Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Mon Nov 26 2018 Valid 12Z Thu Nov 29 2018 - 12Z Mon Dec 3 2018 General Weather Pattern ----------------------- Quasi-zonal flow aloft encompasses much of the continental U.S. after the departure of the large storm system from the Northeast states on Thursday, with a low amplitude perturbation crossing the Plains. This quickly changes going into Friday as a potent Pacific disturbance crosses the Intermountain West and then reaches the central plains by Saturday. After a brief respite, a second storm system will likely affect the Pacific Northwest and allow for the trough to rebuild across much of the West by Sunday, which may eventually affect the central U.S. by the end of the forecast period next Monday. Model Guidance Evaluation ------------------------- The evolution and track of the Pacific low pressure system reaching California late Thursday and then the central U.S. is becoming better refined by the deterministic guidance compared to 24 hours ago. Although the 00Z GFS remains more progressive with the trough axis compared to the 00Z ECMWF with similar amplitude by Saturday, the differences are not as pronounced compared to yesterday's model runs, and both of these models appear to depict a reasonable evolution and the 12Z/18Z versions were incorporated into the WPC forecast. This also has some support from the 00Z CMC. There is also growing consensus for a second Pacific storm system to affect the northwestern U.S. by Saturday, despite some placement and timing differences. The CMC is more amplified than the GFS and ECMWF regarding the upper level trough by Sunday morning over the Intermountain West. The ensemble means provided a reasonable placement for this feature for the Sunday and Monday time frame. Sensible Weather ---------------- Across the western U.S., the first round of heavy precipitation is expected Thursday night and into Friday as the strong Pacific low moves onshore, with a potential atmospheric river event for central and northern California. Enhanced rainfall is becoming more probable for the topographically favored areas of California, especially for the lower elevations of the Sierra Nevada where a few inches of rainfall will be possible, and heavy snow for the higher ridges. Inland heavy snow is likely for the mountain ranges of the Intermountain West as the moisture surge advects eastward. The second storm system that will likely affect the Pacific Northwest this weekend should also bring period of heavy rain and high elevation snow. Active weather is also anticipated from the southern plains to the Ohio Valley Friday night into Sunday as a surface low is forecast to develop and intensify as it tracks northeastward, and this feature would be related to the large scale trough from the first Pacific low previously mentioned. There is an increasing signal in the model guidance for a band of moderate to heavy rain in the warm sector of this low, and SPC is monitoring the potential for severe weather given the strong dynamics aloft that are expected in combination with enhanced moisture advection. Snow is becoming more likely to the northwest of the low track across the central plains and Upper Midwest, however placement differences limit confidence as to where the band of heaviest snow would become established. Windy conditions in the wake of the departing storm system also appear likely for parts of the Plains by Sunday. 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