Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Dec 01 2022 Valid 12Z Sun Dec 04 2022 - 12Z Thu Dec 08 2022 ...Below normal temperatures over the West with rain/mountain snow generally trending lighter... ...Heavy rainfall potential over the east-central U.S. early next week... ...Overview... Expect a majority of the lower 48 to see fast flow aloft between a mean low over central Canada and a ridge over the Gulf of Mexico. Some energy from an upper low initially near the West Coast may get incorporated into this fast flow but a positively tilted trough may persist over the eastern Pacific and western U.S. with only a gradual eastward drift during the early-mid week time frame. An upstream ridge should move into the Northwest U.S./western Canada by midweek or so, followed by a shortwave trough approaching the Pacific Northwest next Thursday. 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. On the other hand it looks to be a favorable setup for heavy rainfall over portions of the east-central U.S. early next week as multiple moisture sources interact with a couple fronts. The far southern tier states should see mostly above normal temperatures. ...Guidance and Predictability Assessment... Various aspects of the forecast have been in flux over recent runs. These include the shortwave details within the Canada/northern tier U.S. stream, specifics of the West Coast upper low that opens up and residual troughing, and how the combination of energy will contribute to a frontal system/low pressure over the East by Tuesday-Wednesday. Operational models have been fairly chaotic with some of the specifics across southern Canada and the northern tier during the period, with the ensemble means providing somewhat more stability. Among the 12Z/18Z guidance evaluated for the updated forecast, the 18Z GFS strayed the most away from consensus for the northern stream during the latter half of the period. Meanwhile the past 24-36 hours of guidance has trended toward a much slower progression of what troughing remains over the eastern Pacific/western U.S. as a portion of the system's energy shears out into the progressive flow downstream. This trend has generally led guidance to depict somewhat weaker waviness along the front that reaches the eastern U.S. by Tuesday-Wednesday, but considerable spread for details due to the variability in the northern stream. There is some improved clustering as an embedded wave may reach New England and the Canadian Maritimes. For the shortwave nearing the Northwest next Thursday, 12Z and new 00Z GFS runs are a bit on the slow/amplified side while the 18Z GFS fit more in line with the model/ensemble mean majority. The starting blend incorporated the 12Z/18Z operational models early in the forecast and then gradually phased out the 18Z GFS while increasing 18Z GEFS/12Z ECMWF means and hanging onto some ECMWF/CMC input. ...Weather/Hazard Highlights... Low pressure near the West Coast through Sunday should produce some enhanced rain and mountain snow along favored terrain of the Sierra Nevada with lighter totals over California and parts of Oregon. Some of the moisture from this system will also extend into the Great Basin and central Rockies. Moist southern stream Pacific flow aloft ahead of this system may continue to produce some rainfall over parts of Arizona and New Mexico through Sunday as well. Some of this activity could be on the locally moderate to heavy side. The southern half of the West should trend drier early next week as the West Coast system opens up, though it is possible for a little moisture to return eventually over far southern areas if residual upper troughing is slow enough. A frontal system approaching the Northwest may bring an increase of precipitation by next Thursday. Farther east, a front settling near the Gulf Coast on Sunday may return northward as a warm front early next week while a system (currently expected to be of moderate strength) emerges from the Plains with a trailing cold front. West-southwesterly upper level flow of moisture from the Pacific and a gradual increase of low level Gulf inflow may interact with the fronts to produce an area of heavy rainfall over the east-central U.S. early next week. Currently the signal is favorable enough to depict a Slight Risk area on the experimental day 5 Excessive Rainfall Outlook (valid Monday-Monday night) centered primarily over most of Tennessee and southern Kentucky. Fairly wet ground conditions will make this area sensitive to significant rainfall as well. Rainfall could persist over parts of the south-central U.S. through at least midweek if the wavy front crossing the region is slow enough. Any snow should be confined to far northern latitudes of the East. Expect below normal high temperatures to persist over much of the West through the Sunday-Thursday period, with most locations 5-15F below normal. The northern Plains will generally be below normal as well, with Monday-Wednesday having the coldest highs of at least 15-20F below normal at some locations. The southern tier will tend to see above normal temperatures, and especially for morning lows that could be up to 20-25F above normal early-mid week. A few daily records for warm lows may be possible. System progression should bring variable temperatures to the northern half of the East. 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 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