Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Jul 02 2022 Valid 12Z Tue Jul 05 2022 - 12Z Sat Jul 09 2022 ...Overview... Expect a persistent upper level ridge/high to be in place over the southern tier of the U.S. with a northward extension of this ridge reaching into the Rockies/High Plains, leading to hot temperatures across much of the central CONUS. The core of the ridge should shift in emphasis from the lower half of the Mississippi Valley into the Four Corners region around Friday-Saturday. Surrounding the mean ridge, a reloading upper low/trough just off the West Coast will persist until possibly ejecting northeastward toward the end of the week while leading energy should flow across the northern tier and feed into a broad mean trough covering eastern Canada and the northeastern U.S. This flow pattern is fairly typical for summer, with the large scale features just a little stronger than the climatological average. 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... The two medium to larger scale discrepancies that arise in the guidance involve upper level energy that may lift north-northeast from Baja California into the Four Corners states around mid-late week and the timing of eastern Pacific upper trough ejection into the Pacific Northwest/British Columbia by around next Saturday. The first feature will be very important with respect to the rainfall forecast over the Southwest/Rockies and then the convective forecast as this energy becomes embedded in the southern fringe of the westerlies and interacts with the surface front to the east of the Rockies. Unfortunately the feature is small enough to have fairly low predictability, and in addition latest models have been very different and variable. GFS runs had been on the strong and progressive side of the spread but the new 12Z UKMET/CMC have adjusted toward bringing this feature farther north in somewhat more GFS-like fashion. The 12Z ECMWF tries to bring it north but is slow enough that the energy gets deflected farther west by the building Four Corners ridge. More adjustments are certainly possible in future runs. As for the trough off the West Coast, preference continues to lean more toward moderately progressive ejection at the end of the week versus some slower runs of the GFS/GEFS mean. The 00Z GFS was much closer to consensus while the 06Z GFS built a lot more ridging into the North Pacific/Aleutians and the new 12Z run still gets fairly extreme with its North Pacific ridge by next Saturday. Given better comparison of the GEFS mean with the majority for flatter flow over the North Pacific, it seems the GEFS mean could be a little slow with the trough ejection. Otherwise, confidence is low for shortwave specifics around the northern periphery of the CONUS ridge and into the eastern Canada/Northeast U.S. mean trough. This is highlighted by current and recent spread in model specifics, leading to meaningful differences in surface front/wave details. It will likely take until the short range period to resolve these details, and even then some aspects of the forecast may have very short lead time given how challenging it can be for models to latch onto mesoscale features that will matter greatly for QPF placement and amounts. Continue to monitor the forecast for updates going forward. Based on the latest array of guidance, the updated forecast incorporated 00Z/06Z operational model runs for about the first half of the period and then added in some 06Z GEFS/00Z ECMWF means (up to about one-third total weight) late in the period. By day 7 Saturday the aforementioned preferences led to tilting GFS input more to the 00Z run than the 06Z version and ensemble mean input slightly more to the ECMWF mean than GEFS. ...Sensible Weather and Hazards... Scattered showers and storms are likely across much of the U.S. at times 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. There is a Slight Risk in the day 4 WPC experimental Excessive Rainfall Outlook in the central Appalachians where there is some signal for this activity could focus and greater sensitivity to heavy rainfall exists. Some enhanced rainfall could occur farther northwest of this area as well as over parts of the Northeast but with lesser confidence in details. Rainfall that could be sufficiently heavy to cause flash flooding may persist beyond the day 4 time frame but with considerable uncertainty in the placement of heavy rain amounts. Scattered thunderstorms are also likely in the Southeast through the week, and some additional Mid-Atlantic to Northeast rain events are possible as well depending on frontal positions. 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 Desert Southwest/Great Basin should be the most consistently dry areas during the period. The greatest concern for temperatures is with the heat expected to build across the central U.S. in response to a strengthening upper level ridge. Highs of 10-15 degrees above normal are likely over the central Plains to Middle Mississippi Valley at least during Tuesday-Thursday, equating to temperatures in the mid 90s to low 100s. A southward-drifting surface front and associated convection could erode this heat somewhat toward the end of the week. However warmer than average temperatures should build into the northern Rockies/High Plains late in the week, leading to some hot readings of up to 10-15F above normal by next Saturday. Southern Plains locations will see hot temperatures through the period but with less extreme anomalies. Meanwhile the West will see cooler than normal highs through at least midweek due to the initial eastern Pacific upper trough and leading moisture over the Four Corners states. Then temperatures should moderate closer to normal, and even above normal in the Desert Southwest and California by Friday-Saturday. The East is likely to be near average temperature-wise. Rausch/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