Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Dec 11 2022 Valid 12Z Wed Dec 14 2022 - 12Z Sun Dec 18 2022 ...Major storm system will continue to bring possibly heavy snow into parts of the Upper Midwest with a heavy rain/severe weather threat for southern areas next week... ...Overview... A deep upper level low will be in place over the central Plains by the start of the period Wednesday and is expected to track steadily eastward into the Northeast by this weekend. The associated surface low pressure system should weaken over the Midwest later this week in favor of a new developing coastal low off the Mid-Atlantic Coast. This system will continue to bring moderate to heavy snowfall across portions of the northern Plains, Upper Midwest, and Great Lakes through Thursday, with increasing confidence for some heavy snowfall to impact interior portions of the Northeast as well by Friday-Saturday. To the south, a heavy rain threat will continue across the Southeast and the eastern/coastal Mid-Atlantic and Northeast as well. Otherwise, a couple of weaker shortwaves from Canada should drop in and along with troughing into the Western U.S. early next week, this should maintain mean troughing supporting below normal temperatures across much of the CONUS. ...Guidance/Predictability Assessment... T period begins with good enough agreement on the upper low placement/system timing for a general model blend (weighted towards the ECMWF/CMC) through day 4. UKMET was faster than the better consensus and was not included. After day 4, however, model guidance begins to diverge rather quickly across the board. The GFS continues to be more elongated with the upper low over the Midwest late in the week and has an overall slightly less amplified pattern (though it does look better than it did in previous runs). This allows for a much quicker and more offshore coastal low/possible nor'easter off the Northeast Coast next weekend. There are some timing differences between the ECMWF and CMC but overall, those models continue to be most in line with the ensemble means. Out West, there's also considerable differences in a shortwave dropping into the northern tier with the GFS much more bullish with this and wanting to drop a closed low across the Western U.S.. The other models and ensembles suggest a weaker more progressive northern stream shortwave with a deeper closed upper low initially well off the West Coast and slowly drifting eastward into the Western U.S. by the end of the period. A quick look at the new 00z guidance (available after forecast generation time) shows even more uncertainty both across the East and the West. Given this, it seemed safest to lean increasingly more on the ensemble means beyond day 5, with some inclusion of the ECMWF and CMC for better system definition. This also maintained good continuity with the previous WPC forecast. ...Weather/Hazard Highlights... Deep ample moisture will continue to stream northward ahead of the upper low over the central U.S. to fuel heavy to excessive rainfall into the Deep South and eventually up the East Coast. The latest day 4 WPC experimental Excessive Rainfall Outlook highlights a slight risk over parts of the Tennessee Valley/southern Appalachians and northern parts of the Southeast where above normal streamflows and soil moisture could enhance flooding potential. The Storm Prediction Center also continues to monitor and highlight the potential for strong to severe convection across the central Gulf Coast on Wednesday. On the backside of the surface low, cold temperatures and wrap-back moisture will continue to support moderate to heavy snowfall across parts of the northern Plains into the Upper Midwest and Great Lakes with some mixed precipitation/freezing rain potential in the transition zone as well as along the eastern edge of the precip shield near/north of the warm front. There is increasing confidence for coastal low development off the Mid-Atlantic coast around next Friday, with some increasing potential for modest snowfall across interior New England. As the system exits the East next weekend, some light lake effect snow showers may continue downwind of the lower lakes. The West to High Plains should see below normal conditions through the period, with the most widespread readings of generally 10-20F below normal for high temperatures behind and underneath the upper low. On the other hand, temperatures will initially be above normal centered across eastern parts of the Plains and the Mississippi Valley Tuesday and shifting east. Anomalies will be greatest for morning lows, with some plus 15-25F anomalies across the southern tier through Thursday. Highs are forecast to be around 10-15F above normal for the central and then east-central U.S., locally higher ahead of the storm system. Much of the country looks to be near or below normal by next Friday and Saturday after the cold frontal passage as high pressure overspreads much of the country. 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 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