Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Jun 11 2019 Valid 12Z Fri Jun 14 2019 - 12Z Tue Jun 18 2019 ...Overview and Guidance/Predictability Assessment... Guidance agrees fairly well with the idea of broad and low amplitude cyclonic mean flow downstream from ridging that rebuilds into western Canada and the eastern Pacific. Within this pattern there continue to be multiple embedded uncertainties that provide challenges for resolving important specific details and significantly lower forecast confidence. The most prominent question marks involve the upper low forecast to be over or near northern Ontario as of early Fri, multiple weak shortwaves flowing through the West and continuing eastward, and the character of western U.S. troughing aloft by early next week. Although there is ample opportunity for best consensus to change, guidance has established a fair degree of persistence in suggesting that the overall pattern evolution may be favorable for another episode of heavy rainfall from the south-central Plains to locations at least as far east/northeast as the Ohio Valley and central Appalachians. Some areas will be highly sensitive to any significant rainfall given ongoing flooding. Compared to yesterday the solution spread is not quite as dramatic for the upper low near northern Ontario on Fri, but models still differ enough to cause havoc with the forecast of frontal progression across the northern U.S. Even the ensemble means have adjusted somewhat, holding the feature farther west than indicated 24 hours ago. However they have been the most consistent in suggesting that the low/trough aloft would remain far enough north that any corresponding cold front would not push extremely far south over the eastern half of the lower 48--in contrast to one or more runs from various global models over recent days. Based on data available through the 18Z cycle a model consensus transitioning toward a blend of the 18Z GEFS/12Z ECMWF means with lingering input from the corresponding GFS/ECMWF runs provided a good representation of the most likely scenario. This majority cluster would have a general area of low pressure track from the northern Plains toward eastern Canada or New England with a trailing front reaching down to and stalling over the south-central Plains during the weekend into early next week. New 00Z guidance still suggests high uncertainty over the upper low of interest and frontal progression. Over the eastern Pacific/western U.S. solutions continue to diverge after Sat, as weak shortwave energy reaches the coast followed by a strengthening of the eastern Pacific ridge aloft. For forecasts valid around day 5 Sun multi-day trends seem to leaning more in the ECMWF direction versus some previous days' GFS runs that had held back or deflected the energy. Then for the rest of early next week the GFS/GEFS become noticeably deeper than most other guidance with the trough centered over California. Teleconnections relative to the agreeable core of positive height anomalies off the Pacific Northwest in latest D+8 multi-day means favor the weaker trough seen in the non-GFS majority. Thus the aforementioned blend tilts a bit in the ECMWF/ECMWF mean direction, but more toward its mean as the operational run may build the Pacific ridge a little far eastward. ...Sensible Weather Highlights/Threats... It will take more time to determine the details but guidance as a whole continues to signal a threat for heavy rainfall from the south-central Plains northeastward, with the most probable locations for highest five-day totals between Oklahoma/southeast Kansas and the Lower Ohio Valley. Some significant amounts may extend into the Mid-Atlantic/Northeast. Some areas within the potential threat area will be very sensitive to additional rainfall due to ongoing flooding or recent wet conditions. Multiple ingredients should contribute to areas of enhanced rainfall--including establishment of a multi-day period of low level flow from the Gulf, potential stalling of a wavy northeast-southwest oriented front, and shortwaves aloft progressing out of the West. While over the western states these shortwaves will encourage episodes of showers and thunderstorms of varying intensity from the Great Basin and possibly Sierra Nevada into the northern-central Rockies. The Florida peninsula should see diurnally enhanced showers/storms for most of the period. Expect above normal temperatures to persist over a majority over the West through the period. The primary axis of plus 5-15F anomalies for highs should extend from the northern half of California into the interior Northwest/extreme northern Rockies. Coverage of similar anomalies for morning lows will likely be more expansive. The eastern U.S. will see a cool Friday with some areas 5-15F below normal. From the weekend into next week most areas over the eastern half of the country should see readings within a few degrees on either side of normal. Some low-confidence pockets of greater anomalies (mainly for warm lows or cool highs) will depend on location of clouds/precipitation on any particular day. 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