Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
234 AM EDT Tue Apr 17 2018

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

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

00z model evaluation and final preferences
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...Deep occluded cyclone tracking into Ontario/Quebec Tue, over
New England Wed, exiting Thu/Fri...
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Preference: General model blend
Confidence: Above average

Both the 00z NAM/GFS are close to the consensus weakening the
surface low over Ontario/Quebec Tue into Wed, then tracking the
next surface low across western ME Tue into eastern Quebec Wed.
The same is true of the mid level circulation, as the broad system
meanders over New England Wed/Thu. Given the tight clustering of
the operational models with the ensemble means, a general model
blend is preferred, with above average confidence.


...Shortwave exiting the Central Rockies into Plains late
Tue/early Wed with a rapidly developing/occluding surface low
tracking into the Great Lakes late Wed/early Thu, reaching New
England Thu/Fri
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Preference: 00z NAM/00z ECMWF/12z ECMWF ensemble mean blend
Confidence: Average

The 00z GFS, while still a bit further north and faster with the
closed mid level circulation that develops at the base of a
negatively tilted early Wed over western IA, is closer to the
consensus with this feature than previous GFS runs. The 00z NAM
remains close to the consensus with the surface and mid level
features into Thu.

After that time, there is a split in the guidance concerning how
surface low pressure crosses the Northeast late Wed into late Thu.
The 00z GFS (supported by the 00z GEFS mean and 00z CMC) tracks
the surface low across southern NY state into southern New
England. Much of the remainder of the guidance (including the 00z
NAM/00z ECMWF/12z ECMWF ensemble mean) develops a secondary
surface low on a low level baroclinic zone over the northern Mid
Atlantic. The secondary low then tracks near the benchmark before
the two camps converge over eastern Nova Scotia early Fri (though
the 00z ECMWF is a bit slower with the surface low track at
20/12z, despite the mid level forcing being further east).
Conversely, the 00z UKMET becomes faster than the consensus with
the surface low late Thu into early Fri, so it was dropped from
the preference.

The southern solution appears to be the more likely scenario, so
the 00z GFS/00z CMC were not included in the preferred blend.  


...Pacific long wave trough reaching the West Coast late Tue and
closing over the Great Basin Thu/Fri...
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Preference: General model blend
Confidence: Above average

Both the 00z NAM/GFS continue to cluster well with the consensus
taking a closed mid level low from central CA Wed night to UT/AZ
by Fri morning. The 00z UKMET has once again clusters tightly with
the consensus. Since there is very good agreement among
operational models and ensemble means, a general model blend is
preferred with above average confidence.


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

Hayes