Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sat Dec 29 2018 Valid 12Z Tue Jan 01 2019 - 12Z Sat Jan 05 2019 ...Overview... The medium range period (Tue-Sat) continues to feature a positively tilted trough stretching across the central U.S. and into the Southwest moving eastward as the subtropical ridge across the Southeast retreats into the Atlantic. To the west, upper riding will build across the Pacific Northwest and shift inland later in the week with a downstream upper trough. This pattern will bring rainfall across the eastern U.S. with some precipitation increasing across the Pacific Northwest. ...Guidance Evaluation/Preferences... There was good agreement between the 12Z GFS/18Z FV3-GFS and 00Z ECMWF for the first two periods of medium range (Tue/Wed). The 12Z GFS overall was in step with the ECMWF with having a closed mid-level low across the Four Corners and moving into the southern Plains by late Wednesday into Thursday. The 18Z GFS was very fast with this trough in comparison to the 12Z GFS, 18Z FV3-GFS and the 00Z ECMWF. By Day 5/Thursday and beyond, the timing of how quickly the trough progresses eastward/the upper ridging in the western U.S. becomes an issue especially for the operational models. The 12Z GFS was faster than the 00Z ECMWF with the trough in the east--but still slower than the 18Z FV3-GFS. By Friday and into Saturday, as the trough in the Pacific begins to approach the Pacific Northwest, differences between the GFS and ECMWF are noticeable since the ECMWF has the upper ridge more amplified and progressing slower than the GFS. The ensemble means were more reasonable with the detail in the timing and details. Thus, the blend used in this portion of the medium range was based on the 18Z GEFS mean/12Z ECMWF ensemble mean/12Z NAEFS. ...Weather Highlights/Hazards... With the trough digging eastward midweek, a cold front will move across the eastern U.S. on Tuesday and retreat northward as a warm front by Wed-Thu along the Gulf Coast. Deep southerly flow ahead of the trough combined with the boundary being a focal point for convection will give way to an active period for the Southeast--especially through Thursday. The best signal for the highest 5-day QPF totals stretch from the central Gulf Coast to the Carolinas. 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. In the Southwest, an initial upper low early in the week will likely result in another round of precipitation for parts of eastern Arizona and New Mexico, with the highest potential for accumulating snows confined to the higher mountains. 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. Reinhart/Santorelli 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