Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 2 2024 Valid 12Z Thu Dec 05 2024 - 12Z Mon Dec 09 2024 ...Another blast of arctic air to bring more heavy lake- effect/enhanced snows and possibly snow squalls near the Great Lakes, high winds and arctic cold into the central and eastern U.S... ...Return moisture from the Gulf of Mexico could bring enhanced rainfall across the western to central Gulf Coast states late this week... ...Overview... A synoptic pattern that favors upper-level troughing over the eastern part of the North American continent will support additional outbreaks of arctic air into the eastern two-thirds of the U.S. The rush of arctic air behind an intensifying Alberta clipper will favor another outbreak of heavy lake-effect snow across the Great Lakes through Thursday along with the possibility of snow squalls and high winds. Meanwhile, moist return flow from the Gulf will support daily enhanced rain chances set to spread from the western to central Gulf Coast states and parts of the Southeast through the weekend into next Monday with uncertainty on the timing and inland extent of the rain. Elsewhere, high pressure will persist in the interior West before a frontal system works through next weekend. Moderate rains will focus over the Pacific Northwest later this week, with snow chances spreading inland across especially favored terrain from Cascades through the Northwest and northern Rockies next weekend with system/colder flow progression. ...Guidance/Predictability Assessment... Guidance today continues with a general trend for the arctic air to rush in faster behind the intensifying Alberta clipper system that is forecast to track east through the Northeast on Thursday. Guidance also shows a faster southward plunge of the Arctic High and the associated arctic air mass into the Plains. Focus will then shift to the Gulf Coast where there is a noticeable difference between the ECMWF and GFS solutions on timing and inland extent of the moisture returning from the Gulf of Mexico into the western Gulf Coast states. The ECMWF tends to hold back the inland progress of the rain while GFS is more aggressive. The CMC is in between these extremes. The uncertainty may be attributed to the uncertainty on the timing and amplitude of the trough/upper low ejecting across northern Mexico toward Texas. Out West, the guidance shows the upper ridge shifting east next weekend to usher in a more cool and unsettled pattern with some lingering uncertainty in the timing of a main/wavy cold front to work its way across the Northwest/West. Accordingly, the WPC medium range product suite was mainly derived from a composite blend of 40% from the 00Z ECMWF/EC mean, 40% from the 06Z GFS/GEFS, and 20% from the 00Z CMC/CMC mean, transitioning to include more of their respective ensemble means starting next weekend amid steadily growing embedded system forecast and continuity variances. This blend yielded a solution rather compatible with the previous WPC medium-range package. The slower ECMWF solutions with the QPF across the western Gulf Coast region were preferred late in the forecast period. ...Weather/Hazards Highlights... Another outbreak of heavy lake-effect snow is likely across the Great Lakes behind an intensifying clipper low pressure system on Thursday. Influx of colder thickness behind the system is also raising the possibility of snow squalls downwind from the Great Lakes, as well as high winds across a wider domain into the Ohio Valley, Appalachians, and across the Mid-Atlantic. Locally very heavy snow accumulations will be possible in favored downwind locations, and travel conditions will likely be hazardous, especially on Thursday. Snow will also be possible elsewhere in the Northeast as the clipper system moves through, especially in the higher terrain of the Appalachians. Conditions should improve briefly late this week, but another low pressure system will bring another chance for wintry precipitation to the region next weekend. The rest of the nation will be mainly dry this week with a couple exceptions. Moist returning from the Gulf will result in daily rain chances along the western Gulf Coast and into portions of the South. While there will likely be some instability to support locally heavier downpours, no threat areas are depicted in the WPC Excessive Rainfall Outlook. Any risk of heavy rainfall will decrease through the end of the week as instability decreases and precipitation becomes more spread out across the Gulf Coast region as fronts move through. Precipitation chances will also return to the Northwest by next weekend as a Pacific frontal system approaches and works steadily inland. This will act to produce a more cooler and unsettled airmass across much of the West, with moderate rains over the Pacific Northwest and some terrain enhancing snows inland across the Northwest and northern Rockies. Temperatures will drop below normal for parts of the north-central to eastern U.S. this week as a cold front ushers in another blast of Arctic air. Daytime highs from the Upper Midwest into the East could be 10-20 degrees below normal. Elsewhere, temperatures will be mild for this time of year, generally near to slightly above average with this milder air gradually expanding east with time. Kong/Schichtel 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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