Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 AM EDT Sat Sep 25 2021 Valid 12Z Tue Sep 28 2021 - 12Z Sat Oct 02 2021 ...Overview... The medium range period begins on Tuesday with a mid/upper-level trough axis along the Pacific Northwest, troughing across the Northeast stemming from an upper low in eastern Canada, with mainly ridging in between in the central U.S., with the exception of a small vort max in the Southern Rockies. This general trough-ridge-trough pattern should maintain itself through the rest of the week while amplifying, as additional energy strengthens both troughs and narrows the ridge, and potentially creates closed lows over portions of both the West and the East by the end of the week. ...Guidance Evaluation/Preferences... While model guidance is generally agreeable with the overall trough-ridge-trough pattern described above, differences in placement of the features as well as their evolution certainly remain. In the East, models are well clustered with the upper low position in Canada and subsequent troughing on day 3/Tuesday, but the 12Z UKMET became an outlier by Wednesday by dropping a shortwave into the northeastern U.S. unlike other guidance. For the latter part of the week, most deterministic guidance is now suggesting that an upper low will close off somewhere along the Eastern Seaboard/western Atlantic. There is low confidence with the positioning and movement of this upper low, however, with the 12Z ECMWF and CMC showing the upper low centered over the Mid-Atlantic on Friday and retrograding on Saturday, while GFS runs have the low's centroid remaining offshore. Cluster analysis of the multi-suite ensemble members shows that these are both possibilities, along with showing latitudinal variations and even some solutions without a closed low. So attempted to take a middle ground approach for now in the upper low position and strength, with changes expected in future forecasts with newer guidance. In the West, there is good model agreement through midweek as the trough pushes through the West. By around Thursday, differences begin to arise with the potential northern/southern stream separation as energy spins into the Southwest, with some guidance forming a closed low at times. Then additional shortwaves stemming from the northeast Pacific vary with their timing and tracks by Friday/Saturday, indicating lower predictability by then. The WPC forecast utilized a blend of the 12/18Z GFS and 12Z ECMWF and CMC deterministic guidance for the early part of the period, and reliance on the GEFS and EC ensemble means increased by the end of the period to reduce some of the differences between individual models. ...Weather and Potential Hazards... Showers and thunderstorms are likely across the Southern Plains beginning around Tuesday, but rain should become heavier and more widespread there for the latter part of the week, as moisture from both the Gulf of Mexico and the Pacific streams into the region ahead of the second round of energy in the Southwest. Some rain is also forecast to spread farther north in the Plains especially around midweek focusing along and ahead of a cold front. 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 next week, with snow possible in the highest elevations. Meanwhile, a round of light to moderate rain is possible along a front in the Mid-Atlantic on Tuesday into Wednesday. Then depending on the eastern upper low's track, moisture may spread into the Northeast by the end of the week. 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 on Tuesday. 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. 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