Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Thu Nov 29 2018 Valid 12Z Sun Dec 2 2018 - 12Z Thu Dec 6 2018 ...General Pattern Overview... The forecast period from Sunday through Thursday morning will feature a pattern change across the continental U.S. as a large scale upper level trough encompasses much of the central and western U.S. with a distinct southern stream and northern stream flow. The upper ridge over the eastern U.S. will be displaced offshore by next week as the trough builds in from the west. The first storm system over the Great Lakes on Sunday, followed by a weaker low along the main frontal boundary, will herald much cooler temperatures courtesy of Canadian high pressure building into the region. By the middle of next week, a new Pacific storm system is expected to slowly approach the California coast with a return to rain/snow for that region. ...Guidance Evaluation/Preferences... Models and ensemble means are in relatively good agreement on Sunday across most of the continental U.S., including the representation of the two main storm systems that will affect the U.S. through Tuesday. However, model spread increases by Monday regarding the evolution of the second low pressure system over the Gulf Coast region along the trailing cold front from the first storm system. The GFS and GEFS mean continue to favor a more amplified surface low over the southeast U.S., whilst the UKMET and ECMWF are more suppressed and are more indicative of a subtle wave along the front. The 12Z ECMWF run on Wednesday was closer to the model consensus compared to the 12Z run on Tuesday, which depicted considerably more upper level ridging over the southwestern U.S. compared to the model consensus. The preference with this system is to stay close to previous WPC continuity along with a blend of the GFS/ECMWF/means, with the low likely tracking near the Gulf Coast and exiting off the southeast coast. Regarding the West Coast system for the middle of next week, the trend has been for a slightly faster progression compared to yesterday. The GEFS and EC means are relatively close together near the northern coast of California by Wednesday morning, and the ECMWF is slightly farther south than the GFS. The CMC is a western outlier and not supported by the ensemble means, so it was not part of the forecast blend. The first half of the forecast incorporated mainly a blend of the ECMWF, UKMET, and GFS, and the second half leaning more toward the EC and GEFS means for placement of main features. Given the lingering uncertainty regarding the second low pressure system over the southeast U.S. early next week, additional revisions to the forecast are likely as models continue to resolve this feature. ...Sensible Weather and Hazards... An active weather pattern is expected across the southeastern U.S. to end the weekend with heavy rain and some strong storms near the Gulf Coast and into the Southeast states in association with the wave of low pressure along the front, but given the uncertainty in the evolution of this system, predictability remains limited regarding the magnitude and placement of the heaviest precipitation. The next system approaching California by the end of the forecast period next Wednesday should bring another round of heavy rain and mountain snow for much of California. Lake effect snow will be likely downwind of the Great Lakes early-mid next week courtesy of strong cold air advection behind the departing low Sunday night. In terms of temperatures, readings are expected to be on the order of 10 to 15 degrees above normal ahead of the weekend storm system from the Gulf Coast to the Mid-Atlantic region on Sunday. Much colder weather will settle in across the central and 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 of the Intermountain West. An arctic front reaching the Upper Midwest by next Thursday will bring an even colder surge, with some high temperatures struggling to reach 10 degrees across parts of North Dakota and Minnesota. 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