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FXUS10 KWNH 231649

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1248 PM EDT Mon Apr 23 2018

Valid Apr 23/1200 UTC thru Apr 27/0000 UTC

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

12Z NAM/GFS evaluation including preliminary preferences

...Closed low over the lower OH valley today, gradually opening
into a trough and lifting into the Mid Atlantic and Northeast,
phasing with northern-stream shortwave digging through the Great
Lakes Tuesday Night into the Northeast...
...Weak surface low over the interior Southeast, redeveloping
along the Atlantic coast by mid-week...
Preference: 75% 12Z GFS / 25% 00Z ECMWF blend
Confidence: Below average

Ensemble spaghetti height trends show that the primary trough axis
reaching the Mid-Atlantic region Wednesday morning was reasonably
consistent over the past 4 cycles (valid 12Z/25) but the latest
cycle adjusted slower/south. This may be a temporary shift, but
currently, the 12Z NAM/GFS are faster with the progression (NAM
fastest), while the 00Z ECMWF/UKMET/00Z CMC are slower. The
GEFS/ECMWF ensemble means are between the faster and slower
deterministic runs, but with a weight toward the faster 12Z

This timing difference is reflected in the ensemble scatter low
plots as well and is greatly influenced due to timing differences
with an upstream shortwave crossing the Great Lakes Wednesday into
Thursday with similar timing differences as the initial wave along
the East Coast (NAM/GFS faster, ECMWF/UKMET/CMC slower). Given the
shift in the ensembles, previous consistency and the latest
ensemble means pulling toward the middle of the spread, the
preference is to be between the 12Z GFS and 00Z ECMWF, with a
weighting toward the faster GFS. Confidence is below average given
greater than average spread and complex interaction of multiple

...Shortwave entering northern Great Basin, amplifying into the
northern Rockies through Monday Night, and then digging through
the Central Plains Wed/Lower MS Valley by Thurs...
Preference: 12Z GFS, 00Z ECMWF, 00Z CMC blend
Confidence: Average

Timing differences here are such that the faster and slower ends
of the spectrum are not preferred with weighting in the middle
best represented by a 12Z GFS...00Z ECMWF...00Z CMC blend. The 00Z
UKMET is significantly faster/flatter with the wave, with the 12Z
NAM slowest. A blend of the 12Z GFS, 00Z ECMWF and 00Z CMC is
toward the middle of the spread and considered best at this time.

...Northern stream portion of Gulf of Alaska trough breaking off
and digging through the Canadian Rockies Wed, continuing into the
northern Plains Thursday, with associated surface cold front
pushing southeast...
Preference: 12Z GFS, 00Z ECMWF, 00Z UKMET blend
Confidence: Average

Ensemble means support the somewhat slower timing of the 12Z GFS,
00Z ECMWF and 00Z UKMET with this shortwave while the 00Z CMC and
12Z NAM are faster as the trough axis reaches the Plains. The 00Z
CMC is also noted to be stronger with the upper wave resulting in
a more defined surface reflection across the Midwest. Given decent
agreement in the ensemble means and with the associated cold
front, a middle of the road approach is favored at this time.

...Southern stream portion of Gulf of Alaska trough breaking off
and developing into a closed low around 35N/135W by Tuesday...
Preference: General model blend
Confidence: Average

The 00Z ECMWF is slower/west with the closed low off of the West
Coast of the U.S., with the 12Z GFS faster. This is an often seen
bias between these two models, except that the ECMWF is not
supported by the 00Z UKMET/CMC, with the UKMET/CMC favoring a more
GFS/NAM-like solution. The ensemble spaghetti plots support a
middle of the road approach, and given overall differences are not
too large through 00Z/27, a general model blend is recommended at
this time.

Model trends at
500 mb forecasts at


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