC product continuity and the National Blend of Models. This solution also has good multi-model ensemble support. Added substantial input from the GEFS/ECMWF ensemble means along with the generally compatible GFS/ECMWF by days 6/7 (Sunday/next Monday) for a bit more detail, albeit amid growing model forecast spread. The last few runs of the Canadian offer less than stellar run to run contiuity at these longer time frames, especially within northern stream flow. This general plan tends to smooth detail consistent with individual system predictability through medium range time scales. Overall, predictability seems near average for a early warm season pattern. ...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 introduced 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 latest version of all these ERO products only tend to slightly decrease areal coverage of the prior threat areas as guidance signals/predictability becomes steadily more focused over time. 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. Schichtel/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