Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Jan 20 2019 Valid 12Z Wed Jan 23 2019 - 12Z Sun Jan 27 2019 ...Overview and Guidance Evaluation/Preferences... Confidence remains above average with the forecast of a strongly amplified eastern Pacific/West Coast ridge and eastern North American trough aloft. Expect this pattern to bring significant precipitation to the eastern third of the country as the mean pattern starts to become established and then below to much below normal temperatures into parts of the eastern half of the U.S. For about the first three days of the period (Wed-Fri) most guidance displays common themes for significant features albeit with lingering detail questions. Then forecast specifics rapidly become more uncertain regarding shortwaves progressing around the top of the West Coast ridge and then dropping into downstream mean trough. The medium to small scale nature of these shortwaves would tend to yield low predictability. During the first half of the period when features cluster decently, the manual forecast started with a 70-80 percent operational model blend with highest weighting on the 18Z GFS/12Z ECMWF. Great Lakes low pressure as of early Wed still has unusually high sensitivity to exact details of shortwave energy aloft, so this feature's evolution from Wed into Thu is more ambiguous than would typically be the case for a 3-4 day forecast. Solutions are gradually becoming less diverse for the trailing frontal wave tracking northward along the East Coast around Thu but meaningful timing/track/strength differences persist. The forecast for this system also depends on smaller scale detail issues that can take into the short range time frame to be resolved. A blend/ensemble mean approach provides the best option for a single deterministic forecast and yields reasonable continuity from previous forecast. Farther to the west expect a strong Canadian shortwave to drop into the northern U.S. by Wed night-Thu with a leading cold front pushing rapidly south and east from the northern Plains. An average of guidance provides a reasonable starting point for this front, minus the 12Z CMC which was a slow extreme. By days 6-7 Sat-Sun the blend rapidly shifted to 70-100 percent 18Z GEFS/12Z ECMWF mean weighting. Thus far operational model runs have been quite varied with the expected surface pattern in response to differences in shortwave evolution to the east of the Rockies as well as influence from upstream energy rounding the ridge. As a result confidence in any specific operational scenario is very low at this time. The one general signal that the guidance provides is for some type of surface system to affect the central/eastern states, producing an area of precipitation of uncertain coverage and intensity. ...Weather Highlights/Hazards... Primarily during Wed-Thu the front trailing from initial Great Lakes low pressure, and then a wave developing along the southern part of the front and tracking northward, will provide a focus for widespread precipitation from near the Mississippi Valley to East Coast. Expect highest totals within an area from the Gulf Coast region and southern Appalachians into 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. Behind this system the Great Lakes will likely see periods of lake effect/enhanced snow of varying intensity. 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. A focused band of moisture will produce snow and low elevation rain from the Pacific Northwest into northern Rockies during Wed-Thu with activity tapering off from west to east. 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. This shortwave energy and associated surface reflection may then produce an area of precipitation spreading across the eastern half of the Lower 48 next weekend. At this time confidence 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. The wavy front heading into the East will bring a brief surge of warmth ahead of it Wed-Thu with anomalies exceeding plus 10F especially for morning lows. A strong cold front pushing southeast from the northern Plains during the latter half of the week will expand coverage of below to much below normal temperatures over the eastern half of the country by Fri-Sun. The most persistent core of coldest anomalies (15-25F below normal) should exist over the Midwest/Upper Mississippi Valley. The upper ridge settling near the West Coast will support plus 5-10F or so anomalies in its vicinity. Warm anomalies may extend into the northern Rockies and High Plains locations that remain west of a mean frontal boundary forecast to meander over or near the High Plains for multiple days. 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