Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EDT Sat Aug 17 2019
Valid 12Z Tue Aug 20 2019 - 12Z Sat Aug 24 2019
...Overview and Guidance/Predictability Assessment...
Guidance agrees that some amplification will likely occur
downstream from a system forecast to be over the northeastern
Pacific as of early day 3 Tue. The upper trough that ultimately
crosses eastern Canada and northeast quadrant of the lower 48
should push a cold front to the East Coast by the latter half of
the week, with the front stalling as it reaches the South. The
trailing part of the front will likely extend back to the High
Plains and then move east/northeast as a warm front. Progressive
Pacific flow extending into the northern CONUS/southern Canada
should bring a front into the Northwest and then the extreme
northern Plains. To the south of the main band of westerlies,
expect initially strong Rockies/Plains ridging to weaken gradually
after midweek.
Within the agreeable pattern evolution there is some lingering
spread for specifics of the upper trough amplifying into/crossing
the northeastern U.S., timing of leading height falls associated
with incoming Pacific flow, and then the character of energy
reaching the Northwest/northern Plains late in the period. Recent
trends toward a more amplified Northeast trough seem to favor
leaning away from the flatter 12Z GFS/CMC for that part of the
forecast while the 12-18Z GFS/12Z CMC become faster than most
other solutions for upstream energy crossing southern Canada and
northern tier U.S. The 00Z GFS has backed off from previous runs'
fast timing of southern Canada flow, a trend supported by other
00Z guidance. 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 by Fri-Sat. In
differing ways the 12Z and new 00Z GFS runs show more troughing
over the Northwest versus the majority of guidance so it may be
reasonable to downplay their exact solutions. They are still
within the full ensemble envelope though.
Based on forecast considerations the updated forecast started with
an operational model blend early in the period (18Z run for the
GFS component) and then increased 18Z GEFS/00Z ECMWF mean input
mid-late period so that ensembles comprised 70 percent of the
blend by day 7 Sat.
...Weather Highlights/Threats...
There may be pockets of locally heavy rainfall with the cold front
moving from the Plains through the East but frontal progression
should keep totals from being too extreme. However the central
Plains may have a little 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 bring a brief period of 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 into midweek. The hot
weather ahead of the front should 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
Fri-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) should be on Wed.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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