Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
211 PM EST Fri Nov 29 2019
Valid Nov 29/1200 UTC thru Dec 03/0000 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...
Final 12Z Model Evaluation with Preferences and Confidence
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...Deep closed low forming in the Plains...
...Handing off to a Northeast Coastal Low on Day 3...
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Preference: Blend of 12Z GFS / ECMWF
Confidence: Slightly Above Average
A deep upper trough pulling into the plains will spawn
cyclogenesis, resulting in a deep closed system that is forecast
to slide eastward through the short range period. As height falls
cross the Appalachians we should see a handoff to a newly forming
coastal low / Nor'easter on Day 3, Monday. In recent days the
ECMWF and ECMWF Ensemble have proven more consistent and served as
a baseline for this storm. The NCEP solutions, however, were
becoming more usable. The 12Z NAM and GFS show a large scale
evolution that is very similar to the ECMWF. The thermal fields
are of interest, given the anticipated winter weather impacts.
Certainly by 48 hours, 12Z Sunday, the NAM strongly indicates its
typical cold bias, with 850 mb cold air of greater magnitude and
more expansive than the other models. Noting that the thermal
fields in the operational ECMWF are strongly supported by the
ECMWF ensemble mean, we have more trust in the European as a
baseline. The GFS, though, does look increasingly reasonable,
especially given just a very subtle warming in the 12Z ECMWF that
brought the two models very much into agreement overall. The main
question mark is slightly warmer air, 850-700mb, the GFS indicates
along the warm front in the Midwest and then near the low track
throughout its journey to the East Coast. It would not be unusual
for air to verify a bit on the warm side of the spread near the
low track. For now, blending toward a consensus seems the best
approach.
...Western U.S. and Northern Rockies/Plains...
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Preference: Blend of 12Z GFS / ECMWF...emphasis on the ECMWF
Confidence: Average
Another deep trough and wet storm system will dig into place near
the West Coast. Through Monday this system sits beneath mean
shortwave ridging over the Pacific Northwest, with northern stream
shortwaves amplifying downstream over the Northern Rockies and
northern Plains. Looking at the northern stream flow pattern
across the Pacific and North America, despite a noisy flow with
numerous shortwaves the models are relatively tight, with the only
outlier being an awkward NAM solution over the northern tier of
the U.S. by Days 2-3. Within the more stationary deep trough along
the West Coast the 12Z NCEP models began a trend in holding onto
the definition of a tight upper low that retrogrades southwest out
of Oregon. The ECMWF and UKMET seem to show a more realistic
trend, while the GFS and Canadian were much slower and represented
a stark change from the previous model cycles. The NAM is one of
the more reasonable solutions within the trough, but its solution
downstream and in the northern stream is so messy and unsupported
that it is hard to recommend in our preference. Fortunately, the
NAM/GFS thermal fields are very similar to the ECMWF over the
western U.S., for forecasting precip type. But for shortwave
timing and large scale forcing, we recommend emphasis on the ECMWF.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
Burke