Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Thu Dec 27 2018 Valid 12Z Sun Dec 30 2018 - 12Z Thu Jan 03 2019 ...Overview... Positively-tilted troughing will reload over the northern Plains and the West while subtropical upper ridging slowly retreats eastward past the Bahamas. This will allow northern stream flow to progress along the U.S.-Canadian border to the south of an upper low over Hudson Bay. With time a strong northern Pacific jet will sag southward, pushing an initial eastern Pacific/northwestern North America ridge into western parts of the Lower 48 with corresponding progression of the downstream upper trough. This pattern evolution will likely produce heaviest precipitation (the majority in the form of rain) over the eastern half/southern two-thirds of the country. ...Guidance Evaluation/Preferences... A general model blend represents current consensus well for days 3-4 Sun-Mon. Previous spread has narrowed considerably with GFS/GEFS guidance in particular trending over the past day to a more amplified/westward forecast with the trough energy digging into the western half of the country. Earlier trends with this evolution had been suggesting an attempt to converge on an intermediate solution but still closer to ECMWF-CMC model/ensemble solutions. Based on the current consensus, expect ejecting southern Rockies/northwestern Mexico trough energy to support a wave that lifts northeastward ahead of U.S.-Canadian border low pressure tracking toward the Great Lakes. Days 5-7 requires some refinement of operational model input and increasing ensemble mean weight as guidance diverges. The 00Z UKMET becomes one of the more extreme solutions for westward extent of upper troughing over the West, leading to its removal from the blend after Mon. In addition the 06Z GFS begins to stray somewhat faster than consensus with the upper trough axis, leading to a transition from 06Z to 00Z GFS runs for the GFS component of the blend into Wed--after which time the 00Z GFS also becomes somewhat faster than the 06Z GEFS/00Z ECMWF means (which are fairly similar). Multi-day trends have been in favor of slowing down GFS progression so continue to lean toward a somewhat slower solution. On the other hand, while guidance is sometimes biased toward breaking down ridges too quickly there is so far not much evidence that the eastern Pacific/western North American ridge will be able to resist the strong Pacific flow to the extent depicted in the 00Z CMC mean. Days 6-7 Wed-Thu increase total ensemble mean weight to 50-70 percent respectively, with aforementioned GFS considerations leading to excluding it after Wed. This time frame used components of the past two ECMWF runs due to differing details over the East and off the Pacific Northwest coast. ...Weather Highlights/Hazards... Multiple waves over the eastern half of the country will promote heavy rainfall potential from areas just west of the lower half of the Mississippi Valley to parts of the Mid-Atlantic/Southeast. Some moisture will likely extend farther northward at times. One pronounced episode should occur around Sun-Mon as a wave tracks northeastward from the western Gulf of Mexico through the lower Mississippi Valley and into the Northeast. A trailing wavy frontal boundary making only gradual progress over the Southeast will likely provide a focus for additional bands of enhanced rainfall from New Year's Day (Tue) onward. The best signal for highest 5-day totals currently extends from the central Gulf Coast into the Tennessee Valley/southern Appalachians. Some moisture reaching the Great Lakes/New England may be in the form of snow, especially with lake effect activity behind low pressure exiting New England on Tue. Farther west, during Sun-Mon amplifying upper troughing and leading northern tier low pressure will spread an area of precipitation--mostly snow--southward through and just west of the Rockies as well as light amounts into parts of the northern Plains/upper Great Lakes. The Pacific jet should reach far enough south for the Pacific Northwest to see an increase of rain/high elevation snow by next Thu. Highest amounts should remain north over Vancouver Island but meaningful totals may extend at least into the Olympics/Washington Cascades. After a brief surge of warmth on Sun (plus 10-20F anomalies), the northern Plains will see a couple very cold days with temperatures 15-30F below normal followed by a rapid warm-up by Thu. Many areas from the Interior West through the rest of the Plains will also see a period of chilly temperatures, generally 10-25F below normal. Cold anomalies should be most widespread in the Mon-Wed time frame. In contrast the East will see above normal temperatures until cooler air moves in Wed-Thu. Greatest anomalies should be in the plus 20-30F range for low temperatures on Tue, while high temperatures should be 10-20F above normal Mon-Tue. Expect Florida to be the last to see the late-period cooling trend over the East, while the West Coast states should experience temperatures within a few degrees of normal on most days. 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