Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Sun Jul 08 2018
Valid 12Z Wed Jul 11 2018 - 12Z Sun Jul 15 2018
...Pattern overview/Guidance evaluation/Preferences...
Latest models and ensembles continue to show a broad area of mean
ridging across the central and western U.S., with the possibility
of an embedded weakness for a time in the vicinity of the central
Rockies/High Plains. To the east of the ridge expect an upper
trough to support a fairly far southward extent of a cold front
that should eventually stall and dissipate, while over the
Atlantic the upper trough should encourage northeastward
progression of Tropical Storm Chris. There has been a stray
solution or two (such as the 00Z UKMET) that show greater effects
on the East Coast but thus far such a scenario has very low
probability. Consult NHC products for latest information on T.S.
Chris. As the eastern upper trough lifts away, guidance is
hinting that some weak energy could be left behind near the East
Coast along with a modest lingering surface reflection consisting
of a trough with one or more embedded waves. Meanwhile the
consensus forecast led by the ensemble means has maintained good
continuity in principle for two vigorous Canadian systems whose
fronts will extend into the northern third to half of the
contiguous U.S. Compared to yesterday guidance clustering has
improved for North Pacific/Alaska flow compared to yesterday so
latest operational runs are generally closer to the means from the
northeastern Pacific into western North America in the late
week/weekend time frame. By day 7 Sun the only items of note are
that the 00Z GFS leans toward the amplified side of the envelope
for northern U.S. troughing aloft while the 00Z ECMWF/CMC bring
more shortwave energy into the West than most other solutions.
Given the better than average agreement in today's guidance, the
latest forecast blend utilized a majority weight of operational
models (00Z-06Z GFS and 00Z ECMWF/CMC) versus 00Z GEFS/ECMWF means
through day 5 Fri and then trended toward an even model/ensemble
blend among GFS/ECMWF runs and their means. The 00Z GEFS was
selected over the 06Z run due to better definition of the second
Canadian system by day 7 Sun but otherwise the 06Z run provided a
comparable forecast.
...Weather threats/highlights...
Expect three general areas of focus for significant rainfall
during the period. Over and just west of the Four Corners states
an active monsoonal flow (with precipitable water values well
above climatological averages) will promote periods of showers and
thunderstorms with locally heavy amounts possible. The area
extending from the northern Plains into the Midwest/and upper
Great Lakes will first see episodes of convection along a leading
cold front crossing the Northern Tier. This front may continue to
serve as a focus for rainfall as it stalls over the north-central
Plains/MS Valley, with more activity to follow as the front lifts
back to the north and a second wave/cold front approach by next
weekend. Meanwhile locations over the South will see diurnally
enhanced convection with some enhancement provided by localized
boundaries and the eastern U.S. front that should dissipate by the
latter half of the period.
Clouds and rainfall should keep high temperatures below normal
over the Southwest/Four Corners region through the period while
the second northern U.S. front will bring a push of cool air into
the northern Plains by Sat-Sun. Otherwise the ridge aloft over
the central/western U.S. will tend to keep readings above to well
above average. The interior Pacific Northwest and parts of
California/Nevada are likely to see highest anomalies exceeding
plus 10F for min and/or max readings on multiple days. Surface
high pressure behind the eastern U.S. front will bring a couple
days or so of near or slightly below normal temperatures. Expect
eastern U.S. temperatures to trend somewhat above normal by next
weekend.
Rausch
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