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FXUS10 KWNH 200429
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1229 AM EDT Tue Mar 20 2018

Valid Mar 20/0000 UTC thru Mar 23/1200 UTC

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

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

...Lower Ohio Valley strong shortwave and associated surface wave
in the TN valley transferring to a coastal low near VA by late
Tuesday... reinforced by rapidly amplifying upstream shortwave
evolving into deep eastern US trof...producing a secondary sfc low
in the Carolinas late Tues becoming a Nor`easter paralleling the
Preference: 00z GFS/12z ECWMF blend
Confidence: Slightly below to below average

Goes-16 WV loop shows strong compact vorticity maxims across the
lower Ohio valley at this time with an eastward trof extension
associated with more positive tilted warm conveyor belt.  An
associated surface wave is tracking across the TN Valley but with
time/due to tilted warm conveyor/height falls will form a weak
coastal low off the mouth of the Chesapeake midday today.  The
compact mid-level center will weaken across the upper Ohio Valley
and give way to upstream development. This upstream feature is
also detected in Goes-16 WV as a low amplitude shortwave emerging
from the southern Colorado Rockies with an associated strong jet
streak.  In combination with weak/broad cyclonic shortwave energy
currently in the central Missouri River Basin will become much
more important in the evolution of an amplifying larger scale deep
eastern US trof by Wed/Thurs.  With this amplification a secondary
surface low will rapidly develop out of the Carolinas becoming a
Nor`easter.  While these features are becoming a bit better
resolved, small variations lead to moderate to large sensitivity
toward the larger scale trof evolution.  Hence there remains some
run to run inconsistency in the longer term (last 2 days or so),
keeping confidence quite low in the precise details, locations of
thermal, precip and other important sensible weather conditions;
however recent trends over the last day or so are providing much
higher confidence in the eventual deep Nor`easter along the east

The 00z NAM which has been most aggressive over the last 2 days or
so, especially with the lead wave staying strong in the Ohio
Valley and building the eastern trof has backed off suggesting a
flatter devolution of the lead height falls and slightly stronger
coastal reflection on late Tuesday bringing it back into the fold
of the more consistent consensus at least for the lead
wave/surface low.   However, in shifting the wave east, the
secondary low has greater spacing and increased shortwave ridging
between the systems resulting in a southward shift of the inner
core of the trof as well as the surface low.  As the surface low
rapidly deepens off the Delmarva, the NAM shifts west and leads to
an earlier wobble as upstream energy carves out the upper level
trof.  This provides increased confidence in other guidance
solutions (mainly the ECMWF) that have presented a surface wobble
toward the coast.  While still plausible (especially with the dry
air in place over the northern mid-Atlantic/New England filtering
south), this the run departure from prior NAM runs as well as the
increasing clustering in the ensemble suite makes it hard to make
such a shift at this time.

While the ECMWF had been inconsistent over the last day or
so...the last 2 runs have set a trend with most guidance trending
toward it.  However, the ECENS cluster is well west toward the
coast during a tricordial wobble and quite a bit stronger through
the depth of cyclone (up to 30 DM).  To offset this potential
western, potential over-development, recent 00z GFS run shifts
closer but along with 12z/18z GEFS members are set a bit south and
east of the ECENS members (particularly on Thur into Fri);
although with similar timing/evolution are consistent enough to
have some confidence in a blend of the two solutions.  The 12z
UKMET is a bit too strong with the lead energy offshore which also
reduces best shape of the upper jet pattern and disrupts the best
rapid cyclogenesis off the coast with the second low.  The 12z
CMC, while being most consistent with the longer term ensemble
trend, seems to have missed the recent shift in the last day
cycles that has been picked up by the remaining guidance.   All
considered will suggest a 00z GFS/12z ECMWF blend, likely
stabilizing the blend with some very low percentage of the
GEFS/ECENS mean and 00z NAM to account for the uncertainty.
Confidence is high for a strong Nor`easter and eastern US trof to
develop but slightly below to below average as the precise details
and the model selection to accomplish these details remains

...Deep trough/closed low offshore the Northwest by Wednesday...
Preference: Non-UKMET blend
Confidence: Slightly above average

Large closed upper level ridge over the western and central Gulf
of AK will retrograde Tuesday and two shortwave features: one
lingering along the SE AK coast and an upstream wave over western
AK begin a binary interaction early Wed in the eastern Gulf of AK
beginning to carve out a deep layered closed upper low (with the
prior wave becoming the core, while the other digs southward.  As
the digging wave rounds the base of the other, a weak surface wave
drops south and begins to merge with the frontal zone lifting
north from a closed central Pacific system (see section below).
Overall, the guidance has been consistent and agreeable with
respect to the evolution of the pair with exception of the 12z
UKMET which is much stronger (typical of bias) and more elongated
N-S, i.e. narrower W-E.  The other departure is more related to
the interface/interaction with the southern stream wave/frontal
zone which is better described below.  As such a Non-UKMET blend
is supported at Slightly above average confidence.

...Closed Pacific low near 34N/140W drifting east breaking down
under influence of digging/northern stream closed low providing
deep layer southwest flow/atmospheric river into Southern CA with
associated surface low and frontal zone.
Preference: 00z GFS and 12z ECMWF/CMC blend
Confidence: Average

A broad closed low around 33N142W is consolidating with strong
inner core and secondary/triple point/interface of warm and cool
conveyor belts near 37N138W...starting to elongate a bit under
some upstream ridging dropping south.  This will flatten the
overall flow but also strengthen it providing an excellent
corridor of moisture flux from the subtropical/tropical regions of
the central Pacific supporting a very strong atmospheric river
event directed at south-central to Southern California from early
Wed to Fri.  As mentioned in section above, strong deep closed low
will dig south out of the Gulf of AK influencing the shortwave as
well, potentially shearing it into the northern stream toward late
Wed into Thurs.  However, there remains mild disagreement in the
timing, magnitude of the shearing of the low toward the northeast
(with associated surface wave along sharpening frontal zone).  The
12z UKMET digs a bit further south with northern stream system
leading to increased shearing early, but given this narrow
solution is not favored for the northern system, its effects on
this system appear equally dubious.   The 00z NAM also showing
increased influence of the northern stream but is also
uncharacteristically quick to weaken/shear this system, allowing
it to migrate toward the West Coast a bit too quick too.  The GFS
and continued with the 00z run, has been trending slower to
interact with the northern stream, nearing in the ECMWF/CMC
timing, but this is mainly due to the aforementioned triple point
shortwave rotating much further west, then south to be affected by
the northern stream eventually rotating back as a weaker wave
(than the ECMWF) and therefore faster than the ECMWF/CMC toward
the West Coast by late Thursday.  The 12z operational ECMWF is
very strong/consolidated and very slow to break down and advance
eastward with time, even more so than a bulk of its ensemble
counterparts.  Additionally, the ECMWF resolves a very strong/deep
surface low.  The 12z CMC seems a more reasonable solution
compared to the ECENS members and the mean as a whole.  Still, a
blend of the 00z GFS along with the 12z ECMWF/CMC should represent
a good blend of the GEFS/ECENS means which are preferred.  Given
spread of timing, confidence is average in the mass fields though
quite high for the resulting atmospheric river, placement and

Model trends at
500 mb forecasts at


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