Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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, will all 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 Western U.S. 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 monsoonal moisture to spread into the West along with moderating temperatures. To the north of the upper ridge, progressive shortwaves moving across Canada and the U.S. northern tier will push a mean frontal boundary into the northern states. Accompanying rounds of showers and thunderstorms will be possible from the north-central U.S. to Northeast. ...Guidance/Predictability Assessment... Dynamical and machine learning (ML) guidance agrees fairly well with the large scale pattern evolution during the period, with primary differences involving low-predictability shortwaves within southern Canada/northern tier U.S. flow and late-period specifics of Rockies/Plains upper ridging. Within the early period mean trough over the Great Lakes/Midwest, guidance is coming into focus for individual shortwaves, namely a leading one that progresses from the Great Lakes through Northeast Friday-Saturday and followed by the next shortwave reaching the Great Lakes by early Sunday. To the south of these shortwaves, models continue to show a strong Atlantic ridge (anchored by a 600+ dm high near 60W longitude by Friday- Saturday) building across the southern half of the East. From the weekend into early next week, the Southeast ridge may weaken a bit while broader mean troughing develops over southern Canada into the Upper Midwest through Northeast as ridging develops over western Canada. Not surprisingly, guidance has varied somewhat for shortwave details by early next week but generally support Canadian low pressure in some form helping to push a surface front gradually southward over the northern tier U.S. At the moment there are no prominent clusters among the various dynamical/ML solutions for specifics. Meanwhile the upper ridge over the West should eventually drift into the central Rockies and then weaken a bit as it extends into the Plains. ECMWF runs have been maintaining ridge strength a little longer than other solutions while the majority suggests the GFS may be a little quick to shift the strongest part of the ridge into the Plains. The relative lack of large-scale/higher-predictability discrepancies allowed for an operational model composite among 00Z/06Z models to comprise the updated forecast during the first half of the period and then a transition to a model/ensemble mean mix thereafter. This approach maintained the theme for the agreeable large scale pattern while tempering details that have lower confidence. ...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. These should lead to the best potential for heavy rain from the Carolinas into the Mid-Atlantic and parts of the Northeast. A Slight Risk area in the Day 4 Excessive Rainfall Outlook represents where the heaviest rain is expected, across the eastern half of North Carolina into the coastal Mid- Atlantic, with a surrounding Marginal Risk extending 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. Pockets of lingering moderate to heavy rain may focus over eastern North Carolina but the sandy soils do not make the region too susceptible to flooding, thus no Marginal Risk area has been depicted. However, expect scattered shower and thunderstorm chances to continue and expand through the Southeast over the weekend into early next week. An area of anomalous moisture moving into Florida from the east may also support some locally heavy convection but with low- predictability small scale upper level features likely playing a role as well, tempering confidence in specifics. By Friday night through the weekend, the combination of northern tier U.S./southern Canada shortwaves and a couple surface fronts may promote one or more clusters of showers/storms from the Upper Midwest into the Great Lakes. Guidance signals for this activity are not coherent enough in terms of magnitude and location for a Marginal Risk area in the EROs yet, but much of this region will be sensitive to any heavy rainfall so future model trends/clustering will require monitoring. Expect this activity to progress into the Appalachians/Northeast by early next week. 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 supports maintaining a Marginal Risk for Day 5 over 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. Expect the West to see some moderation of temperatures closer to (but still above) normal by the weekend and early next week. Highs over the northern half of the region may still be 5-12F above normal through Tue while the monsoonal moisture may help Desert Southwest highs to decline to within a few degrees of normal. 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. Best potential for some areas of plus 10-15F anomalies will be over the Plains. 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 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