Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1210 PM EST Mon Nov 22 2021 Valid 12Z Thu Nov 25 2021 - 12Z Mon Nov 29 2021 ...Potential heavy rainfall for portions of eastern Texas and the Pacific Northwest may impact travel for the Thanksgiving holiday... 17Z Update: The deterministic guidance is in above average agreement through the end of the week on the synoptic scale pattern, with the main difference being a slightly more amplified CMC solution with the building trough over the Midwest and Ohio Valley, and a stronger upstream ridge over the northern Plains. Except for the East Coast region where the UKMET is stronger with the offshore upper low, this model is also in good agreement across most of the nation. By the weekend, the CMC is stronger with a northern stream shortwave tracking towards the Great Lakes, and this stronger solution does not have as much ensemble support as the GFS and ECMWF. One of the main changes for this forecast cycle was increased QPF downwind of the Great Lakes given greater prospects for lake effect snow owing to strong cold air advection in place. The WPC forecast was primarily derived from a GFS/ECMWF/UKMET/ECENS blend through Friday, and then mainly GFS/ECMWF and their respective means for Saturday through Monday. The previous forecast discussion is appended below for reference. /Hamrick ...Overview... Amplified and elongated troughing across the Central U.S. should be splitting into two separate streams as the medium range period begins. The southern energy should drop southward and linger off/over Baja California through next weekend while the northern energy closes off a low over the Great Lakes and shifts into eastern Canada. Additional energy from Canada should act to reload and amplify the trough over the eastern U.S. by early next week as upper level ridging builds across the Northwest/Rockies the second half of the period. ...Guidance/Predictability Assessment... A closed low off the Northeast coast may hang around for a day or two before it weakens and the energy becomes absorbed by northern stream energy from the West. With this, the latest suite of guidance shows reasonable agreement in the large scale as a low closes off over the Great Lakes, with only some minor timing/placement differences. There's more uncertainty with secondary energy diving into the western fringe of the trough (CMC much stronger than the GFS/ECMWF) and then evolution and detail differences as the trough re-amplifies over the East day 6-7. The models actually show really good agreement with the southern stream closed low over Baja California which may try to inch into the Southwest by next Monday. Elsewhere, there are some differences in the East Pacific with the next trough towards the West Coast with the CMC beginning to get out of phase with the GFS/ECMWF and ensemble means. WPC used a blend of the deterministic guidance (more weighting towards the GFS and ECMWF) for days 3-5, with increasing contributions from the agreeable ensemble means by late period which should help mitigate the differences discussed above. This combination helps maintain good continuity with the previous WPC forecast for days 3-6. ...Weather/Hazards Highlights... Moderate rainfall, possibly heavy at times, is expected over parts eastern Texas into Thanksgiving Day. The guidance still shows the best chance for heavy rainfall may be along the western Gulf Coast/far south Texas where instability should be greatest and at least a localized flood threat may develop. 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. Less intense and more scattered rain may extend northward along the cold front into parts of the Lower Mississippi, Tennessee and Ohio Valleys Thursday and eventually the Mid-Atlantic/Northeast on Friday. Shortwave energy should 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 next weekend. One day of near normal temperatures are expected over the East, but after Thanksgiving temperatures should once again trend much cooler from the Midwest into the East underneath of amplified upper troughing. Periods of 10-15 below normal are possible from the Gulf Coast to the Midwest/Ohio Valley on Friday, with generally minus 5-10 degrees into the East next weekend. Meanwhile, amplified troughing into the West will promote a warming trend across the West and into the Plains through the period. By next Sunday and Monday, daytime highs and morning lows across the northern/central High Plains could be 10 to 20 degrees above normal. Santorelli/Hamrick 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