Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1250 PM EDT Tue Mar 13 2018

Valid Mar 13/1200 UTC thru Mar 17/0000 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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...Nor'easter Impacting New England...
...Deep, Occluded Low Evolution over the Northeast and Eastern
Canada through Thursday...
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Preference: General Model Blend through 36 Hours;
            12Z GFS, 00Z ECMWF after that
Confidence: Slightly Above Average

Models are generally in good agreement through the first 24-36
hours of the forecast, with tight clustering of low positions and
height fields aloft. The preference is for a general model blend
during that time frame given only minor model variations. After
that, there are some increasing detail differences that do lead to
slightly different forecasts. The 12Z NAM shows the low filling
faster over eastern Canada, and results in a surface low position
that is to the west of all other models (180km west of ensemble
means) by 15/06Z. With the secondary low developing offshore along
the cold front, the NAM shows a much deeper surface low. This
can't be ruled out, but given that the NAM is stronger with the
upper level low overall than other models, it may be a less likely
scenario.

The 00Z UKMET and 00Z CMC show a notably slower northward motion
with the secondary surface low developing along the cold front,
which may be related to a latitudinal positioning difference of
the upper level low. The CMC and UKMET have the upper level low
and base of the trough slightly further south by Wednesday, which
places the initial surface cyclogenesis further south as well. The
preference is to lean toward the ECMWF and GFS, which have better
support from the global ensembles.


...Reinforcing Shortwaves Digging Through the Great Lakes into the
Northeast Thursday and Friday...
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Preference: 12Z GFS, 00Z ECMWF
Confidence: Average

This is really a continuation of the deep occluded low over the
Northeast, as several shortwaves should dig through the Great
Lakes and reinforce the trough across the region. The track,
timing and amplitude of the shortwaves, therefore, should become
increasingly affected by the structure of the larger trough to the
east. The 00Z CMC and UKMET, which show slightly more
amplification to the trough and a position of the upper level low
to the S/SSW of other models, tend to show a more meridional
orientation to the sheared vort max digging into the Great Lakes.
Given that this is likely to be tied to the model preference for
the overall trough, the preference will be to continue with a
blend of the GFS and ECMWF.


...Large, Amplified Trough Pushing into the West Coast...
...Closed Low Forming over the Pacific Northwest by Thursday and
Becoming Increasingly Cut Off From the Polar Jet...
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Preference: General Model Blend, Weighted to GFS/ECMWF
Confidence: Slightly Above Average

Through Thursday, the global models, ensembles and the NAM all
show a similar amplitude to the trough and closed low along the
west coast given fairly good agreement on the position and
amplitude of the upstream and downstream ridges that will be
restricting the position of the low. The more substantial
difference appears to be related to timing -- how fast the closed
low moves down the West Coast Wednesday Night into Thursday. The
12Z NAM, 06Z GEFS Mean and 00Z CMC show a faster initial
progression down the coast. The preference is to lean toward a
slower progression given the closed and increasingly cutoff nature
of the low, which is closest to the 00Z ECMWF Ensemble. The
operational models closest to this progression are the 00Z ECMWF
and 00Z GFS, with the 00Z UKMET also fairly close.


...Shortwave Kicking Through the Four Corners Thursday Night and
into the Central Plains on Friday...
...Surface Cyclogenesis in the Central Plains...
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Preference: 12Z GFS, 00Z ECMWF, Weighted to the ECMWF
Confidence: Slightly Below Average

A shortwave will kick through the Four Corners, quickly become
negatively tilted, and contribute to surface cyclogenesis in the
lee of the Rockies over the Central Plains. A variety of global
models show a closed low developing aloft over the Plains, with
the 12Z GFS and 00Z ECMWF further south over Kansas by 16/18Z, and
the 00Z CMC further north over Nebraska. In general, the CMC,
along with the 00Z UKMET and 12Z NAM show a faster progression to
the trough, with the NAM showing a much weaker surface low as
compared to some of the global models. Given the negative tilt
nature of the trough, the preference is for stronger cyclogenesis.
And given that the overall pattern has a fair degree of blocking,
with a highly amplified ridge downstream of the closed low along
the West Coast, the preference is for a slower trough progression.
This is most similar to the 12Z GFS and 00Z ECMWF, and the ECMWF
is slowest. Therefore, the preference is for a blend of the GFS
and ECMWF, with a slight lean to the ECMWF.


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

Lamers