Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EST Tue Dec 31 2024 Valid 12Z Fri Jan 03 2025 - 12Z Tue Jan 07 2025 ...Broad chances for impactful 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 possibly impactful snow accumulations in the north-central Plains to Mid- Mississippi Valley and Ohio Valley this weekend, with increasing concerns for ice 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 and uncertainty, however. Through the new 12z guidance for today (which all came in after the forecast generation time), the GFS is the most suppressed and farthest south with this system, while the CMC and ECMWF are farther north. Compared to guidance a day or so ago though, there does seem to be a slight southern trend in heavy snow/ice axes so this is worth monitoring and maybe not completely discounting the GFS at this point. Ensemble means are generally in the middle at this point and not as extreme as their deterministic counterparts, which further emphases the uncertainty not only in north-south placement of the low track but also with the timing. At this point, the WPC blend, which used the older 00z/06z runs, trended towards a general model blend with increasing ensemble mean influence in an attempt to mitigate uncertainty with this system. This generally is consistent with previous WPC forecasts as well. Regardless, there is an increasing signal from all of the guidance for a possibly impactful system with wintry weather. Elsewhere across the CONUS, the guidance shows sufficient agreement with other less impactful systems and the general model blend see, it is mostly a precipitation type uncertainty. med to work well as a good starting point. ...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 continued for the Day 4/Friday 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. There is increasing concern that this could be particularly impactful and significant in some locations (but those locations remain highly uncertain still). 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 in the amounts. 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. Santorelli/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