Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Sat Dec 21 2019 Valid 12Z Tue Dec 24 2019 - 12Z Sat Dec 28 2019 ...Overview and Guidance/Predictability Assessment... Expect a mean trough aloft to persist over the West Coast and Great Basin for most of next week but with some progression toward the Rockies/Plains possible by around Fri-Sat as an eastern Pacific ridge builds toward the West Coast. Moderately progressive flow will prevail downstream across the central and eastern U.S. as shortwave energy ejects from the western trough and is deflected around a broad eastern U.S. mean ridge which some guidance suggests may be anchored by an upper high settling over Cuba and the Bahamas by days 5-7 Thu-Sat. A persistent upper low across the Arctic Ocean well north of Alaska along with weakly positive height anomalies across Canada should continue to minimize potential for any significant outbreaks of cold air across the lower 48 through the end of the week. There is still decent initial clustering for the leading shortwave that rapidly weakens as it ejects from the West into the Plains Tue-Wed. Exactly what happens after Wed has been up for debate in the models for multiple days though. 00Z-06Z runs changed yet again from prior solutions, now suggesting that the combination of enough energy lingering over the northern tier and stronger eastern ridging aloft will divert Plains frontal waviness into Ontario by day 6 Fri--instead of a path somewhere across the northeast quadrant of the lower 48. This leads to the trailing front taking longer to pass through the Northeast and supports stalling of the trailing part of the front over the Plains. Thus far the 12Z model runs are maintaining continuity with the 00Z-06Z cycles for this overall evolution. Two similar runs have not meant a lot so far but hopefully the forecast projection is starting to get near enough for future adjustments to be less dramatic. Over the past day the GFS has made a pronounced adjustment toward the slower consensus for the next significant shortwave expected to reach the West Coast by Wed-Wed night, finally allowing for somewhat more use in the forecast than in some previous days. Other guidance has still varied on the specifics of a potential embedded closed low whose track would most likely be near the California coast. As expected the ensemble means are less defined than the operational models but show a decent signal for flow separation and the 00Z ECMWF mean/12Z GEFS mean briefly close off an upper low center in the vicinity of southern California/Arizona. Faster trend of the 00Z ECMWF brought it more in line with the means. Models and ensembles remain varied on specifics of upstream northeastern Pacific flow that should reach western North America by late this week and have some influence on the trough moving into the West. In contrast to most trends in recent guidance, the 00Z ECMWF becomes quite aggressive in how it amplifies the Pacific energy over the West late week into the weekend. On the other hand GFS runs are fairly strong with energy that reaches the northern Plains by day 7 Sat--but at least they are closer to the ensemble mean trough axis and the old 12Z/20 ECMWF. CMC runs have been out of sync with one or more aspects of North Pacific into North America flow. Lack of confidence in operational model specifics would recommend a rapid increase of ensemble mean emphasis late in the forecast. A compromise among the 00-06Z GFS and 00Z ECMWF/UKMET best represented the most common ideas of guidance from day 3 Tue into day 5 Thu. The starting blend incorporated half total weight of the 06Z GEFS/00Z ECMWF means for Fri and 80 percent for Sat. ...Weather Highlights/Threats... The combination of a leading shortwave ejecting from the West and some weak trailing energy will produce areas of rain and higher elevation snow over the Southwest/Interior West and Rockies Tue-Wed. Best potential for a brief period of enhanced activity would be over favored terrain of Arizona on Tue. By Wed-Thu some of the energy aloft and corresponding surface wave(s) may produce generally light precipitation (likely more snow than rain) from the northern Plains eastward. Expect the next Pacific system to bring rain and high elevation snow to parts of California and the Southwest mid-late week. Best potential for highest precip totals currently exists over terrain near the southern California coast but this is very sensitive to the system's exact track which is not yet fully resolved. Precipitation should expand and become heavier over the central U.S. by Fri-Sat as the western trough aloft approaches and moisture carried by low level Gulf inflow interacts with a stalled front over the Plains. Thus far variability with progression of the upper trough has lowered confidence in timing the onset of enhanced precipitation. Wintry weather will be possible in the northern/western periphery of the moisture shield. The Northwest will see an episode of light-moderate precipitation into midweek. After a brief break, northeastern Pacific flow should bring more rain/higher elevation snow to the region though with uncertainty over precise intensity and southward extent. Highest totals should be over the Olympics. The mean trough aloft will keep highs over a majority of the West near to below normal with central/southern areas most likely to see some minus 5-10F anomalies. Extreme northern areas may see min/max readings drift a little above normal with time. East of the Rockies, recent guidance adjustments for the pattern at the surface and aloft result in greater persistence of central-eastern U.S. warmth and higher peak anomalies. Highs of 10-20F above normal should be common through the period and one or more days with morning lows at least 20-25F above normal will be possible from the Midwest into the Appalachians. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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