Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sat Dec 28 2019 Valid 12Z Tue Dec 31 2019 - 12Z Sat Jan 04 2020 ...Significant winter storm expected to continue Tue across northern New England... ...Heavy rainfall potential over the Lower Mississippi Valley next Thursday... ...Overview... A lead upper low will move along the US/Canadian border Tue/Wed which will spread a swath of snow through northern New York and New England. In the West, a deep upper low will swing through Baja California and old Mexico Wed-Fri before eventually lifting northeastward. This may support a heavy rainfall potential from eastern Texas into Louisiana/Arkansas and perhaps farther east late in the week. In the Pacific Northwest, a couple frontal passages will bring in modest precipitation to the region. ...Guidance/Predictability Assessment... Models and ensembles showed good clustering on the first couple days of the forecast, and a deterministic blend of the 12Z/18Z GFS and 12Z ECMWF/Canadian/UKMET served as a good starting point. The quicker GFS runs in the east were offset by a slower progression in the West/Southwest/Mexico compared to the ECMWF/Canadian/UKMET. By about next Thursday, the largest differences among the guidance were over Texas/Mexico with the upper low. The 18Z/12Z GFS were slower than the UKMET which was slower than the Canadian. The 12Z ECMWF was quickest of the deterministic models and even among the quickest 1/5th or so of ensemble members (and quicker than its previous two runs). Given the strength of the system over Mexico, was reluctant to believe the quickest solution was plausible. The ensemble means were just a bit quicker than the GFS and somewhat like the Canadian, and formed the starting point for that feature next Thu-Sat as it lifts northeastward. Building ridging over Florida (500mb height anomalies over +2 sigma near Cuba) should favor a slower solution as well. Back to the West, 12Z GFS was not preferred as the ensembles showed increased troughing into the West with a cold front progressing eastward and southward. 18Z GFS was a bit lower with heights but still higher than the ensemble means. The 12Z ECMWF was more aggressive with height falls so an intermediate solutions was preferred given the uncertainty leading up to next Sat. ...Weather Highlights/Threats... Exiting surface low off New England (and trailing parent low over the Great Lakes) will leave a trail of snow for parts of northern NY and much of NH/VT into Maine. Temperatures will trend upwards after the storm departs as heights rise from the west. Focus will then be on the Pacific Northwest with and incoming front on Tuesday pushing through the northern Rockies Wednesday and onto the Plains by early Thursday, where it may be rather moisture-starved. Highest precipitation will be over coastal areas and the Cascades with additional snow along the Divide. To the south, upper low will lift out of Mexico toward the Lower MS Valley. Cold front will link up with the Gulf warm front on the tail-end of the departing New England front which may help tap Gulf moisture and bring in an area modest to locally heavy rainfall by Thursday. After that, it remains unclear how resilient this system may be but at least some rainfall will spread eastward through the Southeast on Friday. By then, another front will likely enter the Pacific Northwest which will bring in another round of lowland rain and mountain snows, along with cooler temperatures behind it. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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