Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 15 2018 Valid 12Z Tue Dec 18 2018 - 12Z Sat Dec 22 2018 ...Significant precipitation likely over the Northwest and possible along the East Coast... ...Overview... Guidance agrees in principle that flow aloft will rapidly evolve toward a sharp and amplified western ridge/east-central trough pattern by Thu, as incoming Pacific shortwave energy (contributing to the heavy Northwest precipitation) digs southeastward after rounding the strengthening western ridge. By the late week/weekend time frame the arrival of upstream Pacific shortwave energy should begin to lessen the amplitude of mean flow. However the specifics of this energy are still very uncertain, affecting the forecast not only over the West but also tempering confidence in timing/amplitude of the eastern trough in that time frame. The surface evolution associated with the eastern trough may be somewhat complex due to combined influence of the amplifying energy itself and a leading shortwave reaching the Plains into Mexico by Tue-Wed. ...Guidance Evaluation/Preferences... Latest models and ensembles are still fairly agreeable and consistent for the Tue-Wed period thus allowing for use of an operational model blend for that part of the forecast. After Wed the operational models and individual ensemble members continue to diverge fairly quickly with respect to Pacific shortwave energy reaching North America along with the amplitude/timing of the upper trough over the East. The current array of guidance favors holding onto a scenario fairly close to continuity, and which tilts about 3/4 toward the 12Z GEFS/ECMWF means by day 7 Sat. For the Pacific shortwave energy expected to head into North America late in the week, the initial problem is resolving exactly how flow may separate after the overall trough reaches 130-140W longitude by early Thu. By Fri the most prominent trend in the ensembles versus 24 hours ago is the trimming away of prior ECMWF members that had shown a rapid weakening of the western ridge. Thus the ECMWF mean has significantly flattened the amplitude of its incoming shortwave, which the operational runs have done to some degree as well. On the other hand some guidance (including FV3 GFS runs) still shows more and/or faster shortwave energy than 18Z and new 00Z GFS runs, and the GEFS mean is starting to hint at this feature. UKMET runs thus far appear to be on the extreme side for timing/amplitude of the shortwave through their 144hr ending time. Apparent trends toward an intermediate solution--balancing the strength of the western ridge as of Thu versus the established progressive nature of Pacific energy--favor holding onto a model/mean blend (more ensembles late) for this energy. Farther east, the most notable trend over the past day has been toward keeping more of the leading Plains into Mexico trough energy farther northward thus allowing for more interaction with flow that digs into the east-central U.S. by Thu. This is increasing the potential for what could be a complex evolution involving a Gulf Coast/East Coast wave and Great Lakes low pressure/trailing front. Spread with upstream flow ultimately contributes to significant amplitude and timing differences for the eastern trough and associated low pressure. At the very least would like to maintain progression that is slower than the 12Z GFS/CMC mean that are on the fast side of the spread. The slow 18Z GFS/GEFS mean may have somewhat higher probability than the fast edge of the envelope but confidence in them is still not that great as trends/remaining guidance suggest the upstream ridge may weaken a little faster than in those solutions. This leaves the 12Z GEFS/ECMWF means as the best basis for the forecast later in the period, with the 12Z/18Z FV3 GFS runs averaging in that neighborhood and the 12Z ECMWF a little slower. ...Weather Highlights/Hazards... A frontal system approaching/reaching the Northwest on Tue will provide one episode of enhanced rainfall and higher elevation snow to the region with activity likely extending into extreme northern California. Another front, anchored by a wave whose strength and track are not yet well-resolved in the guidance, should bring another episode of meaningful precipitation to the Northwest late in the week. Some of the moisture from these events will extend into the northern Rockies. Another area of moisture could reach the Northwest next weekend but with lower confidence. Parts of the Northwest may see periods of strong winds. The upper trough initially from the Plains into Mexico along with leading weak surface wave may produce areas of rainfall from the southern Plains eastward mid-late week, while an approaching northern stream frontal system may also provide a focus for at least light-moderate precipitation. Interaction of these features may lead to strengthening low pressure near the East Coast by Fri-Sat, spreading a sizable moisture shield across the eastern states. Currently the best signal for highest rainfall totals exist over parts of the Florida Peninsula and along the North Carolina coast. Depending on details of system evolution there may relative maxima over other parts of the East. Expect most wintry weather to be confined fairly far to the north and west. Temperatures will be above normal over a majority of the Lower 48 during the period. The most extreme readings versus normal should be over the northern Plains where anomalies may exceed plus 20F for one or more days, especially for min temps which could approach or exceed daily records at some locations. One exception to the warmth will be New England which will be on the chilly side Tue behind a departing upper low. 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