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FXUS02 KWBC 211538

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1137 AM EDT Mon May 21 2018

Valid 12Z Thu May 24 2018 - 12Z Mon May 28 2018

...Heavy rain again possible from Florida into the Southeast &
southern Appalachians...

Guidance and Predictability Assessment
Teleconnections with the strong positive anomalies southwest of
the Aleutians and east of Iceland favor troughing in the West and
the Labrador Sea, which the guidance agrees upon.  Persistent
upper troughing is expected to migrate northward from the central
Gulf of Mexico into the lower MS valley.  One issue lies near the
Gulf coast, where the 00z ECMWF is decidedly northwest of the
remainder of the guidance due to its more retrogressive upper
trough -- went on the slow, southeast side of the guidance with
this system as it has yet to form and track the low on an
intermediate route between where a deep layer/cold core cyclone
would track (central Gulf) and a more tropical/warm core feature
would track (near the western FL coast).  The big issue lies in
the northern stream, where there is a sudden lack of clarity on
whether or not a trough moves through eastern Canada.

The main fly in the ointment flow pattern-wise for the Northeast
is the latitude of the mid-level vortex towards the North Pole.
More southerly guidance like the 00z ECMWF/00z Canadian allow this
system to bridge southward to the system transiting the Great
Lakes, using another shortwave moving across west-central Canada.
The GFS/GEFS mean are more northerly and become out of phase with
the Canadian/ECMWF guidance building a ridge over Ontario and
Quebec.  This leads to profoundly different pressure patterns
across New England, with the 00z ECMWF and 06z GFS on opposite
extremes.  Since the guidance has shown a bit of variability with
the Ontario/Quebec ridge, the preferred 500 hPa heights, surface
winds, and pressures were based on the consensus.  This led to an
even blend of the 00z UKMET, 00z ECMWF, 00z Canadian, and 06z GFS
through Saturday before using increasing amounts of the 00z NAEFS
mean and 00z ECMWF ensemble mean thereafter, with some preference
for the 00z Canadian amongst the deterministic guidance for Sunday
into Next Monday over the 00z ECMWF/06z GFS solutions.  This
allowed for reasonable continuity and opens the window to later
guidance shifts.

Weather/Threats Highlights
Areas from the northern Plains eastward will see
rainfall/convection in association with the shortwave energy and
surface low pressure/frontal system initially emerging over the
northern Plains on Thu.  Some activity may be locally moderate to
heavy.  Areas from the lower MS Valley through the Southeast
including Florida will likely see a fairly wet period with the
upper trough deepening along 90W longitude promoting a northward
flow of moisture aloft and surface low pressure possibly tracking
toward/settling over, near, or inland of the central Gulf Coast.
Later in the period there will be a question of how much of the
moisture over the Southeast spreads northward ahead of the front
approaching the eastern states because of uncertain cyclone
track/timing from the Gulf of Mexico.  Heavy rainfall potential
will require close monitoring given the significant totals already
observed over some locations in the past week.  Areas from the
central/northern West Coast into the Rockies will likely see
another episode of rainfall of varying intensity with the upper
low forecast to come inland from the eastern Pacific.

Much of the lower 48 will see above normal temperatures during the
period.  Exceptions will be over the Southeast/Florida where highs
will be lower due to clouds/rainfall and California into the Great
Basin under the eastern Pacific upper low that tracks inland.  The
most prominent warm anomalies should exist ahead of the system
emerging from the northern Plains, and then from the Northwest
through most of the Rockies and eventually into the Plains in
association with upper ridging.  Within both of these areas
scattered locations may see daily record values for highs/warm

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

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