Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 AM EDT Sat May 12 2018 Valid 12Z Tue May 15 2018 - 12Z Sat May 19 2018 ...A wet and convectively active weather pattern... ...Pattern overview and guidance/uncertainty assessment... It remains the case that an upper low over Hudson Bay Monday will slide eastward toward southern Greenland by the end of next week which favors broad cyclonic flow to its south along the US/Canadian border from the Upper Midwest eastward to New England. Upper ridging will slowly bridge atop an active northern Great Basin upper low Tue/Wed. This low is expected to meander across the Great Basin/Rockies into midweek, with weakening energies then drifting into the plains to fire local convection. Farther east, the west Atlantic subtropical ridge will continue to exert influence from the lower Mississippi valley and mid/deep south to the Mid-Atlantic as an active and wet upper low and tropical moisture plume will be temporarily trapped in between just west of Florida. This low will slowly lift northward into the Southeast midweek and be slow to dissipate, but will bring widespread and locally heavy downpours. In the region of relatively weak upper flow between the western trough/upper low and the southeastern ridge, expect a wavy surface front to linger from the Rockies to the Mid-Atlantic through much of the period and remain a focus for pooling moisture and periodic convection. An amplified mid-upper level trough is expected to track from the eastern Pacific inland into northern CA and then the south-central Great Basin next Wed-Fri, with ejecting energy/height falls slated to breach the Rockies/Plains Fri-Sat. which will offer the potential for widespread precipitation along and to the north of its path. Accordingly, with models/ensembles continuing to show above average consensus for much of days 3-5 (Tue-Thu), it seems sufficient to justify use of a multi-model and ensemble blend as mainly derived from the GFS/ECMWF and NAEFS/ECMWF ensembles. Increase blend weighting toward ensembles days 6/7 consistent with growing forecast spread/uncertainty. ...Sensible weather/threats highlights... It remains evident that 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. 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 in the likelihood for heavy rainfall across the Florida peninsula. An upper low developing across the northeastern Gulf of Mexico will advect 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 along the I-95 corridor from Miami to Jacksonville. 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. 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