FXUS02 KWNH 030716 PREEPD PRELIMINARY EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 315 AM EDT THU MAY 3 2018 VALID 12Z SUN MAY 6 2018 - 12Z THU MAY 10 2018 ...Pattern overview... The forecast period begins on Sunday with a broad upper level trough over eastern North America, a ridge over the Intermountain West, and a smaller scale trough near the West Coast. The flow pattern becomes quasi-zonal across most of the continental U.S. going into the first half of next week as the eastern U.S. trough lifts out some and the western U.S. ridge becomes less amplified with time. The faster flow aloft should be mostly relegated to Canada as the polar jet stream lifts north. By the end of the forecast period Wednesday night, a stronger trough is likely to affect the western U.S. ...Guidance and uncertainty assessment... Model guidance is in relatively decent agreement at the beginning of the forecast period. However, given the lower amplitude flow aloft, the timing and magnitude of individual shortwave disturbances and surface lows becomes less certain going into early next week. The CMC becomes more amplified with a trough over the southeast U.S. and over the northern plains by Tuesday. The ECMWF is stronger with the next Pacific storm system reaching the West Coast by the middle of the week, with the GFS tracking it farther north. The WPC forecast was based mainly on the deterministic GFS/ECMWF/UKMET along with some previous WPC continuity through Tuesday, and then increasing percentages of the EC and NAEFS means for the final two days. ...Sensible weather... Showers and storms are likely for portions of the East Coast for Sunday with the frontal boundary slowly exiting the coast. The heaviest rainfall appears likely offshore for now, but any trend farther to the west will need to be monitored. The other area that will have increased chances for rain and thunderstorm activity will be the northern plains to the Upper Midwest towards the middle of next week as a surface low develops and tracks across the region. Elsewhere, precipitation appears to be isolated to scattered at best. In terms of temperatures, expect readings to generally be above average for the Plains to the Intermountain West, including the Desert Southwest where triple digit highs are likely to be realized for the lower elevations. Near normal temperatures are expected to return to the East Coast region after several days of unseasonably warm conditions. Some areas of below normal temperatures are likely near the Great Lakes owing to the trough over southeast Canada and the cold lake temperatures. D. Hamrick ---------- 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 $$