Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1205 PM EST Wed Feb 20 2019

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

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

12Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
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...Overall Pattern Across the CONUS...
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Preference: 12Z NAM/12Z GFS/00Z ECMWF blend, least weight on GFS
Confidence: Above average

The flow pattern across the lower 48 will continue to feature
strong mid-level ridging over the Caribbean with mean troughing
over the West. A shortwave over the Upper Midwest today will shear
northeastward toward New England early Thursday morning while a
second, powerful closed low translates into the Southwest. The
closed low in the Southwest will reach the Southern Plains on
Saturday with a negative tilt as another trough begins to slide
down the coast of the Pacific Northwest.

The 12Z GFS stood out from the consensus early with a weaker
secondary surface low near coastal New England Thursday morning,
though placement appeared reasonable given agreement with the
remaining model consensus. The tail end of the cold front
extending southwest from the departing New England surface low
will lay across the Gulf Coast states Friday into Saturday where
the 12Z GFS again is a bit different with a more southward
displaced frontal boundary and stronger ridge in the 850-700 mb
layer off of the Southeast coastline. While the 00Z UKMET/CMC are
also located farther south with the west-east oriented front
across the Gulf Coast states, there are some minor differences
aloft which may be leading to these differences in frontal
placement.

As the closed low from the Southwest reaches the Plains on
Saturday, the 00Z UKMET stands out as a little slower than the
deterministic/ensemble consensus, with recent trends showing
faster with the trough axis as it reaches the Plains. The model
differences are not too great here, but the surface low from the
12Z GFS on Saturday is on the west side of the latest ensemble
spread in the Midwest, which has also been trending faster/east
over the past 4 12Z/00Z cycles. A weight closer to the 12Z NAM/00Z
ECMWF is preferred over the 12Z GFS.

Across the Northwest on Saturday, another broad mid-level trough
is expected to slowly advance southward across the Pacific
Northwest. The 12Z GFS continued an overall guidance trend to be
slower but stands out on the slowest edge of the latest ensemble
guidance for 00Z/24 over Washington/Oregon. Given tighter
clustering in the ensemble guidance today versus yesterday,
increased confidence is in place for a consensus blend with the
least weight on the 12Z GFS.

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

Otto