Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Jul 07 2018
Valid 12Z Tue Jul 10 2018 - 12Z Sat Jul 14 2018
...Pattern overview/Guidance evaluation/Preferences...
A persistent mid/upper-level ridge across the western/central U.S.
will remain fixed over the region while flattening in time in
response to amplification along the international border with
Canada. While 500-mb anomalies should weaken from the expected 2
to 2.5 sigma values early in the week, the anticyclone will likely
set up across Four Corners into the Southern Plains by Day 6/7,
July 13/14. To the north of this positive height anomaly, an
active pattern is anticipated as a pair of amplified systems track
from the Pacific Northwest/southern British Columbia toward Hudson
Bay. Each will likely be fairly impressive in nature with
attendant deep surface cyclones tracking through central Canada.
Ultimately this will have the influence of flattening the northern
extent of the prevailing ridge across the western/central states.
Meanwhile, what is expected to become Tropical Storm Three should
gradually migrate northeastward away from the Gulf Stream after
stalling during the short range period. The parent elongated upper
trough which helps steer this tropical cyclone out to sea which
linger over the eastern U.S. through Thursday before moving
offshore.
Overall guidance has been struggling with the handling of the
tropical cyclone with a few random solutions taking the system
toward the Eastern Seaboard. However, the 00Z guidance has shifted
eastward which more closely mimics the forecast from the National
Hurricane Center. Considering the remainder of the country, models
have bounced around a bit with the timing of the lead system
tracking across central Canada mid-week although they agree on the
evolution of a rather defined surface low. In its wake, the
guidance has trended deeper with the trailing upper trough which
accelerates into British Columbia/Alberta by Friday with some
southward extension into the northwestern U.S. At this point, the
ensemble means sufficiently smooth out the pattern but are in
agreement in maintaining the tightest height gradient along the
international border with Canada. In terms of preferences,
generally followed a combination of the 18Z GFS/GEFS mean, 12Z
ECMWF/ECMWF ensemble mean, and 12Z NAEFS mean with decreasing
operational influences deeper into the forecast period.
...Weather threats/highlights...
While a vast majority of the country can expect above average
temperatures next week, the exception will be the Desert Southwest
eastward into the Southern Plains. An active monsoonal flow regime
should maintain enhanced cloud cover and widespread
showers/thunderstorms across the Four Corners. This should keep
numbers several degrees below climatology for much of the period.
Meanwhile, the most pronounced warmth will likely focus over the
Pacific Northwest starting Wednesday with widespread 90s expected
inland with cooler readings closer to the coast. In terms of
precipitation, the most organized area of convection will be via
the active monsoonal flow while diurnally forced activity is
expected across the southern U.S. To the north, some frontal
precipitation is possible over the north-central states in
response to warm advection ahead of the next potent shortwave.
Rubin-Oster
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are 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
http://www.wpc.ncep.noaa.gov/heat_index.shtml