Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 AM EST Wed Jan 12 2022 Valid 12Z Sat Jan 15 2022 - 12Z Wed Jan 19 2022 ...Wintry weather is becoming increasingly likely for parts of the Appalachians and Eastern Seaboard this weekend into early next week, but with ample 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. Deterministic models tend to have a westward/more inland track with the surface low compared to the ensemble guidance, so uncertainty is still an issue, but overall guidance is trending 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 model guidance shows fairly good agreement for the large-scale pattern, but with some lingering uncertainties in some impactful details. Models agree that a strong shortwave will track across the Central Plains into the Southeast this weekend, causing surface low(s) to develop and strengthen, with some uncertainty in their placement and phasing. But Sunday into Monday, guidance shows good consensus for surface low pressure to consolidate and deepen, but with variability in terms of its track. The 12Z/18Z model cycle, and the incoming 00Z model cycle, continued to show a notable westward shift compared to previous guidance, which was true for both deterministic and ensemble guidance. However, the deterministic models are generally even farther west than their ensemble counterparts--the 12/18Z GFS runs and the 12Z CMC and ECMWF (12Z UKMET was south/suppressed) clustered very well with an onshore track over the northern Mid-Atlantic Monday morning, but it is possible their supposed agreement is deceptive. When looking at the ensemble means and individual ensemble members, the deterministic guidance was well on the western side of the spread. Felt it was prudent at the time the medium range forecast was made to take a low track in between the 12Z deterministic and ensemble mean positions, which led to a surface low near the coast of Delaware/southern New Jersey but offshore in the western Atlantic on 12Z Monday. The incoming 00Z guidance has generally continued with a westward trend, including a good proportion of 00Z GEFS members and the 00Z CMC ensemble mean, suggesting the deterministic guidance may have been leading the ensemble means in a track consensus. However, it should be stressed that the energy leading to this potent shortwave are still in the data-poor Pacific Ocean at this time and model and forecast changes, perhaps large, could still occur. In the West, ridging is likely to have hold of the region through the weekend, other than a southern stream upper low centered offshore of California meandering. 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. The WPC fronts/pressures forecast was based on a blend of the 12Z deterministic guidance and a small proportion of the GEFS and EC ensemble mean guidance, eliminating the UKMET and increasing the proportion of ensemble mean guidance to 40% by day 5/Monday and holding that blend through day 7. This led to a more inland track of the low pressure system and its associated precipitation/winter weather impacts compared to the previous forecast. ...Weather Highlights... Moderate to possibly locally heavy rain could move across the Southeast quickly this weekend given the low pressure and frontal systems in the vicinity. On the northern side of the precipitation shield, the low pressure system could create some potential for wintry weather across portions of the Middle/Lower Mississippi Valley into Tennessee and Kentucky on Saturday, which should be mainly light, but confidence is low for amounts there especially given the surface low will not have consolidated yet. Better chances for heavier snow are likely in the Southern Appalachians beginning Saturday, and snow is currently expected to spread northeastward along/on the backside of the deepening low track over the Eastern Seaboard this weekend into Monday. There is still uncertainty for the details of this wintry weather that will need to be resolved in future forecasts, but the trend continues for impactful weather from the system. Elsewhere, after the low exits northeast of the U.S. on Tuesday, westerly flow and another shortwave should cause good 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 except possibly southern Arizona/New Mexico Saturday. Rain could develop across the Lower Mississippi Valley beginning Tuesday with return flow from the Gulf of Mexico. Temperature-wise, chilly weather with much below normal temperatures by 15-30 degrees 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 lows. 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, 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