Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 AM EST Wed Jan 10 2024 Valid 12Z Sat Jan 13 2024 - 12Z Wed Jan 17 2024 ...Another major storm to impact the Eastern U.S. with snow/rain/wind threats followed by an arctic blast through much of the Central U.S.... ...Overview... An upper low, responsible for the Eastern U.S. storm, will negatively tilt on Saturday then get wound up over southeastern Canada through the middle of next week. A conveyor belt of shortwave energy will pivot across the southern periphery of the newly formed parent low, propagating through the Midwest and East Coast. An East Pacific shortwave trough will likely swing through the West by way of the Pac NW, followed by an amplifying upper ridge over the West in response to the deepening upper low in Canada. Such a setup will facilitate the descent of arctic high pressure over the Central and Eastern U.S. through next week. ...Guidance/Predictability Assessment... The non 12z UKMET guidance were in reasonable agreement regarding the overall evolution of the synoptic pattern over the lower 48. The models are in general agreement that a trough will amplify over southeast Canada, an upper ridge over the West will deepen in response to that low and that a shortwave trough will enter the West on day 3. First on day 3, the 12z and 18z GFS diverged a bit from the closed low over the Great Lakes that previous runs as well as the CMC and EC have consistently identified. That being said, reasonable clustering with respect to the East Pac shortwave between the three operational models' latest runs supported a general model blend of them on day 3. The 18z GEFS was introduced into the blend a bit earlier than usual to account for variance in the handling of an emerging East Pac ridge and a disturbance within the southern periphery of the aforementioned upper low in Canada. The 12z CMC was dropped from the day 5 blend for the 12z ECE to account for displacement of the upper low farther south in the 12z CMC compared to the the EC and GFS suites. The European and GFS deterministic and ensemble means cluster well on day 6 so the day 5 blend is continued. The 12z EC is replaced with the 12z CMC on day 7 due to over amplification of the western ridge compared to the other guidance. ...Weather/Hazards Highlights... The Mississippi Valley to East Coast storm will be well underway by Saturday with snow, rain and wind threats likely from the Midwest to the East Coast. An occluded sub 980mb surface low centered over the Upper Midwest will produce heavy snow and strong winds over the Midwest and Great Lakes with northwesterly flow over Lakes Huron, Superior and Michigan early on in the day followed by lake effect snow downwind of Lakes Erie and Ontario in the afternoon through Sunday as the system departs into the Canadian Maritime. Lingering snow showers are also possible over upslope areas of the Central Appalachians on Saturday. Sustained winds of 20-30mph and gusts of over 40mph are possible from the Midwest to the Northeast on Saturday. Winds calm down considerably on Sunday across the East. Heavy rainfall is expected to continue across the New England coast Saturday morning into afternoon with a low-end Marginal excessive rainfall risk over the Maine coast. In the West, shortwave energy will spread tropical moisture over the Northwest on Saturday. Heavy rainfall is possible for portions of the central Oregon coast down into northern California where a Marginal Risk of Excessive Rainfall is in effect. Heavy snow is likely over the Cascades of Oregon, southern Washington, the Klamath Mountains and the Shasta Siskiyous. The Intermountain West and Northern/Central Rockies will also experience heavy snow Saturday night into Sunday morning. The deepening upper trough over eastern Canada will act to strengthen the upstream ridge over the West early next week which will allow arctic high pressure to spill out into the Great Plains and Midwest/East later in the week. Extremely cold overnight temperatures are likely from the Northern Rockies into the Central Plains with many night time lows struggling to get out of the negative digits through Tuesday at the earliest. Low temperatures in the -20s and -30s will represent 30-50 degree temperature anomalies over parts of northern Idaho, Montana and Wyoming this weekend into early next week. High temperatures in the negative to single digits will represent similar anomalies down into the Southern Plains early next week. The cold blast expands through the Mississippi Valley and into the Gulf Coast on Monday and Tuesday when low temperatures may drop below freezing in many places. Tuesday morning lows may bottom out into the 20s for south Texas and the Central Gulf Coast Monday night. These anomalous freezing temperatures will be dangerous, particularly for those without adequate heating or shelter. Models have begun suggesting a possible Mississippi Valley to East Coast storm next Tuesday into Wednesday with rain impacts for the Gulf Coast and Southeast as well as snow potential for the Midwest and Northeast/Mid-Atlantic. There remains plenty of model uncertainty, especially with respect to the timing of the embedded shortwave propagating through the central U.S.. This detail will need to be resolved before a more confident forecast can be issued. 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