Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 AM EST Tue Jan 11 2022 Valid 12Z Fri Jan 14 2022 - 12Z Tue Jan 18 2022 ...Overview... Mean ridging is likely over the West during the medium range period, though with the possibility of shortwaves tracking through the mean flow while southern stream troughing lingers near or offshore the California/Baja California coast. Meanwhile in the north-central to eastern U.S., rounds of upper troughing are expected, with surface lows developing in response. These features could cause meaningful snow over parts of the Midwest on Friday before model solutions diverge with the shortwave/low tracks, which keeps confidence low for any impactful wintry weather farther east in the Appalachians and Eastern Seaboard. ...Guidance/Predictability Assessment... Model guidance for the 12/18Z cycle shows okay agreement for the large-scale pattern but with differences in some impactful details. At the beginning of the period Friday, a surface low in the Atlantic demonstrates reasonably good agreement that it will be well off the Eastern Seaboard given a trough axis basically over the East Coast. That surface low should deepen considerably and may affect the Canadian Maritimes by Saturday. Farther upstream, models agree that shortwave energy will be spilling into the Midwest on Friday and continuing to dig Saturday. However, the shortwaves and associated surface low(s) show more variability in their tracks. Overall the trend continues to be southward in the deterministic models especially, as shown by the 12/18Z GFS and ECMWF, while the 12Z CMC was well north (00Z CMC has come south). Guidance for the most part takes the surface low across the Southeast before turning northeastward over the Atlantic. Ensemble mean guidance is a bit farther north with the low center, and felt like following closer to their track for the surface low was a safer bet given the ensemble member spread. After that on Sunday-Tuesday, there is decent clustering in the guidance for a southern Canada/Great Lakes system with associated fronts, as another shortwave originating from the northern Pacific drops into the north-central and then east-central U.S. This shortwave should remain more consolidated as it rounds the Pacific ridge, which hopefully will provide better predictability (at least for the front if not low pressure) than has been the case for other shortwaves. For the West, even on day 3/Friday there is uncertainty about shortwave energy breaking up the mean ridge. The 12Z EC/CMC centered a mid-upper low over the northern CA/NV border, right over the ridge axis in the GFS runs. It looks like the newer suite of 00Z guidance demonstrates better agreement. Southern stream energy also shows differences in positioning among model guidance as it meanders, from well offshore in the Pacific to the western U.S. The 12Z CMC appeared to be a particular outlier as it takes the energy eastward even into the Southern Plains by Tuesday, against consensus and basically all ensemble members. For the medium range forecast, favored an initial blend of the 12Z ECMWF, UKMET, CMC and 18Z GFS favoring the ECMWF somewhat. Then as the period progressed, increased weighting of the GEFS and EC ensemble means to about half the blend by days 6/7, while taking out the 12Z CMC from the blend. This provided the most coherent overall evolution, and while significant further changes may occur, continuity from the previous forecast on days 3-6 was fairly good. ...Weather Highlights... While deepening Atlantic low pressure late this week should stay far enough offshore to keep a majority of precipitation east of New England on Friday, strong winds will be possible due to the tight gradient between this storm and eastern Canada high pressure. The shortwave and surface low dropping southeastward from the northern tier late this week will likely bring a swath of wintry precipitation across portions of the Midwest, followed by some of this precipitation extending across the eastern U.S. this weekend. Guidance signals are improving for a band of significant snow that may cover parts of Minnesota/Wisconsin/Iowa/Illinois around Friday-Friday night, albeit with some uncertainty remaining for location and magnitude as well as exact timing. Confidence remains fairly low for coverage and amounts of any wintry precipitation farther east. Latest trends continue to suggest a farther south axis over the East with lower probabilities over the Northeast initially, though if the low turns significantly northward, snow could spread into the Northeast Sunday on the backside of the low track. The south-central and southeastern U.S. should see mostly light to moderate rain along the southern side of the precipitation shield. An area of light snow may extend across the Upper Midwest into the Great Lakes around Sunday-Monday with the system moving into the region. The most likely forecast scenario has generally light and scattered precipitation over the West during the period. There is a lower probability of more organized activity over a larger area if upper flow details end up gravitating to the less likely side of the guidance spread. Temperature-wise, the central U.S. will see mild conditions on Friday, with lows 10-20F above average while 10-15F above normal highs are expected for the Southern Plains. Warmer than average temperatures should continue into early next week for portions of the Plains. Meanwhile, colder than average temperatures are likely for the Great Lakes to the Northeast through the weekend, with Saturday the coldest day in the Northeast with temperatures 15-25F below normal. The West should see temperatures within about 10 degrees of normal, with a tendency toward slightly warmer than average conditions especially for lows. Tate/Rausch 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