Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 PM EST Thu Jan 13 2022 Valid 12Z Sun Jan 16 2022 - 12Z Thu Jan 20 2022 ...Heavy snow likely from the Applachians to the interior Mid-Atlantic and Northeast with significant icing possible for parts of the Carolinas... ...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, some lingering uncertainty remains with the exact low track, lending 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... The model guidance for the 00z/06z cycle show reasonable agreement regarding the Southeast/East Coast surface low Sunday-Tuesday. The new 12z guidance sticks pretty close to previous runs, with just the typical run-to-run wobble variance. All of the deterministic solutions now keep the low inland, with the 06z/12z GFS a hair east, and the UKMET continuing to stick farther south. The ensemble means are also a bit east of the deterministic solutions, though still mostly along or just inland of the coast. Interestingly enough, the 06z/12z GEFS mean continues to keep it's low center offshore and displaced well to the east of its deterministic counterpart, though there are a fair number of ensemble members with an onshore low. WPC prefers a track a little closer to the deterministic solutions, given the trend over the past couple of days has been westward. The closest proxy to the forecast was a blend of the 00z ECMWF and CMC, which places the low over the Philadelphia region on Monday morning, and very close to that of the previous WPC forecast as well. Given the spread in the individual ensemble members, there still a fair amount of uncertainty in the exact low track and even subtle shifts east or west could create large changes in the impacts for especially the major metropolitan cities along the I-95 corridor. 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. which should track eastward and eventually join with the main flow by around Tuesday. 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 on the next well defined cyclone to traverse the Upper Midwest/Great Lakes and into eastern Canada/northern New England by next Thursday. The WPC medium range forecast was based on an initial blend of the 00Z/06Z deterministic guidance favoring the ECMWF and CMC for days 3-4. After this, added introduced increasing amounts of the ensemble means to help mitigate some of the later period smaller scale details differences. Overall, this presented a forecast very close to previous WPC continuity. ...Weather Highlights/Hazards... The surface low lifting from the Southeast and up the East Coast will bring with it a full array of weather hazards and threats. At this point, heavy snows are most likely in the Tennessee Valley/southern Appalachians beginning this weekend, 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 Sunday night into Monday. For locations east/along I-95 (including the major metro areas of DC, Philadelphia, and New York City), precipitation may start out as a quick burst of accumulating snow, with a changeover to sleet and/or plain rain as warmer air filters in from the east. Chances are also increasing for a potentially significant icing event, especially from parts of northeast Georgia, through parts of the Carolinas, and into 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 todays hazard outlook. To the south, moderately to locally heavy rain likely moves 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 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. 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 Hazards: - Heavy precipitation across portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jan 16-Jan 17. - Heavy rain across portions of the Southeast and the Mid-Atlantic, Sun-Mon, Jan 16-Jan 17. - Heavy snow across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Sun-Mon, Jan 16-Jan 17. - Freezing rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Sun, Jan 16. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Pacific Northwest, and the Ohio Valley. - High significant wave heights for coastal portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jan 16-Jan 17. 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