Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Dec 09 2021 Valid 12Z Sun Dec 12 2021 - 12Z Thu Dec 16 2021 ...An atmospheric river will spread southward along the West Coast early next week bringing coastal rain and heavy mountain snow... ...Overview... The medium range period will begin Sunday with upper troughing across the East that will move quickly into the Atlantic, with upper ridging behind it expanding from the central U.S. into the eastern U.S. as the workweek starts, causing unseasonably warm temperatures. Meanwhile, a slowly digging and reloading trough in the eastern Pacific will direct a persistent and moist inflow (atmospheric river) to the West Coast, with coastal rain and heavy mountain snows that should spread south and eastward with time as the trough moves. By Wednesday into Thursday, an upper low may spin up from the trough along with a surface low, which could bring another round of precipitation to the north-central U.S. but with low confidence in the details. ...Guidance/Predictability Assessment... The 12/18Z (yesterday) cycle of model guidance continues to show very good agreement especially through the first part of the period with the overall pattern described above. There remain some minor model differences initially with the eastern Pacific trough regarding the distribution of energy and the potential for/placement of a closed low to develop within it. By around Tuesday, there are some slight model differences with the timing of the trough axis shifting eastward. The deterministic guidance was generally within the center of the guidance envelope, while GEFS ensemble members generally take a faster/more eastern track compared to EC ensemble members. But overall guidance agrees in a track of the trough axis southward and then eastward, bringing persistent moist southwesterly flow into the West Coast early in the week and spreading inland by Tuesday. Greater differences arise by Wednesday-Thursday with this trough as it ejects farther eastward and with upstream shortwave energy dropping south through the eastern Pacific. While there are some variations in strength and track of these features, the 12/18Z deterministic models have good consensus spinning up a closed mid-upper low in the north-central U.S. by Thursday (though some incoming 00Z guidance shows a compact shortwave that is not closed). A surface low associated with this feature varies somewhat in depth and track as well, but at least there is consensus for the existence of these features by the end of the medium range period. Meanwhile, models agree with the broad ridging across central and eastern parts of the U.S. through much of the week. The WPC forecast utilized a multi-model deterministic blend of the 12/18Z guidance early in the period, and was able to hold onto a majority deterministic models blend through the period given reasonably good agreement, though increased reliance on the ensemble means especially by day 7. This maintained excellent continuity with the previous WPC forecast for days 3-6 as well. ...Weather/Hazards Highlights... Moist onshore flow with a weak atmospheric river will cause widespread precipitation in the western U.S. through the first half of next week. Precipitation is forecast for the Pacific Northwest, northern Rockies, and especially northern California on Sunday, which will spread southward into southern California Monday and east into the Great Basin and Four Corners states on Tuesday. Enhanced totals are likely along favored terrain, with heavy mountain snow especially in the Sierra Nevada and portions of the Rockies. Coastal rain is expected in California, and some locally high rainfall rates causing flooding issues, especially for sensitive burn scar areas, cannot be ruled out. Temperatures will be near to somewhat below normal in the West underneath the upper trough, with temperatures 10-20F below average for parts of California and Nevada. Farther east, the main story is increasing warmth in the central and eastern U.S. through the period giving the strong ridging aloft. Mild to warm temperatures about 10-20F above average begin in the central U.S. Sunday, while temperatures should only increase from there, reaching widespread 20+ and some locations with 30+ temperature anomalies by Wednesday--currently forecast in central parts of the Plains/Mississippi Valley where highs could reach into the 70s. Record warm minimum and maximum temperatures are possible/likely from the Southern Plains to Great Lakes region. Temperatures 10-20F above average are forecast to expand into the Eastern Seaboard as well. Any widespread precipitation is not likely in the central/eastern U.S. until next Wednesday as a low pressure system comes into the Midwest, with snow possible in the northern latitudes and rain possible along its trailing front, but specific precipitation totals will have to be ironed out in future forecast updates. 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