Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Tue Dec 14 2021 Valid 12Z Fri Dec 17 2021 - 12Z Tue Dec 21 2021 ...Guidance/Predictability Assessment... Model and ensemble forecast spread remains less than normal for the majority of the systems of concern through the weekend, bolstering forecast confidence. In this period, the WPC medium range product suite was primarily derived from a composite of best clustered guidance from the 18 UTC GFS, the 12 UTC ECMWF and the 01 UTC National Blend of Models. Ensemble support is also strong and WPC product continuity is well maintained. These models solutions diverge more significantly early next week for Days 6/7 with the timing/amplitude of embedded systems, but most notably with the multi-stream flow over the East/Western Atlantic where the GFS develops a major coastal low while the ECMWF suppresses development. 18 UTC GEFS and 12 UTC ECMWF ensembles offer member clusters in both camps. Given growing uncertainty, opted to blend the more compatible GEFS/ECMWF ensemble means and show modest potential for a coastal system and associated precipitation potential to spread/develop across the region. 00 UTC guidance remains split on development potential at these longer time frames, but the ECMWF has trended in support of a deep low, just delayed. The flow offers that potential. ...Weather/Hazards Highlights... A strong surface low will lift into Canada Thu. Well to the south, upper ridging should support some record warm temperatures ahead of the attendant cold front, with positive anomalies upwards to 15-25F shifting southward over the East to the South/Southeast into Fri/Sat. The trailing portion of the front will meanwhile stall and regroup from the South, with an axis of enhanced rainfall and potential for local runoff and flooding/flash flooding issues from the southern Plains through the Mid-South/Ohio Valley with return flow from the Gulf of Mexico. A developing frontal low will bring a swath of wintry precipitation from the Great Lake states through northern New England into the weekend as the trailing enhanced rainfall axis drops southward across the Gulf Coast and Southeast as cooling high pressure builds across the Plains and to the East. There is then potential for an early next week East Coast system that if realized would spread rains up the East Coast, present a maritime threat, and even offer some Northeast snow potential. Upstream, a moisture surge with moderate precipitation will work into California and across the Northwest Thu ahead of an approaching Pacific system. This system will gradually weaken into Friday. Moisture from another Pacific system may then impact the Pacific Northwest with moderate to possibly locally heavy precipitation amounts Fri into the weekend, with additional energies digging southward over the eastern Pacific early next week that would eventually affect CA. 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 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