Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Mon Jul 04 2022 Valid 12Z Thu Jul 07 2022 - 12Z Mon Jul 11 2022 ...Overview... A strong upper level high over the southern tier of the contiguous U.S. will extend ridging farther north into the Rockies/High Plains and lead to hot temperatures across much of the central CONUS. Surrounding this mean ridge, a reloading upper low/trough just off the West Coast should persist until ejecting northeastward this weekend along the U.S./Canada border, and a mean trough covering eastern Canada and the northeastern U.S. is expected to deepen gradually over the weekend. A mean surface frontal boundary (possibly reinforced by one or more Canadian fronts) draped from the north-central Plains to the Mid-Atlantic, combined with shortwaves embedded in the flow aloft, will likely produce multiple episodes of convection with locally heavy rainfall. ...Guidance Evaluation/Predictability Assessment... Model and ensemble guidance remains in good agreement for the large scale evolution of the strengthening ridge in between the troughing features that show less certainty with the details. Model differences early in the period around Thursday-Friday are mainly limited to mesoscale features, including energy in the central U.S. that has some differences in strength and placement as it rounds the upper high. GFS runs have been among the strongest but the 18Z and 00Z runs have weakened the features somewhat and appear more consistent with other guidance. Unfortunately in this type of pattern, even the smaller scale features will have considerable implications for sensible weather, including frontal positions and rainfall amounts and placement, in this case across the Rockies into the central Plains/Mississippi Valley, while other various shortwaves embedded within the eastern trough also present challenges for the East. Nevertheless a blend of the 12/18Z deterministic guidance worked well for the early part of the medium range period given no notable outliers and larger scale agreement. By the weekend into Monday, some synoptic scale differences begin to arise especially with the western trough, as multiple impulses embedded within the trough diverge and the differences amplify downstream. More specifically, the energy distribution within the trough shows more division 12Z Saturday in the 12Z ECMWF/UKMET compared to the GFS and CMC. The 12Z ECMWF thus ended up closing off an upper low just offshore California while weaker energy races east across southern Canada. Meanwhile the GFS runs showed more potent energy forming a compact closed low as it shifts into south-central Canada, and the new 00Z CMC is similar. The newer 00Z ECMWF does not close off a low like the 12Z run did, adding evidence that the 12Z EC may have been too aggressive with energy left behind in the Pacific. But individual ensemble members show considerable differences with the depth and track of the trough, so ample uncertainty remains. The WPC approach did not favor the 12Z deterministic EC but did prefer the EC ensemble mean to provide some hint toward a more amplified trough remaining offshore compared to the GFS suite. Thus for the latter part of the medium range period, transitioned pretty quickly to a blend favoring the GEFS and especially the EC means as a compromise solution given the spread in the deterministic models. ...Sensible Weather and Hazards... Scattered showers and storms are likely across much of the U.S. from the Rockies eastward at times this week into early next week, with enhanced totals in the vicinity of a couple of frontal boundaries stretching generally west to east across the northern half of the country. Northwesterly flow aloft and abundant moisture and instability will create a favorable pattern for mesoscale convective systems and high rainfall rates within a broad area from the northern Plains/Midwest/Upper Great Lakes into the Ohio Valley and Appalachians to the East Coast. Areas of excessive rainfall/flash flooding are certainly possible through the weekend, but considerable uncertainty with the placement of heavy rain amounts precludes delineating Slight Risk areas in the experimental day 4-5 Excessive Rainfall Outlook at this point. Scattered thunderstorms are also likely in the Southeast through the week, and the current forecast shows the eastern part of the main front sinking southward from meandering over the Mid-Atlantic into the Carolinas and eventually the Gulf Coast by next weekend, leading to more focused rainfall in the Southeast and clearing for the Northeast behind the front. At the same time a frontal system emerging into the northern Plains next weekend may provide added convective focus over that region once again. Meanwhile monsoonal moisture could persist through much of the week with moist southwesterly flow/embedded shortwaves leading to mainly diurnally driven showers and storms over the Four Corners states. The southern Plains and California into the Great Basin should be the most consistently dry areas during the period. Hot weather is expected to continue across the southern half of the Plains/Mississippi Valley underneath the upper ridge/high, with high temperatures approaching or exceeding 100F (about 5-15F above average). The highest heat index values could stretch towards the Middle Mississippi and Ohio Valleys where high humidity will contribute. There could be some daily records set especially for warm lows across the south-central to southeastern U.S. through early next week. Above normal temperatures are also forecast to spread across the northern Rockies to northern Plains late week and increase over the weekend, before possibly moderating by Monday depending on the specifics of a front. Meanwhile, the West Coast could see cooler than normal conditions late week before moderating closer to average, while the Desert Southwest should see near normal temperatures becoming a few degrees above average. Parts of the East may see slightly below normal temperatures spreading south this weekend into Monday behind a cold front. Tate 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