Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 AM 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 by 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 lift away from the East this coming weekend as another Pacific shortwave comes into the West and amplifies as it moves east through the Rockies and the Central U.S. into early next week. ...Guidance/Predictability Assessment... The latest models and ensembles are agreeable and consistent with the large scale pattern evolution, but are showing some spread and recent refinements for specifics. There is a fairly strong consensus now on evolution of Great Lakes low pressure early in the period. The past day or so of model runs have been showing a stronger signal for an upper low to close off somewhere near the Great Lakes, though placement remains a question. The most recent runs of the UKMET/ECMWF are south of the GFS/CMC/ensemble mean consensus, however there has been a southward trend in most of the guidance with this system. Despite these differences, a general model compromise actually serves well for this system which would bring a nice deep surface low into the Upper Great Lakes on Friday and into either far southern Canada or New England by Saturday. Once the trailing front clears the East Coast, some recent model runs of the ECMWF and CMC show potential for wave development in response to sharper shortwave energy in the base of the trough, while the GFS is more rounded with the upper trough and cleanest with its frontal passage. Given the lack of confidence in shortwave specifics, a preference towards minimizing a defined wave near the East Coast is preferred until there is a more coherent signal. 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. The biggest forecast concern with this is there are indications from some models (most notably the GFS and CMC) for separated southern stream energy tracking into the Southern Plains/Gulf Coast states. This would support a well defined surface low in the northern Gulf of Mexico by day 7 (Monday). The details at this point in time remains highly uncertain, but recent runs of the GFS are mostly consistent in showing this. The ECMWF is more suppressed/less enthusiastic about this scenario, as are the ensemble means which is expected. Therefore, at this time, prefer a blend closer to that of the ensemble means with smaller contributions from the deterministic GFS and ECMWF. This shows at least a surface low/front in the Gulf on Day 7, but not nearly as defined as the GFS or Canadian runs would indicate. ...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 still considerable spread for coverage and amounts. In the West, the next shortwave to arrive should bring another round of precipitation over the Northwest, though 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 expand into parts of the Gulf Coast states as well. Above normal temperatures should be fairly widespread across much of the country (with the exception of the Southeast and the Rockies) to start the period, with the greatest anomalies of +10-15F from the northern Plains to the Northeast Thursday and Friday. Amplification of the shortwave reaching the West will suppress the warmth southward by next weekend, while the upper trough/front reaching the East late this week will moderate temperatures back towards or modestly below 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