Extended Forecast Discussion NWS Weather Prediction Center College Park MD 329 PM EST Thu Dec 15 2022 Valid 12Z Sun Dec 18 2022 - 12Z Thu Dec 22 2022 ...Bitterly cold temperatures likely to settle over the northern Plains from Sunday onward... ...Overview... Guidance suggests the mean pattern over the lower 48 should become less amplified for about the first half of next week, as the upper low over the Great Lakes/southeast Canada on Sunday continues eastward and elongated mean troughing (with one or more embedded lows) prevails across southern Canada. Meanwhile a couple Pacific shortwaves may track to the south of the strongest westerlies expected over the northern tier. Expect Pacific flow to support episodes of rain and higher elevation snow over the Northwest and one or two periods of rainfall over the Gulf Coast region. There is good agreement in principle that an upper ridge building into the eastern Pacific/West Coast by Wednesday-Thursday will encourage downstream trough amplification into the Plains/Mississippi Valley. This trough should increase the potential for another period of active weather over the East after midweek and provide a more southward and eastward push for the cold air initially persisting over the northern Plains. Climate Prediction Center outlooks indicate the cold pattern east of the Rockies is likely to persist well beyond the end of the medium range period. ...Guidance/Predictability Assessment... The models are slowly improving their clustering for the system whose influence on New England lingers through the first part of the period. Latest runs appear to follow through on yesterday's emerging trends for the surface low off Nova Scotia (as of early Sunday) to curl back to a position just northeast of Maine by Monday as the upper low tracking out of the Great Lakes interacts with/reinforces leading dynamics. Then the system should finally depart by Tuesday. As in previous days, guidance continues to have difficulty in resolving the precise details of flow downstream from a strong upper high northwest of Alaska. Primary issues have involved the ultimate progression/shape of an initial trough/closed low over southwestern Canada, and how much troughing persists over western Canada through midweek along with amplitude of flow extending into the western U.S. 00Z through 12Z guidance runs in general have oscillated back to the idea from a couple days ago with respect to initial southwest Canada energy progressing eastward in a more defined fashion while trailing flow is a bit less amplified but with lower Northwest heights than forecast by the 06Z/12Z GFS. How much troughing lingers over western Canada through Tuesday will determine some of the details of downstream trough amplification as upper ridging reaches near the West Coast by Thursday. Currently the 12Z UKMET has strayed from other latest solutions, shearing the leading energy more while amplifying the western trough more by the end of its run early Wednesday--but some sporadic model runs in the recent past did show some variation of that evolution. Forecast preference continued to incorporate an operational model blend early and then transitioned to a model/ensemble mean mix, reflecting the most common adjustments in latest guidance while toning down increasingly uncertain details later in the period. The eventual evolution toward an amplified large scale configuration should ultimately lead to greater predictability but that may not apply as well to important embedded details over the eastern half of the country around the end of the forecast period and beyond. ...Weather/Hazard Highlights... Heavy snows across New England associated with a nor'easter moving northward should be coming to an end by Sunday. Favorable westerly flow on the backside of the main upper low will continue to support some lake effect snows over downwind areas for multiple days. A shortwave tracking across the southern tier along with an associated frontal wave over the Gulf of Mexico could produce a period of enhanced rainfall near the Gulf Coast and into Florida during Monday-Tuesday. The Northwest should see multiple episodes of rain and higher elevation snow with highest totals over favored terrain. Confidence is moderate at best for day-to-day details regarding amounts and coverage but there is a general signal that totals should increase toward Wednesday-Thursday. Also around that time, upper trough amplification over the Plains-Mississippi Valley and associated low pressure/frontal system could bring another threat of snow to parts of the Plains and Upper Midwest into Great Lakes, with rain farther south. Lower latitude waviness late in the period, possibly evolving near the Southeast coast by Thursday, could also bring some moisture to parts of the South and East. Confidence in any specifics is low at this time, given the variability that guidance has had for pattern details over recent days. The most widespread and extreme cold anomalies will likely be across the northern Plains from Sunday onward, with daytime temperatures as cold as 30-40 degrees below normal (locally even colder across parts of Montana), equating to widespread and bitterly cold single digit temperatures with some below zero readings across eastern Montana and North Dakota. Morning lows could fall to at least 10-20 degrees below zero (locally colder in spots). Surrounding areas should see less extreme anomalies of 10-20F below normal. After midweek expect the cold air to surge farther south and east, bringing highs down to 10-25F below normal over the southern half of the Plains and an increasing portion of the Mississippi Valley by next Thursday. The East will be chilly to finish the weekend and moderate for a time, before starting to trend colder again mid-late week. The West will tend to see readings 5-15F below normal through Sunday and then central/southern areas should see a warming trend. Moderately above normal readings will likely become more common over the southern two-thirds of the West by Wednesday-Thursday as an upper ridge builds toward the West Coast. Rausch/Santorelli 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 the Pacific Northwest and the Northern Great Basin, Wed-Thu, Dec 21-Dec 22. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon, Dec 19. - Heavy rain across portions of the Southeast, Tue, Dec 20. - Heavy snow across portions of the Northeast, Sun, Dec 18. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Northern/Central Plains, the Northern/Central Rockies, the Upper/Middle Mississippi Valley, the Northern Great Basin, and the Great Lakes, Sun-Thu, Dec 18-Dec 22. - Much below normal temperatures across portions of the Central/Southern Plains, the Mississippi Valley, the Tennessee Valley, the Great Lakes, and the Ohio Valley, Thu, Dec 22. 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