Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Mon Dec 30 2024 Valid 12Z Thu Jan 02 2025 - 12Z Mon Jan 06 2025 ...Colder weather in the central and eastern U.S. allows for broadening wintry precipitation chances over the weekend... ...Overview... A broad upper trough will be in place across the central and eastern U.S. later this week, allowing for near to below average temperatures and multiple days of lake effect snow in favored areas. Meanwhile upper ridging will amplify over the West for warmer temperatures while an atmospheric river produces precipitation in the Northwest ahead of an eastern Pacific trough. This trough is forecast to move inland and toward the central U.S. over the weekend, which will support a developing low pressure system over the Plains and east-central U.S. early next week. Moisture accompanying this low could lead to heavy rainfall currently expected in the Tennessee Valley on Sunday, while the cool temperatures in place farther north may produce a broad area of snow in the north-central Plains to Mid-Mississippi Valley and Ohio Valley this weekend, with possible ice in the transition zone. ...Guidance/Predictability Assessment... Model guidance remains agreeable with the large scale pattern of the western ridge and the eastern trough for the early medium range period. Small impulses riding the ridge into the Northwest show some model spread that affects QPF, and there is also some spread with the details of a weak wave producing some light snow from the Mid-Mississippi Valley to the Ohio Valley and central Appalachians. These smaller scale differences will have to be resolved during the short range period. By Friday into Saturday, the eastern Pacific trough entering the West shows more considerable model variability with its timing. The 12Z and new 00Z CMC runs were slower/farther west with the trough axis than almost all ensemble members. The 12Z ECMWF was also on the slow side though--despite all AI/ML models (based on the EC initial conditions) faster like the GFS runs and the UKMET. Favored the faster solutions, which was similar to the previous forecast as well, and fortunately the new 00Z ECMWF has trended faster. The depth of the trough at least is more agreeable than in previous days, as models have trended away from the flattest solutions. The trough should set up atop the Rockies to Plains early next week with reasonable model agreement. A surface low is likely to form in response, likely over the southern Plains this weekend and then tracking east. The 00Z ECMWF and GFS happen to agree fairly well on a 12Z Monday surface low position over Louisville, Kentucky or so, but models are far from locked in on this position. Continue to monitor future forecasts as the low track will impact precipitation position and type. The WPC forecast was based on a blend favoring the 12/18Z GFS runs with some inclusion of the 12Z ECMWF, 12Z UKMET, and GEFS and EC ensemble means early in the period. Quickly ramped up the proportion of ensemble means to over half by Days 6-7 amid increasing model spread. ...Weather/Hazards Highlights... Lake effect snow is likely later this week with cold and brisk flow across the Great Lakes, likely peaking Thursday-Friday with some continuing into Saturday. Meanwhile shortwave energy and a weak frontal wave may produce a band of light snow late this week somewhere within an area encompassing the Midwest/Ohio Valley to central Appalachians. Farther south, parts of the western/central Gulf Coast could see one or more periods of light rain. Ahead of the eastern Pacific upper trough, moisture in the form of a moderate atmospheric river will be directed into the Pacific Northwest to northern California, likely peaking Thursday night into Friday morning. There are remaining timing and placement differences for the positioning of peak moisture transport and heaviest rainfall. Thus continued to hold off on any excessive rainfall areas in the Days 4-5/Thursday-Friday EROs, but some areas within this region may be more sensitive than normal to heavy rainfall with the wet antecedent conditions. Modest precipitation including higher elevation snow is likely into the Intermountain West and northern/central Rockies. Additional rounds of light to moderate precipitation are possible in the Northwest into the weekend. As the upper trough moves into the central U.S., surface low pressure is forecast to develop over the southern Plains over the weekend. While the details remain nebulous, precipitation chances look to increase generally across the Plains and Mississippi Valley Saturday and stretching farther into the east-central U.S. Sunday- Monday as the low moves east. This precipitation would be snow in the northern half or so of the U.S. given the cold temperatures, with rain in the south. There may be a transition zone of ice in between the rain and snow, and some locations could transition from snow to rain or vice versa. The central Plains into Middle Mississippi Valley appears most likely for transitioning precipitation types at this point. Heavy rain and thunderstorms are possible in the Tennessee Valley on Sunday. Keep an eye on future forecasts as details are likely to change. Generally, the mean upper ridge over the West will promote above normal temperatures from the Great Basin and Southwest to the southern/central Plains late week into the weekend. Highs will reach well into the 70s in the Desert Southwest, and temperatures could warm into the low 80s in southern Texas by next weekend. Meanwhile the mean trough in the central/eastern U.S. will lead to cooler than normal temperatures from the north-central U.S. into the Midwest to Eastern Seaboard. The northern Plains is forecast to see the most anomalous temperatures, around 10-20F below normal for highs and around 5-15F below normal for lows. This translates to highs not reaching or barely reaching 0F and lows between -10F and -20F in some areas. Tate 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