Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
146 AM EDT Thu Aug 16 2018

Valid 12Z Sun Aug 19 2018 - 12Z Thu Aug 23 2018

...Pattern Overview and Preferences...

Early in the medium range, the guidance continues to agree on an
amplification of the large scale pattern toward a central/eastern
North America trough, and a western U.S./Canada ridge. This
evolution is in response to a deep upstream upper low/trough which
develops south of Alaska. Late rin the medium range towards the
end of next week, the patternd eamplifies with reduction in the
amplitude of the eastern Pacific trough, reduction in the
amplitude of the northwest ridge, and reduction in amplitude of
the eastern US trough.

A shortwave aloft moves across the Great Plains this weekend and
spins up a defined surface low pressure  over the central U.S.
which lifts into the upper Great Lakes by Day 5/Tuesday 21 Aug.
Low rpessure then departs into Canada Wed with the trailing front
crossing eastern states to the east coast.
The models are in good agreement on the development of low
pressure but intensity differences remain.  The 12-18z GFS runs
were a bit further south and weaker than the 12z Canadian ECMWF. 
The ECMWF/GEFS/Canadian ensmeble means persist in having better
agreement on the low possitions and intensity through the
forecast.  More weighting was given to the ensmeble means of the
18z GEFS and 12z ECMWF as a result, with low weighting of the
operational 12z ECMWF and Canadian to boost the intensity of the
low pressure intensity a bit.

Out west, the guidance initially centers the upper ridge over the
Southwest. There is some question in the details of a 500 mb
shortwave crossing through the Pacific Northwest, northern
Rockies, and northern Great Basin early in the week. Each of the
ECMWF/Canadian/GFS have a different amplitude and the models are
not quite in synch with the means, so a multi-model/ensemble mean
blend was used until better agreement develops on the evolution of
the wave. The consensus is once the wave moves east it deamplifies
as it crosses the intermountain west and it moves into confluent
flow.

The mid level ridge becomes resatablished over the northwest by
next Thu with the approach of an east Pacific trough.   

...Weather Highlights and Hazards...

Shortwave energy that reaches the Plains on Sunday will provide
focus for organized convection and locally heavy rainfall from the
High Plains to the mid-Mississippi Valley this weekend. Low
pressure moving northeast from the central Plains/MS Valley
towards the Great Lakes brings with it likely showers and storms,
with precip also likely along the trailing cold front moving
through the lower Lakes/Appalachians/Southeast centered on
Tuesday.  Monsoonal moisture into the Southwest drives diurnally
driven showers and storms across the region and into the Rockies,
with the best chance for highest rainfall totals over parts of
Arizona/New Mexico due to greater available moisture.

The core of the coolest highs (10 to 15 degrees below normal) is
forecast to be centered over the north-central Plains
Sunday-Monday. Meanwhile, temperatures over the Interior West
should trend gradually warmer with time as the ridge aloft builds,
with daytime highs of 5 to 10+ degrees above normal likely.

Petersen

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