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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0519Z Jul 13, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
119 AM EDT Fri Jul 13 2018

Valid 12Z Mon Jul 16 2018 - 12Z Fri Jul 20 2018

...Overview...

The flow pattern across the contiguous U.S. will become a bit more
amplified during the medium range period with an upper-level ridge
expanding across the southern Great Basin/Four Corners region, and
a series of upper waves gradually carving out a trough across the
Great Lakes/Northeast. Heights are also expected to fall across
the Pacific Northwest by late next week as an upper low digs south
from the Gulf of Alaska toward the region.


...Guidance Evaluation and Preferences...

With shortwave energy passing south of Hudson Bay and skirting the
Upper Midwest/Great Lakes days 3-5, models showed a range of
solutions with respect to both timing and amplitude. The GFS and
ECMWF were generally well centered within the spread, while the
12Z CMC was on the slow/deep side and the UKMET was on the
faster/weaker side. Based on the gradually increasing flow
amplification through the period, the preference was to go near or
slightly slower/deeper than the consensus, and this was reflected
in the WPC forecast. Models show additional shortwave energy
crossing the northern/central plains and Midwest by Wed-Thu. The
18Z became more amplified than the consensus with this feature
across the Mississippi Valley by Thu-Fri. Models suggest the
potential for this energy to interact with additional energy
farther north across Canada to amplify a broader trough across the
Great Lakes/Ohio Valley by late next week, although confidence is
low in the details with many potential factors affecting the
eventual outcome, so an ensemble approach was preferred by that
time. Farther west, with a shortwave/upper low diving southeast
across the Gulf of Alaska mid to late next week and lowering
heights across the Pacific Northwest, there is uncertainty as to
how deep the feature will be and whether it will dig farther south
or move more quickly eastward. Ensemble means were in general
agreement at 12Z Fri on showing a mean trough axis across British
Columbia. Given the relatively low confidence in the details, an
ensemble mean approach was preferred here as well.

The WPC forecast initially based on a multi-model deterministic
blend during days 3-5 (Mon-Wed), with most weight placed toward
the ECMWF and GFS. By days 6-7 (Thu-Fri), weighting of ensemble
means (ECENS and NAEFS) was gradually increased to comprise a
majority of the weighting.


...Weather Highlights/Threats...

Showers and storms with locally heavy rain will accompany a cold
front across the Ohio/Tennessee Valleys to the
Southeast/Mid-Atlantic/Northeast Mon-Wed. Monsoonal moisture will
also produce scattered afternoon/evening thunderstorms across from
the southern Great Basin/Southwest to the southern/central Rockies
through the next week, with locally heavy rain possible. The
building upper-level ridge may somewhat reduce the coverage of
convection by later next week. Temperatures will initially be 5 to
15 deg F above average across the Northeast ahead of the cold
front early next week, with a decrease closer to seasonal averages
by mid to late next week. In the wake of the cold front, high
temperatures are expected to be 5 to 10 deg below average across
the central/northern plains and portions of the Midwest through
much of next week. Finally, hot temperatures across the Pacific
Northwest (highs 5 to 15 deg above average) should moderate some
by later next week as a Pacific cold front moves inland.

Ryan


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at:

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
http://www.wpc.ncep.noaa.gov/heat_index.shtml