Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EST Mon Dec 28 2020 Valid 12Z Thu Dec 31 2020 - 12Z Mon Jan 04 2021 ...Western Gulf Coast through New England/southeast Canada system to bring a heavy rainfall threat and cold sector snow late this week... ...Significant rainfall/mountain snow from the Pacific Northwest and northern California into the northern Rockies... ...Weather Pattern Overview... Within an amplified upper pattern featuring central U.S. into northern Mexico troughing and Atlantic into northwestern Caribbean ridging, expect strengthening surface low pressure to track from near the Texas coast through the Great Lakes and New England/southeast Canada late this week. This system will bring a broad area of rainfall--some heavy--to the eastern half of the country along with snow around its northern/western side. As the mean flow aloft trends toward a more zonal regime by late weekend/early next week, multiple Pacific systems will bring an increasingly dominant focus for precipitation to areas from the Pacific Northwest into northern California as well as the northern Rockies. ...Guidance/Predictability Assessment... The majority of guidance is clustered fairly well but uncertainties remain for some aspects of the forecast given some notable trending over the past couple days or so as well as embedded features that typically have lower predictability. The latest GFS/ECMWF/UKMET runs show good agreement in principle for the system expected to track from the Texas coast into southeast Canada/extreme northern New England during the first half of the period. CMC/CMC ensemble guidance has been weaker and more suppressed, with the new 12Z CMC trending toward the majority but not quite there yet. Trends over the past two days have been slower/westward for the surface low and trailing cold front. There has been a decent signal for energy dropping in behind this system to produce a shortwave crossing the East during the weekend. This feature has lower confidence in light of run-to-run variability for details and timing, with an average of the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean representing the best ideas of recent guidance fairly well. Farther upstream, one shortwave should reach the Northwest around late Fri followed by another early Sun with a third set to approach offshore next Mon. Latest GFS/ECMWF runs and their means agree fairly well for these features. Even with the current consensus, the fast prevailing flow may still temper predictability somewhat. Based on guidance comparisons the updated forecast emphasized the 06Z GFS and 00Z ECMWF/UKMET during the first half of the period and then transitioned to a blend of the GFS/ECMWF and their means by days 6-7 Sun-Mon with the typical increase in detail uncertainties. ...Weather/Threats Highlights... Another impactful weather system is likely to affect the central and eastern U.S. to close out 2020 and start the new year. Recent consolidation/trending of guidance is adding support for a corridor of moderate to possibly heavy snow from near the south-central Plains through the Midwest into the central Great Lakes region as the surface low tracks from near the Texas coast into far southeastern Canada/northern New England. Some snow is possible over the Northeast ahead of an advancing front as well. Depending on the eventual strength of the low and the resulting pressure gradient, blowing snow/blizzard conditions may once again be possible for some portions of the Midwest states. Some lake effect snow is likely once the surface cyclone exits the Northeast by the weekend. In the warm sector of the low, expect a broad area of rainfall from the Gulf Coast northward. One area of heavier rain may be over the southeastern Plains/parts of the Mid-Lower Mississippi Valley, with another extending from the Florida Panhandle north/northeast including the Appalachians and Mid-Atlantic. Meaningful totals may reach the Northeast as well. Ground conditions are already saturated across many of these areas within the eastern corridor and the additional rainfall may result in some instances of flooding and rising river levels. Given some of the similarities from last week's system, there could also be some severe thunderstorms over parts of the South/Southeast. Check the Storm Prediction Center outlooks for latest info regarding the severe weather threat. For the Northeast, one important difference from last week should be a lack of Atlantic inflow this time. The Pacific Northwest and northern California will see episodes of moderate to heavy rain and mountain snow from late this week onward as multiple Pacific systems in progressive flow aloft periodically enhance onshore flow. Favored coastal terrain as well as the Cascades and Sierra Nevada should see the highest precipitation totals and some locations may see up to 5-10 inches liquid for the five-day period. Less extreme but still significant totals are possible over the northern Rockies. Moist southerly flow ahead of the late week east-central U.S. storm will push temperatures well above normal over the East. Highs should reach 10-20F above normal for a day or two while morning lows could be as much as 20-30F above normal over/near the Southeast and southern Mid-Atlantic. Such anomalies could yield record warm low values if the approaching cold front is slow enough to prevent cooling before the end of the calendar day. In the cold sector, parts of the Midwest and southern Plains may see highs at least 10F below normal on Thu. Transition of the upper pattern to a more zonal regime later in the period will bring a warming trend to the central U.S. by Sun-Mon and strengthen the warm anomalies already expected to be in place over the northern Plains late this week. Areas close to the Canadian border could see min and/or max temperatures up to 15-30F above normal by next Mon. Rausch/Hamrick Hazards: - Heavy precipitation from the eastern portion of the southern Plains, across the mid-Mississippi Valley, into the central Appalachians and southern New England as well as, portions of the Pacific Northwest, Thu-Fri, Dec 31-Jan 1. - Heavy precipitation across portions of northern California and into the Pacific Northwest, Sat-Mon, Jan 2-Jan 4. - Heavy precipitation across portions of interior northern New England, Fri-Sat, Jan 1-Jan 2. - Heavy rain from portions of the Deep South into the interior Southeast, across the southern to central Appalachians, and much of the Mid-Atlantic, Thu-Fri, Dec 31-Jan 1. - Heavy snow across portions of the northern Rockies, Sat-Mon, Jan 2-Jan 4. - Heavy snow across portions of the central Plains, into the upper Midwest and the central Great Lakes, Thu-Fri, Dec 31-Jan 1. - Severe weather across portions of the Deep South, Thu, Dec 31. - Severe weather across portions of the Southeast and the Mid-Atlantic, Fri, Jan 1. - Flooding possible across portions of Illinois and Missouri. 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