Extended Forecast Discussion NWS Weather Prediction Center College Park MD 333 PM EST Wed Jan 12 2022 Valid 12Z Sat Jan 15 2022 - 12Z Wed Jan 19 2022 ...Wintry weather is becoming increasingly likely from parts of the Tennessee Valley into the East this weekend into early next week, but with plenty of uncertainty... ...Overview... The most notable weather feature during the medium range period, and one of the most uncertain, is a developing surface low pressure system that slides across the Southeast and then pivots northward near the East Coast ahead of a potent mid-upper shortwave trough. Uncertainty in specific impacts and weather type remain high, but overall guidance continues to trend toward a low pressure system possibly inland or at least near enough to the East Coast to cause impactful wintry weather. 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 latest 00z/06z (and incoming 12z guidance) continues to show fairly good agreement on the large scale pattern across the CONUS, with plenty of lingering uncertainty in some impactful details. The main system to note is a strong shortwave tracking across the central Plains and into the Southeast this weekend before it pivots near the East Coast by Monday. This will induce cyclogenesis at the surface with better agreement, compared to previous days, in a surface low moving through the Southeast and deepening as it lifts up near the East Coast. There remains some placement differences in the surface low, especially in the East around day 5, but overall, much of the guidance continues to trend farther west, now bringing the low over or even inland from the East Coast by early Monday. The GFS (including todays 12z run) has consistently been the farthest west with the low amongst the deterministic solutions, although its hard to call it an outlier just yet given the latest trends with the new incoming 12z guidance. On the other side, the UKMET is more suppressed to the south and a look at the ensemble guidance from the GEFS and ECENS shows plenty of uncertainty in exact low placement but members are tending to cluster at this point somewhere in between the GFS and UKMET. Both the 00z ECMWF and CMC (and the new 12z runs) are a little closer to the ensembles and thus a preferred starting point for the WPC forecast at this time. This led to a surface low over southern New Jersey which is pretty close to previous WPC continuity. It should, however, be stressed that the energy leading to this potent shortwave is still out in the Pacific Ocean at this time thus, data sampling is poor. As the energy reaches the Northwest U.S. and encounters more observations, additional shifts (potentially large) in the models are likely. Elsewhere, ridging is likely to have hold of the West this weekend, other than a meandering southern stream upper low centered offshore of California. This feature is forecast to track eastward early in the workweek and join with the main flow, and possibly interact with northern stream energy entering the Pacific Northwest around Monday. Model agreement for this overall pattern was fairly good, but with increasing differences especially with embedded shortwaves as the period progressed. Energy progresses downstream early next week with some uncertainty in amplitude across the Great Lakes next Tuesday into Wednesday, and potential interaction with lingering energy from the East Coast low. The blend for the updated WPC medium range forecast used a general model consensus (leaning more towards the ECMWF) for days 3-4. After that, began incorporating some modest amounts of the ensemble means to help temper the late period uncertainties. Overall, this led to a fairly consistent forecast with the overnight WPC package. ...Weather Highlights... The surface low lifting from the Lower Mississippi Valley, across the Southeast, and up the East Coast will bring with it a full array of weather hazards and threats. On the north side of the initial system on Saturday, there could be some potential for wintry weather across parts of the Middle/Lower Mississippi Valley into Tennessee and Kentucky on Saturday, but confidence in amounts is low given the surface low will not have consolidated by that point. More 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 VA 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 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 low exits northeast of the U.S. on Tuesday, westerly flow and another shortwave should may bring favorable conditions for lake effect/enhanced snow over the Great Lakes. 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 beginning Tuesday with return flow from the Gulf of Mexico. Temperature-wise, much below normal temperatures (-15 to -30F) is forecast for the Northeast over the weekend, with slightly cooler than average weather through early next week for the southeastern quadrant of the country. Temperatures should generally be above average in the Plains particularly from Sunday onward after a cooler day Saturday. The West should see temperatures within about 5-10 degrees of normal, with a tendency toward slightly warmer than average conditions especially for morning lows. 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 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, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Sat-Sun, Jan 15-Jan 16. - Heavy snow across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Sat, Jan 15. - 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, the Southern Appalachians, and the Tennessee Valley, Sat-Sun, Jan 15-Jan 16. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Pacific Northwest, and the Ohio Valley. - Flooding likely across portions of the Pacific Northwest. - Much below normal temperatures across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Sat, Jan 15. - High significant wave heights for coastal portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jan 16-Jan 17. - Heavy precipitation across portions of the Alaska Panhandle, Sat, Jan 15. 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