Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM EDT Wed May 27 2020 Valid 12Z Sat May 30 2020 - 12Z Wed Jun 03 2020 ...Much above normal temperatures to progress from the Interior West into the northern/central Plains... ...Overview and Guidance/Predictability Assessment... Through the period, the synoptic pattern across the CONUS is forecast to evolve from strong upper ridging over the Southwest U.S. and Rockies that will spread slowly eastward and gradually settle over the south-central Plains and lower MS Valley. On each coast, stubborn troughing is forecast to remain in place, resulting in a somewhat blocky pattern over North America. In the large scale sense, most of the period has average to slightly above average agreement and forecast confidence, minus late in the period off the western U.S. coast. The compact closed low off the central California coast at the start of the period will weaken and open up as it lifts northeast toward the interior Pacific Northwest by day 4. In its wake and due to persistent ridging over the central US and downstream troughing over the eastern US, a longwave trough is forecast to settle on or just offshore the west coast. By day 6/7, despite above normal uncertainty, a piece of that energy on the southern end may close off and stall off the southern California coast. Model guidance starts off with good agreement in the synoptic pattern such that a near equal blend of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z UKMET was incorporated. The main areas of disagreement lie across the northern Rockies with the west coast closed low opening up where the GFS is slower with the energy and lifts it further north, compared to the ECMWF, which is subsequently faster as it allows it to ride the weaker ridge in place, reaching central Canada faster. As that energy eventually translates eastward, a rather anomalous negatively titled shortwave is forecast to develop off the Northeast U.S. coast early next week with large discrepancies in the depth and degree of any surface low development that could pivot back into the coast early to mid next week. Forecast confidence is below normal here. Finally, southern stream energy of the western U.S. longwave trough could pinch off and form a closed low (00Z ECMWF solution) or remain more open and gradually move eastward into the Southwest U.S which the deterministic other models advertise. Given the overall good agreement yet uncertain, important details and implications for sensible weather, the preferred blend for the period ranged from mostly deterministic guidance day 3/4 to a heavy weight of the ECMWF and ECENS for day 5-7, though the 06Z GFS/GEFS was utilized as well. ...Weather/Hazard Highlights... The main weather hazard and highlights for the period will be the much above normal temperatures across the interior West and Rockies at the start of the period, where across the interior Pac NW and northern Rockies daily max temp anomalies are forecast to be 25-30F above normal. These abnormally warm temperatures will spread eastward across the northern and central Plains by early next week, though with some moderation (anomalies 10-20F above normal). While not extreme, the Great Lakes to Mid-Atlantic and Northeast should see cooler than normal readings for late May and early June with highs anywhere 5-10F below normal, especially early in the period. Extreme northern California and the Pacific Northwest may see locally heavy rainfall during Sat-Sat night as an upper low tracks northward along the West Coast. The upper trough settling near the West Coast will likely produce areas of mostly light to moderate rainfall, with some sensitivity to exactly how energy within the overall trough evolves. The central and southern Rockies should see diurnally favored rainfall through the period. Much of the East will see dry weather from the weekend into early next week, after some lingering East Coast rain on Sat departs with its associated cold front. Parts of Florida and southern Texas may see additional rainfall since those areas will remain south of the front. A warm front crossing the northern tier may generate some showers/thunderstorms from the Great Lakes into parts of the East by Tue-Wed. Farther west a front settling over the Northern Plains could provide a focus for some rainfall but so far guidance is providing mixed signals for specifics of if/where/when it occurs. Rausch/Taylor Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml