Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
144 PM EST Wed Feb 13 2019

Valid Feb 13/1200 UTC thru Feb 17/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidences
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...Central to Eastern CONUS...
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Preference: Blend of the 12Z GFS, 12Z UKMET and 00Z ECMWF
Confidence: Average

..19Z update...

Minor adjustments were made with the southern stream shortwave
from the Great Plains to the Southeast, and the related surface
low across the Mid-Atlantic. The 12Z CMC trended toward the
non-12Z NAM consensus and the 12Z ECMWF/UKMET adjusted south of
their previous 00Z cycles. Given the 00Z UKMET was already on the
southern side of the guidance (not counting the NAM), a farther
shift to the south is not preferred given the 12Z UKMET's
placement on the edge of the latest ensemble scatter low plots.

...previous discussion follows...

A developing coastal low near New England will depart toward the
northeast today/tonight followed by a developing closed low
forecast to track from near Lake Superior Friday morning to
southern Quebec for Saturday morning. Meanwhile a southern stream
shortwave emanating from the Intermountain West will track into
the Great Plains early Friday and help to form a weak surface low
which will track across the southern Mid-Atlantic states.

Differences in timing with the northern system to affect the Great
Lakes and Northeast show up in the 12Z NAM and 00Z CMC as
initially slower with the track of the mid-level closed low. The
12Z GFS becomes a bit faster and tracks slightly south of the
consensus while the 00Z ECMWF is slightly north of the consensus
near the Northeast. A blend of the 12Z GFS, 00Z ECMWF and 00Z
UKMET allows a reasonable compromise with this system.

Farther south, the 12Z NAM is an outlier with the handling of
mid-level shortwave energy out ahead of the main shortwave
crossing the Plains on Friday. This feature is not in the other
models and is an outlier with respect to nearly 100 ECMWF/GEFS/CMC
ensemble members across the southern Appalachians late Friday. The
00Z CMC ends up rather weak with the mid-level shortwave but there
is decent ensemble support for a blend between the 00Z ECMWF, 00Z
UKMET and 12Z GFS concerning the mid-level heights and surface low
track. There have been general trends slightly south over the past
day but continued north/south wobbles can be expected with the
current preference to be near the middle of a 12Z GFS/00Z
ECMWF/00Z UKMET compromise.

...Western CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z NAM/GFS/ECMWF/CMC
Confidence: Average

..19Z update...

Changes in the 12Z ECMWF/UKMET/CMC were minor relative to their
previous 00Z cycles, but enough of an adjustment was made by the
12Z CMC to allow its inclusion as part of the overall preference
for the West given shortwave placement and timing. Only the 12Z
UKMET stands out with a slower shortwave progression into the
Plains and with less amplitude of mid-level troughing off of the
Pacific Northwest Coast near 00Z/17. Otherwise, no changes in
thinking are needed to the previous discussion.

...previous discussion follows...

After the departure of a progressive southern stream shortwave
across California and Nevada late Thursday, an expansive closed
low will set up just offshore of the Pacific Northwest and open up
as it tracks eastward into the Rockies Friday into Saturday.
Additional shortwave energy feeding down the west side of a
longwave trough over the western U.S. into the weekend will follow
suit with a broad trough extending from just offshore of the West
Coast to the central Plains by Saturday evening.

There is generally good agreement with this evolution in the
models, but the 12Z NAM, 12Z GFS and 00Z ECMWF have the most
similar timing of the smaller scale embedded shortwaves and
exhibit agreement with the latest ensemble means. However, run to
run continuity in all of the deterministic models has not been
great out West over their past 3 cycles. Therefore, confidence is
above average on the larger scale evolution given strong agreement
in the ensemble spaghetti heights with the large scale flow, but
below average with the details and timing of the individual
shortwaves.

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

Otto