Prognostic Meteorological Discussion
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FXUS02 KWBC 171607

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1206 PM EDT Sat Aug 17 2019

Valid 12Z Tue Aug 20 2019 - 12Z Sat Aug 24 2019

...Overview and Guidance/Predictability Assessment...
Recent global guidance consensus is for phasing low pressure to
rapidly develop Monday night into Tuesday in the northeast Pacific
and shift into southeast AK Tuesday night. The only runs to not
agree with this is the 00Z CMC and 12Z/16 GFS which are considered
outliers. The 00Z UKMET is slower with this system and is a bit of
an outlier despite developing a similar deep low. This system is
key to the overall patter as it will promote amplification of the
downstream ridge which will extend over the western Canadian
Prairies and northern U.S. Rockies. Downstream of this ridge is a
deep trough and shifts along the south side of Hudson Bay Tuesday
into Thursday before shifting east to Labrador through Friday
night. There is good agreement among global guidance on the upper
trough south of this low that pushes east across the Great Lakes
Wednesday to across the Northeast by Friday. An associated cold
front reaches the East Coast Thursday and stalls over the
southeast Friday and Saturday. The trailing part of the front will
likely stall over the south-central Great Plains Thursday before
lifting back east/northeast as a warm front Friday into Saturday.
An atmospheric River south of the northeast Pacific low will
diminish as it reaches the Pacific Northwest ahead of a cold front
Wednesday. To the south of the main band of westerlies, expect
initially strong Rockies/Plains ridging to weaken gradually/become
more zonal after midweek.

The main differences in deterministic guidance (besides the
northeast Pacific low mentioned above) are with the timing of the
eastern Canadian low as it traverses Labrador Day 6/7, so ensemble
means were preferred for Friday/Saturday. Shortwave details are
noted around these main lows, though the overall pattern is
maintained well through Day 5. A very limited CMC/UKMET general
deterministic blend that greatly favors the GFS/ECMWF was chosen
for Days 3-5 with similar ensemble means warranting greater
inclusion for Days 6/7. Ensemble spread by late in the period and
the low amplitude of expected mean flow yield low confidence in
shortwave details across the Northwest/northern Plains and eastern
Canada by Fri-Sat.

...Weather Highlights/Threats...
There may be pockets of locally heavy rainfall in the eastern U.S.
with the cold front moving from the northern Plains through the
East Tuesday to Thursday, but frontal progression should keep
totals from being too extreme. However the central Plains look to
have more potential for areas of enhanced rainfall due to the
trailing end of the front stalling over the High Plains and then
returning as a warm front. Eventual deceleration of the front over
southern latitudes of the East may lead to more rainfall over
parts of the southern Mid-Atlantic/Southeast as well. Expect
diurnal convection with generally localized heavy amounts over the
central/eastern Gulf Coast region. The front reaching the Pacific
Northwest Wed-Thu may looks to only bring brief rainfall to
northern areas.

The cold front pushing through the central/eastern states will
bring a noticeable cooling trend, with some highs up to 10F or so
below normal over the northern Plains Tuesday to Thursday and for
the northeast on Friday. The hot weather ahead of the front will
persist over central U.S. into Tue and over the East into Wed. A
majority of the central-eastern U.S. should see temperatures
within a few degrees of normal by Sat. Expect above normal
readings to prevail over most of the western U.S. through the
period. The greatest anomalies (with some record highs possible
over the Southwest) looks to be on Wed.


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:

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

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