Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Tue Mar 20 2018

Valid Mar 20/1200 UTC thru Mar 24/0000 UTC

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

12Z Model Evaluation...Including Model Preferences and Confidence
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...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
coast...
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Preference: Lean toward the GFS
Confidence: Average

Compared to its 00Z run, the NAM is faster with the initial
surface wave moving off of the Mid-Atlantic coast on Tue.  Then
with respect to the second low developing off of the Mid-Atlantic
coast late Tue into Wed, the NAM becomes faster than its previous
run as the upper low moves off the Mid-Atlantic coast and begins
to track northeast Wed night.  This brings the surface low more
quickly into the Canadian Maritimes by Thu evening. 

The GFS has remained fairly consistent with its 00Z run until Thu
morning when it becomes a little more amplified with the upper low
and in turn has shifted its surface low a little further west as
it approaches Nova Scotia on Thu.

Even with the shift in the GFS, the NAM is a little more amplified
than the model consensus with the 700-500mb low as it tracks
northeast from off of the northern Mid-Atlantic coast to east of
New England on Thu, and in-turn tucks the surface low closer to
the New England coast than the remainder of the deterministic
guidance.

In contrast to the NAM, ECMWF has trended a little weaker and
faster aloft -- making it a relatively weak outlier as the low
moves along the New England coast Thu morning.  With the majority
of the deterministic and ensemble guidance supporting a deeper
solution, recommend leaning closer a solution more similar to the
GFS.



...Deep trough/closed low offshore the Northwest by Wednesday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

Overall, the NAM has remained fairly consistent with its 00Z run
with this system.  Compared to its overnight run, the GFS digs a
more amplified shortwave south of the low toward the Pacific
Northwest Thu evening.  This not only results in lower surface
pressures along the associated frontal band as it moves across the
northwestern U.S., but also appears to influence the timing and
amplitude of the shortwave described below. 

The UKMET, and to a lesser degree the ECMWF, have shown trends
similar to the GFS -- lessening the differences noted in the
previous issuance, while allowing any lingering differences to be
addressed with a general model compromise.


...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....
...Remnant shortwave trough moving into the central U.S. on Fri...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

Overall, the NAM has remained fairly consistent with its 00Z run
with this system. The GFS however has made a notable shift toward
a more progressive solution.  As the remnant shortwave associated
with the low approaches California, it appears to come under
greater influence of the more amplified shortwave digging south of
the previously described upper low to the north -- ushering a
less-amplified shortwave more quickly to the east into California
on Thu.  The 12Z GFS remains more progressive through the
remainder of the period, in-turn developing a low more quickly in
the lee of the central Rockies and then tracking it east into
central Kansas by late Fri.  

As with the system to the north, the UKMET and ECMWF have shown
trends similar to the GFS.  While the GFS is still along the less
amplified and progressive side of the guidance with the shortwave
as it moves across southern California into the Great Basin on
Fri, differences are less pronounced than those noted in the
previous issuance.  Here too, any remaining differences can be
largely addressed with a model compromise.   

Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Pereira