Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Sep 01 2018 Valid 12Z Tue Sep 04 2018 - 12Z Sat Sep 08 2018 ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance continues to advertise the establishment of a mean trough extending from northern/western Canada into the northeastern Pacific, in response to strong upstream ridging likely to extend through the eastern Bering Sea for much of the period. The expected mean pattern compares fairly well to what is suggested by teleconnections relative to the ridge's corresponding positive height anomaly center by the D+8 time frame. Perhaps more of a concern shortly after the current forecast period, the northern part of the envelope for where the positive height anomaly may exist could allow for some undercutting flow that would ultimately flatten the pattern a bit over the northwestern U.S.. Another wild card will be the extratropical transition of current Super Typhoon Jebi and its downstream effects. Current guidance indicates that the ridge to the east of this evolution is what reinforces the eastern Bering Sea mean ridge late in the week. Meanwhile anomalously strong eastern U.S. ridging aloft through midweek will gradually weaken later in the period. There is still a fair degree of shortwave detail uncertainty within the band of progressive flow between the eastern ridge aloft and axis of the developing mean trough upstream. Thus far there have been some daily oscillations for timing of the leading shortwave forecast to cross central Canada and extreme northern tier of the lower 48 Tue-Wed, with the past day of guidance trending faster in contrast to opposite trends that were seen from the prior 24 hours. Guidance clustering has improved out to about day 4 Wed before details diverge over eastern Canada. Latest trends likewise lead to a faster progression of the leading cold front across the northeastern quadrant of the lower 48. Behind this feature, most solutions agree in principle on amplifying the northeastern Pacific trough aloft (with embedded upper low). This feature would then eject inland late in the period. Exact timing becomes very uncertain due to question marks with important upstream details, but for now there is actually decent agreement regarding surface low pressure reaching the vicinity of the northern Plains/southern Canada by day 7 Sat. To the south of the eastern ridge aloft, GFS/ECMWF runs have converged decently--at least out to Wed/Thu--with respect to the mid level energy and weak surface reflection expected to track westward near the Gulf Coast. Other models and some ensemble members still offer alternate details, suggesting ongoing potential for eventual changes in the forecast relative to current consensus/continuity. During days 3-4 Tue-Wed a blend of operational model guidance (including the 12Z FV3 parallel GFS) represented consensus well while resolving detail/timing differences that were within typical guidance error. The 12Z CMC trended out of phase versus consensus for some aspects of flow aloft mid-late period while increasing detail differences among model runs favored incorporation of some ensemble mean guidance. Thus days 5-7 Thu-Sat used 30-60 percent total of the 18Z GEFS/12Z ECMWF means to go along with the gradually decreasing weight of 18Z GFS/12Z parallel GFS/12Z ECMWF. ...Weather Highlights/Hazards... Moist flow around the eastern ridge aloft will bring episodes of meaningful rainfall across Florida and the Gulf Coast region as well as the southern High Plains and extending northeastward through the upper Midwest/Great Lakes, and eventually the Northeast/Mid-Atlantic as the ridge weakens. The weak surface wave and energy aloft progressing westward along the Gulf Coast should enhance activity over that region while a surface trough may approach Florida late in the period. Weak impulses aloft will play a role in locally heavy southern High Plains convection. Currently the most coherent signal for heavy rain potential appears to be from the central Plains into upper Great Lakes, with a frontal system approaching/crossing the region early in the period and then stalling over mid-latitudes of the central-eastern U.S. before the central U.S. part begins lifting northeast as a warm front. Some locations within this area have already received significant rainfall recently, increasing the potential for hydrologic concerns from this coming week's rainfall. Expect some showers/storms to reach the Northeast/Mid-Atlantic by late week/weekend. New England may see some lighter rainfall early in the period with a leading front. The Pacific Northwest may see light precipitation late in the period with the arrival of shortwave energy aloft. Warmest temperatures relative to normal will extend from the Midwest into the Great Lakes/Mid-Atlantic/Northeast with plus 5-15F anomalies. Departures from normal should generally be a little greater for morning lows, with Tue-Wed being the most likely days when a number of locations may see new daily records for warm lows. Meanwhile the cold front initially tracking east/southeast from the northern Plains will bring below normal readings to parts of the central U.S. with some minus 10-15F anomalies over the northern Plains in particular Tue-Wed. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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