Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Jan 13 2022 Valid 12Z Sun Jan 16 2022 - 12Z Thu Jan 20 2022 ...Wintry weather is expected across the Appalachians and parts of the Eastern Seaboard, but uncertainty remains with precipitation type in some areas and with the location of the heaviest snow... ...Overview... The most notable weather feature during the medium range period is a developing surface low pressure system that slides across the Southeast on Sunday and then pivots northward near/along the East Coast ahead of a potent mid-upper shortwave trough. This will cause impactful wintry weather to spread across much of the East early next week. However, uncertainty remains with the exact low track, which leads to low confidence in precipitation type (which could change in some locations as the event progresses), the axis of the heaviest snow, and the westward extent of the snow. Meanwhile in the West, mainly dry conditions are expected underneath surface high pressure, with the exception of light precipitation in the Pacific Northwest and Northern Rockies as weak fronts track through. ...Guidance/Predictability Assessment... Model guidance for the 12/18Z cycle (and the incoming 00Z cycle) shows a similar idea to the previous couple of cycles of model guidance with the impactful surface low in the East--that is, a low track as described above staying onshore in the deterministic guidance, but with the GEFS and EC ensemble means showing a track offshore, while individual ensemble members demonstrate considerable spread in low placement, with certainly some ensemble members along the coast or inland. Again, while model guidance exhibits good agreement for a strengthening low, the exact low track will create significant differences in weather type at certain locations, especially near the Atlantic coast to about the I-95 corridor. The 12/18/00Z GFS along with the 12/00Z CMC all show a track basically on the western side of the guidance envelope (well inland over the northern Mid-Atlantic Monday morning), with the 12Z ECMWF a bit farther east but still onshore. Felt it prudent to continue with a low track in between the western deterministic guidance and the eastern EC and GEFS ensemble mean guidance, but somewhat favoring the fairly well clustered deterministic models. The best proxy for this at the time of forecast creation was close to the 12Z ECMWF and the 12Z CMC ensemble mean; the 12Z CMC ensemble mean along with the incoming 00Z CMC mean are the first ensemble means to show an onshore track with the low. This led to a surface low basically over Philadelphia on Monday morning, which was a very minor shift farther inland/west compared to continuity. Hopefully models will converge and confidence will increase within the next day for the exact low track, as the shortwave energy leading to this system will enter the Pacific Northwest/southwestern Canada and have better data sampling for input into the model guidance. Elsewhere, ridging is likely to have hold of the West early this week, other than a meandering southern stream upper low centered offshore of California. The southern stream low is forecast to track eastward and join with the main flow by around Tuesday, with some possible interaction with northern stream energy in the eastern Pacific, but most guidance other than the 12Z CMC now has more separation with these pieces of energy. Then there is good model agreement for an amplified ridge over the West Coast by Thursday drifting eastward from the Pacific, with some model differences in possible embedded shortwaves particularly shown by the GFS runs. Meanwhile, additional shortwaves reinforce troughing centered over the Great Lakes Wednesday-Thursday, for which there is reasonable model agreement. The WPC medium range forecast was based on an initial blend of the 12/18Z deterministic guidance favoring the ECMWF somewhat, adding the CMC ensemble mean by day 4 with its favorable low position in the East and increasing the proportion of the EC ensemble mean for the latter part of the period as well, to help temper minor variations in model guidance. ...Weather Highlights/Hazards... The surface low lifting from across the Southeast and up the East Coast will bring with it a full array of weather hazards and threats. Concerning chances for heavy snow are likely in the southern Appalachians beginning Saturday, spreading northward along/on the backside of the deepening low and generally west of the I-95 corridor in the Mid-Atlantic and the Northeast. Chances are also increasing for a potentially significant icing event, especially from parts of far northern Georgia, through the western Carolinas, and into south-central Virginia where warm and moist Gulf of Mexico inflow will overrun initially cold air at the surface. WPC has both the snow and ice threats highlighted in the most recent hazard outlook. To the south, moderately to locally heavy rain could move across the Southeast this weekend, and possibly up the immediate East Coast Sunday into Monday. Gusty winds surrounding the surface low may create high waves off the coast leading to possible coastal flood concerns along the Northeast coastline. Of course, all of this is highly dependent on the exact track of the surface low and even small adjustments back to the east could bring heavy snow potential into even coastal parts of the East. Elsewhere, after the main low exits northeast of the U.S. on Tuesday, another (weaker) low pressure system is forecast to track over the north-central U.S. in response to a mid-upper shortwave and bring some snow to the Upper Midwest and Great Lakes, with possibly higher totals near typical lake enhancement areas. The West could see some light precipitation during the period, particularly over the Pacific Northwest and into the Northern Rockies early next week, with mainly dry conditions elsewhere. Rain could develop again across the Lower Mississippi Valley and Southeast beginning Tuesday and increasing Wednesday with return flow from the Gulf of Mexico. Cooler than normal temperatures are expected generally over the eastern U.S. early next week, with particularly chilly lows below 0F in the interior Northeast Sunday morning, about 15-25 degrees below average. The West to the Plains can expect generally above average conditions through Tuesday. Then as a cold surface high drops into the north-central U.S. Wednesday and Thursday, temperatures will fall to 10-20F below normal in the Upper Midwest. Tate/Santorelli 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml