Extended Forecast Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Mon May 03 2021 Valid 12Z Thu May 06 2021 - 12Z Mon May 10 2021 ...Weather Pattern and Model Guidance... An amplified upper level pattern will be in place across the nation through Saturday with one closed low crossing the Great Lakes Thu, northeast Fri, and departing Saturday. The 06z GFS remains stronger and farther west with the low offshore from New England Friday night compared to the 06z GEFS mean, 00z ECMWF/Canadian/UKMET, so a more offshore solution is indicated following the majority cluster. A closed low over the northeast Pacific is forecast to move east southeast to near Vancouver Island Friday and onshore into the northern Rockies Saturday, then Wyoming Sunday morning before shearing out as it drifts southeast. The 06z GFS was an outlier in forming a wave on the trough in the northern Plains Sunday and amplifying it and moving it into the upper MS Valley Monday. The 06z GEFS mean keeps the wave from amplifying, much like the ECMWF, Canadian, and their respective ensemble means. Again, the majority cluster was preferred. Models and ensembles remain in above average agreement through early next week and offer good run to run continuity. There is some variance with the timing and strength of smaller scale systems, but composites of the 06z GEFS Mean, 00z ECMWF Ensemble Mean, 00z Canadian Ensemble Mean, and operational 00z ECMWF were used to mitigate these differences. ...Weather Highlights/Hazards... On Thursday a 700 mb wave crosses the Great Lakes, producing a round of showers, moving into the northeast on Friday. On Friday, more showers are expected with the next wave approaching the Pacific Northwest, which move into northern Rockies over the weekend. On Saturday confluent flow across the Ohio Valley may lead to showers, then extending into the Mid Atlantic Sunday. Another area of enhanced rainfall is becoming more likely across portions of the middle Mississippi River valley as deep Gulf moisture intersects a stationary front, with 1-2 inches of rain forecast by Sunday morning. The showers continue as the front remains quasi stationary from the mid MS Valley to the Ohio Valley, with locally enhanced rain as wave move along the front in the OH Valley. Days 6-7 rainfall forecasts show potential for locally heavy rain near the confluence of the lower Ohio River Valley and mid MS River Valley. A forming closed low over Wyoming Sunday should lead to lower elevation rain showers over the high Plains and mountain snows, which should extend down into Colorado Monday 10 May. Above normal temperatures are likely from the Desert Southwest to the Great Basin on Thu with temperatures 10-20 degrees above normal. This area of above normal temperatures move east from the Great Basin into the central and southern Rockies on Friday, and then the southern Plains on Saturday Early next week, a dipole pattern sets up where temperatures will fall below normal in the northern Plains (temperatures 10 to 20 degrees below normal), and above normal in the lower MS Valley to the southeast. Petersen/Hamrick 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 Hazards: - Heavy rain across portions of the Central/Southern Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Great Lakes, and the Central/Southern Appalachians, Sat-Mon, May 8-May 10. - Flooding occurring or imminent across portions of the Southern Plains. - Much above normal temperatures across portions of the Northern/Central Great Basin and the Northern/Central Rockies, Thu, May 6. 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