Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Mon Jun 18 2018
Valid 12Z Thu Jun 21 2018 - 12Z Mon Jun 25 2018
...Guidance/Predictability Assessment...
Models show broad agreement on the large scale flow pattern across
the CONUS during the medium range, with a modestly progressive
pattern, but with at least some continued tendency for at least
occasionally blocked flow and amplification. An upper ridge across
central Canada persisting through the forecast period will support
trough amplification across the northeastern U.S. (initially on
Thu and then again by early next week), as well as across the
Pacific Northwest. Meanwhile, energy crossing from the Rockies
into the central U.S. will see potential for deepening and/or
cutting off, moving through a region of relatively weak upper flow
beneath the Canadian ridge.
The WPC forecast during days 3-5 (Thu-Sat) was based on a majority
deterministic blend including the 00Z ECMWF/06Z GFS. These
solutions represented consensus will with respect to the trough
lifting out of New England and the upper low moving from the
central plains toward the Mississippi Valley before transitioning
to an open wave across the Great Lakes. This blend also
sufficiently represents additional shortwave energy crossing the
northern Rockies Thu-Fri.
Model differences begin to grow after 12Z Sat as energy crosses
the Rockies and moves toward the plains. The GFS has been
relatively consistent in trying to close the energy off either as
it crosses the Rockies or as it nears the High Plains Sun/Sun
night. The ECMWF on the other hand has generally kept a more
progressive solution, but with more significant run-to-run
variability. Given the amplified nature of the flow across the
higher latitudes from the North Pacific across Canada, the pattern
seems, at least conceptually, to also favor greater potential for
shortwave energy to close off across the CONUS. The ensemble means
also seem to support this idea, with a significant number of
NAEFS/GEFS/ECENS members showing a solution at least somewhat
closer to the GFS (the 00Z ECENS mean was slower and more
amplified than the deterministic ECMWF). Farther downstream,
additional uncertainty enters the forecast with respect to trough
amplification across eastern North America by Sun-Mon. The 00Z/06Z
GFS runs were highly amplified by Mon, with significant height
falls getting into New England. The ECMWF was similar but a bit
less amplified than the GFS. Ensemble means showed good consensus
at least with respect to the general timing of any trough
amplification, however, with the main differences pertaining to
the amplitude. Given the increased uncertainty, weighting of
ensemble means (00Z ECENS/NAEFS) was boosted to a slight majority
of the forecast weighting during days 6-7. Among the deterministic
components, the 06Z GFS was weighted a bit more relative to the
00Z ECMWF, to support at least somewhat more amplification of
features.
...Weather Highlights/Threats...
A frontal boundary with multiple waves of low pressure will
persist from the central U.S. into the Ohio Valley and
Mid-Atlantic through much of the forecast period, promoting
scattered to numerous showers and thunderstorms, with areas of
heavy rainfall potential. Meanwhile, above average temperatures
will continue for much of the western U.S. as well as the
Southeast, with high temperatures 5 to 10 deg F above average
possible across these areas through at least a portion of the
medium range. The largest high temperature anomalies are expected
across portions of California and Oregon Fri-Sun, where highs may
reach 10 to 15 deg F above average.
Ryan
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4