Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM EST Wed Nov 30 2022 Valid 12Z Sat Dec 03 2022 - 12Z Wed Dec 07 2022 ...Below normal temperatures over the West with rain/mountain snow gradually trending lighter... ...Heavy rainfall potential over the east-central U.S.... ...Overview... Most guidance suggests that the initial pattern consisting of strong west-southwesterly upper flow between a Gulf of Mexico ridge and central Canada upper low, with a mean trough near the West Coast, should transition to broadly cyclonic flow as the West Coast trough moves inland and an upper ridge builds in its place by next Wednesday. This pattern should promote below normal temperatures over most of the West and northern Plains on most days, while the rain and mountain snow affecting some parts of the West should steadily decrease in coverage and intensity with time. The first of two primary systems of interest will be tracking away from the Upper Great Lakes on Saturday while the second will initially be near the West Coast and then move inland. The setup ahead of this second system may support a period of heavy rainfall over parts of the east-central U.S., while the southern tier states should see above normal temperatures. ...Guidance and Predictability Assessment... The latest guidance through the 00z/06z cycles shows fairly good agreement on the initial strong system through the Northeast as well as an upper low dropping down the Northwest Coast. The 00z CMC and UKMET both become too amplified over the Northeast around Sunday-Monday, with the CMC out of line wanting to develop a closed low over the Canadian Maritimes which lingers for a few days. Meanwhile the UKMET is much too amplified with some energy coming in on the western side of an upper low over north-central Canada at the same time. By as early as day 5, there are some important detail differences which arise surrounding energy rotating around the Canadian upper low across the northern tier of the U.S., as well as differences in energy as it ejects from the Western U.S. trough. This leads to some fairly impactful differences in frontal placement and heaviest QPF axis across the east-central U.S. days 5-7. The ECMWF is more amplified/sharper with a northern stream shortwave leading to a more southern shift with the heavier QPF. The rest of the models, along with the National Blend of Models and the ensemble means, would suggest a little farther north footprint across the Tennessee Valley. At least the 12z GFS however, available after forecast generation time, did come in farther south, so any potential southward trend should continue to be monitored. By days 6-7, there continues to be a wide ensemble member spread for how the West Coast system eventually progresses inland as well as in the character of upper troughing to the north/northeast. The good enough agreement in the early period allowed for a general deterministic model blend, led by the GFS and ECMWF. After day 4 though, had to incorporate the ensemble means in place of the CMC and UKMET due to the differences described above with a nearly 50/50 deterministic/ensemble mean blend by day 7. This approach kept fairly close to the previous shift, with mainly only minor changes needed from continuity. ...Weather/Hazard Highlights... Low pressure near the West Coast during the weekend will likely bring the highest rainfall/mountain snow totals to areas from the southwest corner of Oregon through northern California and the Sierra Nevada. Lighter totals are possible over the Pacific Northwest, southern California, and portions of the Rockies. The flow aloft ahead of this system may carry in some added moisture from the eastern Pacific, leading to areas of rainfall across Arizona and New Mexico. Some of this activity could be on the locally moderate to heavy side but confidence in the details is still fairly low. The southern half of the West should trend drier by early next week as upper troughing moves farther inland but some precipitation may persist over the Northwest/Rockies. The West Coast system may produce gusty winds over some areas on Saturday. Areas east of the Rockies should see a fairly active pattern but with some uncertainties given the spread in the guidance for specifics. Best continuity is with a vigorous system tracking away from the Upper Great Lakes after early Saturday with a brief period of brisk to strong winds crossing the northeastern quadrant of the lower 48. The associated cold front could bring some light to locally moderate rainfall over the East. This front should stall and then return northward as a warm front ahead of the next system emerging over the Plains/Midwest, serving as a focus for increasing rainfall over the middle latitudes of the east-central U.S. Sunday onward. Both low level flow from the Gulf and the west-southwesterly flow aloft should contribute moisture for this event that could produce areas of moderate to heavy rainfall. Some rain may extend back into the Plains as well. The system's cold front could maintain an enhanced rainfall focus into Tuesday. It should be noted that while the general rainfall signal has been persistent over recent days, confidence in the specifics is not great in light of the guidance spread for how the surface system evolves/tracks early next week. Some snow could fall in the northern/northwestern periphery of this system Much of the West should see below normal high temperatures from the weekend into the middle of next week, with most locations 5-15F below normal. The northern Plains will generally be below normal, with Sunday likely being the most moderate day closest to normal. Then highs should drop to 15-25F below normal over some areas for Monday-Tuesday over the Northern Plains with a slight warming trend by Wednesday. Southern/eastern areas will tend to see near to above normal temperatures, with various areas of plus 10-15F anomalies likely during the weekend into Tuesday (then dropping as a cold front reaches the East by Wednesday). The southern tier should see multiple days of plus 15-25F anomalies for morning lows and some daily records for warm lows may be possible Monday-Tuesday. Santorelli/Rausch 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, and the Ohio Valley, Sun-Tue, Dec 4-Dec 6. - Heavy rain across portions of the Northeast, Mon-Tue, Dec 5-Dec 6. - Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, and the Southwest, Sat-Sun, Dec 3-Dec 4. - Heavy snow across portions of the Central Great Basin and the Northern Great Basin, Sun, Dec 4. - Heavy snow across portions of the Southern Rockies, the Central Rockies, and the Central Great Basin, Mon, Dec 5. - Flooding possible across portions of the Southeast. - High winds across portions of the Great Lakes, Sat, Dec 3. - Much below normal temperatures across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, California, the Northern Great Basin, the Upper Mississippi Valley, and the Great Lakes, Mon-Tue, Dec 5-Dec 6. - Heavy precipitation across portions of mainland Alaska, Mon-Tue, Dec 5-Dec 6. - Heavy precipitation across portions of mainland Alaska, Sat-Sun, Dec 3-Dec 6. - Heavy snow across portions of mainland Alaska, Sat-Mon, Dec 3-Dec 5. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Dec 3-Dec 4. - High winds across portions of mainland Alaska, Sun-Tue, Dec 4-Dec 6. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml