Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EDT Wed May 09 2018 Valid 12Z Sat May 12 2018 - 12Z Wed May 16 2018 ...Pattern overview... An upper low is expected to meander over the Great Basin this weekend into early next week with zonal flow across the central/eastern states. An upper low just north of Hudson Bay will promote broad cyclonic flow over southeastern Canada and along our shared border into/through the Great Lakes and Northeast. At the surface, a front associated with the upper low in the west will slowly push out of the Rockies by next week as a Pacific front finally approaches the Pacific Northwest coast around next Tuesday. The eastern extent of the front will ebb and flow northward and southward in the east as successive waves exit off the east coast. ...Guidance/uncertainty assessment/preferences... Models/ensemble have quickly to come a consensus over the past 24 hours, keeping an upper low back across the Great Basin for much longer than most guidance previously indicated. Model/ensemble spread during days 3-5 (Sat-Mon) was sufficiently low to justify a multi-model deterministic blend (including the 12Z ECMWF/UKMET/CMC and 18Z GFS) as a forecast starting point. This blend also resolves the stationary front (and a few waves along it) well, extending from the Great Basin/Rockies east across the plains/Ohio valley to the Mid-Atlantic. One wave of low pressure develops across the Ohio valley on Sat and then quickly crosses New England by Sun morning. A couple additional waves of low pressure will traverse the front across the central U.S. Sat-Sun, the strongest of these likely crossing the central plains on Sat. Spread does increase during the days 6-7 (Tue-Wed), with some continued disagreement on how quickly the upper low moves east into the plains. The ECMWF remains on the fast side of the spread, outpacing the GFS and CMC, and even (slightly) the 12Z ECENS mean. The 12Z ECMWF was excluded from the blend for days 6-7 given its acceleration of shortwave energy into the Upper Midwest while the consensus (and the trend) has been toward a slower progression. Additional shortwave energy and associated surface front are also expected to reach the Pacific Northwest by Tue, and models show some timing differences, with the 12Z ECMWF on the fast side and the GFS slower (also showing a closed upper low). Favored a shift to an ensemble approach during this time frame, with increased weight placed toward the 12Z ECENS/NAEFS means. ...Sensible weather/threats highlights... The wavy/stationary frontal boundary extending from the central Rockies and adjacent plains to the Great Lakes/Northeast from Sat onward will serve as a focus for scattered to numerous showers and thunderstorms over a large area. Models continue suggest the potential for areas of heavy rain (and high elevations snows) across portions of the central Rockies Sat-Sun (especially northeastern Nevada eastward into Wyoming) as the vigorous upper trough amplifies to the west, and a surface low develops along the front across the central plains, enhancing low-level easterly upslope flow. A number of deterministic solutions show the potential for localized multi-day totals of at least a few inches. A the front exits toward the plains next week, precipitation will expand from northern Texas northward to Kansas and eastward along the frontal boundary. By next Tue-Wed, convective activity may once again expand north to the northern plains/Upper Midwest as upper energy finally begins to exit the Rockies, enhancing upper-level support for precip, and the central plains surface front lifts north as a warm front. Farther east, broad southerly flow across the plains will transport moisture north and lift it across the frontal boundary, and models show potential for multiple rounds of convection along the front Sat-Sun across portions of the Midwest/Great Lakes and perhaps even into portions of the Northeast or Mid-Atlantic, with the potential for areas of locally heavy rainfall. Confidence in the specific locations of any heavy rain is relatively low at this time, and would likely be determined by mesoscale/convective processes. Farther south, a dissipating frontal boundary will focus increasingly widespread shower and thunderstorm activity across the southern/central Florida Peninsula from late this week into early next week. Areas of heavy rainfall will be possible given the multi-day nature of the event and high Pwat values (+2 to +3 standard deviations). High temperatures are expected to be 10 to 20 deg F above average across much of the southern tier on Sat, with the center of greatest anomalies shifting east to the lower Mississippi valley, Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F anomalies). Record high temperatures are quite possible from the Mississippi valley to portions of the Southeast Saturday-Monday and maybe into Tuesday. Temperatures to the north near the Canadian border will be below average on the cool side of the front. Ryan/Fracasso Associated WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet probabilities can be found at... http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4