Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EDT Thu May 10 2018 Valid 12Z Sun May 13 2018 - 12Z Thu May 17 2018 ...Pattern overview... Upper ridging will remain dominant along the North America west coast through the medium range, with a persistent upper low just northeast of Hudson Bay. In between, a broad area of modestly cyclonic upper-level flow will remain in place from the Canadian prairies to New England, with relatively active flow and frequent passage of frontal boundaries. A cutoff upper-level low in place across the Great Basin at the start of the forecast period (Sun) is expected to meander across the Great Basin/Rockies into at least Tue before drifting east into the plains. Farther east, the west Atlantic subtropical ridge will exert its influence from the lower Mississippi valley to the Mid-Atlantic. 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 use of a multi-model deterministic blend including the 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 a shortwave/upper low that develops across the northeastern Gulf of Mexico, a trough/upper low remaining off the U.S. West Coast (the 18Z GFS was much farther south with the system than consensus, and was excluded in favor of the 12Z run), 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), majority weighting was given to the 12Z ECENS/18Z GEFS ensemble means, but with continued use of the ECMWF/CMC as well. The ECMWF/CMC showed decent consensus with the large upper low approaching the West Coast by the middle of next week (the GFS was likely too fast), and with additional northern stream energy approaching the Great Lakes, and energizing a surface frontal wave tracking from the Great Lakes to New England. ...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. 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 north, and locally heavy rains will be possible. Farther west, precipitation may begin to increase across the West Coast states by the middle of next week as the aforementioned trough/upper low approaches the region and a cold front moves ashore. Temperatures will be the other big story during the medium range, with subtropical ridging leading to what may be perceived as the first 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) look 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. 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