Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Sat Aug 10 2019
Valid 12Z Tue Aug 13 2019 - 12Z Sat Aug 17 2019
...Overview and Guidance/Predictability Assessment...
The upper pattern at the beginning of the medium range period/Tue
remains quasi-zonal, but with embedded shortwaves tracking through
and some indication of troughing along the West Coast. As the week
progresses, expect flow to transition to more amplified as a
deeper trough digs into the West Coast and ridging strengthens in
the Southern Plains Thu/Fri. This would likely lead to troughing
downstream in the eastern U.S. By Sat it appears that energy from
the western trough may split, with part moving eastward and part
hanging back along the western U.S.--the 00Z and 06Z GFS and the
00Z ECMWF happen to be in good agreement with this, though the
scenario has low predictability. But it does seem reasonable and
similar to the pattern lately that energy will be more progressive
when moving into the central U.S. from the West. A multi-model
blend incorporating the 06Z GFS/GEFS mean and 00Z ECMWF/EC
ensemble mean was able to be used, without leaning heavily on any
one model run due to the continued smaller-scale uncertainties in
the flow. Did lean away from the 00Z CMC after midweek for two
reasons: first, the eastern trough is deeper than other guidance,
and other guidance like the EC and EC mean has trended weaker with
this troughing. Second, it is slower with the western trough by
Thu, which means it does not move the energy eastward by Sat as
expected. Overall this blend led to good continuity with the
previous forecast issuance.
...Weather Highlights/Threats...
The main rain maker in the medium range should be a frontal system
moving southeastward through the eastern and central CONUS early
in the period, stalling out over the Southeast late in the week.
Widespread showers and thunderstorms are expected to move
southward from the Northeast and Ohio Valley/Mid-Atlantic on
Tuesday to focus in the Southeast/Gulf Coast from midweek onward
as Gulf moisture streams into the vicinity of the front. Periods
of rainfall and thunderstorms will also be possible ahead of
shortwaves in the north-central U.S.
Before the aforementioned front passes, warmer than average
temperatures are forecast for the Southeast, Tennessee and Ohio
Valleys, and Mid-Atlantic states. Additionally, the Southwest to
Southern Plains should see hot temperatures through the period
under the upper ridge. Thus, widespread record high minimum
temperatures are expected in the southern tier of the CONUS and
excessive heat is possible. On the other hand, with troughing
making its way into the Northwest and shortwaves traversing
through the Northern Plains, below average temperatures are
forecast in those regions.
Tate
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