Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 AM EDT Mon Aug 13 2018

Valid 12Z Thu Aug 16 2018 - 12Z Mon Aug 20 2018

...Pattern Overview and Guidance/Predictability Assessment...

The prevailing westerlies should remain confined to the vicinity
of the international border with Canada. Multiple features dipping
southward within this active flow will be key features to hone in
on during the period. On Thursday morning, the guidance show some
combination of shortwave energy lifting out of the Central Plains
and a feature elongating out of the Upper Midwest. A surface wave
attendant to the lead height falls is forecast to gradually deepen
while progressing from the Ohio Valley into New England. Recent
GFS runs remain at odds with other models as they favor one
primary shortwave across the Upper Great Lakes on Thursday.
Additionally, forecast spread is noted with surface low
progression as the 18Z/12Z GFS are slower than the consensus while
the 12Z ECMWF sits well ahead of its ensemble cluster. Regarding
intensity, the 12Z UKMET is easily 5 to 10 mb deeper than most
other solutions by Saturday morning although its most recent 00Z
run did significantly back off on this.

Looking farther west, as a general longwave trough pattern sits
over the northeastern U.S. into the western Atlantic, models show
weak height falls from the Pacific Northwest interacting with
amplification within the northern stream. This should ultimately
lead to some level of amplified flow across the middle of the
country during the weekend. Ensemble spaghetti plots have
routinely favored general west-northwesterly flow aloft over this
region with varying degrees of amplification suggested in several
members. By Day 5/Saturday onward, the 12Z GFS does stand out as
being the most amplified among the 90 ensemble member comparison.
What ever feature emerges within the flow regime should advance
downstream with a position near or east of the Mississippi River
by Day 7/August 20 as suggested by ensemble means. In its wake,
heights significantly build across western North America with the
00Z GFS 500-mb height anomalies around 2 to 2.5 sigma above
climatology across the Pacific Northwest into British Columbia. It
is on the stronger side of the envelope with the ECMWF ensembles
depicting a weaker ridge, generally around a 1 sigma departure.
While outlying in nature, it is still worth noting the 00Z/12Z CMC
show a longwave trough in a similar location. Their solution has
nearly zero ensemble support so it can be rejected here.

Through Day 4/Friday, there was enough operational model support
to use an array of solutions, primarily led by the 18Z GFS/12Z
ECMWF with smaller contributions from the 12Z CMC/UKMET. Beyond
this period, the 12Z UKMET grew out of tolerance being deeper with
the northeastern U.S. cyclone while the 00Z/12Z CMC diverged
significantly from ensembles back toward the northeastern Pacific.
Thus, leaned more in the direction of the 18Z GFS/12Z ECMWF with
an increasing influence of the trio of ensemble means, 18Z
GEFS/12Z ECMWF/12Z NAEFS. Confidence throughout the period is
around average with global solutions at least in agreement with
the general synoptic pattern.
 

...Weather Highlights and Hazards...

With the expectation of a deep upper trough pushing across the
Northern Rockies into the Northern/Central Great Plains during the
Day 5-7, August 18-20 time period, much cooler conditions can be
anticipated. With expected highs only in the 60s across the higher
terrain and 70s across the lower elevations, such readings would
be around 10 to 15 degrees below average. Before this takes place,
it should be quite warm across the north-central U.S. through
Friday as high temperatures sit in the low/mid 90s. Elsewhere, a
building upper ridge along the West Coast will allow temperatures
to soar each day with most anomalous numbers over the Pacific
Northwest. By Day 6-7, August 19-20, Portland and Seattle may push
into the 90s which would be 10 to 15 degrees above mid-August
climatology.

Multiple rounds of showers and thunderstorms are likely from the
middle of the country eastward into the Great Lakes and New
England. The best focus will initially be near the lifting surface
wave while vast warm sector convection should prevail to the
south. With the flow expected to amplify this weekend across the
Rockies, robust QPF amounts are noted in many model solutions with
a particular focus across the Central Plains into the Middle
Mississippi Valley. Across the Four Corners region, active
monsoonal flow will aid in daily thunderstorm chances with heavier
amounts potentially over southern Arizona where higher
precipitable water air is to reside.


Rubin-Oster

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