Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1153 AM EDT Fri Aug 24 2018

Valid 12Z Mon Aug 27 2018 - 12Z Fri Aug 31 2018

...Pattern Overview and Weather Highlights/Hazards...

Amplified mid-upper level troughing over the north-central Rockies
and Intermountain West Monday will trek east-northeastward over
the north-central U.S. and then through eastern Canada/Northeast
Tuesday-Thursday. This will support a wavy frontal system across
the northern tier which will focus periods of organized
convection, particularly from the mid-Mississippi Valley and
Midwest/Great Lakes eastward. To the south, less predictable
mesoscale processes within the hot/humid summertime airmass
(temperatures well above average for late August) and deep
moisture/instability will also give rise to locally heavy rainfall
which may produce local runoff issues with repeat/training of
cells. Upstream, a strong upper high south of the Gulf of Alaska
will retrograde westward and allow another cooling trough into the
Pacific Northwest mid-late week with enough onshore flow to
initiate some precipitation. Temperatures will be cooler than
average in the Northwest under the upper trough.

...Guidance Evaluation/Preferences...

Models and ensembles show a similar larger scale pattern in the
3-5 day period (Mon-Wed), but smaller scale differences that will
drive convective evolution will remain unresolved until the short
term. A blend of the 00Z/06Z GFS and 00Z ECMWF/UKMET/Canadian was
a good starting point as their consensus low positions fit the
trends nicely. Incorporated more ensemble weighting by next
Thu/Fri with some details provided by the 06Z GFS and 00Z ECMWF as
there remained uncertainty with the upstream flow (ripple effects
from the entrainment of extratropical lows associated with
weakening west Pacific tropical systems Cimaron and Soulik) as
well as the southward extent of the cold front in the east as it
battles the upper ridging in place midweek. Teleconnections to
upstream anomalies over northern Alaska (negative) and the
northeastern Pacific (positive) were conflicting for the Eastern
US, thus the wavering of the models/ensembles.


Fracasso/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