Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Thu Dec 26 2019 Valid 12Z Sun Dec 29 2019 - 12Z Thu Jan 02 2020 ...Significant winter storm expected to impact parts of the northern Plains into Upper Great Lakes/Northeast into early next week... ...Heavy rainfall potential from the western-central Gulf Coast region northeastward mid-late next week... ...Overview and Guidance/Predictability Assessment... Guidance agrees fairly well at least with respect to the large scale mean flow. The upper low over the Plains as of early day 3 Sun will track gradually east-northeast while another vigorous upper system drops southward along or just inland from the West Coast. As the feature near the West Coast reaches over or near northwest Mexico, strong North Pacific flow should reach into western North America. Consensus expects ridging over the eastern Pacific to strengthen by Wed-Thu and encourage amplification of the incoming Pacific flow--leading to an amplified trough over the central U.S./northern Mexico by next Thu and ejection of the leading upper low. Embedded details have exhibited notable spread and/or trending though. Multi-run trends have generally favored the slower ECMWF cluster for the initial Plains low and a corresponding longer duration of precipitation across the northern tier of the central and eastern U.S.. Ahead of this system the slower trend allows for more high pressure to build into eastern Canada and the Maritimes for a time, leading to slower northward progress of a triple point wave forecast to track from off the northern Mid-Atlantic coast into the Canadian Maritimes. Meanwhile locally significant differences persist for the energy dropping southward near the West Coast. Behavior of guidance thus far seems to favor an intermediate track as it makes its descent, with GFS runs a bit westward of the guidance majority. This is opposite of historical biases so it will be interesting to see how future runs cluster. After early Mon the westward position of GFS runs becomes even more noticeable, though at least the 12Z run has adjusted a bit east to near the 06Z/12Z GEFS mean runs. Latest CMC runs are a bit slower than the ECMWF/ECMWF mean and UKMET but faster than the GFS/GEFS mean. This difference has a significant influence downstream, as GFS and to a lesser extent GEFS mean runs bring lower heights through the eastern U.S. and into the western Atlantic versus other guidance from early week onward. Resulting sensible weather differences involve how quickly moisture will spread from near the Gulf Coast northeastward Tue night onward with the GFS scenario being slowest (but the 12Z GFS trending a bit faster than the 06Z/00Z runs). Differences appear more modest with the strong North Pacific flow that comes into western North America and then amplifies in response to the ridge off the West Coast, with most aspects of the GFS/ECMWF and their means actually fairly similar from the eastern Pacific into the western half of the U.S. by just after the end of the forecast period. Based on latest trends and clustering the updated forecast emphasized the 00Z ECMWF/UKMET/CMC with only modest combined input of the 06Z GFS/GEFS mean into mid-period. Thereafter a model/ensemble blend consisting of even weight of the 00Z ECMWF/CMC and 06Z GEFS/00Z ECMWF mean provided the best representation of the most likely evolution relative to data available through the 06Z cycle. ...Weather Highlights/Threats... The storm system initially over the Upper Midwest should make slow progress into the Upper Great Lakes while a developing coastal wave tracks near the northeastern coast. This evolution will support a lengthy period of precipitation (wintry weather in northern areas) from portions of the northern Plains into the Northeast. Please see the medium range winter weather outlook products for more details on areas that will see the greatest probability of significant snowfall. Other areas in the East will see a more brief episode of rain as the cold front passes through. Some locally heavy rainfall may be possible over/near the Tennessee Valley on Sun. Farther west, expect the energy dropping southward near the West Coast to bring areas of rain and higher elevation snow of varying intensity from Sun into early next week. Exact amounts at some locations will be very sensitive to exact track of the upper system, with additional time needed to refine this fine-scale detail. Then the strong jet energy moving into the Northwest and pushing southeastward Tue-Thu may bring a burst of fairly intense terrain-enhanced rain/mountain snow to areas across the Pacific Northwest/northern Rockies and to a lesser extent into the central Rockies. As the West Coast U.S. into northwest Mexico upper low energy comes under the influence of the strong flow moving into the Northwest/Rockies, ample moisture from the Gulf of Mexico is likely to surge northward across the eastern half of the country by the mid-late part of next week. Thus expect a period of widespread showers and thunderstorms across portions of the western/central Gulf Coast and lower Mississippi Valley, with the potential for areas of heavy rain extending northeast to some degree as well. While confidence is not yet great for precise timing and duration, there is a decent signal for a meaningful rainfall event over this general area. The warm sector ahead of the initial Upper Midwest storm will contain much above normal temperatures into the start of next week, with some readings as much as 20-35F above normal over the eastern half of the lower 48. Frontal passage will bring a cooling trend but only to near/moderately above normal levels. Central U.S. trough amplification aloft late in the period should encourage a warmer trend again over the East by next Thu. Areas from the central/southern Rockies into the Southwest are likely to see the most persistent below average temperatures with some areas averaging 5-10F below normal for highs through the Sun-Thu period. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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