Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Tue May 08 2018 Valid 12Z Fri May 11 2018 - 12Z Tue May 15 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... After several runs of varied solutions across the CONUS, the 00Z models/ensembles made a significant jump toward a consensus solution best advertised by the Canadian from 24-36 hrs ago which was considered an "outlier" solution -- sending a reminder that ensembles are underdispersive at all lead times and that "extreme" solutions can verify. The consensus now lingers the western upper low west of the Divide this weekend and only slowly allows it to move eastward by next week, in line with climatology in what is the peak of closed low frequency over the CONUS in the atmospheric spindown transition period from winter to summer. With relatively little disagreement, used a blend of the deterministic models (GFS/ECMWF/UKMET/Canadian) Fri-Sun with a trend toward a GFS/ECMWF and GEFS/NAEFS/ECMWF ensemble mean blend for next Mon Tue with manual edits in order to keep the upper low better defined in the west. This resulted in a few changes from the previous forecast when the models were not yet in agreement. ...Sensible weather/threats highlights... The wavy/stationary frontal boundary extending from the central Rockies and adjacent plains to the Great Lakes/Northeast Fri-Mon 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 across portions of the central Rockies Fri-Sat (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, likely in favored upslope areas. 00Z GEFS M-Climate QPF shows 48-hr "MAX" values indicating a relatively rare forecast for QPF values this high this time of year. A the front exits toward the Plains next week, precipitation will expand from northern Texas northward to Kansas and eastward along the frontal boundary. 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 Fri-Sun across portions of the Midwest/Great Lakes and perhaps even into portions of the Northeast, with the potential for areas of 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) and another area of "MAX" values per the 00Z GEFS M-Climate QPF in the MIA-PBI-FPR region. High temperatures are expected to be 10 to 20 deg F above average across much of the southern tier on Fri, 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 east of the Divide on Friday and in the Mississippi Valley 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. 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