Extended Forecast Discussion NWS Weather Prediction Center College Park MD 220 PM 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 Wed-Fri... ...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 later this week. 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 a cooling trend over that region during the weekend. Near to moderately below normal temperatures should prevail across the southern tier and especially around the Southwest early in the period. ...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. Aside from the apparent errant 06Z GFS run, the 00Z models were in good agreement and a blended solutions sufficed to start. 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. Upstream, ensembles differ on the shape and southeastward extent of lower heights into/toward coastal California stemming from uncertainty in the north Pacific system nearing the Gulf of Alaska. With no obvious trends and a split in the mid-latitude spread maxima, opted to rely on a broad consensus until a clearer picture emerges. Meanwhile guidance is still 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. Again the 00-06Z GFS/GEFS were southwest of the 00Z ECMWF/ECENS and Canadian/CMCE mean over the Maritimes, so relied mostly on the better ECMWF-led consensus there but with continued changes in the frontal progression. Finally, in the Gulf of Mexico, consensus remains on a developing area of low pressure (or two) moving eastward in tandem with the upper trough into/across Florida later this week. Aside from the still likely overdone 00Z UKMET, a general consensus was preferred though the Canadian and ECMWF were on the deeper side compared to the GFS. Overall a preference among the 00Z models/ensembles was used as a starting point which 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 experimental Excessive Rainfall Outlook (12Z Wednesday-12Z Thursday) highlights the areas 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) continues 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, which may be contingent on any Day 3 rainfall. 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 as temperatures fall back to near normal early June values. 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. Fracasso/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