Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 AM EST Wed Jan 13 2021 Valid 12Z Sat Jan 16 2021 - 12Z Wed Jan 20 2021 ...Overview and Guidance/Predictability Assessment... A deep upper cyclone over the lower Great Lakes/Ohio Valley to start the period (Saturday) will lift quickly northward through New England on Saturday. The amplified pattern should induce a wave of low pressure along the surface front off the Northeast coast, which looks to rapidly deepen as it wraps north and westward into far eastern Canada. Overall good agreement leads to high confidence in the setup, with lower confidence in the smaller scale details. Meanwhile, a couple of shortwaves moving through the Western U.S. during the period will amplify and maintain troughing across the Central U.S.. Southern stream energy moving across the southern Plains will encourage a couple of low pressure waves off the Texas coast. The first wave on Sunday should be more subtle, but the guidance seems more excited about the second wave as it dips into the Four Corners region on Tuesday. Latest model runs show potential for a cut off low into northern Mexico by Wednesday. This could bring a stronger area of low pressure into the far western Gulf on Wednesday. Run to run variability on this scenario is high, so there is a lot to be worked out still with regards to timing and intensity. Therefore, prefer a more modest approach towards the ensemble means in the day 6-7 time frame on this system until details can be further resolved. The WPC forecasts were derived from a blend of mostly the deterministic 18z/Jan 12 GFS and the 12z/Jan 12 ECMWF for days 3-5, with increased weighting towards the ensemble means out into days 6 and 7. This maintains good continuity with the previous forecast. The CMC was not included at all with this package because its synoptic evolution was noticeably different, especially northern stream energy through the Ohio Valley mid-period, and also with the cut off low scenario early next week. ...Weather/Threats Highlights... Deepening low pressure through New England on Saturday will bring heavy precipitation to much of the region. Latest guidance suggests heavy rainfall along the coast, with the better potential for accumulating snowfall across interior portions from upstate New York through northern Maine. This of course, is highly dependent on the exact low track and so lingering uncertainties could mean heavy rainfall for areas farther inland as well. Gusty winds will likely be associated with this system too. Lingering mainly light lake effect snowfall on the backside of this system will continue through the weekend. Shortwave energy moving through the West will bring some light rain or mountain snows through the period. As troughing deepens over the Plains this weekend, precipitation should expand into parts of Texas and the lower Mississippi Valley. Greater moisture transport ahead of amplifying energy into Northern Mexico early to mid next week would increase the potential for moderate to heavy rainfall into parts of the central Gulf Coast, though confidence in exact placement and amounts is low given increased model differences. Above normal temperatures should persist most of the period across California and the Desert Southwest, while initial above normal temperatures on Saturday in the Northeast, get replaced by more typical January like values Sunday and beyond. Near or below normal temperatures can be expected for parts of the Southeast this weekend, and the Rockies/southern High Plains next Tuesday and Wednesday. Another round of above normal temperatures should also move into the Northern Plains by next week, with daytime highs 10 to 15 degrees above normal. 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