Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
332 AM EDT Tue Sep 24 2019

Valid Sep 24/0000 UTC thru Sep 27/1200 UTC

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

00Z Final Model Evaluation with Preferences and Forecast Confidence
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...CONUS...
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Preference: 00Z NAM/UKMET/ECMWF and 12Z ECENS
Confidence: Above average

07Z Update:
There is little change in the new suite and updated global
consensus, so the preferred blend remains primarily unchanged. The
00Z CMC did speed up a bit to better match the consensus, but it
still lags with the trough axis and front on D3. While this is an
improvement, due to most of the remaining guidance being so well
clustered opted to leave the CMC out of the blend for now.
Additionally, the GFS is now a fast outlier with the eastern
trough axis while the GEFS mean remains on the slow side of the
envelope. The 00Z GEFS has caught up a bit to the consensus,
similarly to the CMC, but remains on the slow side of an otherwise
well clustered suite. For this reason opted to leave the GFS and
GEFS out of the blend until better consistency can arise from that
model.

Previous Discussion:
The guidance is in exceptionally good agreement in the longwave
pattern across the CONUS the next 3 days. A large closed low over
the Hudson Bay, Canada, will shed vorticity impulses across the
northern tier in a fast northern stream through late week. Further
south, a general flat ridge will encompass much of the region from
35N latitude and south, the exception being a slow moving closed
low dropping through the Southwest and into Mexico before
returning to the northeast and slowly becoming absorbed into the
westerlies Friday morning.

For the Southwest closed low, there is little spread whatsoever
until the end of day 3 when the UKMET outpaces the remaining
guidance in its absorption into the fast flow to the north. The
difference in timing and speed is minimal even by day 3, so a
general model blend is sufficient. However, further east, a
shortwave driving a cold front across the middle of the country
and then into the east coast D2-3 begins to feature more
uncertainty as the CMC and GEFS mean slow the progression to
become well west of the mean trough axis by Friday morning. This
seems to be due to a weaker and more strongly kinked 250mb jet
streak in the vicinity of the Great Lakes. While the GEFS and CMC
are actually in pretty good agreement, they are much slower than
the global consensus, and with a fast northern stream anticipate
the faster solutions are more likely.


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

Weiss