Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Fri Sep 24 2021 Valid 12Z Mon Sep 27 2021 - 12Z Fri Oct 01 2021 ...Overview... The medium range period will begin on Monday with a closed upper low offshore of British Columbia and another in southeastern Canada, spreading low amplitude ridging to the Northwest and Northeast. Besides a compact upper low moving across the Southwest to Southern Plains, ridging should dominate the center of the country. This general trough-ridge-trough pattern should maintain itself and even amplify during the period as the initial trough/low is forecast to push into the western U.S. and amplify ridging over the central U.S. during the week, while additional energy strengthens the trough in the East as well. ...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, 12/18Z GFS runs were somewhat out of line with consensus regarding the structure and placement of the upper low in southeastern Canada, splitting some energy eastward and shifting the center of the main upper low to the west because of the split, though less so than in its previous runs. Favored the other deterministic guidance for this upper low placement, and noticed the 00Z GFS came in more in line with other models, showing one low. As the week progresses, all guidance generally shows amplification of troughing in the East, including the GFS, which had been out of phase in previous runs. In the West, overall good agreement exists for the first part of the period with troughing coming in and with the small upper low shifting from the Southwest into the Southern Plains and eventually getting absorbed into the eastern side of the trough. However, uncertainty increases from midweek onward given a complex pattern with multiple shortwaves potentially tracking through the trough stemming from the lower predictability northern Pacific. There are questions of northern/southern stream separation or phasing as another vort max may come into the Southwest later in the week, with the 12Z EC and CMC separating the streams more quickly than GFS runs. The GFS runs also produced troughing across the Northern Rockies/Northern Plains for the latter part of the workweek, unlike other guidance. Confidence in the evolution of these features is low. The WPC forecast began with a blend of 12Z/18Z deterministic guidance and shifted to include more ensemble mean guidance as the medium range period progressed to lessen the differences between individual models. ...Weather and Potential Hazards... Increasing moisture from the Gulf of Mexico is forecast to spread into Texas by Monday/Tuesday, causing increased chances of rain/storms there and farther north into the Plains later in the week, focusing ahead of a frontal boundary. Scattered showers in parts of the Four Corners states could become more widespread and heavier later in the week as well. The Pacific Northwest should see periods of orographically enhanced precipitation with the upper trough, especially Monday, and even high elevation snow for the Cascades after a cold frontal passage. Generally only light precipitation is forecast elsewhere in the country, with a possible round of rain moving south across the Eastern Seaboard with the front, and scattered showers across southern parts of Florida. 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 influencing the western U.S. will lead to below normal temperatures there, 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. Tate 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