Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Aug 19 2019 Valid 12Z Thu Aug 22 2019 - 12Z Mon Aug 26 2019 ...Overview... From late this week into the weekend expect an upper low crossing eastern Canada to anchor a trough that crosses the Great Lakes/Northeast. The trough should depart after Sat. The leading cold front reaching the East as of early Thu will likely stall over the Southeast while the trailing part of the front over the Plains returns as a warm front (though with slower multi-day trends). To the west low amplitude mean flow should prevail from the North Pacific into western-central parts of the northern CONUS/southern Canada, with a trend toward some troughing by Sun-Mon. Specifics will depend on the yet-to-be determined relative strength and position of ridging over the eastern Pacific and southern half of the western U.S.. Individual shortwaves within the flow will bring at least a couple fronts into the northwestern states and northern Plains. Some guidance still hints at energy aloft lifting across the extreme western Gulf of Mexico toward the Gulf Coast. ...Guidance/Predictability Assessment... For the northeastern upper trough there is decent consensus in principle. However shortwave differences between models and in consecutive runs lead to some spread/variability in specifics of low pressure forecast to track just north of New England and the trailing wavy front that sinks through the Mid-Atlantic into the Southeast. Meanwhile guidance is having a difficult time resolving the details of shortwave energy and associated surface reflection reaching western North America by the start of the forecast early Thu and continuing east/northeast thereafter. The primary forecast consideration in the larger scale was to discount the 12Z GFS due to effects from a disagreeable short-range evolution over the eastern Pacific extending into the western states. There is some relative agreement that the next Pacific shortwave should reach the coast around late Fri-early Sat. Beyond that time solutions have differed regarding progression of this incoming energy as well as for strength of ridging over the southern half of the West and over the eastern Pacific. There has been a modest slower trend in consensus for the evolving northern stream trough over the past couple days but latest ECMWF/CMC ensemble means have been more stable with their depiction of the trough relative to the latest GEFS mean runs that are somewhat slower (corresponding to a weaker western U.S./eastern Pacific ridge). Recent ECMWF runs and the 12Z/18Z GFS also support the faster scenario. Finally, teleconnections relative to the core of positive height anomalies expected just off the West Coast late in the period favor a downstream trough far enough east to favor the majority non-GEFS cluster. From Thu into Sat a blend of mostly operational guidance represented the most common ideas over the East while it provided an intermediate approach and reasonable continuity aloft over the Northwest into northern Plains. Some surface details differed though. The 12Z NAEFS/ECMWF means balanced the 12Z ECMWF/18Z GFS later in the period, with the NAEFS preferred for its inclusion of CMC ensembles and only partial account for the GEFS. ...Weather Highlights/Threats... The front dropping through the Mid-Atlantic into the Southeast and extending back into the Plains continues to serve as a likely focus for areas of heavy rainfall, with best potential for highest totals along and just inland from the southern Mid-Atlantic/Southeast coast as well as parts of the central/south-central Plains into Mississippi Valley. The central and possibly western Gulf Coast region may see some enhanced rainfall from western Gulf energy aloft and possible associated surface trough. Confidence is below average due to the small scale of feature(s) involved. Expect scattered showers/thunderstorms on a daily basis over the rest of the Southeast/Gulf Coast region. Areas from the northern Plains east into the Mississippi Valley may see one or more episodes of convection with fronts reaching the region. It will take added time to resolve enough of the specifics at the surface and aloft to gain more confidence in pinpointing location/timing of any enhanced rainfall. Expect above normal temperatures to be most persistent from southern Oregon and the northern two-thirds of California into the southern High Plains with upper ridging in place during the period. Scattered locations within this area may see max and/or min temperatures at least 10F above normal. Frontal passages will lead to more variation over northern parts of the West. Central/eastern states should tend to see moderately below normal to slightly above normal readings, with the central/northern Plains into Great Lakes seeing the best opportunity for below normal highs from Thu into Sat. Rausch 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