Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 AM EDT Mon Jul 8 2024 Valid 12Z Thu Jul 11 2024 - 12Z Mon Jul 15 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... What is left of Beryl (namely a post-tropical/frontal low) is forecast to push across the Lower Great Lakes region as the medium range period begins Thursday. A stalling front, energy and moisture from Beryl, and a favorable upper pattern with Great Lakes troughing and a retrograding shortwave south of western Atlantic ridging will promote a wet pattern for the Eastern Seaboard for late week with heavy rain and flooding concerns. Meanwhile, a persistent upper ridge over the West will continue to support a dangerous and likely record-setting heat wave across much of the region. This ridge looks to combine with the westward expanding Atlantic ridge, while progressive shortwaves move across Canada and the U.S. northern tier. ...Guidance/Predictability Assessment... At the start of the period Thursday, models agree that troughing will be in place over the Great Lakes, in part from Beryl's remaining energy, with some spread that affects Beryl's (frontal) surface low. The 18Z GFS was among the faster/farther northeast guidance from the 12/18Z model cycle, but the 00Z model cycle had the GFS a bit farther west while the ECMWF was east. A model blend favoring the 12Z ECMWF and both the 12Z and 18Z GFS runs provided a compromise position. There is general model agreement that this trough should steadily weaken through early Saturday as strong Atlantic ridging (anchored by a 600+ dm high near 60W longitude by Friday-Saturday) builds into parts of the East. There is still a signal for some weak energy to approach the East Coast from the Atlantic on the western periphery of the core of strongest ridging, with some potential influence on rainfall late in the week. By Sunday-Monday, consensus suggests that shallow cyclonic flow may expand into the Northeast while ridging persists over the South. Low predictability for specific details mid-late period would continue to favor a composite/mean approach to represent the larger scale pattern for this part of the forecast. Dynamical/ML guidance continues to agree fairly well for the eastward drift of the western U.S. ridge from California and Nevada into the Four Corners/Central Rockies region over the course of the period. This drift should allow for weak troughing to develop near the West Coast by the weekend. There is still some spread for shortwaves rounding the top of the ridge, so there is low to medium confidence in the specifics across the northern tier of the U.S., but there is some clustering for a frontal system to meander and then finally press southward into the Upper Midwest by next Monday. The WPC forecast thus used a deterministic blend favoring the 12Z ECMWF and 12/18Z GFS early in the forecast period, including the GEFS and EC ensemble means by Day 5 and increasing to around half means by Day 7 to minimize individual model differences within a generally agreeable large scale pattern. ...Weather/Hazards Highlights... Several ingredients will come together for heavy rain and flash flooding potential in the Eastern Seaboard for late week. A positively tilted trough with its axis over the Great Lakes will provide lift in the right entrance region of the jet. Beryl's low will be tracking northeastward as well, and ample moisture (precipitable water values over the 90th-95th percentile) will be in place while a front stalls. Marginal Risks are in place across the region for the Days 4-5/Thursday-Friday EROs, with an embedded Slight added over parts of the Interior Northeast on Thursday especially if the main rainfall axis related to Beryl lingers from Day 3/Wednesday into Thursday morning. Within this broad risk area there are various antecedent ground conditions, such as drought over the Mid-Atlantic and wet conditions over parts of the Northeast, so various levels of impacts are possible. One change to the previous ERO for Thursday was to limit the Marginal to northeastern portions of Florida where urban areas may increase sensitivity to heavy rain in an otherwise swampy/high Flash Flood Guidance area. Some showers and storms may linger into the weekend for portions of the East, but rain amounts do not look as focused or heavy. Shortwaves and frontal boundaries could also promote rounds of rainfall across the Midwest to Great Lakes. Farther west, some moisture may feed into the Four Corners states in a monsoonal pattern. A Marginal Risk is in place on Thursday mainly for New Mexico--especially considering sensitivities due to burn scars and areas that have seen ample rainfall recently. Less overall coverage of showers/storms is currently forecast on Friday along with less focus in the most sensitive areas of New Mexico (Sacramento Mountains), so do not have a Marginal delineated in the Day 5/Friday ERO. The chance for flash flooding may be nonzero but it seems to be less than 5 percent, the Marginal threshold. However, expect increasing moisture to promote more convective rains in the Four Corners states this weekend. 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, with typically hotter locations exceeding 120F. 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. By the medium range time frame, temperatures will have moderated in the Pacific Northwest with possible troughing aloft, though may still be a bit above 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. 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 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