Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1057 AM EST Sun Jan 20 2019 Valid 12Z Wed Jan 23 2019 - 12Z Sun Jan 27 2019 ...Overview... Guidance continues to agree with the overall amplified pattern with a ridge across the eastern pacific/West Coast and trough over the central-eastern portions of CONUS aloft. This will give way to a wet pattern for the eastern half of the U.S. especially in the beginning of the medium range period with some precipitation in portions of the Pacific Northwest and Intermountain West. However, the details regarding the shortwaves moving over the top of the ridge and downstream across the mean trough still remain highly uncertain. ...Guidance Evaluation/Preferences... During the first portion of the medium range period, models are agreeable with the features aloft and at the surface. The upper trough will begin to push from the Plains eastward into the Mississippi Valley while the upper ridge builds across the West Coast. A shortwave will slide over the ridge into the Pacific Northwest/Intermountain West during this time. On Day 3/Wednesday, the 06Z GFS/00Z ECMWF and the 00Z CMC/UKMET were in good agreement with the timing of the front sweeping across the eastern U.S. Thus, day 3 had a blend of these with most weight given to the GFS and ECMWF. However, by day 4, slight differences in the details aloft give way to big differences at the surface. The 00Z CMC becomes too deep with the surface low in the Mid-Atlantic, and the UKMET is too slow with the front. The GFS/ECMWF were still comparable--but already begun to introduce their ensemble means by this time. By Day 5, differences in the ECMWF and GFS start to increase but still were reasonable enough to use in the blend. The 06Z and 00Z GFS were faster and more intense with the surface high pressure dropping across the central U.S. than the 00Z ECMWF. However, the previous run of the ECMWF was more aligned with the GFS. Because of run-to-run variability with the operational models, increased the means by this time. However, the days 6-7 time period (Sat-Sun) becomes even more varied with the surface pattern. Because of the low confidence in this time period--followed suit from the previous shift by decreasing the use of operational models to 20 percent on Day 6, and went 100 percent with the 06Z GEFS/12Z ECMWF mean by day 7. ...Weather Highlights/Hazards... Most of the precipitation occurring during the medium range is expected on Days 3-4 (Wed-Thu). Heavy precipitation will follow along the cold front advancing from the Mississippi Valley to the East Coast. Expect the highest totals along portions of the southern Appalachians and southern New England. There is still uncertainty in strength/timing/track of the frontal wave but currently the best potential for at least some wintry weather extends from west of the Appalachians northeastward into New England. This system will exit the East Coast by Friday, with some lingering lake effect snow across the Great Lakes. Low pressure and a strong cold front passing through the region may bring a period of synoptic scale snow during the latter half of the week. Another feature could enhance snowfall for a time during the weekend. The Pacific Northwest/Intermountain West will experience rain and higher elevation snow on Wed-Thu. Locations along the eastern slopes of the northern half of the Rockies may see additional periods of light snow depending on how shortwaves evolve downstream from the West Coast ridge aloft. As the shortwave energy and surface reflection moves south and east, precipitation will also follow and could extend into the central Plains/Mississippi Valley by the weekend. Confidence remains low in coverage/intensity of precip is very low. Within the moisture shield the rain-snow line may reach fairly far south given the cold pattern. A warm up across the eastern U.S. is expected on Wed-Thu ahead of the incoming wavy cold front--with high temperatures nearing 10-15F above normal. However, cold, Arctic air will quickly spread across the northern tier states, Upper Midwest and into the eastern U.S. by Fri-Sat. On Friday, high temperatures in the Upper Midwest will be 15-25F below normal--with some temperature readings not getting above zero. By Saturday, the Mid-Atlantic and Northeast will have high temperatures 10-15F below normal. The West Coast will stay 5-10F above normal through the medium range due to the upper ridge. Reinhart/Rausch 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