Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Wed Jun 12 2019 Valid 12Z Sat Jun 15 2019 - 12Z Wed Jun 19 2019 ...Overview and Guidance/Predictability Assessment... The models and ensembles remain fairly similar and consistent with the large scale pattern evolution from the weekend through the first half of next week. From early Sat into Mon expect low amplitude broadly cyclonic mean flow over the lower 48, supported by an upper ridge that builds into western Canada and mean ridging over the eastern Pacific (consisting of a leading ridge initially nearing the West Coast and a much stronger ridge that builds farther west). Once this stronger ridge becomes established it appears likely that troughing will amplify over the west-central U.S. by next Tue-Wed. Guidance continues to show a heavy rainfall potential that is most pronounced from the south-central Plains into the Ohio Valley and central Appalachians--along and south of a front that settles into a northeast-southwest orientation over the eastern two-thirds of the country from Sun into early next week. Some areas within this threat region will be very sensitive to any significant rainfall in light of ongoing flooding or wet ground. Aside from still-oscillating timing for the wave/frontal system crossing the Lower Great Lakes/New England, the updated forecast maintained reasonable continuity by starting with an operational consensus on day 3 Sat and then incorporating gradually more 06Z GEFS/00Z ECMWF ensemble mean input such that day 7 Wed consisted almost entirely of the means. The combination of the upper low over the eastern half of Canada and individual smaller scale shortwaves tracking along the U.S.-Canada border continue to cause difficulties in resolving specifics at the surface over the northeastern quadrant of the lower 48. On the positive side, solutions are slowly becoming less divergent for the upper low itself. However the shortwaves are fairly small in scale and it may take well into the short range time frame to achieve good clustering and continuity. Latest 06Z/00Z GFS runs were still faster than most other models and the GEFS means with a leading surface wave Sunday, so opted to stay nearer to the larger consensus around the 00Z ECMWF or 06Z FV3-GFS (the last run in parallel before the switch with the 12Z cycle today). Even with these differences and variability there is still good agreement in principle that the trailing front will stall over the central U.S. by Sun and possibly the Northeast toward midweek. Farther west, in the Sat-Tue time frame, typical depth/timing differences arise for the upper trough expected to deepen over the west-central U.S. days 6-7 Tue-Wed. Relative to typical errors at that time frame the differences do not appear great enough to favor one side of the spread or the other for timing. An additional complication that comes into play is the upper low tracking just south of Alaska, with the ECMWF and its ensembles generally quicker than the GFS/GEFS members. Each ensemble system has trended stronger with the Pacific ridging off California, but its shape differs between the ECMWF ensemble and GEFS mean--the former being elongated east-west while the latter oriented more as a circle. Without some conflicting trends downstream, preference was to trend in between the means late in the period to lean away from the various low-confidence specifics offered by individual model runs. The deepening upper trough should help to push the stalled Plains boundary northward as a warm front toward the latter part of the period. ...Sensible Weather Highlights/Threats... For the time being not a lot has changed with the heavy rainfall threat from the south-central Plains into the Ohio Valley/Appalachians, aided by moist flow from the Gulf interacting with a wavy/stalling northeast-southwest front and multiple shortwaves embedded in flow aloft. The best consensus of guidance maintains highest potential for greatest five-day totals from Oklahoma/southeast Kansas into the Ohio Valley. There are some hints that this favored area could extend into parts of the Lower Great Lakes but historically northern biases for some convective events suggest less confidence in that idea. More localized significant activity may extend into the Mid-Atlantic/Northeast. The overall forecast over the West is fairly consistent as well, with shortwaves aloft promoting showers/thunderstorms that should produce highest totals in the vicinity of the northern-central Rockies. Some activity could extend back into parts of the Great Basin/Sierra Nevada. Specifics of flow aloft become more uncertain toward next Tue-Wed, thus lowering confidence in location/coverage of rainfall by then. Energy that amplifies into the west-central U.S. should begin to spread more showers/thunderstorms into the northern-central Plains from later Mon into Wed. The Florida peninsula will likely see diurnally-enhanced convection through the period. The West will continue to see above normal temperatures through the weekend with plus 5-15F anomalies for highs extending from the northern half of California into the interior Northwest/extreme northern Rockies. Such anomalies for morning lows should cover a greater portion of the West. Expect gradual moderation in temperatures next week, especially over the Northwest. The developing west-central trough aloft may bring below normal temperatures to the northern/central Rockies by midweek. Over the eastern half of the country readings should be fairly close to normal for highs but near to above normal for lows. Depending on the exact surface pattern parts of the Mid-Atlantic could see one or more days of above normal highs. Greatest uncertainty in temperatures next week will be in the West due to the timing of height falls and eastward extent of the upper ridge perhaps into California rather than remaining offshore. Fracasso/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