Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 PM EST Sun Nov 21 2021 Valid 12Z Wed Nov 24 2021 - 12Z Sun Nov 28 2021 ...Potential heavy rainfall for portions of the Southern Plains, Lower Mississippi Valley, and Pacific Northwest may impact travel for the Thanksgiving holiday... ...Overview... An amplified pattern should prevail through much of the medium range period (Wed-Sun) beginning with a trough axis moving through the interior West, a closed low off the Northeast Coast, and ridging in between. The developing closed low off the Northeast coast may linger for a few days but likely remaining far enough east not to impact the Northeast U.S. significantly. Out west, guidance continues to advertise the trough will likely split into two separate streams, with southern energy digging into Mexico and northern stream energy sliding across the Northern Plains and Great Lakes. The latter may eventually combine with a shortwave in central Canada to form a closed low over eastern Canada next weekend. The southern stream energy and moisture flowing into a frontal system could produce locally heavy rain across the south-central U.S. during the late week. ...Guidance/Predictability Assessment... The 00Z/06Z guidance continues to show the leading trough/closed low linger near the Northeast before pulling away from the coast further out into the Atlantic Ocean. This feature will mostly impact the Canadian maritime region. Meanwhile another longwave trough moves into the central CONUS and transitions to a split flow pattern. The northern stream is expected to continue to track across the Mississippi Valley to the Mid-Atlantic and Northeast regions while the southern stream gets hung up near the Baja Peninsula for a few days. The southern stream energy continues to trend weaker in the models however this leads to uncertainty on when energy slides southeast into the Southern Plains. The CMC is the quickest to bring the energy into the Southern Plains next weekend and is quite generous with the associated QPF. The GFS/ECMWF have slower timing, but also differing placements of the maximum QPF. As noted on the last cycle, the ensemble means continue to support the parent ECMWF/GFS solutions. The northern stream has had persistent, run-to-run consistency and clustering with the evolution as the low closes off near eastern Canada, likely a result of stronger shortwave energy dropping in behind it from western Canada. The next shortwave should begin impacting the West by Saturday-Sunday. The CMC is much slower with the shortwave as it drops southward from Alaska late this week and quickly becomes out of phase with the GFS, ECMWF, and the ensemble means showing strong ridging over the West Coast on day 7 rather than troughing like the better consensus would suggest. The WPC model blend consisted of 00Z CMC/ECWMF/00Z and 06Z GFS and the their ensemble means through the extended period - weighting of the means increased through the middle and later periods. This combination helps maintains continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Moderate rainfall, possibly heavy at times, is expected over parts eastern Texas into the Lower Mississippi Valley on Wednesday and into Thanksgiving Day as a cold front passing through the Southern Plains. The best chance for heavy rainfall may be Thursday along the western Gulf Coast where instability should be greatest and at least a localized flood threat may develop. Less intense and more scattered rain may extend northward along the cold front into parts of the Ohio Valley Wednesday-Thursday and eventually the Mid-Atlantic/Northeast on Friday. The degree of risk for excessive rainfall will be dependent on the how quickly the cold front progresses; if it slows or stalls the risk for local flooding concerns will elevate. Shortwave energy may help direct an elongated plume of moisture into the Pacific Northwest to produce terrain-enhanced, moderate to heavy rain and mountain snows across eastern portions of Washington and Oregon on Thanksgiving Day. Unsettled conditions will persist across the West through the weekend, with the precipitation weakening in intensity for a few days. The system to follow has the possibility of ushering in another round of locally heavy rains to the Pacific Northwest. For temperatures across the CONUS, sprawling high pressure in the East on Wednesday should continue well below normal daytime highs and morning lows, particularly for locations in the Southeast and Mid-Atlantic states. Daily low reading are expected to be near or below freezing. Temperatures may briefly warm back towards normal on Thursday and Friday but could trend chillier again next weekend as the next cold front moves through. Through the extended forecast, locations across the West are expected to trend warmer, which will spill over into the Plains next weekend. Daytime highs for the northern and central High Plains could be 10 to 20 degrees above normal by next Sunday. Campbell 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