Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
148 PM EST Fri Feb 15 2019

Valid Feb 15/1200 UTC thru Feb 19/0000 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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...Overall Pattern Across the CONUS...
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A difficult forecast will set up over the lower 48 as the mean
upper trough axis currently just off of the West Coast shifts to
the desert Southwest for Monday evening. As a shortwave emerges
into the Central Plains on Saturday/Sunday, it will become sheared
with a powerful upper level jet axis oriented from the Southwest
to the Northeast by Sunday evening, located between anomalous
ridging over the Caribbean and upper troughing over southeastern
Canada.

...Low amplitude shortwave reaching the Mid-Atlantic on Saturday
with corresponding surface low across the Southeast...
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Preference: General Model Blend
Confidence: Slightly Above Average

Despite a southward shift in the guidance compared to yesterday,
there is good agreement with the track of the mid-level shortwave
but the 12Z GFS is a bit faster, while the 12Z UKMET slightly
slower 12Z Saturday. The 12Z ECMWF/UKMET/CMC showed convergence
toward a common solution with only minor differences remaining.
The zonal flow in place supports a slightly faster track which
would go against the 12Z UKMET but again, differences have become
rather small. There is decent agreement in the surface low track
as well, which has tightened compared to yesterday. A slight jog
to the north cannot be ruled out but model agreement is fairly
good at this point.

...Lead mid-level shortwave crossing through the Central Plains
and Midwest...
...Weak surface low tracking across the Appalachians Sunday night
...
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Preference: Blend of 12Z GFS, 12Z ECMWF and 12Z UKMET
Confidence: Below Average

Differences appear with a closed low across the Central Plains by
Sunday morning with the 12Z NAM on the stronger/deeper side of the
latest deterministic/ensemble guidance. The 12Z UKMET is on the
weak side of the guidance with the remaining models somewhere in
the middle but the UKMET did strengthen compared to its previous
00Z run. Ensemble scatter low plots do not show good agreement but
even still, the 12Z NAM shows on the far northern edge of the
ensembles 00Z/18 and it contains a rather strong 850 mb low on
Sunday. The 12Z ECMWF on the other hand is displaced north with
its mid-level vorticity max on Sunday relative to the remaining
deterministic and ensemble mean guidance, and is likely too weak
with its 850 mb low but it has trended toward the stronger
guidance compared to its 00z cycle. By Monday morning,
redevelopment of low pressure offshore of the Northeast is
expected, but the 12Z GFS is much stronger than other available
guidance. No single model or blend appears ideal here, but
blending the 12Z ECMWF, 12Z UKMET and 12Z GFS is closest to the
little ensemble clustering that is observed both north and south
with this system.

...Mid-level trough re-amplification across the West Coast on
Sunday...
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Preference: General Model Blend
Confidence: Average

Differences in the guidance across the West by Monday morning are
smaller than other parts of the U.S. and rather typical for the
time frame observed. By late Monday, the 12Z NAM is a bit faster
to bring the trough axis toward the east compared to the
consensus, but when comparing with ensemble spaghetti heights,
does not appear unreasonable despite a slight slowing trend in the
guidance over the past four 12/00Z cycles. A general model blend
appears okay with this system through the short range period
(through 00Z/19). The 12Z ECMWF/UKMET/CMC made minor timing
adjustments here so no change in thinking applies to the final
preference.

...Low amplitude mid-level shortwave trough nearing southern
British Columbia late Monday...
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Preference: 12Z GFS/12Z UKMET/12Z ECMWF blend
Confidence: Below Average

The models advertise a low amplitude shortwave moving
south-southeastward on the lee side of a large eastern Pacific
ridge, near the British Columbia coast Monday morning.  The
solutions vary between weakening the trough as it approaches the
U.S. (12Z NAM), keeping the energy to the north into Canada (12Z
CMC) or south (00Z ECMWF) and splitting into two (12Z GFS). The
12Z ECMWF/UKMET keeps the energy in the middle of the spread but
consolidated. Given poor run to run consistency with this feature
and low confidence given its origins over the north-central
Pacific, confidence is limited, but a 12Z GFS/UKMET/ECMWF blend is
a middle-ground approach that is closest to the latest broad model
consensus.

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

Otto