Extended Forecast Discussion NWS Weather Prediction Center College Park MD 242 AM EDT Mon Sep 27 2021 Valid 12Z Thu Sep 30 2021 - 12Z Mon Oct 04 2021 ...Overview... The medium range period begins Thursday with an amplified upper pattern across the contiguous U.S., as ridging in the east-central U.S. separates a closed low centered over the Northeast and mean troughing over the West. Initial western trough energy should split, with the southern part forming a Southwest upper trough or low and the rest tracking into the northern tier U.S./southern Canada. Heading into/through the weekend the pattern should flatten somewhat as a Pacific shortwave reaching western North America continues on toward the mean ridge position and heights over a majority of the West generally rise. Late this week the Southwest upper low and moist inflow will lead to heavy rainfall potential over southern portions of the Rockies and Plains. ...Guidance Evaluation/Preferences... The 12/18Z model guidance continues to consolidate around the idea of a closed upper low centered over the Northeast U.S. Thursday with a gradual drift northward Friday and into the weekend. Some differences remain with the roundness or narrowing of the low as well as the timing of its movement north, with the 12Z GFS as well as the GEFS means perhaps northern outliers by Saturday. The 18Z and new 00Z GFS keep the low farther south. On the other hand, the 12Z UKMET split energy southward toward the Mid-Atlantic region by Friday into Saturday, but its new 00Z run came in closer to consensus. Thus leaned toward a 12Z EC/ECens mean/CMC and 18Z GFS solution for the upper low in the East through the weekend. In the West, guidance has tended to remain somewhat weaker with energy and the potential for a closed southern stream low in the Southwest for the latter part of this week. There is still potential for a closed low at times, but some guidance (particularly GFS runs) appear to keep the northern and southern streams a bit more connected rather than closing off a low. But overall a multi-model deterministic blend worked for this feature. Meanwhile, a couple of rounds of shortwave energy in the northern stream with the western trough continue to have some agreement issues among the guidance. An initial shortwave could move through the north-central U.S. and/or south-central Canada for the latter part of the week, but with track and speed differences. Then with the second shortwave, there is general agreement for it to move into the Pacific Northwest on Friday, but with some differences beyond that time. Initially the major outlier is the 12Z CMC, which has the potent shortwave spinning southward and closing off an upper low near northern California by early Saturday, unlike other guidance, which eventually splits energy across the Pacific but weaker and farther offshore. Through the weekend and into early next week, the shortwave differences impact the central CONUS quite a bit. 12/18/00Z GFS runs have energy from both shortwaves interacting to produce troughing across the Dakotas and Midwest by Sunday and even forming a closed low over those areas by Monday. There is some semblance of a north-central U.S. trough particularly in the GEFS means, with the 12Z EC ensemble mean showing some mild evidence for it as well. Thus the forecast trended in the direction of a trough, but not nearly as strong as the operational GFS runs, and confidence is quite low by early next week. The WPC forecast quickly trended toward using the ensemble means by the latter part of the medium range period. ...Weather and Potential Hazards... Southern parts of the Rockies and Plains can expect multiple days of widespread and potentially heavy rainfall for the latter half of the week, as the upper low in the Southwest provides moist inflow from the Pacific and the low-level pattern brings moisture from the Gulf of Mexico. The specifics of rainfall duration and intensity remain somewhat uncertain given model differences in the timing and track of the Southwest upper low, but the current forecast shows 2 of 4 inches of rain with locally higher amounts in the southern High Plains. Periods of rain are also possible farther north toward the rest of the Plains and the Midwest in the vicinity of a couple of frontal boundaries. The Pacific Northwest should see precipitation lasting through Thursday, with orographically enhanced amounts in the Olympics and northern Cascades, and some snow is possible at the highest elevations. Scattered showers and thunderstorms are likely across southern Florida as a couple of fronts move through. The rest of the East should be generally dry underneath high pressure, other than showers in the Northeast mainly Thursday due to the nearby upper low, and some possible moisture approaching the Ohio Valley into the Mid-Atlantic by Sunday. Warmer than average temperatures are forecast across the north-central tier of the CONUS through the period, with highs in the 70s, even 80s in the Midwest underneath upper ridging. On the other hand, high temperature anomalies of 10-20F below average should be centered across central/southern portions of the Rockies and High Plains given the clouds and precipitation over the region. The West Coast should warm up above normal by the weekend, while slightly below average temperatures are expected for the Northeast for as long as the forecast upper low persists over the region. 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