Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Tue Jul 31 2018

Valid 12Z Fri Aug 03 2018 - 12Z Tue Aug 07 2018

...Pattern Overview and Guidance/Uncertainty Assessment...

Guidance continues to be in agreement with the overall pattern: a
strong upper ridge south of the Aleutians will lead to an
amplified trough over the eastern Pacific this weekend. However,
models still have to work out the details with respect to this
trough.  Towards the east, a weakening trough across the
Mississippi Valley will eventually be eroded by the Bermuda high
building across the Southeast which will drift toward the Plains
by early next week. Because of this set up, most of the jet energy
will flow across the the Northern Tier states from the Pacific
Northwest to the Great Lakes and southern Canada.

In terms of the eastern Pacific trough, models are in agreement
through the weekend the evolution of a mid-level closed low
slipping southward along western British Columbia this weekend. 
By Sunday and into Monday, the 00Z ECMWF begins to split off with
what the 06Z GFS and 00Z CMC show--with an elongated trough over
the West Coast of Canada and into the Pacific Northwest. The 00Z
ECENS and 00Z GEFS trend toward both the 06Z GFS and the 12Z ECMWF
from yesterday. Consequently, by Tuesday, the 00Z ECMWF does not
have a ridge building across west-central Canada and does not show
a trough pushing across the Great Lakes. The 00Z ECENS and 00Z
GEFS show this evolution as does the 06Z GFS and the older (12Z)
ECMWF.   

Due to the zonal nature of the mean flow along the northern U.S.
and southern Canada, the detailing with the shortwaves aloft is
still difficult to resolve. Models still show a disturbance
skirting across the Plains this weekend that will absorb into the
trough along the Great Lakes and then diverging later on in the
period.  However, guidance in general is showing that by late in
the period and beyond some of this trailing energy will ultimately
settle into a modest eastern North America trough mostly aligning
in the Upper Midwest/Great Lakes region that would be consistent
with the pattern upstream.

For the WPC blend, because of the eastern Pacific trough and the
handling of the upper ridge building across western Canada/trough
along the Great Lakes, used the 06Z GFS/00Z ECMWF and a small
sample of their means on day 3 and 4; by day 5 and beyond,
weighted more on the means and used the 12Z ECMWF.


...Weather Highlights/Threats...

The amplifying upper trough off the West Coast will likely bring
more moisture across the Pacific Northwest. An increase of
rainfall will occur early in the week and into Tuesday. Farther
south, diurnally driven thunderstorms along the terrain in the
Southwest/Four Corners region can be expected throughout the
medium range period. In the East, the wet pattern continues across
the Southeast, Mississippi Valley and into the Northeast due to
the weak trough along the MS Valley. Once the Bermuda high builds
across the Southeast, precipitation will become more scattered in
lieu of widespread and organized. Convection is also expected
across the Plains and Mississippi valley with the energy moving
across the Plains. 

Overall, high temperatures will be below normal for the Pacific
Northwest and along the Southeast into the Mid-Atlantic for
beginning of the weekend due to active weather during that time
period. High temperatures will be above average for the northern
and central Plains along with the Great Lakes and Northeast. Most
of the country will be near or above normal later on in the
weekend and into early next week. New England could see high
temperatures over 10 degrees above normal by Sunday and Monday.
The Pacific Northwest will continue to be much cooler than normal
as the trough moves across the region.


Reinhart/Rausch


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml