Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM 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 into early next week... ...Overview... Quasi-zonal flow over the CONUS to start the period will transition toward increased troughing over the eastern 1/2 of the Lower 48, due in part to strong ridging over the Davis Strait/southern Greenland as well as over the eastern North Pacific/western Canada. Trough entering the Pac NW this weekend is poised to move across the southern tier with a potential mess of wintry weather (snow/sleet/freezing rain) on the north side of its precipitation shield and heavier rain in the warm sector across portions of the Mid-South. Temperatures are expected to be below normal from the northern Plains into the Northeast with above normal temperatures likely for the Southwest early in the period that may trend back toward more typical values by next week. ...Guidance/Predictability Assessment... Latest models/ensembles remain in good agreement through the period with the overall longwave pattern and pattern transition, but vary on some system details. The 00Z Canadian was slower with the incoming western shortwave than the consensus and was not preferred henceforth. The 00Z/06Z GFS differed in their handling of the same shortwave after passing through the Rockies and neither was a good fit CONUS-wide to the better clustering of the ECMWF EPS/control (ex-HRES), GEFS mean (mostly), and the excellent continuity from the prior overnight shift. This maintained a developing cyclone over the southern Plains later this weekend with a track into/through the Ohio Valley. The guidance does show a fair amount of along and cross-track spread, with the ECMWF-AIFS notably quicker. For now, ensemble mean consensus near the 00Z ECMWF control seemed prudent. ...Weather/Hazards Highlights... No excessive rainfall areas are depicted during days 4-5, but by day 6-7 there may be a need for one over the Mid-South. Please see our latest day 3-7 hazards chart for a graphical depiction. Lake effect snow is likely later this week into the first part of the weekend with cold and brisk flow across the Great Lakes, likely peaking Thursday-Friday. 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 near a lingering frontal boundary. 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, but increased amounts a bit into the favored areas of southwestern OR and northwestern NorCal. Still 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. As currently forecast, this precipitation would be snow in the northern half or so of the U.S. given the cold temperatures, with rain in the South. NBM weather type probabilities show an overlap of wintry p-types (snow/sleet/freezing rain) in between the rain and snow areas, with some locations transitioning between each type. 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. The Winter Weather Outlook depicts a large area of 10-40% probabilities of "plowable" snow/sleet from the Plains eastward across the Midwest early next week. 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 are 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. Daily records there for this time of year are generally in the -30s and colder (low mins) and around -20F for low maxes. While the probability of reaching records at most long- term stations over the Northern Plains is very low (through next Monday), wind chill values may dip below -20F to -30F. Fracasso/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