Extended Forecast Discussion NWS Weather Prediction Center College Park MD 220 AM EST Thu Jan 11 2024 Valid 12Z Sun Jan 14 2024 - 12Z Thu Jan 18 2024 ...An Arctic airmass blankets much of the Northern and Central U.S. through early next week... ...Overview... An upper-level low will get pretty wound up near James Bay, Canada while waves of shortwave energy rotate through its southern periphery (the Midwest and Northeast U.S.). Meanwhile, an upper ridge will develop and amplify over the eastern Pacific and West. This upper pattern will result in Arctic high pressure at the surface flowing through the Northern Rockies/Plains and into much of the Central U.S.. ...Guidance/Predictability Assessment... The ensemble means capture the synoptic evolution of the upper level pattern reasonably well in the medium range. The deterministic guidance struggles to consistently cluster around any one solution. A blend of the 12z and 18z GFS as well as the 00z and 12z EC were used on day 3 since the 12z UKMET/CMC struggled to present anything similar to what the ensemble means and EC/GFS have. On day 4 the 18z GEFS and 12z ECE were introduced to the blend to mitigate some of the over amplification of a positively tilted trough in the West by the 12z EC. The Canadian is introduced on day 5 since it sort of catches up to the general EC/GFS/ensemble mean solution with respect to the shortwave propagating through the Mississippi Valley. The previous EC/GFS runs were removed. The Canadian ensemble mean is introduced on day 6 and replaces the deterministic 12z CMC. Only the three ensemble means remain in the blend by day 7. The GFS consistently has a stronger mean low over James bay throughout the medium range period. It is also more progressive with its embedded shortwave energy compared to the EC on day 5, but the Canadian is the deepest and most progressive of the operational models. The UKMET has it's shortwave still moving through the Intermountain West on day 5. The operational Euro and GFS have both slowed down considerably over their last few runs and now has the shortwave moving through the eastern third of the country on day 6, while the 12z Canadian has that same energy out into the Western Atlantic. The ensemble means, especially the GEFS and ECE, have relatively low spread across the lower 48 throughout the medium range period. ...Weather/Hazards Highlights... The main sensible weather highlight during the medium range will be the Arctic blast likely to impact the Northern Rockies, Northern Great Basin, Great Plains, Mississippi Valley and Midwest. The synoptic setup, including a strengthening upper ridge in the West and an massive upper low in the East, will support the descent of surface high pressure through the Central U.S. this weekend through early next week. The coldest parts of this airmass will be over portions of northern Montana where low temperatures could bottom out around 40 degrees below zero Sunday morning. Parts of the Northern Rockies down through the Central Plains and Middle Mississippi Valley may drop to below zero degrees Saturday, Sunday and Monday nights. This will make for very dangerous outdoor conditions. Minimum temperature anomalies are forecast to be between 20-40 degrees below average for these areas. Freezing temperatures will also make their way to the Gulf Coast Monday and Tuesday night. Temperatures will begin to rebound on Wednesday as another disturbance enters the Northern Plains/Upper Midwest and shifts surface winds out of the South. The shortwave trough responsible for advecting tropical moisture into the Northwest leading to the production of heavy snow will then spread that same moisture regime into parts of the Central/Southern Rockies and Intermountain West producing snow for those areas on Sunday. Snow probabilities remain high for the Wasatch, Uinta, Colorado Rockies, San Juan and Sangre de Cristo Mountains. A swath of snow showers may also spread into the Central/Southern Plains and Middle Mississippi Valley on Sunday. Snow will continue over the Colorado Rockies into Monday while some more snow showers develop over the Tennessee/Ohio Valleys. A coastal low is expected to develop just off of the Southeast Coast later on Monday and generate showers and thunderstorms across the central Gulf Coast and much of Florida through Midweek. The exact track of this system has yet to be ironed out but there seems to be some agreement between the Euro and GFS for some snow showers extending from the Mid-Atlantic up into the Northeast Tuesday into Wednesday afternoon. The European ensemble mean has been a bit more aggressive with snow probabilities down the I-95 corridor compared to the GEFS and CMCE, but they've all trended higher with their snow/sleet probabilities. A new system enters the Northwest next Wednesday, bringing with it mountain snow and coastal rain. Westerly winds will advect the frigid arctic airmass over the Great Lakes leading to persistent Lake Effect snow this weekend through the middle of next week. Kebede 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw