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326
FXUS10 KWNH 231832
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
232 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
ingest...

12Z model evaluation including final 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: 50/50% 12Z GFS / 12Z 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 12Z ECMWF/UKMET/CMC are slower. The GEFS/ECMWF
ensemble means are between the faster and slower deterministic
runs, but with a weight toward the faster 12Z GFS/NAM. The trends
in the 12Z ECMWF/CMC have been faster and the 12Z UKMET has
adjusted away from the better ensemble/deterministic clustering
with an upstream wave valid Wednesday, shifting south/deeper with
the core of the 500 mb low.

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 (referenced at the end of the last paragraph above) 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 12Z ECMWF, with equal weighting given
recent 12Z deterministic trends. Confidence is below average given
greater than average spread and complex interaction of multiple
shortwaves.


...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, 12Z ECMWF, 12Z 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...12Z ECMWF...12Z CMC blend. The 12Z
UKMET remains significantly faster/flatter with the wave, with the
12Z NAM slowest. A blend of the 12Z GFS, 12Z ECMWF and 12Z CMC is
toward the middle of the spread and considered best at this time,
with no significant changes noted with the 12Z ECMWF/UKMET/CMC
compared to their previous 00Z runs.


...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: non 12Z NAM blend
Confidence: Average

Ensemble means support the somewhat slower timing of the 12Z
GFS/ECMWF/UKMET/CMC with this shortwave while the 12Z NAM is
faster as the trough axis reaches the Plains. Given decent
agreement in the ensemble means and with the associated cold
front, a middle of the road of the ensemble spaghetti plots
approach is favored at this time, best represented by a non 12Z
NAM blend.


...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 12Z ECMWF remains slower/west with the closed low off of the
West Coast of the U.S., with the 12Z GFS faster/east. This is an
often seen bias between these two models, except that the ECMWF is
not supported by the 12Z UKMET but the 12Z CMC adjusted to be
closer to the ECMWF compared to the 00Z CMC.  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 www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Otto


$$





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