Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1149 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 Washington/Oregon coast around next Tuesday. The eastern extent of the front in the Rockies will ebb and flow northward and southward in the east as successive waves exit off the coast into the Atlantic. ...Guidance/uncertainty assessment/preferences... Model/ensemble consensus remains quite well-clustered to star the medium range period, keeping an upper low back across the Great Basin Sat-Mon and a multi-model deterministic blend (00Z GFS/ECMWF/UKMET/CMC and 06Z GFS) was used as the starting point. This blend also resolved the stationary front (and a few waves along it) well as some expected timing and latitudinal differences remain. One wave of low pressure will exit the Northeast on Saturday as another will push through the Ohio valley on Sunday. The GFS was first to depart from the good clustering well northwest of the CONUS just south of the Gulf of Alaska and dropped it from the consensus by after next Monday due to its differences in system evolution there as well as downstream through southern Canada where it appeared much too strong with height falls into the Great Lakes next Tue-Wed. The 00Z ECMWF/Canadian best represented the ensemble clustering of the 06Z GEFS and 00Z ECMWF ensemble mean which was used as the starting point for next Tue/Wed as the upper low in the west weakens and lifts out due to the kicker system in the Pacific just off the west coast. The 00Z ECMWF ensemble mean still appeared too quick with the weakening lead system and approaching Pacific system but a blend with the slower ECMWF and much slower Canadian offset it. With an overall trend toward slower system progression, opted to rely more on the slower solutions again today through the mid-latitudes. ...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 to 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 (straddling the short range into the medium range). As the front exits toward the plains next week, precipitation will expand from northern Texas northward to Kansas and northeastward along the frontal boundary into the Corn Belt. By next Tue-Wed, convective activity may once again expand northward 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 northward as a warm front. Farther east, broad southerly flow across the plains will transport moisture north and lift it across the frontal boundary. Models show potential for multiple rounds of convection along the front Sat-Sun across portions of the Midwest/Great Lakes and into the Northeast/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 and developing upper low 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 Saturday, with the center of greatest anomalies shifting eastward to the lower Mississippi valley, Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F anomalies). Record high temperatures will likely shift eastward out of the southern Plains Saturday (near 100 degrees along the Red River between TX/OK) and into the Mississippi valley Sat-Tue next week. Temperatures to the north near the Canadian border will be below average on Saturday with a trend toward average or slightly above average by next Monday as the colder temperatures remain confined to Canada. Fracasso/Ryan 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