Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1204 PM EST Thu Feb 28 2019
Valid Feb 28/1200 UTC thru Mar 04/0000 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...
12Z NAM/GFS Model Evaluation...with Preliminary Preferences and
Confidence
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Overall CONUS-wide Pattern
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Preference: 12Z GFS/00Z ECMWF, more weight toward ECMWF
Confidence: Slightly Below Average
The 500 mb flow across the lower 48 will stay zonal through the
weekend but with highly meridional flow across Canada and the
transition along the northern tier. Model confidence in this type
of a pattern is a little below average given subtle changes in the
flow can result in larger differences at the surface downstream
and with time. A 12Z GFS/00Z ECMWF blend appears to work well for
most systems given greater ensemble support in the middle of the
spread, except where there are some larger GFS/ECMWF differences
in which case more weight toward the 00Z ECMWF is preferred.
Across the East, disorganized upper level energy will allow a low
along a surface front to track toward the southern Appalachians
through Friday morning at which point a secondary low will begin
to develop along the frontal boundary off of the Mid-Atlantic
coast and offshore of New England. The ensemble scatter low plots
have made large changes over their past 4 12/00Z cycles and
currently, the ECMWF members are closer to the coast and the GEFS
members are weaker and a bit farther southeast/offshore compared
to the ECMWF members. There is greater deterministic support for
an ECMWF-like solution, including the 00Z CMC and 12Z NAM to some
degree. The 12Z NAM however, ends up tracking the surface low much
quicker off to the north and east than all of the deterministic
models and many of the ensemble scatter low plots. Here, a 00Z
ECMWF/12Z GFS blend is recommended overall, but with more weight
on the 00Z ECMWF.
Along the West Coast, a powerful closed low offshore of
Washington/Oregon separates into two parts, with one piece
ejecting east across the Northern Plains through Friday while the
remnant portion weakens across the Intermountain West into
confluent mid-level flow through Saturday morning. Differences
with the portion over the western U.S. show the 12Z NAM/GFS faster
than the 00Z ECMWF/UKMET/CMC. A middle ground is best between the
12Z GFS and 00Z ECMWF.
Another shortwave reaches the West Coast over California by late
Saturday morning with the energy sweeping east to the south of a
large closed low, which will begin to drop south through the Great
Lakes region. At the surface, a low will develop near the Lower
Mississippi Valley Sunday morning and track toward the East Coast
through Sunday evening while deepening. At this time, the 00Z
UKMET/CMC appear weaker/faster with the shortwave energy compared
to ensemble data while the 12Z NAM ends up slower with the
mid-level shortwave later in the day on Sunday. A somewhat more
amplified version between the GFS and ECMWF has greater support in
the latest ensemble guidance but with the ECMWF matching more
closely to the ensembles at 850 mb valid 00Z/04 compared to the
12Z GFS.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
Otto