Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1129 AM EST Sun Mar 04 2018

Valid Mar 04/1200 UTC thru Mar 08/0000 UTC

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12z NAM/GFS Evaluation and Preliminary Preferences
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...Mean Trough Exiting the Mid-Atlantic / Northeast States through
Monday...
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Preference: 12z GFS and 12z NAM
 
In the wake of the deeply occluded storm off the east coast, a
small scale shortwave trough is forecast to dive across the
Northeast and Mid-Atlantic today, reinvigorating the base of the
larger scale trough offshore before the next upstream system
starts to take center stage. Generally speaking, there are some
reasons to favor the NCEP camp of models regarding many of the
shortwave details within the blocky northern latitudes and
semi-progressive more southerly latitudes (see Central U.S.
section below). When the 12z NAM trended slower and deeper with
the afore-mentioned shortwave diving across the Northeast, and the
GFS then fully supported this trend, there appear to be plenty of
reasons to favor a NAM/GFS blend for this system. The 00z ECMWF
certainly appeared too fast, or was the fastest of the major
operational models.


...Central U.S. Storm Passing Off to Developing Nor'easter by
Wednesday...
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Preference: 12z GFS...supported by 00z/06z GFS Continuity, 06z
GEFS Mean, and to lesser extent the NAM

A medium wavelength system ejecting at a negative tilt into the
Plains will drive a winter storm and, in some cases, blizzard
conditions, across that region through Monday night / Tuesday. The
storm then goes through a fairly interesting evolution which the
models agree should lead to a hand-off to coastal low pressure
development and a potential Nor'easter mid week. The mature
cyclone over the Plains/Midwest by Tuesday will sit beneath a
blocky pattern in place across the full breadth of Canada, but
within what could loosely be termed a progressive flow pattern at
the latitudes of the United States, describing the flow from
roughly British Columbia to the central U.S. to the western
Atlantic. The mature cyclone is forecast to be urged along by
energy dropping down the back side of the back side of the upper
trough, pushing height falls to the south and east, while a sort
of unconventional kicker system opens up the trough from the north
side / Ontario and Great lakes / allowing it once again to pivot
around to a negative tilt as it reaches the east coast on
Wednesday.

Models agree to these concepts, but when considering the details,
WPC preferred the more pronounced cyclonic flow over the central
High Plains states on the back side of the cyclone, as well as
sharper troughing into the height weakness over the offshore
waters of British Columbia. This tended to remove the 00z ECMWF
from consideration, and to lesser extent the UKMET, especially by
Day 3. The 00z Canadian did not appear particularly helpful, and
was somewhat out of phase along the east and west coasts by Day 3.
This leaves us favoring the NCEP... GFS/NAM camp. Fortunately, the
GFS showed good continuity between 00z, 06z, and 12z runs, as well
as good support from the 06z GEFS mean. This helps boost our
confidence in recommending the GFS as a majority basis for the
short term forecast. The 12z NAM, while usually not the best model
for the details beyond Day 1, is reasonably close to the GFS for
many aspects of the flow, and could be used, although we would
more strongly recommend the GFS.


...West Coast / Northwest / Northern Rockies...
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Preference: Blend of 12z GFS/NAM with the 06z GFS

Synoptic reasoning applied to the flow regime from the eastern
Pacific to the Atlantic led WPC to favor the GFS as a basis for
the short range forecast (see previous sections). In the West, the
12z GFS may be over-amplifying a shortwave within the eastern
portion of the positively tilted trough by the end of Day 3, as it
approaches the area off northern CA and southwest Oregon. Blending
with the 12z NAM and 06z GFS would provide a better consensus
solution more in line with the relative confidence given a fair
amount of spread among the operational models. Here, as well as
elsewhere on the map, however, the 06z GEFS does offer good
support to the recent operational GFS runs and the 12z NAM.
  
Previous model trends at www.wpc.ncep.noaa.gov/html/model2.shtml  
Previous 500 mb forecasts at  
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml  
  
Burke