Extended Forecast Discussion NWS Weather Prediction Center College Park MD 325 PM EST Thu Dec 19 2019 Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019 ...Heavy rain over parts of the Southeast departing after Monday... ...Overview... The large scale pattern will feature a mean trough near the West Coast. One or more leading embedded shortwaves will eject inland followed by amplifying upstream energy that reinforces the trough. Expect this mean flow to produce a period of unsettled weather over portions of the West. At the same time a potent southern stream system will track across the extreme Southeast/Florida late this weekend and possibly into Mon, then push slowly out to sea. Locally heavy rain will be possible around its circulation between northern Florida and parts of the Carolinas. There is reasonable agreement with the mean pattern but much less confidence over the specifics of individual shortwaves reaching the West along with their influence on surface features/sensible weather east of the Rockies. Uncertainty is also fairly high regarding the extent to which a ridge crossing western-central parts of southern Canada will affect flow over eastern North America mid-late week. ...Guidance/Predictability Assessment... From the eastern Pacific into the western U.S. preferences continue to lean more toward ECMWF/ECMWF mean in general, with slower shortwave progression than recent GFS runs through nearly the entire period. By late in the period the 00Z ECMWF adjusted a bit faster than its prior run while the 00Z CMC seemed a bit slow/cut-off with its upper system so the slowest side of the spread may still be a little too slow. The new 12Z CMC is close to the 00Z ECMWF. Latest GEFS runs have been faster than the ECMWF cluster but slower than operational GFS runs. Downstream there is a general consensus that ejecting western energy will support surface waviness to the east of the Rockies by Thu-Wed Christmas Eve/Christmas Day. By mid-late week guidance becomes increasingly dispersed over the eastern half of the country due to differences in relative strength of southeast Canada/northeast U.S. upper troughing (strongest in the 00Z ECMWF) versus energy that may be ejecting from the western trough. Given preferences toward the slower half of the spread within the West Coast mean trough and the ECMWF/CMC plus their means all showing notable trends toward higher eastern U.S. surface pressures by day 7 Thu, the updated forecast trended somewhat toward the more suppressed eastern scenario at that time. Confidence is fairly low though. The early-period Southeast system continues to have a fair amount of spread in the guidance with the GFS/CMC on the faster side (CMC farthest north) and ECMWF/UKMET slower. The 06Z GEFS mean was a bit slower than the operational run. Overall favor a tilt more toward the slower timing but the pattern may be progressive enough to support slightly faster timing than the slowest edge of the spread--and the new 12Z ECMWF has indeed nudged its system a bit faster. Combining the above considerations led to a forecast blend that emphasized the 00Z ECMWF and UKMET/WPC continuity with modest inclusion of the 06Z GFS/GEFS mean early in the period. Then the forecast transitioned toward more ECMWF mean emphasis versus the operational run while maintaining GEFS mean/continuity input and removing the 06Z GFS. ...Weather Highlights/Threats... The mean trough aloft near the West Coast will support fairly widespread but generally light to moderate rain and higher elevation snow next week, spreading inland as shortwaves eject from the overall trough. Uncertainty over timing and amplitude of individual shortwaves will lower confidence in precise timing/coverage/intensity of precipitation at any particular time. The same is true to the east of the Rockies where preferences maintained a lean toward less precipitation than the GFS/GEFS but with low confidence. For any precipitation that does occur, it would most likely be snow from the northern Plains through the Upper Great Lakes and northern New England. The eastern Gulf/Southeast system will provide varying rainfall totals for much of the Southeast into Sun and/or Mon with locally heavy amounts continuing out of the short range time frame that goes through Sat. A period of robust inflow off the Atlantic may promote heaviest activity along the Southeast coast. The overall pattern will support a trend to modestly below normal highs over the West while the Southeast system should be accompanied by slightly below normal highs early in the period. Otherwise the central/eastern states will see above normal highs with warmest readings versus climatology likely to be over the central Plains on Sun-Mon. High pressure may bring a couple cool mornings to southern Texas early in the period but otherwise expect min temps to be warmer than normal over nearly the entire lower 48. The northern Plains into Upper Midwest will likely see the highest anomalies. 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 Hazards: - Heavy precipitation across portions of California, Thu, Dec 26. - Heavy rain across portions of the Southeast and the Mid-Atlantic, Sun-Tue, Dec 22-Dec 24. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Dec 22-Dec 23. - Heavy snow across portions of the Central Great Basin and the Southwest, Tue-Wed, Dec 24-Dec 25. - Flooding occurring or imminent across portions of the Ohio Valley and the Northern Plains. - Flooding likely across portions of the Pacific Northwest and the Northern Plains. - High winds across portions of the Southeast, Mon-Tue, Dec 23-Dec 24. - High winds across portions of the Southeast, Sun-Mon, Dec 22-Dec 23. - High significant wave heights for coastal portions of the Southeast, Sun-Mon, Dec 22-Dec 23. 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