Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat Jul 6 2024 Valid 12Z Tue Jul 09 2024 - 12Z Sat Jul 13 2024 ...Beryl will spread heavy rainfall across the south-central U.S. as it transitions to post-tropical... ...Dangerous heat wave in the West persists into next week... ...Overview... The most prominent large scale features during the period will be a persistent upper ridge over the West supporting a dangerous and likely record-setting heat wave across much of the region into next week, and a positively tilted Plains/Mississippi Valley/Great Lakes mean trough whose southern periphery will interact with Beryl as it transitions to post-tropical. Heavy rain is likely to spread from eastern Texas northeastward into midweek and beyond as Beryl's low pressure and moisture interact with a frontal boundary. Meanwhile, seasonably hot and humid conditions along with scattered thunderstorms at times should persist across the Southeast to East Coast. ...Guidance/Predictability Assessment... Guidance continues to have difficulty with the forecast of Beryl, as there is considerable sensitivity to exactly how the Midwest/Great Lakes into Plains upper trough interacts with the system. Among the dynamical models, the ICON has consistently been a fast/northeast extreme while recent GFS runs have been on the slow side and tending to weaken the surface reflection more than other guidance. Respectively, these extremes correspond to rapid phasing of trough energy with Beryl versus weakening the shortwave energy and/or reluctance to show much phasing. The new 12Z GFS has at least trended faster into midweek but then still becomes suppressed/diffuse thereafter (though the new 12Z CMC also trends more suppressed). Meanwhile the 00Z ECMWF-initialized machine learning (ML) models favor a fairly progressive evolution, with four out of five MLs clustered between the ICON and slower remainder of dynamical models. By early Thursday the new 12Z UKMET has adjusted considerably faster than its 00Z run to be closer to the ML cluster, though it may stray a little too far east with its upper trough axis. Meanwhile there are still lower-predictability differences for eastern Canada upper troughing that brushes the Great Lakes/New England Tuesday-Thursday, with resulting effects on one or more surface fronts. In varying ways, most guidance suggests that Great Lakes into Plains/Mississippi Valley upper troughing should persist into Saturday but with a gradual weakening trend. This will occur as strong Atlantic ridging (anchored by a 600+ dm high over 50-65W longitude by Friday-Saturday) builds into parts of the East. A modest upper weakness (with predictability low enough not to reflect yet in a deterministic forecast by Saturday) could approach the East Coast from the Atlantic on the western periphery of the ridge. A composite of model/ensemble guidance appears reasonable for aforementioned troughing and the retrograding Atlantic ridge. Aside from typical detail differences, dynamical/ML guidance shows good agreement with eastward drift of the western U.S. ridge from California/Nevada into the Four Corners region over the course of the period. However around the top of the ridge there has been a fair amount of spread and run-to-run variability for Northeast Pacific into western Canada dynamics Wednesday onward. The most common theme is for this energy to be slower to cross southern Canada versus the 06Z GFS by late week, and the 12Z GFS has indeed trended favorably toward other guidance. With the GFS least preferred for the depiction of Beryl, the early-period model blend based on 00Z/06Z guidance started with a more even weight of the ECMWF/UKMET/CMC/GFS than would typically be the case. The latter part of the forecast gradually incorporated some of the 06Z GEFS/00Z ECens means. The QPF associated with Beryl consisted of more 00Z ECMWF/UKMET/CMC input (but tempering max amounts due to ongoing uncertainty) to reflect an evolution close to the official National Hurricane Center track. ...Weather/Hazards Highlights... By the start of the medium range period early Tuesday, the 15 UTC NHC advisory shows Beryl weakening to a tropical depression over eastern Texas. Then Beryl should quickly phase with an upper trough and surface front to the north. These features should lead to the potential for heavy rain causing flash flooding concerns over some areas from eastern Texas northeastward through Tuesday- Wednesday. Generally faster model trends for Beryl continue to favor somewhat of an eastward shift in the Slight Risk area depicted in the Day 4 Excessive Rainfall Outlook. Within the Slight Risk area, there is an improving signal toward the best heavy rain potential being centered over a majority of Arkansas. Poor model behavior for this system so far, and still a lot of spread, preclude introduction of a Moderate Risk area at this point. Then the Day 5 ERO introduces a lower-confidence Slight Risk area over and near the Lower Ohio Valley to reflect the heavy rainfall potential associated with the official track of Beryl. A much broader Marginal Risk area surrounds the Slight Risk, with ingredients for isolated flash flooding like above normal moisture forecast to be in place across the south-central U.S. and northeastward into the Mid-Atlantic and parts of New England. Interaction of moisture ahead of Beryl and a front settling over the Mid-Atlantic/southern New England may favor another area of heavy rainfall potential (and possible risk upgrade) over this region. Leading into this midweek setup, upper troughing pushing one or more cold fronts across the northeastern U.S. will also lead to some thunderstorm chances. Some focus of the convective rain ahead of the front(s) and sufficient instability for heavy rain rates in the northern Mid-Atlantic to Northeast favor maintenance of a Marginal Risk in the ERO for Day 4/Tuesday. Persistent moist conditions will promote scattered storms continuing through late week across the Southeast/Carolinas and into the Mid-Atlantic. The Day 4 ERO introduces a Marginal Risk area over parts of the far Southeast/Florida Peninsula given abundant moisture and some model signals for locally heavy rainfall. Farther west, some moisture may feed into the Four Corners states in a monsoonal pattern. Marginal Risks are in place for Tuesday and Wednesday mainly for New Mexico--especially considering sensitivities due to burn scars and areas that have seen ample rainfall recently. The West will continue to see a major to extreme and long-lived heat wave into next 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 Oregon and Washington and east into the Intermountain West, as temperatures soar 10-20F above normal with locally higher anomalies. 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. The Pacific Northwest looks to moderate closer to normal later week with possible troughing aloft, but interior areas and expanding into the north-central U.S. will be above normal. Meanwhile, warmer than average high temperatures in the Northeast Tuesday should moderate by midweek but the forecast pattern may keep morning lows 5-10F above normal into next weekend. The Southeast can expect near normal heat for this time of year--which is still hot and humid. The south-central U.S. should see some minor heat relief through Thursday or so with below average temperatures given the clouds and rain chances. 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