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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1558Z May 20, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun May 20 2018

Valid 12Z Wed May 23 2018 - 12Z Sun May 27 2018

...Guidance and Predictability Assessment...

In general, most models are in line with the overall synoptic
pattern with the typical differences in detail.  On Day 3/4
(Wednesday/Thursday), shortwave energy will begin to eject into
the northern Plains while the upper ridge weakens and shifts east
toward the Great Lakes. An upper trough in eastern Canada will dip
into the Northeast states by Thursday,  Meanwhile, the next upper
trough will also begin to approach the West Coast.  By late week
into next weekend, the upper trough in the Pacific will begin
approaching the West Coast.  To the east, an upper trough will
slide across the Great Lakes once again while an upper trough
moves northward into the central Gulf of Mexico.

The GFS/GEFS, for the most part, continues to be the slower
solution in regards to the upper trough that will move toward the
West Coast late next week.  The ECMWF and its mean is staying the
course on being more progressive with this system with the CMC
also showing this progressive solution.  However, the 00Z GFS/GEFS
aligned more with the ECMWF/CMC with being faster--with the 06Z
GFS/GEFS flipping back to being slower.  By Day 7 (Sunday), the
00Z GFS/GEFS was much farther north with the trough moving into
the Pacific Northwest while the 06Z GFS/GEFS was farther south
like the ECMWF has consistently been.  Both the 00Z GFS/ECMWF and
their means also agreed with the evolution of the shortwave energy
moving into the northern Plains midweek and eventually toward the
Great Lakes by the weekend.

Farther East, there are some differences in timing with regards to
the upper trough in eastern Canada dropping south over the
Northeast region. However, the 00Z GEFS/ECENS in general showed
the upper trough over New England on Thursday.  As the shortwave
energy moving from the northern Plains enters the Great Lakes/Ohio
Valley, models are consistent showing an upper trough over the
Southeast/central Gulf Coast.  There continues to be a signal that
a weak surface low will evolve from the upper trough by next
weekend.  In addition, the GFS has consistently shown the low from
the northwest Caribbean taking an eastward path with the ECMWF
continually showing a westward trend.  The CMC also favors the
westward path but continues to be quite aggressive with this
surface low.  In general, stayed with continuity by favoring the
eastward (GFS/GEFS) solution. 

The WPC blend for the first part of the medium range forecast was
a blend of the 00Z ECMWF/GFS/CMC.  By Day 5 and 6, weighted more
of the blend with 00Z ECENS/ECMWF and some 00Z GEFS/GFS.  By Day
7, because 06Z GEFS/GFS was trending toward the ECMWF and its
mean, used the 00Z ECENS/ECMWF and the 06Z GEFS/GFS. 

...Weather/Threats Highlights...

The shortwave energy moving over the northern Plains and into the
Great Lakes will bring precipitation across the Intermountain West
and into the Plains by midweek and will increase over the
Mississippi Valley by Thursday/Friday.  By the weekend, rainfall
will spread throughout the Great Lakes and the Ohio
Valley/Northeast.  With both the surface low from the upper trough
in the Gulf along with the surface from the northwest Caribbean,
the central and eastern Gulf Coast region, Southeast and Florida
can expect showers and thunderstorms throughout the weekend. 
Rainfall will also increase through northern/central California
and the Pacific Northwest as the upper trough approaches. 

Most of the country can expect above normal temperatures during
the medium range period.  The biggest exception will be Florida as
cloudy conditions/rainfall will keep temperatures below normal. On
Thursday, due to the eastern Canada upper trough, temperatures
will dip below normal but will recover back to near normal by
Friday and last through the weekend.

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