Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM EDT Tue Jul 9 2024 Valid 12Z Fri Jul 12 2024 - 12Z Tue Jul 16 2024 ...Heavy rain and flooding concerns for the Eastern Seaboard linger into late week... ...Dangerous heat wave in the West persists through late week... ...Overview... Troughing over the Great Lakes and a shortwave on the western side of an Atlantic upper ridge, as well as moisture from what was Beryl and a stalling frontal boundary, all will contribute to the possibility of heavy rainfall over much of the Eastern Seaboard on Friday. Meanwhile, a persistent upper high over the West will continue to support a dangerous heat wave that could continue to set records, especially through late week. The West ridge will drift east and could combine at times with the Atlantic ridge across the Southeast, causing heat to spread across the central and eastern U.S. this weekend and early next week, but allow for some moderation of temperatures and some monsoonal moisture to spread into the West. To the north of the upper ridge, progressive shortwaves will move across Canada and the U.S. northern tier for some rounds of rain in the north-central U.S. to Northeast. ...Guidance/Predictability Assessment... Model guidance is reasonably agreeable in depicting the overall pattern, but with more spread in the details especially as quasi- zonal flow begins to prevail with rounds of smaller-scale shortwaves moving through the northern tier. As the period begins, the Great Lakes to Ohio Valley trough seems well handled by a deterministic model blend. Guidance continues to show steady weakening of the Great Lakes/Midwest upper trough into early Saturday as strong Atlantic ridging (anchored by a 600+ dm high near 60W longitude by Friday-Saturday) builds into parts of the East. Meanwhile the upper high in the West shows good agreement in drifting eastward and at times perhaps joining with the western part of the Atlantic ridge. There is some model disagreement in how much/when the upper high weakens early next week, with the EC slower to weaken than other guidance. Weak troughing could set up in the eastern Pacific to the west of the ridge. But the primary model differences are with shortwaves along the northern side of the ridge. These are lower predictability and also affect surface low/frontal systems. The newer 00Z guidance continues to show spread with broad but shallow troughing in the Great Lakes to Northeast early next week, not reaching as far south as the 12/18Z cycle, and showing a weaker surface low in southern Canada. The WPC forecast was based on a blend of the deterministic guidance early in the period, favoring the 18Z GFS and 12Z ECMWF. Included the GEFS and EC ensemble means from Day 5 onward, but was able to maintain majority deterministic guidance, with some inclusion of the means and the blending process serving to lessen too much influence on the smaller model differences. ...Weather/Hazards Highlights... Several ingredients will come together for heavy rain and flash flooding potential along the Eastern Seaboard continuing into Friday. A positively tilted trough with its axis over the Great Lakes will provide lift in the right entrance region of the jet. Ample moisture with precipitable water values over 2 inches should pool near a stalling front. Heavy rain is forecast from the Carolinas into the Mid-Atlantic and parts of the Northeast. A Slight Risk is in place where the heaviest rain is expected, across the eastern half of North Carolina into the coastal Mid-Atlantic, with a Marginal surrounding farther west toward the Appalachians and north into New England. Some showers could continue into Saturday along the Eastern Seaboard but the heaviest rainfall should be over by then; lingering moderate to heavy rain looks to focus perhaps in eastern North Carolina where sandy soils do not make the region too susceptible to flooding. However, expect scattered shower and thunderstorm chances to continue and expand through the Southeast over the weekend into early next week. Additionally, frontal systems and shortwaves will provide support for rounds of rain across the Upper Midwest to Great Lakes and Northeast, with some model spread for amounts and timing details. Farther west, some moisture will feed into the Four Corners states in a monsoonal pattern, especially over the weekend and beyond as the upper ridge finally weakens some and drifts east. A nonzero chance for heavy rain to cause flash flooding is possible across Arizona in particular on Friday, but the better moisture arriving Saturday has prompted a Marginal Risk for Day 5 for portions of the Four Corners states. This moisture plume looks to expand farther north into early next week for increasing rain/thunderstorm chances for the Rockies to High Plains. The West will continue to see a major to extreme and long-lived heat wave through late this week. Highs exceeding 110F should be widespread in the Desert Southwest. Farther north, highs into the 100s are forecast into the Intermountain West as temperatures soar 10-20F above normal. Numerous daily records are likely to be broken during this heat wave, with monthly/all-time records not out of the question. The prolonged nature of the heat wave and record warm overnight temperatures will increase heat stress. Some moderation of the temperatures closer to normal (but still a bit above) is finally forecast in the Desert Southwest over the weekend into early next week. Meanwhile the warmer than average temperatures will be spreading farther east into the northern and central Plains late this week/weekend, and into the Midwest and East this weekend into early next week. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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