Extended Forecast Discussion NWS Weather Prediction Center College Park MD 124 AM EST Fri Dec 28 2018 Valid 12Z Mon Dec 31 2018 - 12Z Fri Jan 04 2019 ...Overview... Positively-tilted troughing in the West will slowly translate eastward as subtropical upper ridging slowly retreats from the Bahamas into the Atlantic. 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 Mon-Tue except for the 12Z UKMET which continued its trend of forecasting a farther westward extend of lower heights in the West than nearly all the ensembles. The 18Z GFS/12Z FV3-GFS and 12Z ECMWF clustered well with the ensemble consensus in light of continuity as the trough moves into the southern Plains late Wednesday. After that, upstream uncertainty in the northeast Pacific moves into the Pac NW and the models/ensembles diverge in how quickly to advance the eastern trough late Thursday (even closed off per the 12Z ECMWF) so opted to rely much more on the ensemble means with only the 12Z FV3-GFS to add some detail as it maintained better system timing/amplitude. ...Weather Highlights/Hazards... Multiple waves over the eastern half of the country will promote heavy rainfall potential from the Mississippi Valley eastward through the Ohio Valley into parts of the Mid-Atlantic/Southeast. Some moisture will likely extend farther northward at times depending on the track/strength of the surface feature. 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 through Georgia. 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, an area of precipitation--mostly snow--will spread southward through and just west of the Rockies with 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/Fri. Highest amounts should remain north over Vancouver Island but meaningful totals may extend at least into the Olympics/Washington Cascades. 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/cold temperatures, generally 10-25F below normal. Cold anomalies should be most widespread in the Mon-Wed time frame until the trough weakens and moves eastward. 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 as the cold front finally moves eastward, while the West Coast states should experience temperatures within a few degrees of normal on most days. Fracasso 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