Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Feb 28 2019 Valid 12Z Sun Mar 03 2019 - 12Z Thu Mar 07 2019 ...Pattern Overview... The forecast of large scale flow has maintained good continuity over the past day. Downstream from persistent mean ridging/positive height anomalies over northwestern North America, a trough will amplify into the eastern U.S. with greatest depth around midweek before the trough axis continues into the western Atlantic. At the same time a mean ridge will build into the west-central U.S. as a mean trough settles near the West Coast. There are embedded details that will take time to resolve though, especially over the East early in the week and near the West Coast from Tue onward. Accompanying this evolution will be an episode of extremely cold temperatures for early March over the central U.S. with some of this cold extending through almost all of the eastern states as well as back into the interior Northwest. A fast moving wave will spread a broad area of various precipitation types across the East early in the week and another wet southern stream Pacific system will bring more moisture into the West starting Tue-Tue night. ...Guidance Evaluation/Preferences... Over the past couple days there has been a pronounced northwestward trend for a wave expected to track from the lower half of the Mississippi Valley into the Canadian Maritimes during Sun-Mon. This trend is due to a combination of faster adjustments for shortwave energy streaming into California on Sat and changes toward slower progression of northern tier U.S. height falls into the Northeast--allowing for more interaction. The ECMWF has been leading this trend over the past couple days while the GFS has been slowest to trend to this scenario. The 12Z ECMWF was in the northern minority among all ensemble members by late Sun-Mon so preference was to follow a track slightly to its south (compromise with latest UKMET/CMC runs), but trends do provide more confidence in the northwest half of the spread versus the southeastern half. New 00Z runs offer hope that the forecast could become a little more stable soon. The eastern Pacific/West Coast region will be a focus for significant detail uncertainty by mid-late period. The full array of model/ensemble guidance shows a variety of possible evolutions involving three separate features: 1. sheared southwest British Columbia energy that will likely form a closed low somewhere between the Pacific Northwest and offshore Vancouver Island, 2. Alaska/northeastern Pacific shortwave energy dropping practically due southward from Sun onward, and 3. a closed southern stream system reaching near 135W by Tue. Thus far recent CMC runs have been farthest south/southeast with the northern upper low thus suppressing surface low pressure nearing the West Coast by midweek--a seemingly low probability solution at this time. In spite of the potential complexity of evolution, the ensemble means have been remarkably stable/agreeable thus far with low pressure nearing extreme northern California by early Wed. The 12-18Z GFS/12Z ECMWF runs were very close to their respective means so those solutions provided the basis for the updated forecast with good continuity. 12-hourly GFS runs have been fairly stable for the past day or so while ECMWF runs have been oscillating between 12Z runs near the ensemble mean consensus and 00Z runs that were 12-24 hours slower. Based on above considerations the updated forecast blend went 70 percent toward the 12Z ECMWF/UKMET/CMC cluster for days 3-4 Sun-Mon and then removed the CMC from consideration due to its suspect solution along the West Coast. At the same time the forecast started to incorporate the 18Z GEFS/12Z ECMWF means on day 5 Tue with a gradual increase of the means to slightly more weight than the 18Z GFS/12Z ECMWF by day 7 Thu. ...Weather Highlights/Hazards... A broad area of precipitation will accompany a vigorous wave tracking northeast over parts of the eastern states and into the Canadian Maritimes Sun-Mon. Northwest trends in the past day have pushed the most likely area for significant wintry weather to an axis extending from the Mid-Mississippi Valley/Ohio Valley into the Northeast. A more consistent theme from yesterday is the potential for heavy rainfall over parts of the Lower Mississippi Valley and Tennessee Valley/interior Southeast. Much of the East will trend drier after the wave's passage though moisture may linger for a time over the extreme Southeast/Florida. Expect periods of lake effect snow downwind from ice-free areas of the Great Lakes and one or two areas of light snow may pass through the Plains/Midwest. Rain and higher elevation snow in progress over the central/south-central parts of the West late in the weekend will taper off from west to east. Then from Tue onward the next system in the series will spread a broad area of moisture across the West, with greater northward extent than with the previous system. Favored terrain over northern and central California should once again see the highest precipitation totals with this system. Moisture reaching northern portions of the West may bring wintry weather to low elevations over some areas given the cold temperatures in place from early in the week. Continue to expect coldest temperatures relative to normal during Sun-Mon with some central U.S. locations seeing anomalies of 30-45F below normal and a widespread area over the Plains/Midwest with readings that will approach or break daily records--more likely a greater number for cold highs. Interior Northwest locations may also see some temperatures at least 20F below normal with local challenges to daily records. The area of potential daily records will likely shift more into the eastern half of the country (Midwest/Great Lakes/Ohio Valley) by Tue. As of Thu expect coverage of minus 20F or greater anomalies to decrease considerably but the eastern half of the country will still be chilly versus normal. The southern 2/3 of the West continues to look warm for morning lows while highs should be within a few degrees of normal. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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