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FXUS10 KWNH 210426
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1225 AM EDT Wed Mar 21 2018

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

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

00Z Model Evaluation...Including Model Preferences and Confidence

...Amplifying synoptic scale trof in Eastern US with development
of Nor`easter off Delmarva coast today that will reach Nova Scotia
late Thursday with significant winter weather impacts along the
Northeast corridor...
Preference: Non-NAM blend leaning 00z GFS in weighting
Confidence: Average

The 00z NAM has trended a bit more amplified with the return wave
emerging from the Ohio valley undercutting the main inner pivot
portion of the vorticity center currently progressing into WV.
This leads to an eventual slowing and secondary tricordial wobble
southeast of Long Island before transferring dominance to the
newer triple point low near Nova Scotia by 18z Thurs. While the
evolution is similar to a slower 12z ECMWF, this run takes it even
further from the stronger clustering, and given the latent
heat/convective influence already manifesting along the Gulf
Stream currently, the likelihood would to favor a slightly earlier
transfer and northeasterly drift Wed into Thurs as suggested by
the 00z GFS, 18z GEFS and to a lesser extent the 12z ECENS mean.
By Thurs into Friday, the 12z ECMWF looks more in line with GFS as
well as the 12z UKMET/CMC which appear serviceable.  As such will
favor a Non-NAM blend leaning toward the GFS solution which
continues to anchor/represent the ensembles well as well as
maintains continuity.  Small scale influences growing upscale a
bit and their importance on mesoscale banding effects and impact
weather conditions still lead to enough uncertainty to have high
confidence in this blend, but is becoming stronger with time to be
considered average at this time.

...Weak broad remaining mid-level troughing across the central
US/Canada border that will dig and amplify through the Great Lakes
toward NY by Friday in the wake of exiting Nor`easter...
Preference: General model blend leaning EC/CMC toward end of Day 3
Confidence: Slightly above average

Goes-WV mosaic depicts a broad loosely defined (multiple internal
shortwaves) troughing conditions over ND/MN/Manitoba at this time.
 As the main eastern US trof exits, this wave will amplify across
the Great Lakes corralled by a strong vort max near James bay that
was the tail of old global scale trof exiting the Northeast
American continent.  The strength and position of this strong vort
max is very important to the evolution of this and the upstream
wave playing a large role in the medium range (please refer to
PMDEPD for additional information) along the Northeast and
Mid-Atlantic.  The 00z NAM and GFS are quick to break down this
corralling wave shearing it northeast; which in turn strengthens
northern stream digging and jet streak through SE Canada into New
England making it about 30-40 DM deeper.  As such the wave is
slightly faster toward southeast transition into the northeast
Atlantic, but more importantly, has reduced upstream shortwave
ridging which negatively affects Day 4 evolution.  As this point,
the spread is mild enough to support a general model blend but
will be hedging weighting toward the more amplified upstream
ridging presented by the ECMWF/CMC in collaboration with WPC
medium range forecast preference.  Confidence is slightly above
average, and would be higher if there was stronger agreement
weakening of the strong vort in Quebec/"corralling wave".

...Deep trough/closed low offshore the Northwest by late Wed into
Thurs with accompanying surface lows...
Preference: General model blend to 23/12z
            00z NAM and 12z ECMWF/UKMET blend afterward
Confidence: Slightly above average

Models have locked into a very common solution for large deep,
slightly elongated north to south closed upper low across the
eastern Gulf of AK.  Eventually, as the lead shortwave round the
base midday Thurs phasing with shearing sub-tropical stream
shortwave energy into the Pacific northwest (more specific
details/preferences about this in section below), the upper low
becomes more symmetric continuing to slowly drop southeast.  At
the surface, guidance is also quite agreeable in the mass fields
especially through 12z Friday, the UKMET and CMC centers are a bit
more flung further from the center of rotation but this appears
minor at this time.   However, after 12z Friday, the instability
of the circulation begins to manifest as it is fully mature, ready
to devolve into its constituent internal smaller scale waves.  The
strongest wave will rotate around the NW and western side and
start angling toward OR/N CA by the end of the forecast period.
The 00z GFS is faster to wind up the system as it rotates through,
but also is faster than the remaining guidance, perhaps a bit too
much to keep in the blend.  The 00z NAM leads the ECMWF/CMC and
GEFS mean slightly but remains tightly clustered to the strong
consensus. By this time the main outlier is the 12z CMC, which has
broken from the tight agreement favoring weakening earlier and
swinging the wave faster and shallower around the parent closed
low`s base toward WA/OR.  Specifically with mass, will support a
general model blend through 23/12z and shift to a 00z NAM/12z
ECMWF/UKMET blend thereafter at slightly above average confidence.

...Combination of sheared southern stream energy and height falls
from internal wave of closed low enters Pacific NW Thurs into
Friday with associated surface wave that transfers to deepening
surface cyclone across SW Canada/Northern High Plains Fri into
Preference: General model blend through 23/00z
            12z UKMET/CMC blend after
Confidence: Slightly above average through 23/00z
            Average thereafter

As mentioned in the parent closed low section above, a combination
of strong height falls/shortwave rotating around its base in
combination with moisture and shearing shortwave energy from the
subtropical stream will phase as it begins to enter coastal OR
with landfall of weakening surface wave along the frontal zone
very late Wed into early Thursday.  This wave translates through E
WA/ID/S BC leading to lee surface cyclogenesis in Alberta late
Friday.  However, there is a bit of discrepancy in the angle and
latitude with the models bring the strongest height falls.  The
12z ECMWF is uncharacteristically a bit faster and injects a but
further north supported by a bulk of its ensemble members.  This
is opposed to the GFS and GEFS members that are slower (though the
00z is a bit faster than the 18z) and further southwest along the
terrain.  The 00z NAM favors the ECMWF solution but is even a bit
faster making it a clearer outlier at this time.  The UKMET shares
similarities with the ECMWF and hedges toward its solution but is
the clear most central solution.  The CMC, like the UKMET is very
central but is even slower than the GFS, but shades to that side
of the guidance.  Ensemble suites/trends would favor the ECMWF
side of evolution/timing but not as far NW with the mass field
solutions.  As a compromise will favor a 12z UKMET/CMC blend
mainly after crossing the higher terrain (Fri 12z) confidence is
higher earlier but average by Fri into Sat.

...Trailing upstream shortwave energy of the shearing central
Pacific closed low, focusing atmospheric river through landfall in
Srn CA Friday morning.  Crossing the Four Corners into the Central
plains with lee cyclone late Fri into Sat
Preference: Non-NAM blend
Confidence: Average

The 00z NAM was generally weaker, north and therefore faster with
the trailing shortwave energy entering southern California.  This
trend continues through late Friday into Saturday, with the
resultant surface cyclone well east of the otherwise moderately
tight clustering in the central Plains.  The 00z GFS also shifted
a bit weaker initially resulting in a slightly faster, weaker
solution in the central Plains, but still within the moderate
clustering of the 18z GEFS.  The 12z ECMWF/CMC are nearly
identical in timing/strength and evolution/shape with the UKMET a
shade faster.  All in all, will support a Non-Nam blend favoring
the continuity of the ECMWF/CMC/UKMET and lower the weighting of
the GFS (but reserve right to shift toward GFS with upcoming
update as guidance may be trending this direction). Confidence is
average in this blend.

Model trends at
500 mb forecasts at


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