Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Thu Jan 28 2021 Valid 12Z Sun Jan 31 2021 - 12Z Thu Feb 04 2021 ...Significant precipitation event likely along the central West Coast from late weekend into early next week... ...Midwest/Ohio Valley system followed by coastal cyclogenesis to spread a broad area of wintry weather and rain across the eastern U.S. this weekend into next week... ...Weather Pattern Overview and Guidance/Predictability Assessment... Overall the latest guidance offers decent agreement and continuity in principle for what will be an active pattern with two dominant systems. An eastern Pacific upper trough will bring significant moisture into the Pacific Northwest and especially the central West Coast from late weekend into early next week. Then inland progression of the trough will produce precipitation across more of the West and by mid-late week develop a large precipitation area over the central U.S. This system will also bring a pronounced cooling trend to the West and eventually Plains next week. The evolution heralds a significant pattern change with a late-period eastern Pacific ridge essentially replacing what had been a long term mean trough over the West Coast/East Pacific. Meanwhile guidance continues to show initial Midwest/Ohio Valley low pressure giving way to coastal development by Mon with the resulting system lifting northeastward with time. An initial supporting upper low/trough may see additional contributions from upstream energy as this evolution occurs. There will be potential for meaningful (and possibly significant in some areas) snow from the Lower Great Lakes/Ohio Valley through the northern/central Mid-Atlantic and Northeast. Rain will fall farther south with highest totals likely over the southern Mid-Atlantic. For the eastern Pacific upper trough that moves into the West and Plains, a general blend (an operational model consensus trending toward a model/ensemble mean mix late) works well in light of most differences being of small enough scale as to have low predictability for the time frame of interest. By early next week guidance has converged over the past day or so by way of the ECMWF trending toward the sharper GFS on Mon followed by the GFS trending toward the faster ECMWF progression for Tue. Late in the period there are some differences over where/when the strongest bundle(s) of energy emerge over the Plains with corresponding effects on surface low pressure. However there is a decent signal for a significant storm system regardless of the specifics. An intermediate solution seems reasonable until better defined clustering and/or trends emerge. For the eastern U.S./western Atlantic evolution, there is good consensus and continuity for a Midwest upper low and trailing trough (incorporating weak energy from upstream) to reach the East Coast by around early Tue. By this time and/or later additional shortwave energy sliding down the eastern side of the upstream ridge may feed into the upper trough, with exact influence on the western Atlantic system yet to be determined. For Sun-Mon recent GFS/GEFS runs have been questionable due to being a fast extreme with the initial Midwest system and then farther north with the boundary on which coastal cyclogenesis occurs. Thus the latest forecast maintained continuity with greater emphasis on ECMWF/CMC/UKMET solutions in this time frame. By Tue the 18Z GFS surface system compared better with the guidance average and continuity. Then by Wed the 12Z ECMWF strayed to the slow side of the spread including the ECMWF mean--favoring a lower ECMWF weight in the forecast by that time. ...Weather/Threats Highlights... The upper trough and leading frontal system approaching the West Coast will bring moderate to heavy rain/mountain snow to the Pacific Northwest and central West Coast during the latter half of the weekend. Heavy precipitation should focus in a more concentrated fashion over the central West Coast/Sierra Nevada early next week. Progression of the upper trough will spread more moderate amounts of precipitation across a majority of the West into midweek, though favored terrain may still see some enhancement. Then by mid-late week the West will see a drying trend while precipitation coverage and intensity should increase considerably over the central U.S. Currently expect the northern half of the Plains and vicinity to see the best potential for meaningful snowfall as low pressure evolves over the central U.S. Also winds could become fairly strong with sufficient development of this system. Some locations farther south could see moderate to heavy rainfall after midweek. Overall the guidance consensus has been fairly stable in depicting the best potential for meaningful (potentially significant) snowfall from the Lower Great Lakes/northern Ohio Valley region through the interior Mid-Atlantic/central Appalachians and into the Northeast. It will likely take a ways into the short range time frame to resolve more precisely the locations of the rain-snow line/transition zone and axis of heaviest snowfall, in light of dependence on fine-scale details at the surface and aloft. As is frequently the case with such systems there may be a small margin for error between a significant snowfall and a modest one. Some areas along or offshore the coast may see a period of brisk to strong winds. Farther south expect the highest rainfall totals with some moderate to heavy activity over or near the southern Mid-Atlantic early in the forecast period as a warm front moves in from the south and coastal low pressure develops. Most of the greatest temperature anomalies will be on either side of the system moving into the West/Plains next week. Into the first part next week an expanding area across the Plains (from north to south) will see readings up to 10-20F above normal. Then this area of warmth should continue into the Mississippi Valley and vicinity. Around midweek some pockets of anomalies greater than plus 20F are possible for morning lows. Meanwhile by Wed-Thu colder air moving into the West/Plains could bring highs down to at least 10-15F below normal from the Great Basin/Southwest into the central Plains. On Sun the system affecting the East will bring above normal highs across parts of the Southeast in contrast to below normal readings over the Mid-Atlantic and portions of the Northeast. Then western Atlantic low pressure will keep temperatures below normal over the southern two-thirds of the East early in the week, followed by a moderating trend as it tracks northeastward. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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