Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Wed Aug 15 2018

Valid 12Z Sat Aug 18 2018 - 12Z Wed Aug 22 2018

...Pattern Overview and Preferences...

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.
Initially, the medium range period starts out with an elongated
trough across the Eastern U.S., kicking a surface low out of New
England, and a trailing front dropping through the Mid-Atlantic.
Meanwhile, a shortwave aloft emerges into the High Plains this
weekend and spins up a defined surface wave over the central U.S.
which lifts into the upper Great Lakes by Day 6/Tuesday. Ridging
building through the period over the Western U.S. which possible
lobes of upper level energy streaking through.

While there is relatively good agreement through the entire period
on the overall synoptic setup, there remains questions with
individual details, particularly regarding the well defined
surface low lifting through the north-central states. The 00z
ECMWF continues to advertise the strongest/deepest solution (a
995mb low over the upper Great Lakes on Day 6), one of which, if
it were to verify, could be record low pressure for parts of the
upper Great Lakes. The remainder of the guidance is varying
degrees weaker, with the 06z GFS coming in the flatest/weakest.
Until a more clear path is paved here, the preference continues to
be a general model blend between the latest GFS/ECMWF/CMC, which
results in something in between the two extremes and a solution
very close to that of the latest ensemble mean guidance and WPC
continuity.

Out west, the GFS/ECMWF/ensemble means initially center the high
amplitude ridge over the Southwest, with likely building into the
rest of the Western states as high amplitude ridging drifts inland
from the east Pacific. There is some question in the details of a
shortwave crossing through the Pacific Northwest in the middle of
the period, with the latest run of the CMC advertising a deep
closed low centered over the region by day 7. The deterministic
GFS/ECMWF and their ensemble means are much more modest with this
shortwave as it races eastward into the northern Intermountain
West and sort of becomes lost/absorbed within the amplifying ridge
over the East. Thus the preference for this system was a non-CMC
blend. 

...Weather Highlights and Hazards...

Showers/storms across the Northeast at the end of this week moves
offshore by the weekend, but should continue across the
Carolinas/Tennessee Valley as the trailing front eventually lifts
northward as a warm front. Shortwave energy that likely 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, modest precip also likely along
the trailing cold front moving through the lower
Lakes/Appalachians/Southeast by the end of the period. 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 large scale troughing in the central U.S. will bring an
expanding area of below normal temperatures, with the core of the
coolest highs (10 to 15 degrees below normal) 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 possible.

Santorelli

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