Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1052 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 the 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 into southern California. A strong jet aimed at the Northwest U.S./extreme southwestern Canada will promote a continued 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 deterministic model runs from the 00Z/06Z cycles to represent the most common ideas of guidance and yield only modest detail adjustments from continuity. By Wed some spread arises with systems along/north of the Canadian border (especially with the UKMET) and off the West Coast but a consensus/blend offered a good starting point. This modulated well the differences between the initially quicker GFS runs with the then quicker ECMWF/Canadian runs. 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 (around 145W) 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 full array of guidance and established progression of the pattern support the idea of some shortwave energy reaching well inland by day 7 Fri into the Northwest but the expected strength of the western ridge (at least S California into the Southwest) as of Thu would favor leaning away from the flatter ECMWF mean (subject to averaging 50 forecasts of the trough/ridge couplet over various longitudes yielding an unlikely zonal flow). Utilized a model/mean blend by next Thu/Fri that maintained at least the southern portion of the western ridge (via the 06Z GEFS mean and previous forecast) and allowed some northern stream height falls across the Pac NW, which seemed to be a reasonable starting point. Over the East the blend suggested more northern stream dominance with eventual separation from energy that descends into Mexico. This would take the surface low out of the northern Gulf Thursday and up through the Mid-Atlantic Friday as a northern stream front barrels into New England from the Great Lakes. ...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 with significant upslope-enhanced precipitation in favored areas. 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 brief lake-enhanced snow is also possible with/behind the frontal passage Monday. 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 a more modest episode of lake effect snow in its wake. The degree of interaction with Gulf moisture is quite uncertain but the potential exists for the infusion of Gulf/Atlantic moisture up the coast late next week into/along the East Coast. 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. Due to relatively cold average temperatures for mid/late December, actual mins are forecast to be in the 20s (average in the single digits) with maxes in the 30s/40s. Fracasso/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