Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1140 AM EDT Thu May 10 2018 Valid 12Z Sun May 13 2018 - 12Z Thu May 17 2018 ...Locally heavy rain possible for much of Florida next week... ...Pattern overview... Upper ridging initially off the North America west coast on Sunday will slowly nudge eastward atop an upper low over the Great Basin. A persistent upper low just northeast of Hudson Bay will favor broad cyclonic flow that will skirt across the US/Canadian border from the Upper Midwest eastward to New England, with relatively active flow and frequent passages of frontal boundaries. The cutoff upper low in place across the Great Basin at the start of the forecast period is expected to meander across the Great Basin/Rockies into at least Tue or Wed 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 to the Mid-Atlantic with another upper low trapped in between just west of Florida. That will slowly lift northward into the Southeast midweek and dissipate, but will bring a widespread modest rainfall to much of the Sunshine state. 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. ...Guidance/uncertainty assessment/preferences... Models/ensembles continue to show above average consensus for days 3-5 (Sun-Tue), and consensus was sufficient to justify continued use of a multi-model deterministic blend including the 00Z ECMWF/UKMET/CMC/GFS. This blend represents a consensus solution well for the western upper low, not transporting it east too quickly by Tue as has been a bias among nearly all deterministic guidance except the CMC in recent days. Additional features well resolved by a multi-model blend during days 3-5 include the upper low that develops across the northeastern Gulf of Mexico (06Z GFS appeared too far west), a trough/upper low remaining off the U.S. West Coast, and northern stream shortwave energy crossing south of Hudson Bay, which will push a cold front into northern New England by Tue. During days 6-7 (Wed-Thu), nearly equal weighting was given to the 00Z ECENS/GEFS ensemble means and the 00Z ECMWF/CMC as the Pacific upper low approaches California. There remains north-south spread in the location of the frontal boundary east of the Mississippi due in part to the strength of high pressure over northwestern Ontario next Thursday and the evolution of both the exiting Great Basin/Rockies upper low and the southeastern weakening upper low. Ensemble consensus provided a best position for now. given the lead time. ...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 Sun-Tue. 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 be possible into Georgia. 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 be the other 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 Sun-Mon of 10 to 15 degrees above average are possible, and a sizable number of record high temperatures (many in the low to mid 90s) may be 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 midweek. Much of the Great Basin will moderate from below average to above average temperatures through the period as the upper low exits the region. Fracasso/Ryan 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