Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 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 Coast. 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 had. On day 4 the 18z GEFS and 12z ECE were introduced to the blend to mitigate some of the overamplification 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 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. On day 7, only the 3 ensemble means were used. 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 East 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 weather highlight during the medium range will be the Arctic blast expected 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 high pressure through the Central U.S. this weekend through early next week. The coldest parts of this airmass will be over parts of northern Montana where low temperatures could bottom out near 40 degrees below zero Sunday morning. Parts of the Northern Rockies down through the Central Plains and Middle Mississippi Valley will struggle to make it above zero degrees Saturday, Sunday and Monday nights. This will make for very dangerous outdoor conditions. Minimum temperature anomalies will 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 shortwave enters the Northern Plains/Upper Midwest and shifts surface winds out of the South. The shortwave energy responsible for spreading tropical moisture over the Northwest leading to the production of heavy snow will spread that moisture and snow into parts of the Central/Southern Rockies and Intermountain West 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 Wednesay afternoon. 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