Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat May 12 2018 Valid 12Z Tue May 15 2018 - 12Z Sat May 19 2018 ...Rather wet pattern with scattered convection over much of the lower 48... ...Pattern overview... An upper low initially over Hudson Bay Tuesday will slide eastward toward southern Greenland by the start of next weekend which favors broad cyclonic flow to its south along the US/Canadian border from the Upper Midwest eastward to New England. A weakening upper low over the Great Basin on Tuesday will dissipate around Thursday while another upper low west of California will move into norCal and through NV into UT next Fri/Sat. Yet another upper low will slowly move into the Southeast Tue/Wed and weaken as it lifts into the TN valley/southern Appalachians late Thu into Fri. In between these features--central/southern Plains into the mid-Mississippi valley--will be a wavy surface boundary that promote scattered showers/storms through the period. ...Guidance/uncertainty assessment... Models/ensembles continue to show better than average agreement despite the active pattern and a blend of the deterministic models (00Z GFS/ECMWF/Canadian/UKMET and 06Z GFS) offered a good starting point for the Tue-Thu period. After that, the model/ensembles diverge in their handling of the northern stream flow across southern Canada along/south of 50N with rising heights behind the departing Hudson Bay low and lowering heights downstream of building ridging over western Canada. With additional ridging nosing westward from the western Atlantic near Bermuda, leftover troughing along the Mississippi should squeeze northeastward through the Great Lakes, lifting a sfc low into the St. Lawrence valley. Trend has been toward increased Atlantic ridging which may help guide this system more northeastward rather than eastward even as a Canadian cold front pushes southeastward Thu-Fri into Ontario. Trend toward increased ensemble weight helped temper any changes from the previous cycle. To the west, ensembles favor taking the upper low into Utah by next Saturday but may get stuck beyond that given the possible Rex Block pattern formation in the west and building heights to its east. Fracasso ...Sensible weather/threats highlights... Scattered to numerous showers and thunderstorms will be possible across a large area in the vicinity of the wavy surface frontal boundary, from the Great Basin/central Rockies to the Mid-Atlantic/Northeast. Several areas of potentially heavy rainfall are possible, with the potential for multiple rounds of convection (though SPC does not indicate the potential for organized severe weather). The specific areas most likely to receive heavy rains will likely be somewhat difficult to pin down until the short term when models begin to resolve mesoscale/convective processes. Farther south, higher confidence exists as we near the event in the likelihood for heavy rainfall across the Florida peninsula owing to very high PWs across the Florida (about +2 standard deviations or around the 98th percentile), and multi-inch rainfall totals are possible across several days with the potential for especially urban flooding. Current indications among the guidance are that deeper moisture may gradually advect farther north into much of the southeastern U.S. as the upper low drifts northward, and locally heavy rains will spread through the Southeast and perhaps into the southern Mid-Atlantic by the end of the week. As a Canadian cold front sinks southward to the Mason-Dixon line, precipitation may expand along this boundary as well, setting up a potential modest/heavy rain event for the mid-Atlantic to southern New England. Farther west, precipitation will begin to increase across the West Coast states by midweek as the aforementioned upper low approaches the region and its attendant cold front moves ashore, focused mostly over northern California/Nevada/Utah and southern Oregon/Idaho. Schichtel 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