Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Wed Aug 01 2018

Valid 12Z Sat Aug 04 2018 - 12Z Wed Aug 08 2018

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

The overall overall has not changed with the eastern Pacific
trough sitting offshore of the Pacific Northwest. In addition, a
trough will develop over the northern Plains and move eastward
over the Great Lakes/Ohio Valley and a strong upper ridge
dominating the western U.S. 

Models have begun to converge on a solution dealing with the upper
low/trough over the eastern Pacific in the first part of the
medium range. The 00Z ECMWF has began to trend toward the 00Z/06Z
GFS and 00Z CMC with regards to the orientation of the closed
upper low through the medium range--being much farther south and
west than in previous runs.  In fact, the GFS, ECMWF, CMC and
UKMet have all shown the trend toward a closed upper low remaining
well off the West Coast through day 7. However, the ensemble means
are consistently showing a trough off the West Coast and closer to
the coast--with the 00Z ECENS hinting at trending toward a closed
low.  WPC used a blend of the previously mentioned 00z runs of the
GFS/ECMWF/CMC/UKMet in the beginning of the medium range period as
the agreement overall was good.  By the end of the period, used
more of the 00Z ECENS and 00Z NAEFS to incorporate both the CMC
mean and GEFS to take into account the uncertainty with the
handling of the upper low/trough. 

The forecast in the eastern U.S. continues to be less sensitive.
The weakening trough over the Mississippi Valley will weaken early
on in the period as the Bermuda High builds across the Southeast. 
The upper ridge will shift and remain over the Southwest--helping
to keep the eastern Pacific closed low/upper trough from moving
onshore. There will also be a transition of southwesterly flow
across south-central Canada/northern Plains to an eastern North
America mean trough that ends up suppressing Atlantic ridging that
builds into the Southeast. The uncertainty for this region of the
forecast remains with the shortwave impulses that impact surface
fronts/waves. The selected blend made for the upstream forecast
provided a reasonable starting point for depicting the most common
elements of the pattern evolution over central/eastern parts of
the country.


...Weather Highlights/Threats...

Because of the position of the upper low/trough in the eastern
Pacific the guidance has began to hint at, this will hint at a dry
trend for the Pacific Northwest as a lot less moisture will be
brought into the region.  For now, much lighter rainfall for this
area--with the possibility of being completely dry. The ensemble
means still support light amounts of rain in the area given the
positions of the trough. Due to this uncertainty, the forecast for
temperatures will also be uncertain--with the possibility of
cooler temperatures with a more active weather pattern or warmer
daytime temperatures if the dry trend should persevere.

Elsewhere, expect diurnally driven showers/thunderstorms along the
terrain in the Southwest/Four Corners region through most of the
period.  From the weekend into early next week a mean frontal
boundary with embedded waves will be a focus for periods of
locally heavy/strong convection from the northern Plains into the
Great Lakes. Toward midweek the gradual development of upper
troughing over the East should bring organized activity into the
Appalachians and Mid-Atlantic.  Before that occurs, the Bermuda
High that will build over the Southeast will promote a drier trend
over the East after a wet short range period. However, diurnally
driven scattered thunderstorms can still be expected. Areas along
the Gulf Coast may see one or more days of rainfall as well. Above
normal temperatures will be fairly widespread east of the Rockies.
 Warmest readings versus normal should exist for morning lows from
the upper MS Valley into New England, with some pockets of very
warm highs from portions of the Plains into Northeast on one or
more days.


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