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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0505Z Jul 16, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
105 AM EDT Mon Jul 16 2018

Valid 12Z Thu Jul 19 2018 - 12Z Mon Jul 23 2018

...Overview...

Confidence is now fairly high that the flow pattern across the
contiguous U.S. will become more amplified during the medium
range, with an upper-level ridge expanding from the Great Basin to
the southern plains, and a series of upper waves gradually carving
out a trough across the Great Lakes/Ohio Valley. A shortwave
originating in the Gulf of Alaska will skirt the Pacific Northwest
by late in the week, briefly lowering heights across that region,
before moving east toward the northern plains/Upper Midwest by
Sun-Mon.

A look at hemispheric teleconnections shows support for this
pattern evolution across North America, further increasing
confidence in the impending pattern change. An anomalous and
persistent Rex block across eastern Asia favors upper ridging
across the southwestern and south central U.S., with a relatively
strong vortex across the Arctic, and an equatorward extension in
the form of favored troughing across Hudson Bay and the Great
Lakes. Given the persistent nature of the aforementioned Rex
block, would expect that the upcoming pattern shift across North
America will not be a transient one.


...Guidance Evaluation and Preferences...

Model spread was sufficiently low to justify use of a majority
deterministic multi-model blend (including the
ECMWF/GFS/CMC/UKMET) as a basis for the forecast starting on day 3
(Thu) and continuing through day 5 (Sat). During days 6-7
(Sun-Mon) weighting of ensemble means (ECENS/NAEFS) was gradually
increased to account for an increase in spread during that time
frame.

Models continue to show some variability with the amplitude of
shortwave energy crossing the Midwest Thu-Sat, although the trend
seen in recent days toward more amplification of the feature seems
to have slowed. In general, continue to favor the somewhat deeper
ECMWF/CMC as a closed upper low develops by Fri-Sat, and this is
reflected in the forecast blend. As the feature amplifies further
across the Great Lakes/Ohio Valley Sun-Mon, the ECMWF/CMC continue
to be the preferred solutions over the less amplified GFS. The
past couple runs of the FV3 also showed a more amplified low over
the Ohio Valley similar to the ECMWF/CMC, lending further credence
to that scenario. Farther west, timing and amplitude differences
remain with respect to shortwave energy crossing British Columbia
and skirting the U.S. Northwest Fri-Sat. No clear outliers were
evident, but in general a solution with timing resembling the
ECENS mean was favored, and the described multi-model blend
yielded such a forecast. By days 6-7 as the system crosses the
northern plains, the ECMWF was perhaps a bit fast with the
eastward progression of the system (the 12Z CMC may actually be
the closest solution to consensus throughout the progression of
this system across the northern tier). The heavier ensemble mean
weighting by that time frame kept a forecast that is in line with
the consensus for this system.


...Weather Highlights/Threats...

A cold front expected to become stationary from the Southeast to
the southern/central plains will focus scattered to numerous
showers and thunderstorms from late in the week into next weekend,
with areas of heavy rain possible. Farther north, the shortwave,
and a developing associated surface low pressure system crossing
from the  northern plains/Midwest to the Ohio Valley will generate
convection with locally heavy rain. The amplifying upper trough
across the Great Lakes/Ohio Valley by next weekend could also
result in the potential for transport of deeper moisture northward
across the eastern U.S., with widespread convection and areas of
heavy rain possible across portions of the Mid-Atlantic and
Northeast by the weekend. Finally, monsoonal moisture will result
in scattered afternoon/evening convection from the southern Great
Basin to the southern/central Rockies through much of the week and
into the weekend.

The expanding ridge from the Southwest to the southern plains will
result in rising temperatures by later in the week. High
temperatures across the southern plains are forecast to be 5 to 15
deg F above average from Thu onward, with temperatures expected to
soar past the 100 degree mark for many areas. A number of record
high temperatures across the southern plains could be in jeopardy.


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