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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0646Z May 24, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Thu May 24 2018

Valid 12Z Sun May 27 2018 - 12Z Thu May 31 2018

...Potentially significant rainfall events possible over the lower
Mississippi Valley/Southeast and northern Rockies/Plains...


The fairly blocky pattern expected through the first part of the
period shows signs of change at least over western areas as
Pacific trough energy heads toward the West Coast, serving to
dislodge the slow moving upper low expected to be over the Great
Basin to start the period.  Teleconnections relative to positive
height anomaly centers over the Pacific (south of Alaska along
45-50N latitude) and south of James Bay in recent D+8 multi-day
means offer support for a mean trough settling along or just
inland from the West Coast.  Away from northern latitudes the slow
evolution of flow will persist somewhat longer over the East, as
surrounding ridges aloft will likely allow for only a gradual
northward drift of a Gulf of Mexico/southern U.S. upper low.

...Guidance and Predictability Assessment...

In principle there is decent agreement and continuity (at least
within the past day) for evolution from the western Pacific into
central U.S.  There are still some significant differences in the
specifics.  Among recent solutions the 12z ECMWF becomes the
lowest confidence scenario from about Mon onward as it is quite
progressive with Pacific height falls moving into the western
North America and thus may be somewhat quick to eject energy from
the Great Basin upper low into the northern Plains.  Thus for the
purpose of an overall blend the old 00z/23 ECMWF run had to be
given greater weight for the operational ECMWF component.  Other
models/means compared better to each other.  The 18z GEFS mean was
a tad slower with the incoming trough versus most of the remaining
guidance leading to greater preference for the 12z version. 
Aforementioned teleconnection support adds confidence in the idea
of a mean trough becoming established near the West Coast but
ensemble spread is high enough to suggest a fair degree of
uncertainty in the details.

Significant track spread continues for the system expected to
emerge from the northwestern Caribbean and track into the Gulf of
Mexico during the short range time frame.  Latest solutions
generally continue to occupy their historical sides of the
east-west envelope thus far, GFS/GEFS/UKMET on the east (though
GEFS west of the GFS) and ECMWF/ECMWF ensembles/CMC ensembles on
the west with the operational CMC a bit eastward of the latter
cluster.  A westward adjustment in the new 00z GFS versus the
18z/12z runs has narrowed the spread though.  Multi-day trends
toward slower northward progression persist.  Trends that have
seen a trimming of the eastern side of the full guidance envelope
over recent days along with the strengthening ridge aloft to the
east of the system seem to continue support for a track in the
middle to western portion of the envelope.  By day 7 Thu it
remains to be seen how much of a surface reflection remains, while
the new 00z GFS may become a little quick to eject the upper
feature in response to flow upstream.

After some recent continuity changes, the forecast for southern
Canada/northeastern U.S. flow aloft and associated surface
evolution over the Great Lakes/Northeast/Mid Atlantic have held
fairly steady over the past day.

Based on above preferences or representation of consensus, the
updated forecast incorporated aspects of the past two ECMWF runs
(more 00z/23 than 12z/23), 12z CMC, 12z-18z GFS, and 12z
GEFS/ECMWF means.  Operational model weight decreased from 80
percent early to about half by the end of the forecast.

...Weather/Threats Highlights...

The possible system tracking toward the Gulf Coast along with
broad moist flow to its east will bring the potential for a
multi-day heavy rainfall event to parts of the Gulf Coast and
Southeast--most likely beginning this weekend and persisting into
the middle of next week.  Preferred solutions would bring some of
the heavy rainfall as far west as the lower Mississippi Valley. 
On the other hand the lower probability scenario of a farther east
track could direct more moisture into the Mid Atlantic where a
frontal boundary could provide an added focus for significant
rainfall.  Farther northwest, expect the upper low tracking out of
the Great Basin to generate areas of locally heavy rainfall from
the northern Great Basin across the northern Rockies and northern
to possibly central Plains.  In both of these areas of focus some
locations have already experienced significant rainfall recently
and will be sensitive to additional rain.  Portions of the Great
Lakes/Northeast may see some showers/thunderstorms and locally
heavy rain with an upper shortwave and associated surface
wave(s)/frontal system.

The Plains will likely see the highest temperature anomalies
during the period with many areas experiencing readings 10-20F or
so above normal for multiple days.  Some daily max/warm min
records are possible.  Cool highs over the Great Basin will trend
closer to normal as the initial upper low lifts to the northeast. 
Over the Southeast persistent cloudiness/areas of rainfall should
result in below average highs while early in the period onshore
flow from the Atlantic will keep New England on the cool side. 
The pattern will be favorable for warm lows over much of the East


WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at: