Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EST Mon Jan 11 2021 Valid 12Z Thu Jan 14 2021 - 12Z Mon Jan 18 2021 ...Overview... Amplified mean upper ridging over the West Coast to start the period (Thursday) will promote a deepening upper trough over the eastern half of the country late this week. Surface low pressure associated with this trough may be fairly deep, tracking into the Upper Great Lakes and Northeast, pushing a cold front into the East on Friday. This will spread an area of precipitation (more snow than rain) into the Great Lakes/Northeast and vicinity late this week into the weekend. The upper trough should slowly lift away from the East this coming weekend which may favor lake-effect/enhanced snow. A Pacific cold front is forecast to come into the West and move through the Rockies toward the Rio Grande and Gulf Coast by early next week. ...Guidance/Predictability Assessment... The latest models and ensembles were mostly agreeable and consistent with the large-scale/longwave pattern evolution, but continue to show differences in the track/depth of the embedded systems. A blend of the guidance served fairly well to start, favoring a the deterministic consensus early before favoring the ensemble means later in the period (ECMWF/Canadian ensembles a bit more than the GEFS). Trend over the past day of runs has been toward a slower/deeper upper pattern unfolding over the East, resulting in a delayed exodus of the Great Lakes/Northeast system which may get wrapped up in Ontario/Quebec Fri-Sun. This resulted in decent continuity but slower overall with the features. With the Great Lakes low pressure Thu-Fri, ECMWF/ECMWF ensemble mean were on the southwest side of the spread into Minnesota (or ND per 00Z ECMWF) along with the UKMET while the GFS/GFSp/Canadian were to the northeast. Split the difference as ECMWF can be too sharp/deep though usually its ensemble mean is much more reserved. A more wound-up system may support a wave of low pressure along the front near the East Coast Fri as the cold front slowly moves eastward, but these and other details remain unclear. The next Pacific shortwave should arrive into the West around Friday-Saturday, amplifying as it moves across the Rockies and into the Central U.S. next weekend. Slower pace of the Eastern system would shorten the wavelength spacing enough to preclude much development of this system heading toward northern Mexico and the northwestern Gulf by next week, but timing remains uncertain overall. With upper ridging expected to build again along 130W by the end of the period, further digging of the trough is possible. As such, majority ensemble weighting was prudent at this time. ...Weather/Threats Highlights... Leading closed low energy into the Great Lakes would support a combination of synoptic and lake effect/enhanced snows from the Great Lakes into the northern/central Appalachians, with precipitation also extending into parts of the Northeast. Expect rain in the southern part of the moisture shield along the leading cold front into the East as well. Some rainfall is also possible over the Southeast/Florida late this week, though coverage and amounts are still rather uncertain. In the West, the next shortwave to arrive should bring another round of precipitation over the Northwest, though much less extreme than the preceding atmospheric river setup now solidly in the short range period. As this shortwave deepens over the Rockies/Plains this weekend, precipitation may begin to push out of the mountains and expand into parts of Texas and the lower Mississippi Valley. Above normal temperatures should be fairly widespread across much of the northern half and southwestern portions of the lower 48. Maximum temperatures may be about 5-15 degrees above normal while overnight lows may be 10-30 degrees above normal. The Southeast and Rockies will see near to below normal temperatures by about 5 degrees. Milder temperatures in the East ahead of the front will be replaced by more typical mid-January values by the end of the period as we near the coldest time of the year for many. Fracasso/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 Northeast, Fri-Sat, Jan 15-Jan 16. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Thu-Fri, Jan 14-Jan 15. - Flooding possible across portions of the Pacific Northwest. - Flooding occurring or imminent across portions of the Mid-Atlantic. - Flooding likely across portions of the Pacific Northwest. - High winds across portions of the Central/Northern Plains, Thu-Fri, Jan 14-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