Prognostic Meteorological Discussion
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FXUS10 KWNH 210635
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
235 AM EDT Sat Apr 21 2018

Valid Apr 21/0000 UTC thru Apr 24/1200 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air

00Z Model evaluation including preference and forecast confidence

...Progressive shortwave moving through New England/N Mid-Atlantic
late Sun...Mon
Preference: General model blend
Confidence: Slightly above average

Lingering troughing exists across the Northeast today into Sunday
with little fanfare or impact.  As the closed low over the South
shifts eastward, a weak shortwave slides through south central
Canada and sharpens slightly as it crosses the Northeast late
Sunday into Monday.  Only the 12z UKMET remains out of phase
already showing signs of deeper and shifted south and west
amplification of the trof crossing the Great Lakes Sunday which
further deepens and lags the growing consensus, further northeast.
 The 12z CMC is also a bit slow at peak sharpening of the trof 12z
Monday in line with its bias.  As such will favor a 00z NAM/GFS
and 00z ECMWF blend across the Northeast through the weekend and
start of next week.  Confidence is slightly above average given
the pattern and fairly tight ensemble suite.

07z update: The 00z UKMET significantly backed off the
deeper/earlier amplification of the shortwave moving through the
Great Lakes and Northeast late Sunday into Monday; this allows for
a faster eastward progression through the Northeast as well.  The
00z ECMWF/CMC both are a bit sharper than the NAM/GFS and the CMC
remains slightly slower (typical of bias)...but at this point a
general model blend can be preferred at slightly above average

...Strong closed low in the Four Corners steadily pushing east
into the Southeast by Monday...
...Inverted surface trough in the western Gulf Coast region, with
weak surface low developing in the central Gulf Coast states
Sunday...shifts to Southeast by Tues
Preference: 00z GFS/ECMWF/UKMET blend
Confidence: Average

Strong closed low appears centered near the Sangre de Cristo Range
with numerous shortwave vorticity centers along the outer
periphery.  This pattern is expected to continue with slow
eastward progression.  The upper low will start to elongate a bit
north to south, though the NAM continues to support a stronger
northern portion of the upper low even as early as late Sat/Sun.
Still the spread is not too bad even through midday Sunday into
the Lower MS Valley, especially with timing of the convection and
frontal zone.  After this period however, there is fairly sizable
departure.  The 12z UKMET which presents a strong shortwave moving
through the Great Lakes aids stronger ridging across the Central
Ohio Valley and Upper TN Valley...directing the upper low further
south and east.  The NAM  departs further north at this time with
some strengthening/upscale feedback, while most guidance suggests
slow weakening.   The 00z GFS supported by the GEFS members
significantly slows across MS/AL especially within the lowest
troposphere levels...this is opposed by the ECMWF/CMC and their
ensemble suites that press the surface low with the deepest
convection/latent heat release along the SC coast by the end of
the forecast period.  The 7H pattern would suggest the ECMWF may
be a bit too flat connecting the conveyor belt further north and
west versus the GFS/CMC showing more of a comma head feature
across the East Central Southern states.  To compromise a blend of
the three will be average confidence (though
confidence is higher earlier on Sat/Sun).

07z update:  The 00z ECMWF trended a bit slower and stronger to
match the 00z GFS quite well, providing additional confidence.
Likewise, the UKMET significantly shifted toward north and west
with a more concentric inner core of the slowly weakening closed
low.  This matches well with the ECMWF/GFS.  The 00z CMC while not
as dramatically displaced as the 12z UKMET or the 00z
NAM...suggests a further south and presses the occlusion further
southeast.  As such a 00z GFS/ECMWF/UKMET blend at increased but
average confidence.

...Strong shortwave/attendant cold front nearing the Pacific
Northwest currently before shifting north of the US border this
Preference: General model blend
Confidence: Above average

Models are in very good agreement with the mass fields for this
system and a general model blend is preferred with high confidence.

...Secondary shortwave reaching the Pacific Northwest early
Sunday; amplifying and pushing into the Northern Rockies Monday.
Potentially combining with northern stream energy by Tues into
Northern High Plains...
Preference: 00z ECMWF/NAM/CMC blend
Confidence: Slightly below average

A shortwave enters the Pacific northwest Sunday and quickly
translates across the Northern Rockies.  In this process slow
phasing/connection starts to manifest across central MT with the
trailing trof from the preceding strong deep wave over Central
Canada.  Concurrently, the wave is situated in a flow regime
supporting increased amplification and cyclonic development even
spurring a modest surface reflection across the Great Basin that
translates to southeast WY/northern CO by late Mon...connecting to
the frontal zone through the Dakotas on Tuesday.  While there is
good timing/shape to this at the surface...the upper level pattern
is a bit more complex due to influences from other
systems/moisture streams.   The 00z NAM has significantly changed
over the last few runs to come into a common evolution/solution
presented by the 12z ECWMF/ECENS mean.  Both are a bit faster and
further south (well presented in the run to run ensemble
suite...where ECMWF members dominate a faster/south trof by 12z
Tues across the central Rockies, and therefore are less connected
to the northern stream digging trof through southern Canada.  The
00z GFS has picked up pace to be better timed with the ECMWF,
however, it is further north and supports winding up the mid-level
circulation through N WY into SD late Monday into Tuesday with
stronger influence from the digging trof out of southern Canada.
This evolution is very comparable to the 12z CMC and the CMCE.
The 12z UKMET seems a middle ground solution between the two
camps, though coming to this solution quite differently in the
evolution.   While the spread is moderate, guidance wants to come
to a common solution reducing overall spread in impact/sensible
weather.  Confidence is not high (slightly below average), but a
general model blend is supported to smooth out these

07z update:  The 00z ECMWF continues to uncharacteristically be a
faster overall solution and while not trending south as much as
the 12z run continues to be east of the mean consensus and
generally broader than the tight/wound up 00z GFS solution by the
end of the forecast period.  The 00z CMC trended a bit less wound
up and while not completely similar to the ECMWF just yet, it is
trending that direction about half way between the GFS/ECMWF.  The
00z UKMET trended a bit stronger in the northern stream and slower
with the base of this trof emerging from the Northern
Rockies...and now looks out of place in the blend.  The overall
trends would suggest a 00z NAM/ECMWF/CMC blend may work best
though confidence remains slightly below average.

Model trends at
500 mb forecasts at


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