Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sat Dec 29 2018 Valid 12Z Tue Jan 01 2019 - 12Z Sat Jan 05 2019 ...Overview... The medium range period features a possibly closed low progessing eastward from the Four Corners region on Day 3 (Tue) to the Southeast/Tennessee Valley by Day 6 (Fri) before becoming absorbed into the larger scale troughing. To the north, additional energy slides along the U.S./Canadian border underneath of deep low rotating over Hudson Bay. Out west, upper ridging across the east Pacific shifts inland as an upstream shortwave trough moves towards the Pacific Northwest. The overall pattern would bring widespread rain to the eastern half of the nation while precipitation again increases in the Pacific Northwest. ...Guidance Evaluation/Preferences... A trend in both global models and ensembles is emerging towards a slower and slightly stronger upper low progressing from the Four Corners to the East next week. The latest run of the CMC favors a more elongated trough type feature across the Central U.S. and as a result is noticeably faster than the rest of the guidance and thus was not included in the blend. There is some question as to the location of the surface low on day 5 with the ECMWF being noticeably farther north than the GFS (northern MS/AL vs central Gulf Coast) but based on the ensemble low plots, somewhere in the middle of the two is probably a good place to be at this point. Out in the west, beyond day 5, timing of the ridge-trough-ridge pattern in the eastern Pacific and Northwest U.S. becomes an issue, especially in the operational models. The ensemble means appeared to present a more agreeable (with obvious supression) solution this late in the period. The blend for this cycle of the WPC medium range progs features a majority deterministic model blend (between the 06z GFS/00z ECMWF/00z FV3-GFS) days 3-5, with increasing weighting of the ensemble means (GEFS/ECENS) from day 5 and beyond. Continues to include a small bit of the 00z deterministic ECMWF through day 7 because it seemed to fit best with the ensemble means. ...Weather Highlights/Hazards... Closed low in the Southwest should bring a second round of precipitation to parts of eastern Arizona and New Mexico, with the highest potential for accumulating snows confined to the higher mountains. As this low progresses across the Southern tier, this will promote heavy rainfall potential from the Deep South into the Eastern U.S. next week. A surface low lifting through the Southeast into the Mid-Atlantic is likely to provide a focus for enhanced rainfall with the best chance for 3+ inch 5-day rainfall totals currently extending from the central Gulf Coast into Georgia.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 mid-week. The Pacific Northwest can expect an increase in rain/high elevation snow by Thursday and continuing into Friday--with some meaningful totals likely in the Olympics/Washington Cascades and the northern portions of the northern Rockies. In regards to temperatures, the northern Plains can expect very cold temperatures on Tuesday--with high temperatures reaching 15-25F below normal. By Wednesday, temperatures will rebound in this region and are expected to be upwards of 10-15F above normal. The central and southern Plains will also be much below normal on Tuesday and Wednesday--becoming near normal by Thursday. The exception will be along the western Gulf Coast as an active period for precipitation is expected. Meanwhile, the East Coast will be anomalously warm on Tuesday--returning to near normal by Wednesday. Later in the medium range period (Fri-Sat), the central U.S. will be above average--with high temperatures 15-20F above normal. The West Coast should be near normal through the medium range period. Santorelli/Reinhart 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