Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Fri Dec 14 2018 Valid 12Z Mon Dec 17 2018 - 12Z Fri Dec 21 2018 ...Significant precipitation likely over the Northwest... ...Overview... Overall expect flow across the Lower 48 to be fairly progressive but it will be amplified over some areas for at least a portion of next week. During the early-mid week time frame strong dynamics digging across New England will serve to pull low pressure initially off the New England coast into the Canadian Maritimes. Meanwhile shortwave energy entering the West early Mon will quickly separate with the northern part continuing along the Canadian border as the rest amplifies into the Southwest/southern Rockies-Plains and northern Mexico due to a ridge building upstream. A strong jet aimed at the Northwest U.S./extreme southwestern Canada will promote a wet pattern over that region. By next Thu-Fri there is increasing uncertainty over details of flow aloft from the eastern Pacific eastward--rapidly lowering confidence in some aspects of the sensible weather forecast. ...Guidance Evaluation/Preferences... For the days 3-5 Mon-Wed period the forecast blend was able to employ operational model runs from the 12Z/18Z cycles to represent the most common ideas of guidance and yield only modest detail adjustments from continuity. By Wed there is some spread that arises with systems along/north of the Canadian border, in particular the new 00Z UKMET becoming quite fast with the leading feature crossing Ontario and new 00Z GFS straying a bit faster than consensus with the trailing system. There is also some spread with frontal waviness off the West Coast. By Thu-Fri spaghetti plots of the full range of model/ensemble solutions descend into chaos with wide differences in how eastern Pacific trough energy may split and how quickly/strongly the northern part of this energy may progress around the periphery of the upper ridge that builds over the West by midweek. These issues also lead to a widening array of possible solutions for the character of upper troughing expected to amplify into the eastern states as well as how much interaction there may be with the trough forecast to be over the southern Plains into Mexico as of Wed. Very loosely there are two clusters, GFS/GEFS runs that want to hold back most of the Pacific energy into day 7 Fri versus the ECMWF/CMC and their means that are quicker/stronger to varying degrees. The ECMWF/ECMWF mean are noticeably stronger with their shortwave energy though, and interestingly the 18Z FV3 GFS is in this cluster too. The full array of guidance and established progression of the pattern support the idea of some shortwave energy reaching well inland by day 7 Fri but the expected strength of the western ridge as of day 6 Thu would favor leaning away from the most amplified ECMWF/ECMWF mean. A model/mean blend that emphasizes the ensemble means by late in the period yields a rather flat shortwave which seems to be a reasonable starting point. Over the East the blend suggests more northern stream dominance with eventual separation from energy that descends into Mexico. ...Weather Highlights/Hazards... The most confident focus for heavy precipitation during the period will be over the Pacific Northwest and may at times extend into extreme northern California. The amplitude of upstream Pacific troughing aloft may promote incorporation of some lower latitude moisture to enhance rainfall and higher elevation snow. Periods of strong winds are possible as well. In less extreme fashion some of this moisture will reach into the northern Rockies. Farther south, separating shortwave energy aloft may bring relatively light precipitation from California and the Great Basin to the southern Rockies early in the week. Then confidence is below average regarding coverage/intensity of rainfall that may spread eastward from Texas Wed onward. Early next week strong dynamics will intensify a western Atlantic storm that tracks into the Canadian Maritimes, leading to a period of fairly strong winds over the Northeast. Some lake effect snow is also possible. Low pressure currently expected to track into the Great Lakes and southeast Canada/New England (along with its associated fronts) late in the week should bring at least light-moderate precipitation to parts of the East with another episode of lake effect snow in its wake. The degree of interaction with Gulf moisture is quite uncertain. Strong dynamics aloft/leading cold front will bring a brief period of below normal temperatures to the Northeast, especially around Tue. Otherwise much of the Lower 48 will see near to above normal readings most days with the Plains likely to see the most extreme warm anomalies, plus 15-25F over some locations--especially over northern areas for morning lows. Rausch 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