Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM 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 12z/18z model runs from yesterday remain in fairly good agreement with the lingering closed low off the Northeast Coast Wed-Fri. Some lingering uncertainty regarding placement of the upper low and surface low, but the bulk of the model guidance does agree that regardless, these features will mainly be a marine and southeast Canada threat. In the West, the guidance agrees the main trough over the Rockies/Southwest should split into two streams. The southern stream energy continues to trend weaker in the models, but should linger for at least a few days near Baja California. Questions begin to arise by later in the period regarding timing of the energy as it eventually slides eastward. The CMC is the quickest to bring the energy into the Southern Plains next weekend while the latest runs of the GFS/ECMWF hold it back through the weekend. The ECMWF and GFS have more support from the ensemble means. The northern stream energy shows better agreement and run to run consistency. By next weekend though, the CMC is faster to lift a newly formed closed low into eastern Canada, likely a result of stronger shortwave energy dropping in behind it from western Canada. Again, the ensemble means are closer to the GFS and ECMWF solutions, both of which are is good agreement with each other. Finally, 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. Overall, the deterministic solutions showed good enough agreement for a general model blend days 3-4. After this, the ensemble means were used in place of the CMC for the various issues listed above. Was able to maintain a 60/40 blend of the GFS/ECMWF with the ensemble means days 6 and 7 which should help give some added definition to systems normally washed out by the means. This approach maintains good continuity with the previous WPC forecast as well for days 3-6. ...Weather/Hazards Highlights... The western U.S. cold front should reach the Plains by Wednesday and begin interacting with Gulf moisture to produce an expanding area of moderate to possibly heavy rainfall from eastern Texas into parts of the Lower Mississippi Valley on Wednesday and especially into Thanksgiving day. 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. Shortwave energy may help direct an elongated plume of moisture into the Pacific Northwest to produced terrain enhanced moderate to heavy rain and mountain snows across eastern ports of Washington and Oregon on Thursday. Conditions remain unsettled into the weekend out West, but precipitation should weaken in intensity for a few days. The next system moving towards the region Saturday into Sunday could bring another period 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 across especially the Southeast/Mid-Atlantic where morning lows near or below freezing are likely. 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. Out West, temperatures should trend warmer through the period and eventually 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. Santorelli 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