Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
124 AM EDT Sat Jun 09 2018
Valid 12Z Tue Jun 12 2018 - 12Z Sat Jun 16 2018
...Pattern Overview...
Upper flow across North America during the medium range is
expected to show a large degree of blocking, particularly at the
higher latitudes. An anomalous upper ridge is expected to be in
place on day 3 (Tue) from the Upper Great Lakes north across
Hudson Bay. As shortwave energy traversing the westerlies across
North America undercuts this ridge, the main anomaly center should
focus near/just northwest of Hudson Bay by the middle of next
week. As with any highly anomalous blocking pattern, a significant
question exists as to how long the setup will persist, and model
solutions continue to vary in that respect. As long as the block
persists, a relatively active flow regime will exist from the
Pacific Northwest eastward into the Midwest, Great Lakes, and
Northeast, south of the blocking upper high. A couple such
shortwaves are expected to cross the Midwest/Great Lakes/Northeast
Tue-Fri. Additionally, models show consensus that an upper wave
should reach the Pacific Northwest by next Wed, with some degree
of height falls pressing into the interior western U.S. through
the end of next week. Meanwhile, an upper ridge is also expected
to remain anchored across the south central U.S., varying in
strength throughout the period.
...Model Guidance/Predictability Assessment...
The WPC forecast was initially based on a heavily deterministic
blend including the 12Z ECMWF/GFS during days 3-4, with increasing
weight placed toward the 12Z ECENS/NAEFS ensemble means from day 5
onward.
Models showed some significant differences and run-to-run
variability throughout the period. The 18Z GFS was much weaker
with a shortwave initially across the Mid-Atlantic on day 3, and
was rejected in favor of the 12Z run which was more similar to the
consensus including the ECMWF and the ensemble means. With
shortwave energy crossing the CONUS northern tier days 3-5,
solutions vary widely due to complex interactions with Arctic
energy rounding the eastern side of the anomalous ridge. The GFS
has been persistent in amplifying a trough across the Northeast by
days 6-7, while the ECMWF shows a similar scenario but is a bit
weaker and quicker to move the feature east. The 12Z GFS was
probably the most similar to the ECMWF in that respect. With the
upper energy reaching the West on day 4, the ECMWF also a bit
weaker and quicker to push the wave into the interior West by day
6. The GFS has been rather variable with the 18Z run even keeping
the system along the West Coast as a retrograding upper low into
day 7, while the 12Z run seemed to be somewhat of a reasonable
middle ground solution (and was relatively close to the ensemble
mean based consensus). Given the unstable nature of this flow
pattern with a high degree of blocking, predictability is low and
no scenario can really be ruled out, especially by days 6-7.
Finally, the GFS continues to develop an area of low pressure in
the Gulf of Mexico by the middle of next week more so than any
other deterministic solutions. The ECMWF and CMC both continue to
show a much weaker and broad area of disturbed weather drifting
westward across the Gulf. At this time, there is a lack of support
for a GFS-like solution among ECENS and CMCE ensemble members.
...Weather Highlights/Hazards...
Scattered to numerous showers and thunderstorms are expected
Tue-Wed from the Mississippi Valley east to the Eastern Seaboard
along a lingering surface frontal boundary and ahead of an
approaching cold front moving from the Midwest on Tue to the
Northeast on Wed. Areas of heavy rain appear possible, but
confidence in the location of any heavy rain is low. As the cold
front moves off the East Coast by Thu, the trailing frontal
boundary will linger across the Ohio Valley and central plains,
where it will continue to focus areas of convection through the
end of next week. The potential shortwave crossing the northern
Rockies by next Fri-Sat may further enhance the
shower/thunderstorms potential across the northern plains.
Temperatures are expected to be well above average through the
middle of next week from the Great Basin east across the central
U.S., where high temperatures may range from 5 to 15 deg F above
average. The arrival of lower heights into the western U.S. by
Fri-Sat will bring temperatures down closer to average values for
much of the West, but expect hot conditions to continue for much
of the central U.S. as the upper ridge persists.
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