Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun May 28 2023 Valid 12Z Wed May 31 2023 - 12Z Sun Jun 04 2023 ...Well above normal to near record temperatures from the Upper Midwest to the Northeast next week... ...Broad multi-day thunderstorm and local heavy rain threat continues for the Plains/northern Rockies... ...Overview... Models and ensembles still advertise the development of a large scale Omega block pattern from midweek through next weekend, as Great Lakes and High Plains ridges merge and amplify over the central U.S. and Canada. Ridging may amplify even more into northwestern Canada and Alaska next weekend. Multiple features will comprise overall mean troughing over the western U.S./eastern Pacific, with a tendency for western U.S. shortwaves/upper lows to become somewhat weaker/more diffuse around next weekend as a reloading northeastern Pacific upper trough/low becomes more dominant. To the east of the ridge, an amplifying trough over eastern Canada should begin to push into the Northeast around Friday and extend its influence farther south and west thereafter. The forecast pattern will support rainfall of varying intensity from the Sierra Nevada through the Great Basin and northern Rockies, while portions of the Plains will see potential for locally heavy rainfall over multiple days. One or more fronts should provide added focus over the northern Plains and a shortwave traversing the southern tier, along with associated surface troughing/possible embedded lows, could enhance activity near the Gulf Coast and eventually into the Florida Peninsula. Areas from the northern Plains through Northwest will see well above normal temperatures mid-late week with the Northeast upper trough leading to a cooling trend over that region during the weekend. Near to moderately below normal temperatures should prevail across the southern tier. ...Guidance Evaluation/Predictability Assessment... The latest guidance runs continue the theme of depicting the large scale pattern evolution reasonably well but with embedded differences and run-to-run variability. Across the western U.S./eastern Pacific, GFS runs have tended to be on the fast side with the upper low tracking inland from California midweek onward. However a combination of gradual slower nudges in successive GFS runs and some new 00Z runs trending a bit faster have narrowed the timing spread. There is decent continuity with the shortwave expected to reach the Pacific Northwest around Thursday and eject northeastward thereafter, as well as the general idea for additional energy behind the California low to reinforce a diffuse negatively tilted trough by Saturday. Consensus shows heights rising a bit over this area into Sunday. The primary issue with the trough amplifying over the eastern Pacific is that recent GFS runs had strayed to the southern extreme with the anchoring upper low. The new 00Z GFS compares much better to other guidance in that regard but may still be overdone with height falls reaching California by next Sunday. Meanwhile guidance is currently waffling with respect to depth/amplitude of the developing eastern Canada into northeastern U.S. trough, related in part to the ultimate path of any embedded upper low. New 00Z runs seem to be minimizing the potential for a leading low offshore eastern Canada to get incorporated into the trough as depicted in the 12Z ECMWF and some earlier GFS runs. The 00Z GFS keeps its upper low associated with the digging eastern Canada energy well north of the 12Z/18Z runs, now aligning better with the ensemble means. The 12Z ECMWF may also become overdone with how much troughing extends into the Great Lakes by next Sunday. Finally, the past couple cycles have become better defined with a surface trough and one or more embedded lows crossing the Gulf of Mexico and Florida in association with the upper shortwave/low drifting eastern over the course of the period. Recent UKMET runs have been a pronounced extreme, depicting a deeper upper low than other solutions with the new 00Z run actually lifting its surface low inland over the South versus the more suppressed track in other models/means. A 12Z/18Z operational model composite early in the period, followed by a transition to a nearly even model/mean weight late in the forecast, represented the most common ideas of guidance while downplaying the more questionable aspects on a regional basis. ...Weather/Hazards Highlights... The forecast pattern evolution should favor multiple days of showers and thunderstorms from near the Sierra Nevada into the northern Rockies and extending south 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, with a general tendency for maximum rainfall totals to increase after midweek. The far western part of this activity could taper off toward the end of the week and weekend though. The Day 4 Excessive Rainfall Outlook (12Z Wednesday-12Z Thursday) maintains the identical Marginal Risk areas from the previous Day 5 forecast, namely from near the Sierra Nevada into the northern Rockies and over the southern High Plains. Some guidance signals for QPF over the latter area have become less pronounced over the past day but the upper pattern and other ingredients appear to be fairly consistent, favoring no change in the outlook at this time. The Day 5 outlook (12Z Thursday-12Z Friday) introduces a Slight Risk area focused over central Montana with a frontal wave crossing the area, moisture anomalies showing a pronounced increase over the course of the period, and instability sufficient to support fairly high rain rates. A surrounding Marginal Risk area encompasses most areas included in the Day 4 outlook as well as most remaining portions of the High Plains. A wavy front extending into over the Upper Midwest may still focus some locally heavy rainfall but guidance still differs enough to depict any risk area. Meanwhile, the system tracking along and south of the Gulf Coast should produce some areas of heavy rainfall but with continued uncertainty in northward extent. The best guidance clustering suggests that Florida and possibly nearby areas will see the highest potential for some locally heavy rainfall by the Day 5 time frame, with a Marginal Risk area depicted in the Excessive Rainfall Outlook. Enhanced rainfall may continue at least into Friday. 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. The upper trough and leading cold front dropping in through the northeastern U.S. around the end of the week will bring a noticeable cooling trend to areas from the eastern Great Lakes/Ohio Valley into New England for the weekend. The Northwest should see near normal temperatures Wednesday-Thursday as an upper shortwave reaches the region mid-late week, followed by a warming trend that could bring highs up to 5-15F above normal by next weekend. 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 from southern California and the Desert Southwest into the southern half of the Rockies/High Plains. 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