Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Sep 27 2021 Valid 12Z Thu Sep 30 2021 - 12Z Mon Oct 04 2021 ... Heavy rain threat over portions of the southern Plains late this week into the weekend... ...Overview... An amplified upper pattern will likely continue across the U.S. mainland for much of the medium range period. This pattern will encourage returning moisture from the Gulf of Mexico to interact with a stalled frontal boundary across the Plains. A threat of heavy rain appears most likely over portions of the southern Plains late this week into the weekend as a shortwave is forecast to lift across the region. Meanwhile, a broad upper low will be slow to exit the Northeast, as the next cold front is forecast to move through the Northwest and then into the northern Plains by early next week. ...Guidance Evaluation/Preferences... Latest models and ensembles show fairly good agreement on the slowly exiting broad upper low over the Northeast, as well as the next trough moving into the Pacific Northwest. It is the weaker and more subtle upper trough extending across the Plains that the models continue to indicate noticable differences and run-to-run variabilities. While the GFS exhibits run-to-run jumps, the ECMWF has generally trended faster toward the mean GFS position of this upper trough over the central Plains during the weekend. A general model blend of the 00Z ECMWF/EC mean, 06Z GFS/06Z GEFS, and some contribution from the 00Z CMC/CMC mean was used to handle this system. Elsewhere, some additional timing issues are seen later this weekend into early next week across the Ohio Valley where the upper trough from the Pacific Northwest is forecast to reach the northern Plains. Meanwhile, near/just off the California coast, models are indicating the formation of an upper low during this period. Models are showing fairly good agreement on its position. More of the ensemble means from the 00Z EC, 06Z GEFS, and the 00Z CMC were used to compose the Days 6 and 7 forecasts. ...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. Kong/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