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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1546Z Jul 15, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Sun Jul 15 2018

Valid 12Z Wed Jul 18 2018 - 12Z Sun Jul 22 2018

16Z Update:

Similar to the previous forecast issuance, there is enough
synoptic scale agreement through Thursday to merit a multi-model
deterministic blend.  By Thursday night, the 00Z ECMWF is west of
the model consensus, including the EC mean, regarding the axis of
the shortwave trough/closed low crossing the northern plains and
Upper Midwest.  This continues through the remainder of the
forecast period as the long wave trough evolves over the eastern
third of the nation.  However, its upper low position falls within
the range of the 576dm ensemble member solutions and is not
considered an outlier at this time.  The 00Z UKMET is on the
faster side of the guidance and it also falls within the ensemble
spread.  There has also been a stronger trend in the deterministic
guidance over the past 24 hours, and the latest WPC forecast takes
this into account.  The feature with the greatest model agreement
is the building upper level ridge over the High Plains and
extending over the Inter-mountain West.  The previous overnight
discussion is appended below for reference.

D. Hamrick
---------------------------
...Overview...

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. The intensifying
upper ridge farther south will help ensure that any such height
falls in the Northwest are short-live, however.


...Guidance Evaluation and Preferences...

Model spread was sufficiently low to start the forecast on day 3
(Wed) with a multi-model deterministic blend (including the
GFS/ECMWF/CMC/UKMET). The trend in the guidance noted in recent
days toward more amplification of features seems to have
continued. Model consensus is now good with respect to the upper
trough initially across the Great Lakes on Wed, which lifts out
across New England on Thu. The aforementioned trend also applies
to shortwave energy crossing the northern plains/Upper Midwest
Wed-Thu and then amplifying further across the Great Lakes/Ohio
Valley on Fri (day 5). Models continue to vary with respect to the
amplitude of this feature, but given the trends, cannot rule out
the more amplified solutions (such as the CMC and UKMET). Thus,
the multi-model deterministic blend continued to serve as a basis
for the forecast through day 5.

During days 6-7 (Sat-Sun), the primary questions become how much
amplification occurs with the eastern U.S. trough, as well as to
what degree and for how long heights fall across the Pacific
Northwest with the shortwave passing across western Canada. Given
the increasingly amplified flow across the CONUS, opted to lean a
bit toward greater amplification of the trough across the Great
Lakes/Ohio Valley. The GFS has been among the least amplified
solutions, but the FV3 is much more amplified, similar to the
ECMWF and CMC. The GFS (prior to the 00Z run) has also been among
the less amplified solutions with the western Canada energy. Thus,
the forecast during days 6-7 was based on a blend of
deterministic/ensemble mean solutions, including the ECMWF/CMC
along with the ECENS/NAEFS means.


...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 midweek into next weekend. Areas of
heavy rain will be possible, with guidance continuing to show a
relatively strong signal for heavy rain across portions of the
Southeast, from the coastal Carolinas to northern Florida. Farther
north, the shortwave crossing the northern plains/Midwest Wed-Thu
will generate convection with locally heavy rain. This feature
should eventually generate a new wave along the aforementioned
surface front, which is forecast to cross the Midwest/Great Lakes
Fri-Sat, producing additional convection and locally heavy rains.
The amplifying upper trough across the Great Lakes/Ohio Valley by
next weekend could also result in the potential for latitudinal 
transport of deep tropical moisture along the Eastern Seaboard. A
couple deterministic model solutions indicate the potential for
widespread convection and areas of heavy rain across portions of
the Mid-Atlantic and Northeast by late in the week into the
weekend. Ensemble probabilities suggest that confidence in this
scenario is relatively low at this time, however. 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. Convective activity should
gradually become more sparse in nature through the week as the
upper ridge strengthens overhead.

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 well past the 100 degree mark across much of Texas. 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