Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Tue Aug 28 2018

Valid 12Z Fri Aug 31 2018 - 12Z Tue Sep 04 2018

...Guidance/Predictability Assessment...

Model guidance continues to be in good agreement with the overall
synoptic pattern across the U.S. through the medium range period.
The upper ridge will remain the dominant feature across the
central U.S., Southeast and East Coast.  Meanwhile, model guidance
continues to show an amplified trough over the Pacific Northwest
and northern Rockies. This trough will slide eastward toward the
U.S. northern tier later in the medium range period.  Meanwhile,
Pacific energy will move inland across California--either in
separate or broader southern stream flow by late this weekend and
into early next week. Model guidance still needs to resolve the
evolution of this energy--although the 00Z NAEFS/00Z ECMWF and the
06Z GEFS show a trough over the West Coast by day 5 but details
are slightly different. The 00Z NAEFS/00Z ECMWF are also in
agreement in Day 6/7 a trough over the West Coast sliding over the
Intermountain West/northern Plains.  WPC used a blend of the 00Z
CMC/ECMWF and 06Z GFS in the beginning of the medium range period
and increased the use of the 00Z NAEFS/00Z ECENS later in the
period to account for the above average forecast spread especially
from the flow evolution upstream over the Pacific.

...Weather Highlights/Hazards...

With multiple fronts moving into the northern Plains and Upper
Midwest--along with a wavy front draped across the Ohio Valley and
Mid-Atlantic, deepened moisture and instability will fuel periods
of heavy rainfall especially across the middle/upper Mississippi
Valley, Great Lakes, Ohio Valley and Mid-Atlantic in the beginning
of the medium range period.  As the next round of heavy rainfall
will be expected for the northern Plains and Upper Midwest later
in the medium range period as another round of fronts move across
these regions.  Farther south, diurnally driven thunderstorms
across the Southeast and Gulf Coast could bring locally excessive
rainfall--but less organized thunderstorms.  A tropical wave
passage over Florida and into the Gulf of Mexico could bring a
potential of heavy rainfall to the state and across the Gulf
Coast.


Reinhart/Schichtel


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