Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Jul 03 2022 Valid 12Z Wed Jul 06 2022 - 12Z Sun Jul 10 2022 ...Overview... The center of a strong upper level ridge/high over the southern tier of the contiguous U.S. will drift from the Mississippi Valley into the Four Corners/High Plains during the latter part of this week into the weekend, while extending ridging farther north into the Rockies/High Plains. This broad area of ridging will lead 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. The eastern North America trough should maintain fairly stable amplitude through late week and then may deepen gradually over the weekend. 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 and ensemble guidance remains in good agreement for the large scale evolution of a strengthening ridge between an eastern Pacific trough/low eventually tracking into the Northwest U.S. and southwestern Canada late week into the weekend, and eastern Canada/Northeast U.S. trough that may amplify somewhat next weekend. Solutions cluster reasonably well in principle into about day 5 Friday. Then they diverge for details of North Pacific flow as well as the distribution, strength, and track of Pacific trough energy as it progresses inland. Part of the challenge in forecasting the Pacific trough is that models suggest the feature will consist of multiple impulses, each of which has low predictability on its own within the more confident mean feature. Of particular note by day 7 Sunday, the 06Z and 12Z GFS runs lean somewhat to the fast/amplified side of the spread (00Z GFS much closer to consensus with the overall trough) with the ejecting trough while the last two CMC runs stray to the flat side with any lingering West Coast troughing due to being fast with the upstream low/trough. Meanwhile there is fair consensus that the mean front over the East should push southward by next weekend as supporting upper troughing slowly amplifies. Other 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 disrupting evolution of the broad upper high at times. Unfortunately the feature is small enough to have fairly low predictability and models continue to differ with what proportion of initial energy reaches the Four Corners states. GFS runs have typically been stronger with this feature with the ECMWF among the weakest (or diverting more energy westward instead of northward). UKMET runs have generally been closer to the GFS in principle while the 12Z CMC seems to split between the GFS and ECMWF ideas. Whatever proportion of energy that lifts northward will play an important role 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. 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 (though with some similarity in the 12Z CMC/UKMET) and thus confidence is low for these features and for surface front/wave details. Still expect to wait 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. The above considerations led to starting the updated forecast with a composite of 00Z/06Z operational model runs for the first half of the period, followed by adding some 06Z GEFS/00Z ECMWF mean input while maintaining the 00Z ECMWF component but transitioning the GFS exclusively to the 00Z run and reducing CMC weight. This approach provided good continuity aside from typical adjustments resulting from guidance changes in smaller-scale frontal wave details. ...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. At the same time a frontal system emerging into the northern Plains next weekend may provide added convective focus over that region. 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 most likely over the central Plains to Middle Mississippi Valley mid-late week, equating to temperatures in the mid 90s to low 100s, while parts of the northern/north-central Plains may experience similarly hot weather by next Saturday-Sunday. Adding in the effect of humidity, highest heat index values should be from the lower half to two-thirds of the Mississippi Valley westward into the Plains. Day-to-day temperatures around the periphery of this heat will depend on uncertain details of a wavy front and associated convection. Southern Plains locations will also see hot temperatures through the period with mostly plus 5-10F anomalies for highs. Most of the East should see highs within a few degrees on either side of normal for most of the period while lows tend to be somewhat more above normal. Some daily record warm lows will be possible over parts of the central/eastern U.S. Eastern U.S. highs may trend slightly below normal next Sunday as upper troughing near the East Coast amplifies. 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. Highs over and near the Desert Southwest may trend slightly above normal by the weekend. 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