Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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) should reach the eastern Great Lakes or vicinity by the start of the medium range period early 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... As of Thursday the guidance shows some lingering detail differences for post-tropical Beryl with solutions waffling with respect to exact upper level details and resulting strength/track of the surface reflection. The 06Z GFS/00Z ECMWF strayed a bit on the weaker/eastern side of the spread for Beryl's surface low but other models and a relative majority of ECMWF-initialized machine learning (ML) models would recommend a stronger/westward solution (but not as slow as the 12Z CMC). A composite of latest models with minor adjustment provided a reasonable starting point reflecting the official NHC track. 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. 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 specifics late in the week. By Sunday-Monday, consensus still depicts shallow cyclonic flow expanding into the Northeast while ridging persists over the South (but weakening a bit by Monday). 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, with the combination of these features beginning to pull monsoonal moisture farther northward over parts of the West during the weekend into early next week. To the north of the upper ridge, guidance clusters well for a leading trough/upper low reaching western Canada late this week and then likely opening up and progressing eastward with time. Then the relative majority of dynamical and ML guidance suggests that another shortwave will reach western Canada by Saturday and possibly amplify gradually as it reaches a position between Hudson Bay and the northern Plains/Upper Midwest by next Monday. At that time corresponding surface low pressure would be over or near Ontario with the trailing front pushing farther south into the northern tier relative to a leading front that may stay near the Canadian border. The 06Z GFS strayed out of phase from Alaska and vicinity eastward by mid-late period, favoring exclusion of its forecast at that time. The 00Z and new 12Z GFS runs compare better to remaining guidance. Above considerations based on 00Z/06Z guidance led to a forecast update consisting of a blend of operational guidance (splitting GFS input among the 00Z/06Z runs) early in the period and then incorporating some 06Z GEFS/00Z ECens means while shifting GFS input to the 00Z run exclusively. ...Weather/Hazards Highlights... Several ingredients will come together for heavy rain and flash flooding potential along 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 surface low will be tracking northeastward as well, and ample moisture (precipitable water values over the 90th-95th percentile) will be in place. A stalled front, possibly beginning to lift north some as Beryl's low approaches, may help to focus rainfall as well. The Day 4 ERO (Thursday) update accounts somewhat more for consensus regarding anomalous moisture, some instability, and average of the past couple ECMWF runs plus ensemble signals in expanding the central New England and vicinity Slight Risk area eastward into portions of Maine. Most non-ECMWF models are less enthusiastic about potential for significant QPF, so confidence is still somewhat below average. Farther south, guidance continues to suggest a favorable setup for significant rainfall near/inland from the Mid-Atlantic coast with the combination of a stalling surface front and a band of enhanced deep moisture that stays in place during Thursday-Friday. Based on a composite of guidance signals and WFO coordination, the Day 4 ERO proposes maintenance of the existing Marginal Risk area extending south into the Carolinas (with initially dry ground conditions being a mitigating factor) while the Day 5 ERO will introduce a Slight Risk area from southern New Jersey into eastern North Carolina given increased sensitivity due to rainfall expected in the Day 4 period. This proposed Slight Risk area represents the best overlap of guidance but there will be heavy rainfall potential within the surrounding Marginal Risk area as well. Some showers and storms may linger into the weekend for portions of the East, but rain amounts do not look as focused or heavy as the surface front/moisture band become more diffuse. 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, guidance agrees upon increasing/northward spread of moisture over the Southwest/Four Corners states this weekend and early next week so expect more convective rains across the region during that time frame. 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. 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