Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1118 AM EDT Sat Aug 24 2019 Valid 12Z Tue Aug 27 2019 - 12Z Sat Aug 31 2019 ...Overview and Guidance/Predictability Assessment... An upper-level ridge axis is forecast to persist along the West Coast through the medium range, with a broad upper-level trough downstream of the ridge, across the north central and northeastern U.S. The trough will be reinforced periodically by shortwaves passing through an active polar jet from the Gulf of Alaska across central Canada, toward the Great Lakes and the Northeast. Cold fronts associated with these waves will bring periodic rounds of cool and dry polar air into the north central U.S. Model consensus has improved a bit with respect to an area of low pressure expected to be a few hundred miles east of the Outer Banks on day 3 (Tue). The ECMWF has slowed from its previous faster progression, and the GFS has shifted the system a bit east, farther away from the coast. The largest differences are now with respect to whether the models deepen the system or now, with the 00Z ECMWF the most aggressive with this and the GFS/CMC the weakest. Please refer to outlooks issued by the National Hurricane Center for more details on this feature. Elsewhere the most significant differences among the models were with the progression of a northern stream shortwave initially near the Alaska Panhandle on Tue, expected to then progress southeastward across western/central Canada toward the north central U.S. The ECMWF showed a significant difference from much of the other guidance for this feature, showing a slower southeastward progression from Tue onward. Ensemble spread is rather large and perhaps somewhat multi-modal here, but ensemble means do support something faster than the 00Z ECMWF, more along the lines of the 00Z UKMET or 06Z GFS. This eventually results in significant timing differences among the guidance for a cold front expected to cross the north central and northeastern U.S. days 5-7 (Thu-Sat). Farther west, spread was somewhat less with respect to an upper-level trough off expected to remain off the coast of the Pacific Northwest through day 7. Given these considerations, a blend of the 00Z ECMWF/UKMET and 06Z GFS was used as a forecast starting point for days 3-5, with more emphasis on the UKMET/GFS solutions by days 4-5 given their more favored northern stream solutions. By days 6-7, increased northern stream spread warranted heavy use of 00Z ECENS/06Z GEFS ensemble means. ...Weather Highlights/Threats... Areas of showers and thunderstorms will accompany a cold front across the Great Lakes/Midwest/Ohio Valley/Northeast Tue-Wed, with moderate to locally heavy rainfall a possibility. Models show a stronger signal for heavy rainfall potential across the Southern Plains Tue-Wed along the stationary trailing end of the frontal boundary, with the front and some areas of convection lingering through later in the week. Meanwhile, an early fall-like air mass is expected to persist from the Northern/Central Plains to the Upper Midwest as multiple cold fronts bring reinforcing rounds of air originating in Canada. High temperatures are forecast to be 5 to 15 deg F below average across these areas through the forecast period, into next weekend, with highs in the 60s and 70s for many locations. Meanwhile, the West Coast and portions of the Great Basin will remain warm beneath the upper ridge, with highs expected to be 5 to 15 deg above average. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml