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FXUS10 KWNH 170645
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sat Mar 17 2018

Valid Mar 17/0000 UTC thru Mar 20/1200 UTC

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

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

...Northwesterly flow regime across New England/SE Canada with
numerous shortwave passages along southwest side of persistent
upper low center...
Preference: Non-UKMET blend
Confidence: Slightly above average

Large scale vortex remains locked across the northeastern portion
of the North American continent and Labrador Sea for the forecast
period leading to persistent northwesterly flow with embedded
strong shortwave features with subsequent cold air surges.   The
lead very deep wave amplifies Sat and while the GFS/ECMWF have
been very locked...the remaining guidance strongly agrees in
totality for this wave.  In its wake...smaller waves break through
as a weaker but important center of upper low develops pushing a
first wave through ME on Sunday with main core of height falls
moving through Monday.  Here the GEFS mean/ECENS mean continue to
serve the forecast well washing out run to run smaller detail
variations for a more consistent forecast...however the 00z NAM
joins with the operational 00z GFS and 12z ECMWF to have solid
enough agreement to bring out the smaller scale enhancements with
some increased confidence.  The 12z UKMET/CMC are not terribly off
but are a bit more amplified and slow as or too weak and fast
respectfully to match the ensemble suite.  A 00z NAM/GFS and 12z
ECMWF blend will represent the means well enough to have average
confidence in it.

07z update: Little variation noted between the 12z ECMWF and the
new 00z to build further confidence in the initial preference.
The 00z UKMET is quite strong with the new inner core/centroid as
it rotates through northern New England Monday keeping it out of
the preference at this time.  The 00z CMC on the other hand is
much more favorable in the shift toward the initial preference to
the point it can be included.  As such a non-UKMET blend is
preferred at Slightly above average confidence.

...MO Valley cyclone weakening as it Shifts east into the Ohio
Valley and Carolinas this weekend...
Preference: General model blend
Confidence: Above Average

GOES-16 WV suite and national radar mosaic depict a banded closed
low elongating NW to SE through the MO valley starting to shift
toward the Ohio Valley currently.  The eastern vorticity center
and associated surface wave will slide ESE today into the
Carolinas with enhanced surface development along the lingering
frontal boundary.  Guidance has been very good in coming to a
common solution though small variations especially in the
mid-upper level circulation are noted with a deeper 00z NAM run
making it a bit slower but not significantly so.  As such a
general model blend remains supported at above average confidence.

07z update: The slight increase in amplification noted by the NAM
and GFS, was also the trend in the 00z ECMWF/UKMET and CMC to
further lock down a high confidence support in a general model
blend for the remainder of this wave.

...Large Closed Low along OR/CA boarder over the Northwest U.S
breaking down and interacting with northwest moving shortwave
energy from high plains deformation zone...melding into weak broad
trof across northern high plains/Dakotas by Tues...
Preference: Non-NAM blend
Confidence: Average

Large scale upper low breaking down across the interior west over
the next day or so remains particularly complex especially with
respect to interaction with other stream flows/shortwave energy.
Currently the closed low is centered along the OR/CA boarder with
undercutting s/w and jet energy.  This wave will lift northeast
and slowly stretch NNE to SSW across W MT into the Great Basin by
Sunday interacting with sheared shortwave energy from the NW
portion of the wave over the central Plains.  A small binary
interaction/combination will evolve across the High Plains of MT
into S central Canada by Monday where northern stream shortwave
weak baggy troughing will also interact leading to a weak broad
trof over the Northern Plains by Tues.  Here the timing and
general evolution is conceptually best represented by the 00z GFS
and 12z ECMWF which are center to ensemble suite consensus.  The
12z UKMET is much too strong with the internal s/w features during
an overall filling setup...that by Tuesday is much too amplified
in the final re-development/consolidation phase of the combined
streams.  The 00z NAM while timed well and generally weaker with
the features than expected (from its know bias) shows a broader
binary interaction with the shortwave features that looks out of
sync with the ensemble suite and expected conceptual evolution.
The 12z CMC shows a very strong wave in the northern stream
potentially showing some bias as well.  All considered a 00z
GFS/12z ECMWF blend should suffice though given the complexity of
the evolution confidence is slightly below average.

07z update: While the 00z UKMET remains a bit more robust with the
remaining interacting shortwave features; there is much less
feedback toward a more intense/wound up solution and now depicts a
similar placement/evolution to the favored 00z GFS with a shade
greater strength.  The 00z ECMWF/CMC both shows similar evolution
favoring the northern stream development over the northern Plains
into day 3 over the GFS/UKMET solutions.  Still this does not seem
out of range from the initial thoughts/preference and and such
will favor a Non-NAM blend.  Confidence has increased slightly to
average; but given the complexities of interactions of weak
features in multiple streams there remains quite a bit of
uncertainty if this blend will come to fruition.

...Strong shortwave across central Canadian Rockies rotates SW
through Vancouver Island and off Pacific NW by remaining static
while shearing/drawing toward approaching central Pacific closed
low Mon/Tues...
Preference: General model blend
Confidence: Average

GOES-W WV depicts a very strong shortwave moving west through the
central Canadian Rockies along the northern periphery of the
larger scale deep cyclone that dominating the western US at this
time.  This wave will continue to drift west before sliding
offshore Vancouver Island and amplifying slightly by early Sunday.
 The wave will then stall off the WA coast through Monday when it
will start to elongate/shear under the influence of the
approaching closed low in the central Pacific.  Given the weakness
of the mean large scale flow...there is a fair amount of spread in
timing/placement of this elongation...with the 12z CMC breaking
first from the consensus shifting west under stronger/broader
ridging lifting north between 120-130W and becomes a clear
outlier.  The remaining guidance depicts further slow dissipation
of the remaining energy with ever so slight southward migration
with time.  Given the remaining agreement of the
GFS/NAM/UKMET/ECMWF will favor this blend.  Confidence is average
given the complexity of the evolution given multiple
streams/approach of closed low.

07z update:  The issues with the CMC have seemed to resolve with a
much weaker solution stalling off the coast Sunday.  As such there
is much less interaction with the closed low offshore and so
slides eastward into the Pacific Northwest by Tues weakening
further as it does so.  the 00z ECMWF trended a bit less
interactive with the upstream closed low as well...leading to an
earlier shift south-east into Tuesday than the 12z run and
GFS/UKMET/NAM.  Still the separate/timing is not significant
enough (especially in sensible wx fields) to remove it from the
preference and as such a general model blend can be supported for
this wave.  With that stated however, run to run variation is
likely to continue given the proximity in the weak flow field/col
overall to only support average overall confidence in the blend.

...Stronger wave on the western periphery of the closed low moves
into California Saturday ejecting into the Plains on Sunday with
Lee Cyclogenesis and eventual eastward progress into the
Mid-Mississippi Valley on Monday...
Preference: Non-NAM blend weighted to 00z ECMWF
Confidence: Average

GOES-W WV depicts a warm zone along 130W along the western
periphery of mean large scale western upper low.  This wave
descends in latitude and rounds the base of the upper low into S
CA/AZ by early Sunday.  The wave accelerates east and under very
favorable upper level diffluent pattern and rapidly deepens/wraps
up across NM into the Southern Plains by Monday with strong
surface cyclogenesis.  Run to run ensemble spaghetti analysis
shows a reduction in model cohesion breaking from strong agreement
yesterday to differences in latitude of ejection into the Plains
as well as strength of influencing upstream shortwave/jet energy.
 The 12z UKMET evolution is typical of its bias...slightly faster
and quite wound up emerging into OK Mon maintaining some strength
of the inner core moving through the Ohio valley.  The 12z ECMWF
remains a southern solution and a bit stronger/wound up than some
other guidance but also is the most consistent solution in
timing/strength and shape showing some eventual filling/weakening
east of the MS River into Tues, which seems conceptually
consistent moving into confluent flow.  The 12z CMC like the ECMWF
is south and wound up but unlike the ECMWF remains compact not
resolving the upstream confluence as well (note section one

As for the newest guidance, the 00z NAM ejects most northerly; and
typical of Day 2.5-3 bias is very strong/wound up eventually
slowing in timing relative to the ensemble mean and by Tuesday is
a clear outlier in mass fields.  The GFS which had shifted north
over the last few cycles has slipped south relative to the 18z run
but remains north of the ECMWF/CMC emerging into the Plains.  The
wave is also a bit faster typical of its bias...both with respect
to the main shortwave through the Ohio Valley to WV by 12z Tues
but also with upstream stronger/faster amplification from fast
moving wave originating from the Pacific across the lower Red
River valley at the same time.  All considered the 12z ECMWF is
favored heavily but will include a portion of the GFS (3/4 to 1/4
GFS) to account for some variability in the ensemble suite.
Confidence is average.

07z update: An agreed upon shift with a slightly weaker evolution
of the wave emerging into the southern Plains was noted in the 00z
CMC/UKMET and ECMWF.  The 00z UKMET still remained faster than the
ECMWF but matches quite nicely with the 00z GFS.  While the ECMWF
remains further south, the timing was faster than the 12z run and
shows an earlier weakening of the initial wave with a slightly
faster/stronger upstream shortwave enhancing from the Pacific flow
across the southern Plains and Lower MS River Valley on Tues.  The
00z CMC is a bit stronger with the inner core moving through the
OH/TN Valleys Mon into Tuesday and lags the ECMWF a bit but fits
with current trends/thinking enough to include it at lower
weighting.  As such this leads to WPC preference of a Non-NAM
blend continuing to favor a slower ECMWF solution in overall
weighting with respect to the GFS and UKMET.

Model trends at
500 mb forecasts at


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