Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Jul 03 2022 Valid 12Z Wed Jul 06 2022 - 12Z Sun Jul 10 2022 ...Overview... Expect the center of a strong upper level ridge/high to drift slowly westward across the southern tier of the contiguous U.S. for the latter part of this week into the weekend, while producing ridging farther north into the Rockies/High Plains and leading to hot temperatures across much of the central CONUS. Surrounding the mean ridge, a reloading upper low/trough just off the West Coast will persist until ejecting northeastward by the weekend 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... Model guidance remains in good agreement for the large scale pattern of an east Pacific low tracking into the Northwest late in the week, reloading troughing over the Northeast, and strengthening ridging in between. Significant differences on the large scale do not really arise until next weekend; while guidance is more agreeable in recent cycles for at least some energy and troughing to come onshore into the Northwest from the eastern Pacific around Saturday, models differ regarding its evolution as it tracks eastward over the weekend, how much energy/troughing is left behind in the Pacific, as well as the flow upstream. The main outlier with at least the latter appeared to be the 12Z GFS that showed an upper low farther south in the Pacific compared to other guidance indicating the low would be near the Gulf of Alaska. However, smaller scale features remain considerably uncertain through the period. One such mesoscale feature is shortwave energy that looks to be somewhere near Baja California/northern Mexico/southern Arizona as the period begins Wednesday morning and tracks north-northeast Thursday-Friday, possibly slightly disrupting the broad upper high at times. Unfortunately the feature is small enough to have fairly low predictability. GFS runs have typically been stronger with this feature with the ECMWF among the weakest, while the 12Z CMC was stronger than the newer 00Z CMC, and the UKMET was on the stronger side but the farthest west in track west of the ridge. The 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, so this is one aspect of the forecast that leads to uncertainty in the details of sensible weather. Additionally, shortwave specifics around the northern periphery of the CONUS ridge and into the eastern Canada/Northeast U.S. mean trough vary considerably between models and thus confidence is low for these features and for 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. Overall, the WPC forecast utilized a blend of the 12Z/18Z deterministic models for the early part of the period, with some inclusion of the GEFS and EC ensemble means later on as differences increase. ...Sensible Weather and Hazards... Scattered showers and storms are likely across much of the U.S. from the Rockies eastward at times this 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 end of the week, 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 by next weekend, leading to more focused rainfall in the Southeast and clearing for the Northeast behind the front. 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. 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 Wednesday-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 for the weekend. 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 and into the weekend. The East is likely to be near average temperature-wise, with a trend toward cooling slightly by the weekend. Tate/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 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