Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
108 AM EDT Wed Sep 25 2019

Valid Sep 25/0000 UTC thru Sep 28/1200 UTC

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

00Z Preliminary Model Evaluation with Preferences and Forecast
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Northwest...
~~~~~~~~~~~~~~~~~~
Preference: 00Z GFS/18Z GEFS/12Z ECMWF/ECENS
Confidence: Slightly above average

Through 60 hours (12Z Friday) a general model blend is acceptable
with less weight on the CMC as it begins to diverge Friday
morning. However, the most impactful weather and largest spread
begins on D3 /Friday and Friday night/ as an anomalously deep
closed low digs into Washington/Idaho/Oregon. This feature will
spawn surface cyclogenesis and lead to an impressive early season
winter storm across the northern Rockies. While all guidance
outside the CMC suggests this closed low, there exists
considerable timing and intensity differences among the rest of
the suite.

Excluding the CMC which is very much an outlier, the UKMET begins
to lag the remaining global models early on D3 with its southward
push of the closed low, likely due to a much weaker and less
amplified 300mb jet diving equator-ward from Alaska and into the
North Pacific. The NAM takes up the other side of the envelope
becoming a fast solution with the southward push of the closed
low. While the spread between the fast (NAM) and slow (UKMET) is
not extreme, the differences in timing are important as it changes
the precipitation onset for snowfall across the region. The
GFS/GEFS/ECMWF/ECENS are all in very good agreement with both the
position and depth of the upper low at 12Z Saturday, as well as
the 850-700mb thermal evolution, and for this reason are preferred
at this time.


...Rest of CONUS...
~~~~~~~~~~~~~~~~~~
Preference: General model blend through 48 hours. Non-CMC
thereafter.
Confidence: Above average

Through 48 hours the global suite features little spread. The
general synoptic evolution is for a closed low across the
Southwest to slowly lift northeast and become absorbed in the
westerlies ahead of the amplifying northwest trough by day 3. In
the east, a ridge will persist with the greatest height anomalies
across the Southeast. This forces the sensible weather and
greatest forecast challenges across the northern tier in fast flow
pinched generally above 40N latitude. Outside of the CMC which is
vastly different in its mid-level wave amplitude and evolution, a
shortwave driving a cold front through the middle part of the
country by D3 features little spread in QPF, placement, or timing.
The minor exception is the ECMWF which lags a bit compared to the
consensus. While it can be included for now, there is potential
with significant QPF possible into D3 and beyond that the slower
solution could pose more of a flooding problem towards the end of
the period. Low forecast skill in that at this time range, and low
confidence as well, but should be mentioned for possible slightly
less weight on the ECMWF at D3.


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

Weiss