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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1559Z Jul 16, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Mon Jul 16 2018

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


Latest guidance continues to provide above average confidence that
the large scale flow pattern aloft will become more amplified
during the period.  A ridge positioned over the Great Basin/Four
Corners as of early Thu will expand and settle over the southern
Rockies/Plains by Sat-Mon.  Meanwhile multiple pieces of energy
will carve out a trough over the eastern half of the country with
a possible embedded low center most likely to track into the Great
Lakes.  Farther upstream a shortwave originating from the Gulf of
Alaska will skirt the Pacific Northwest by late in the week,
briefly lowering heights across that region before continuing
eastward across the Northern Tier Sun onward.

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...

During the early/middle part of the forecast period a blend of
mostly operational guidance represented consensus well and in most
cases provided good continuity.  The one notable change is that
the majority of guidance is showing somewhat more of a signal for
a defined wave along the East Coast boundary that starts lifting
up as a warm front by late Fri-Sat--and with a slower northward
progression.  Also of note, operational 00Z-06Z GFS runs were
weighted less from the start of the period than would typically be
the case with some inclusion of the 06Z GEFS preferred as a
substitute.  GFS runs were faster than consensus (including the
GEFS mean) with timing of the wavy eastern front and also differed
from other solutions for some flow details aloft between Hudson
Bay and the upper Great Lakes.  In addition guidance trends have
been gravitating much closer to what ECMWF/CMC-based guidance had
advertised a couple days ago.  It remains to be seen whether the
upper low within the trough will track as far south as depicted in
recent parallel FV3 GFS runs though.  Elsewhere, the 00Z ECMWF
leaned toward the extreme side of the solution envelope for
troughing over the Northwest by Fri-Sat but was still within the
typical range of error at that time frame.

Increasing detail spread/variability among operational runs and
agreeable solutions among the GEFS/ECMWF ensemble means
recommended trending toward a fairly even model/ensemble mean
blend by late in the forecast.  Timing/details of energy crossing
southern Canada and the northern U.S. will likely provide ongoing
uncertainty--with some slower trends evident over the past couple
days.  The same will be case for specifics of the eastern U.S.
trough including the degree of influence from upstream flow.

...Weather Highlights/Threats...

The evolving pattern will bring the potential for one or more
periods of significant rainfall to locations from the northern
Plains/MS Valley through much of the East.  This activity will be
beneficial for some areas that have received very little rain over
recent weeks (pending activity that is possible in the short range
time frame) while it may be more problematic over other areas.  A
cold front likely becoming stationary from the Southeast to the
southern/central Plains as of the start of the period will provide
one focus for scattered to numerous showers and
thunderstorms--some with heavy rainfall--from late in the week
onward.  This boundary will eventually lift northward as a warm
front.  Farther north evolving trough energy aloft and the
associated surface low pressure system drifting from the northern
Plains into the Great Lakes will generate convection with locally
heavy rain.  The amplifying upper trough over the East may also
promote transport of deeper moisture northward across the eastern
U.S., with widespread convection/areas of heavy rain possibly
reaching portions of the Mid-Atlantic and Northeast by the
weekend.  Finally, expect monsoonal moisture to 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.  Some locally enhanced amounts are possible but
activity should generally be lighter and/or less organized than in
some recent episodes.

The expanding ridge aloft from the Southwest into southern Plains
will be accompanied by multiple days of hot conditions from late
week onward.  The focus for highest max temperature anomalies will
likely be over Texas and vicinity with many 100+F highs and some
locations reaching 10-15F above normal.  Readings may reach or
exceed record high/warm low values at a number of locations.  The
southern 2/3 of the West should see rather warm min temperatures
most days but highs should be less extreme relative to normal. 
Expect some below normal readings over the East at the start of
the period (especially morning lows Thu), and below normal highs
over areas that see clouds/rainfall from the Midwest into the East
during the period as well as the northern Rockies/Plains by early
next week.


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