Extended Forecast Discussion NWS Weather Prediction Center College Park MD 455 PM EDT Fri Sep 24 2021 Valid 12Z Mon Sep 27 2021 - 12Z Fri Oct 01 2021 ...Overview... The medium range period begins on Monday with a closed upper low offshore British Columbia and another across far southeastern Canada, with a compact upper low moving across the Southwest and southern High Plains. This general pattern should maintain itself through the entire week and gradually amplify as the initial trough pushes into the Western U.S.,ridging sharply amplifies over the Central U.S., and additional energy strengthens the trough in the East as well. The latest suite of guidance suggests a cut-off low may form over/just off the Mid-Atlantic coast by Friday, with a second possible cut-off low over the Southwest/Four Corners. ...Guidance Evaluation/Preferences... While model guidance generally is agreeable with the overall trough-ridge-trough pattern described above, differences in placement of the features as well as their evolution remain. On day 3/Monday, mainly minor placement issues remain with the upper low across southeast Canada, with the 00z CMC now the outlier with being farther south with it. Most of the guidance now shows troughing hanging through the week over the East Coast before northern stream energy lifts into Maritime Canada and then possible energy breaking off forming a cut-off closed low which slowly drifts south near the coastal Mid-Atlantic by Friday. Confidence in this feature and associated details remains low however, just given the current spread in the ensembles. Out West, there is overall good agreement as troughing moves into the West Coast and ridging amplifies over the central states. It's not until later in the week when detail uncertainties arise regarding both northern stream energy moving inot the northern High Plains/southern Canada as well as southern stream/cut-off energy in the Southwest. The WPC forecast used a blend of the latest deterministic guidance for the first half of the period. Increased the ensemble means the second half of the week to mitigate some of the detail differences, but continued to maintain at least 50% combined of the GFS/ECMWF for some added definition to Eastern U.S. and Southwest features. This maintained fairly good continuity with the previous shift as well. ...Weather and Potential Hazards... Increasing return flow moisture from the Gulf of Mexico is forecast to spread showers and thunderstorms into Texas early week, with southerly flow ahead of a Southwest closed low sliding eastward continuing the threat for moderate to at least locally heavy rain across the southern and central Plains later in the week. Scattered showers in parts of the Four Corners states may also become more widespread and heavier through the period as well. The Pacific Northwest should see periods of orographically enhanced precipitation during next week with snow possible in the highest elevations, generally after the cold front passage. Rain should begin to break out ahead of the front late week across the northern Plains with continued shower chances across southern Florida associated with a weakening stationary front. Temperatures are expected to be warmer than average for the north-central U.S. throughout the period underneath the upper-level ridge. The Northern Plains in particular could see temperatures around 15-25 degrees above normal, with highs in the mid to upper 80s. Troughing across the western U.S. will result in below normal temperatures, with highs 10-20 degrees below average spreading from the Pacific Northwest into the Great Basin. Meanwhile, temperatures in the East will generally be right around average. Santorelli Hazards: - Heavy rain across portions of the coastal Pacific Northwest, Mon-Thu, Sep 27-Sep 30. - Heavy rain across portions of the western slopes of the Cascades, Mon-Tue, Sep 27-Sep 28. - Heavy rain across portions of the central Plains, Thu, Sep 30. - Heavy rain across portions of the southern High Plains, Thu-Fri, Sep 30-Oct 1. - Flooding possible across portions of the central Appalachians to the northern Mid-Atlantic, as well as portions of the lower Great Lakes. - Flooding occurring or imminent across portions of the lower Great Lakes and central Appalachians. - Much above normal temperatures across much of the northern and central Plains, Mon-Tue, Sep 27-Sep 28. 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, 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