Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Nov 28 2018 Valid 12Z Sat Dec 1 2018 - 12Z Wed Dec 5 2018 General Pattern Overview ------------------------ The forecast period from this weekend through Wednesday will be active with two dominant systems of interest. One vigorous shortwave over the central plains on Saturday will progress northeastward towards the Great Lakes, accompanied by a strong surface low pressure system. Another storm system will reach the Pacific Northwest by Saturday, and associated upper troughing should amplify over the West during the weekend as this feature drops towards the southeast, followed by ejection of leading energy which may generate another surface low over the central U.S. by next Tuesday. During the first half of next week the specifics of the eastern Pacific pattern become somewhat uncertain with individual models/ensembles differing over how flow may separate within the overall trough that will be heading into the upper ridge that approaches the West Coast. Model Guidance Evaluation and Preferences ----------------------------------------- Models and ensemble means are in relatively good agreement on Saturday across most of the continental U.S., including the representation of the two main storm systems that will affect the U.S. during this forecast period. However, model spread increases significantly starting as early as Sunday regarding the evolution of the second low pressure system over the western U.S. with the 12Z ECMWF depicting considerably more upper level ridging over the southwestern U.S. compared to the model consensus, and this becomes even more apparent by Monday morning. This has major implications on the surface cyclogenesis and eventual low track across the central and eastern U.S. for the first half of next week. The 12Z ECMWF is indicating a building surface high over much of the central U.S. by the Day 5 period on Monday that is wildly different from its previous 00Z run, whereas the CMC/GFS/JMA/00Z Tuesday ECMWF are supporting the idea of a low pressure system that will lift towards the northeast. There has been more run-to-run consistency with the past few runs of the GFS with this feature. Taking these considerations into account, the 00Z Tuesday run of the ECMWF was substituted for the 12Z run for the entire forecast period since it is closer to both WPC continuity and the existing model consensus. The first half of the forecast incorporated mainly a blend of the previous ECMWF, CMC, and GFS, and the second half leaning more toward the GEFS mean along with some of the 00Z EC mean and 18Z GFS for placement of main features. Given the uncertainty regarding the second low pressure system over the south-central U.S. early next week, noteworthy revisions to the forecast may be required if more models trend in the direction of the 12Z Tuesday ECMWF, and confidence is quite limited for this portion of the forecast. Sensible Weather and Hazards ---------------------------- An active weather pattern is expected across the eastern half of the nation this weekend as the strong surface low tracks northeastward towards the Great Lakes and the Northeast. Numerous showers and thunderstorms are forecast to develop in the warm sector of this low from the central Gulf Coast to the Ohio Valley, with the potential for a few strong storms. Heavy rainfall across parts of the Deep South and northern Florida may lead to some localized flooding concerns. Farther to the north across the northern plains and the Upper Midwest, a band of moderate to heavy snow is expected to develop within the deformation zone on the northwest side of the low, with some locations possibly getting over six inches of accumulation before the storm system departs the region. There may also be additional impacts from the next system tracking through the south-central U.S. with heavy rain and strong storms near the Gulf Coast and into the Southeast states, but given the uncertainty in the evolution of that second system, predictability remains limited regarding the magnitude and placement of the heaviest precipitation. A third system approaching California by the end of the forecast period next Wednesday should bring another round of rain and mountain snow from central California to western Washington. In terms of temperatures, readings are expected to be on the order of 10 to 20 degrees above normal ahead of the weekend storm system from the Gulf Coast to the Great Lakes on Saturday, and then 5 to 15 degrees above normal for the East Coast on Sunday. Much colder weather will settle in across the western U.S. courtesy of the building upper level trough, with anomalies on the order of 10 to 20 degrees below normal to begin the work week, leading to some subzero overnight lows in the normally colder locations. With the exception of Florida, most of the continental U.S. should have below average temperatures by the middle of next week. D. Hamrick WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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