Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sat Aug 03 2019
Valid 12Z Tue Aug 06 2019 - 12Z Sat Aug 10 2019
...Overview and Guidance/Predictability Assessment...
Retrogression of an upper high initially to the south of mainland
Alaska should allow cyclonic mean flow to develop over the Gulf of
Alaska/extreme northeastern Pacific. By next Sat this evolution
will likely support the ejection of an upper trough/embedded
closed low forecast to persist just off the West Coast for most of
next week. Downstream the Rockies/Four Corners ridge aloft should
drift into the Plains while eastern North America troughing should
likewise shift a bit to the east with time. Guidance consensus
shows the eastern Canada/Northeast U.S. part of the upper trough
deepening by late next week/weekend.
Overall the latest and recent guidance agrees well for the general
pattern evolution with some typical spread and run-run variability
for medium/smaller-scale details. Thus confidence is fairly high
for the mean pattern but lower for some regional specifics.
Primary guidance notes involve the 00Z GFS and some 00Z GEFS
ensembles possibly being quick to eject the eastern Pacific
trough/upper low into western North America by the latter half of
the period and the 00Z GFS being somewhat deeper and/or sharper
than most other solutions within portions of the eastern trough
aloft. The latter issue shows up early in the period as well as
with upstream energy that digs into the mean trough Fri-Sat. The
06Z GFS/GEFS mean compare more favorably to remaining consensus
for both western and eastern features. Also the 00Z CMC strays a
bit south of the guidance majority for the upper low reaching the
Northwest by Sat. In light of these considerations, the updated
forecast started with an operational model blend of 06Z GFS and
00Z ECMWF/UKMET/CMC days 3-5 Tue-Thu and then transitioned toward
an even model/mean weight of the 06Z GFS/00Z ECMWF and their means.
...Weather Highlights/Threats...
Monsoonal moisture should enhance scattered showers and storms
with locally heavy rainfall over/near the Four Corners states
through the period. Interior West locations could see one or more
episodes of scattered convection in response to shortwaves
embedded in southwesterly flow between the eastern Pacific upper
trough/low and Rockies/Plains ridge. Confidence is quite low in
details though. Ejection of the Pacific trough aloft may also
generate rainfall at some locations over the Northwest by Fri or
Sat. To the east of the Rockies two fronts--one weakening as it
progresses from the Great Lakes/central Plains to the East Coast
and a second eventually stronger one reaching the northern tier by
midweek and the East Coast by Fri-Sat--will provide the main
large-scale focus for convection. Some locations within the
northern two-thirds of the Plains/Mississippi Valley may see
higher heavy rainfall potential at times as the fronts will tend
to settle over the central/south-central Plains/MS Valley.
Locally heavy rainfall will also be possible farther east but
frontal progression may temper highest accumulations. The
Southeast will see scattered diurnal convection.
The Pacific Northwest/northern Rockies will see above normal
temperatures (locally plus 10F or so anomalies) into midweek.
Eventual ejection of the initial eastern Pacific upper trough/low
should promote a gradual cooling trend over the West later in the
week. Modestly below normal temperatures (by up to 5F or so) will
become established over the northern Plains after passage of front
dropping out of southern Canada Tue-Wed. By late week/weekend
near to slightly below normal readings should extend into the
Great Lakes toward the Northeast. The southern High Plains will
likely see plus 5-10F anomalies for most of the period.
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