Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1128 AM EDT Fri May 11 2018 Valid 12Z Mon May 14 2018 - 12Z Fri May 18 2018 ...Pattern overview and guidance/uncertainty assessment... An upper low initially centered over of Hudson Bay 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 initially along the west coast Monday will slowly nudge eastward atop an active upper low over the Great Basin Mon-Wed. This cutoff low is expected to meander across the Great Basin/Rockies into midweek before drifting east into the plains and weakening/dissipating. Farther east, the west Atlantic subtropical ridge will exert its 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. Another closed low will track from the eastern Pacific inland into northern CA and then the south-central Great Basin next Thu-Fri, 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 3-7 (Mon-Fri), it seems sufficient to justify use of a multi-model and ensemble blend as mainly derived from the 00Z GFS/ECMWF/Canadian/UKMET to start with a trend toward the 06Z GFS/GEFS and 00Z ECMWF/ECMWF ensemble mean for next Thu-Fri. ...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 the middle of next week as the aforementioned upper low approaches the region and its attendant cold front moves ashore. Temperatures will also remain a big story during the medium range, with subtropical ridging promoting well above average temperatures for portions of the southern/southeastern states. High temperatures Mon-Tue of 10 to 15 degrees above average are possible, and a sizable number of record high temperatures (many in the low to mid 90s and even some upper 90s) are in jeopardy of being tied/broken. Some relief may arrive across the Southeast by Tue-Wed as rainfall becomes more widespread, but the lower/mid Mississippi valley will remain well above average. The only other area for below average temperatures outside Florida after Monday will be in the west (Oregon through California the into Nevada) as the upper low moves ashore later in the week. Much of the Great Basin will moderate from below average to above average temperatures through the period as the upper low exits the region, only to cool again next Friday. Fracasso/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