Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri May 11 2018 Valid 12Z Mon May 14 2018 - 12Z Fri May 18 2018 ...Pattern overview and guidance/uncertainty assessment... A mean upper vortex centered just to the northeast of Hudson Bay favors broad cyclonic flow underneath that will skirt the US/Canadian border from the Upper Midwest eastward to New England, with relatively active flow and frequent passages of frontal boundaries. Upper ridging initially along the North America west coast Monday will slowly nudge eastward atop an active upper low over the Great Basin. 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 deepened moisture will be trapped in between just west of Florida. This low will slowly lift northward into the Southeast into 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. Later week upstream closed trough development and gradual track from the eastern Pacific inland into CA then south-central Great Basin offers potential for widespread precipitation in a pattern with increasing model spread, but generally good overall support/continuity. 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 18 UTC GFS/GEFS, 12 UTC NAEFS/ECMWF ensembles and ECMWF. ...Sensible weather/threats highlights... It remains evidemt 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 (+2 to +3 standard deviations), 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 over the Southeast. Farther west, precipitation may 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 fronts move ashore. Temperatures will also remain a big story during the medium range, with subtropical ridging leading to what may be perceived as the first significant heat wave of the season 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) in jeopardy. 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 after Monday will be in the west (Oregon through California) as the upper low moves ashore mid-later 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. 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