Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon May 29 2023 Valid 12Z Thu Jun 01 2023 - 12Z Mon Jun 05 2023 ...Well above normal to near record temperatures from the Upper Midwest to the Northeast Thursday-Friday... ...Broad multi-day thunderstorm and local heavy rain threat continues for the Plains/northern Rockies... ...Overview... Guidance continues to suggest that the mean pattern should begin to look more like an Omega block configuration, anchored by consolidating Great Lakes/Midwest upper ridging whose center should ultimately retrograde slowly along the U.S.-Canadian border. This ridge should extend well northwestward by the weekend. Model/ensemble runs over the past day have altered some details though. The western U.S. pattern looks complex with multiple pieces of energy/upper lows that could get stuck over the region for a time between the strong ridge to the east and rising heights just inland from the West Coast (as troughing prevails over the northeastern Pacific late week into the weekend. This pattern should support multiple days of potentially heavy showers and thunderstorms from the northern Rockies into the southern Plains. To the east of the upper ridge/high, some troughing may brush the Northeast around the end of the week and into the weekend, with a larger scale trough possibly developing over the eastern half of Canada and approaching the Great Lakes/New England by early next week. Meanwhile, a Gulf Coast upper trough/low may settle near the southeastern coast by the weekend and early next week, bringing the potential for areas of heavy rainfall to parts of the Southeast. Areas from the northern Plains eastward through the Great Lakes to the Northeast will see well above normal temperatures mid-late week with the Northeast upper trough leading to some cooling over that region during the weekend. The Northwest will also see above normal temperatures from Friday onward. Near to moderately below normal temperatures will tend to prevail across portions of the southern tier. ...Guidance Evaluation/Predictability Assessment... First looking at the western U.S. and eastern Pacific, the GFS continues to be a bit on the fast side with the leading upper low forecast to reach the Four Corners region by the start of the period early Thursday. Recent trends have nudged a bit faster, allowing for a compromise at least early in the period. The new 00Z GFS has adjusted a tad slower initially, though by day 5 Saturday it is a deep extreme as the remaining energy reaches the northern High Plains. Additional southern stream energy should come into the West right behind the upper low, with guidance showing various ideas for a resulting diffuse upper trough/low that could linger over the West into the weekend. The new 00Z GFS is fairly extreme, even compared to the GEFS mean, in pushing most of this energy into the High Plains versus keeping it farther west. Meanwhile a shortwave reaching the Pacific Northwest by Thursday should lift northeastward thereafter (without the southern part pulling off as depicted in the 00Z CMC), followed by rising heights if not a small-scale ridge over the Northwest as a northeastern trough/upper low eventually lifts out. Southern stream shortwave energy over the Pacific may close off an upper low as it approaches southern California by day 7 Monday. For the time being, guidance shows better than average agreement given the scale of the feature and being that far out in time. Over the eastern half of North America, one notable trend over the past day has been toward greater dominance of the initial Great Lakes upper ridge forecast to retrograde slowly with time, relative to Northeast troughing. As a result consensus now suggests merely a progressive passage of shortwave energy across the Northeast around Friday-Saturday instead of the deeper and more persistent trough seen in earlier runs. The 00Z ECMWF has reversed this trend somewhat. For the start of next week the 12Z-18Z models/means agreed fairly well on a larger scale trough developing over the eastern half of Canada and beginning to reach into the Great Lakes and Northeast. However the 00Z GFS delays the evolution of this trough by at least a day. Farther south, 12Z/18Z guidance was trending toward a somewhat slower and deeper Gulf Coast upper trough/low ultimately expected to reach around the southeastern coast. Complicating the forecast, some models now suggest that weak energy lingering over the Mid-Atlantic could feed into the overall feature. Highlighting the uncertainty/low predictability, the 00Z GFS/ECMWF have switched to a faster and more open solution compared to the 18Z GFS and 12Z ECMWF. The updated forecast employed an operational model composite for the first half of the period to yield the best compromise or consensus depending on the feature, followed by a trend toward 60 percent total ensemble means (18Z GEFS and 12Z ECens/CMCens) with lingering operational model input to add some detail to the means where reasonable. Latest model trends may favor some adjustments to this blend. ...Weather/Hazards Highlights... Expect the forecast pattern to favor multiple days of showers and thunderstorms from the northern Rockies southward across much of the High Plains and vicinity. One or more surface fronts as well as upper shortwaves/lows moving into and through parts of the West/Rockies will help to enhance the focus for convective episodes. Parts of the Great Basin may see some activity as well late in the week, followed by lighter and more scattered rainfall. The Days 4-5 experimental Excessive Rainfall Outlooks covering the Thursday through Friday night period highlight a Slight Risk area on both days over parts of Montana and northern Wyoming. This area has already seen significant rainfall recently and the combination of upper dynamics, a wavy surface system, sufficient instability, and anomalous moisture should be favorable for producing locally heavy rainfall. Both days depict a Marginal Risk area encompassing much of the High Plains and vicinity with an embedded Slight Risk becoming possible at some point depending on how well guidance signals converge. Parts of the Great Basin could also see locally heavy rainfall in the Day 4 period when a Marginal Risk is depicted there as well. The Gulf Coast/Florida system should produce some areas of heavy rainfall but with continued uncertainty in details. The best guidance clustering suggests that Florida should see the best potential for some locally heavy rainfall by the Day 4 time frame, with a Marginal Risk area maintained in the Excessive Rainfall Outlook. Recent trends have favored trimming the potential northward extent of heaviest rain though. Especially with the trends in the new 00Z guidance, heaviest rainfall may trend more offshore of Florida and the overall Southeast coast after Day 4. Cold fronts dropping into the Northeast late this week and early next week could produce some showers and thunderstorms. Most northern tier locations east of the Rockies plus the Middle Mississippi Valley will see well above normal temperatures through at least Friday, with areas from the northern Plains through Great Lakes and Northeast most likely to see multiple days of plus 10-20F anomalies. High temperatures will likely be well into the 80s to low 90s which could approach or exceed daily record highs. A cold front dropping in through the northeastern quadrant of the U.S. around the end of the week will bring a cooling trend primarily to areas from the eastern Great Lakes/Ohio Valley into New England for the weekend, but only New England highs may drop as low as around normal with supporting dynamics trending weaker. The Northwest should see near normal temperatures Thursday with a shortwave passing overhead. Then the region will likely see a warming trend that could bring highs up to 5-15F or so above normal from Friday onward, peaking by Sunday-Monday. Much of the southern tier will tend to see near to moderately below normal temperatures, with the best focus for highs 5-10F below normal gradually shifting across the Four Corners states into the southern High Plains and possibly the southern California coast next Monday with the approaching Pacific upper low. Similar anomalies are possible over parts of the Southeast depending on the coverage of clouds/rainfall. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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