Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sat Jul 07 2018
Valid 12Z Tue Jul 10 2018 - 12Z Sat Jul 14 2018
...Pattern overview/Guidance evaluation/Preferences...
A mid/upper-level ridge will persist across the western/central
U.S. through the period. While 500-mb anomalies should weaken
from the expected 2 to 2.5 sigma values early in the week, the
anticyclone will most likely set up across Four Corners/Great
Basin into south-central portions of the Plains by Days 6/7, July
13/14. The northern periphery of the mean ridge should flatten
with time due to an active pattern involving a pair of amplified
systems tracking 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. Their trailing fronts should extend into the
Northern Tier states. Meanwhile the National Hurricane Center is
issuing advisories for what is currently Tropical Depression Three
over the western Atlantic, with strengthening anticipated over the
coming days while the system initially stalls and then tracks
northeastward offshore the East Coast. Consult NHC products for
latest information on T.D. Three. An elongated upper trough that
helps to steer this tropical cyclone out to sea should linger over
the eastern U.S. into late week before weakening/moving offshore.
Early in the period 00Z-06Z GFS runs and the 00Z ECMWF/CMC offered
good agreement with significant features (aside from T.D. Three)
and had good support from the ensemble means, so those solutions
provided the basis of the forecast into Day 5 Thu. At that time
guidance began to diverge in response to differences that develop
with North Pacific/Alaskan flow. Of particular note the 06Z GFS
became out of sync relative to the 00Z GFS/ECMWF/CMC and their
ensemble means so the updated blend removed that run after Thu.
Meanwhile by Fri-Sat consecutive operational model runs and
individual ensemble members suggest decreasing detail confidence
within a pattern that at least shows a general theme of broadly
cyclonic flow over the western half of Canada and extending into
the Northern Tier U.S. As a result the forecast trended much more
toward the 06Z GEFS/00Z ECMWF means late in the period. Latest
guidance consensus shows the front that drops into the eastern
U.S. Tue onward pushing farther south than prior forecast as well
as persisting for a longer time before weakening. There is also
more of a signal for some degree of waviness (frontal or
otherwise) near the southern Mid Atlantic coast during the latter
half of the period.
...Weather threats/highlights...
A majority of the country can expect above average temperatures
next week, with the primary exception being 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, keeping high
temperatures several degrees below climatology for much of the
period. The eastern U.S. cold front and trailing high pressure
should bring a couple days of near to slightly below normal
temperatures to the Northeast/Mid Atlantic around midweek. Expect
the Interior West into northern Plains to see well above normal
readings Tue-Wed ahead of the first front progressing into/across
the Northern Tier. Then the Pacific Northwest will see a
pronounced warming trend by Wed-Thu ahead of the next front. In
both cases some plus 10-20F anomalies are possible. In terms of
precipitation, the most organized and persistent area of
convection will be via the active monsoonal flow over and near the
Four Corners states while diurnally forced activity will occur
across the southern U.S. The cold front pushing into and
eventually stalling over the eastern U.S. may add some focus for
convection. To the north, some locally heavy convection is
possible with the front crossing the northern Plains/upper MS
Valley around Tue-Wed with amounts tending to be lighter farther
south/east of this region. There may be another period of
convection over the Northern Tier late in the period as the
trailing part of the front returns northward and the second front
from western Canada approaches.
Rausch/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