Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Wed May 01 2019 Valid 12Z Sat May 04 2019 - 12Z Wed May 08 2019 ...Overview... Upper ridging will remain anchored over the northeastern Pacific during the medium range period which will drive a split flow across the Lower 48. A closed low in the southern stream will move into California Sunday/Monday while northern stream systems skirt along the US/Canadian border. Ridging off the Southeast coast will wobble in place but likely build northward toward the end of the period next Wed. Frontal boundaries in the east will waver as surface waves move through. ...Guidance/Predictability Assessment ... The models/ensembles have come into better agreement off the west coast with the approaching upper low but still exhibit spread in timing with the GFS generally quicker than the ECMWF and the UKMET in between. Trend over the past 24 hours has been slower in the GFS and again prefer a solution near the slower half of the ensembles near the 00Z ECMWF ensemble mean but not as slow as the deterministic ECMWF. By next Tue/Wed, trough axis is forecast to move through the deserts with good ensemble agreement. Blend between the 06Z GEFS mean and 00Z ECMWF ensemble mean by then was a good starting point. In the northern stream, a surface low will move through southern Canada this weekend with a cold front across the High Plains and Upper Midwest that will push through the eastern Great Lakes and Northeast early next week. Differences in timing remain so a blended solution was preferred again today. To its south, a wave or two of low pressure will exit off the mid-Atlantic coast late this weekend and drop its cold front into the Southeast. By next Tue/Wed, that boundary will dissipate as upper ridging increases and forces the thermal gradient to the front to the north. Tail end of that boundary will linger across the central Plains into the Corn Belt and is still unclear if a defined surface low will exit through the Great Lakes or just remain a wavy front into next week. For now, opted to maintain continuity with a wavy front rather than depict a surface low like the ECMWF. ...Weather Highlights/Threats... Front draped across the southern Appalachians back to the lower Mississippi Valley will be a focus for locally heavy rain/thunderstorms in the warm sector with another area of rain to the north of the low across the northern mid-Atlantic and Southern New England. This will keep temperatures below average on Sunday along the PA/NY line eastward through CT/MA/RI in the 50s or low 60s. Cooler than average temperatures will remain in place over the High Plains/Upper Midwest next week by 5-15 degrees F (temperatures only in the 50s). Closed low into CA/Southwest early next week will support scattered showers and some convection from into the Great Basin/Rockies as the height falls move eastward. Precipitation (generally all rain with snow levels quite high) will mostly focus on higher elevations but with some rain into the valleys, dependent on mesoscale details. Moisture influx into the region appears to be modest across southern CA/NV/AZ but drier mid-levels may inhibit that potential rainfall, at least at the onset. Winds will increase in tandem with height falls over especially higher terrain as temperatures trend cooler than average. Precipitation may expand over the Rockies around the middle of next week as the upper low continues eastward. Southwest to westerly flow aloft across the Plains next Tue-Wed will support showers and storms along the northern stationary front to the north and near the Gulf Coast as the southern stationary front moves northward. Fracasso 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