Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM 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 produce warm to hot temperatures in the north-central U.S. to the Northeast. Mean troughing across the Eastern Seaboard is likely to see embedded systems, including lows/fronts that could produce heavy rain over Florida on Thursday and perhaps lasting over the Southeast. In the western to central U.S., some impulses could produce multiple days of potentially heavy showers and thunderstorms across the northern Rockies/Plains and extending southward across much of the High Plains. ...Guidance Evaluation/Predictability Assessment... Model guidance continues to show good agreement with the upper high/ridge to track from atop the Great Lakes region westward across the Midwest and Plains and extending well northward into Canada by the weekend. More uncertain are the patterns over the West and East. 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). Early in the period/late in the week there have been some variations for the timing of a trough tracking east but in terms of the 00Z/06Z guidance the UKMET seemed like a slow outlier, with its axis over California instead of the Southwest like other guidance by Day 4/Friday. After that, there are some small-scale differences in a pattern becoming weaker forced between an upper low in the eastern Pacific off of British Columbia and the upper high to the East. Southern stream shortwave energy over the Pacific may close off an upper low as it approaches southern California by day 7 Monday, and rising heights if not a small-scale ridge over the Northwest is possible early next week. A blend of models and ensemble means seemed to work fine. Across the Southeast, an upper trough/low may form across the Southeast late week along with a surface low pressure system. But this will be affected by positional and phasing differences farther north across the Northeast and eastern Canada that have shown considerable variations in the model guidance. Through the 00/06Z model cycle, the ECMWF (and many of its ensemble members) showed a farther west diving of energy south there by around Sunday compared to the in-between GFS runs and the farther east CMC. The ECMWF picked up the associated surface low and brought it faster/farther into the Atlantic than some other guidance that had the low stalling. These positions have waffled from cycle to cycle, and the incoming 12Z runs of the ECMWF and GFS are even farther west. So this part of the forecast remains low confidence. The updated WPC forecast favored a blend of the 06Z GFS, 00Z ECMWF, and 00Z CMC along with the 06Z GEFS mean and 00Z EC mean. The deterministic models were the majority initially, with gradual increasing of the ensemble means to over half with time. ...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. 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. Farther east and south, 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. For Day 5/Friday for example, 00Z/06Z models showed fairly good agreement on a solution favoring heavy rainfall across parts of the Texas Panhandle into western Oklahoma, but when the 12Z GFS came in farther east held off on any Slight Risk upgrades for now. Parts of the Great Basin may see some locally heavy rainfall as well late in the week, prompting a Day 4/Thursday Marginal Risk, followed by lighter and more scattered rainfall. 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 continuing to trim the potential northward extent of heaviest rain, but other than that it remains uncertain where heavy rainfall may occur within this moist pattern. Particularly if heavy rain looks to focus over the urban areas of southeastern Florida, a Slight Risk may be needed there in future ERO issuances. 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. Well above average temperatures will be common across the northern Plains, Upper/Middle Mississippi Valley, and the Great Lakes/Ohio Valley regions late this week into early next week under the ridge/high aloft. High temperatures in the 80s to low 90s could approach or exceed a few daily record highs. Perhaps more anomalous but shorter-lived warmth will be across the Northeast Thursday-Friday. Temperatures of 15-25F above normal are likely to set some record highs especially on Friday before a cooling trend for the weekend with some upper-level energy and a backdoor cold front. The Northwest should see near normal temperatures Thursday with a shortwave passing overhead. Then the region is likely to see a warming trend that could bring highs up to 5-15F or so above normal from Friday onward, peaking by Sunday-Monday with highs possibly reaching 20F above average. Meanwhile, 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. Tate/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