Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM EST Thu Dec 30 2021 Valid 12Z Sun Jan 02 2022 - 12Z Thu Jan 06 2022 ...Heavy rain threat for the Southeast/Mid-Atlantic Sunday along with snow/ice potential from the southern Appalachians to the Northeast... ...Coastal heavy rain and mountain focusing heavy snow threat Sunday-Thursday for the Northwest/northern California spreads to the northern Great Basin/Rockies... ...Another Arctic Blast for the North-Central U.S. Tuesday-Thursday with lead low snows into the Great Lakes and interior Northeast... ...Guidance/Predictability Assessment and Weather/Hazards Highlights... The WPC medium range product suite was mainly derived from a composite of reasonably well clustered guidance from the 06 UTC GFS and 00 UTC ECMWF days 3/4 (Sunday/Monday) along with the 13 UTC NBM in a pattern with overall above normal predictability. However, there are lingering model differences with the timing/phasing of frontal waves over the Southeast/East and subsequent coastal low potential in this period. This composite develops a surface system seemingly most consistent with mean upper trough ejection, albeit with less stream separation aloft than the 00 UTC Canadian/UKMET and 12 UTC Canadian. WPC progs develop a better defined inland system/coastal low than continuity that acts to hold precipitation back into the Mid-Atlantic. A 12 UTC GFS/ECMWF/UKMET composite is in line with that solution. Expect record breaking warmth to linger into Sunday in advance of the starkly temperature contrasting front over the Southeast/East. The main weather focus into next week turns to another Arctic outbreak down through the north-central U.S. with lead low snows and modest trailing front rains over the east-central states. The pattern will also feature upstream a multi-day precipitation episode for the Northwest and California with the threat of coastal heavy rains and inland/mountain snows, with the snow threat spreading into the northern Great Basin/Rockies with dynamic/amplifying Pacific system energies/height falls and modest moisture influx surges over the Pacific. Model differences, especially at the surface become increasingly problematic Tuesday-Thursday despite a generally compatible larger scale pattern evolution aloft. The WPC medium range product suite in this period was instead mainly derived from a composite of reasonably well clustered guidance from the 06 UTC GEFS mean and 00 UTC ECMWF ensemble mean that forged a solution near the middle of the full envelope of model solutions. The main guidance trend was to limit the southward extent of height falls and heavy precipitation down into California compared to earlier runs and that seems reasonable considering upstream Omega block orientation over the northeast Pacific. Schichtel 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 precipitation across portions of California, the Great Basin, and the Pacific Northwest, Sun-Thu, Jan 2-Jan 6. - Heavy snow across portions of the Northern/Central Plains, the Great Basin, the Northern/Central Rockies, and the Pacific Northwest, Mon-Wed, Jan 3-Jan 5. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Wed, Jan 5. - Heavy snow across portions of the Northeast, Sun, Jan 2. - Flooding possible across portions of the Central Appalachians, the Ohio Valley, and the Tennessee Valley. - Much below normal temperatures across portions of the Plains, and the Mississippi Valley, Sun, Jan 2. - Much below normal temperatures across portions of the Northern/Central Plains, the Upper/Middle Mississippi Valley, and the Northern Rockies, Wed-Thu, Jan 5-Jan 6. - Heavy precipitation across portions of the Alaska Panhandle, Sun, Jan 2. 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