Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Dec 31 2024 Valid 12Z Fri Jan 03 2025 - 12Z Tue Jan 07 2025 ...Broad chances for wintry weather across the north-central U.S. to Mid-Mississippi and Ohio Valleys to the East Coast over the weekend into early next week... ...Overview... As the medium range period begins Friday, broad troughing will be in place across the East, allowing for another couple of days of lake effect and terrain-enhanced central Appalachians snow. Meanwhile upper ridging will amplify over the West for warmer temperatures while a moderate atmospheric river will provide some modest to locally heavy precipitation over the Northwest/northern California continuing into Friday 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 cold 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 ice likely in the transition zone. ...Guidance/Predictability Assessment... Model guidance remains agreeable in showing a West upper ridge and East upper trough late this week. The timing of eastern Pacific troughing moving into the West and then central U.S. and developing surface low pressure shows much more model variability, however. Through the 12Z run, the CMC has been slower than consensus and most ensemble members with the trough by late week and beyond, which WPC has not favored. The new 00Z run has fortunately adjusted faster. Meanwhile, the 12Z ECMWF was on the faster side in a notable change from its previous runs. EC-based AI/ML models and EC ensemble members were generally faster compared to the GEFS and CMC suites, though some ML models were slower than the fast EC control run by Monday-Tuesday with the surface low and upper trough. The 12/18Z GFS and 12Z UKMET had timing that was in between the slow 12Z CMC and fast 12Z EC. So the GFS/UKMET seemed to be an okay middle ground through the weekend forecast, but did not favor the evolution of the GFS runs' low tracks from a slow/west position with the surface low Sunday and then ejecting east with the 12Z run farther north and 18Z run farther south than preferred by Monday. The best proxies for an intermediate forecast were actually the old 00Z (Dec 30) ECMWF, continuity, and perhaps the GEFS mean. Fortunately the new 00Z ECMWF has slowed compared to its 12Z run. There are certainly still model differences in the 00Z model cycle with the timing and position of the surface low, affecting precipitation locations and type, but at least less so than the 12Z cycle. The WPC forecast used a blend of the 12Z/18Z GFS, 12Z and older 00Z run of the ECMWF, and 12Z UKMET plus a bit of ensemble means for the early part of the period. Eliminated the fast 12Z ECMWF from the blend and gradually lessened the proportion of GFS runs while the time limited UKMET phased out, leading to a model blend of well over half ensemble means by Days 6-7 given the ample spread in the 12/18Z cycle. ...Weather/Hazards Highlights... Lake effect snow is likely into Friday-Saturday with cold and brisk flow across the Great Lakes. Favored areas could see heavy snow amounts piling up, especially east of Lake Ontario. Additionally, shortwave energy and a cold front may produce light snow in the Ohio Valley with terrain enhancement of totals in the central Appalachians on Friday. Farther south, parts of the western/central Gulf Coast could see periods of light rain. Ahead of the eastern Pacific upper trough, moisture in the form of a weak to moderate atmospheric river will be directed into the Pacific Northwest to northern California, likely peaking Thursday night into Friday morning. A Marginal Risk has been added for the Day 4/Friday ERO from the continuation of a Day 3/Thursday Marginal ERO. This is likely a low end Marginal Risk but these areas may be more sensitive than normal due to wet antecedent conditions. Rain rates could be half an inch to an inch per hour early Friday. 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. Model guidance continues to show a relatively broad region where there may be a transition zone of sleet and/or freezing rain in between the rain and snow, and some locations could transition between multiple precipitation types. The central Plains into Middle Mississippi Valley to perhaps the Tennessee and/or Lower Ohio Valley appears most likely for transitioning precipitation types at this point. The Winter Weather Outlook depicts a large area of 10-50% probabilities of "plowable" snow/sleet from the north-central Plains eastward across the Middle Mississippi Valley and Ohio Valley this weekend. Some wintry weather may push into the Mid-Atlantic into parts of the Northeast by Monday, but with low confidence. Heavy rain and thunderstorms in the warm sector could also be hazardous with this system, currently forecast to peak 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 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. These areas are forecast to moderate closer to normal early next week, perhaps reaching below average in Texas. Meanwhile the mean trough in the central/eastern U.S. will lead to cooler than normal temperatures from the central U.S. into the Midwest to Eastern Seaboard, with multiple resurgences of cold air. The northern Plains are forecast to see the most anomalous temperatures through the weekend, 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. This should not be record breaking cold, as daily records there for this time of year are generally in the -30s and colder (low mins) and around -20F for low maxes. However, the cold could still be hazardous as wind chill values may dip below -20F to -30F. 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