Prognostic Meteorological Discussion
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933
FXUS10 KWNH 241858
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Tue Apr 24 2018

Valid Apr 24/1200 UTC thru Apr 28/0000 UTC

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

12Z model evaluation including preferences and forecast confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Closed low over the OH/TN Valley today, gradually opening into
a trough and lifting into the Mid Atlantic and Northeast, phasing
with northern-stream shortwave closing off through the Great Lakes
late Wednesday before lifting into the Northeast Thursday...
...Weak surface low over the interior Southeast transition to
Carolinas Wednesday/Thursday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z GFS and 12Z ECMWF
Confidence: Average to slightly below average

The models continue to advertise the phasing of southern stream
troughing/closed low energy over the OH/TN Valley region with the
arrival of a northern stream shortwave dropping southeast across
the Great Lakes region. Differences with the southern stream
energy are quite small as the energy lifts off to the northeast
over the next 24 to 36 hours. However, the 12Z NAM and 12Z GFS are
a bit faster to advance the northern stream energy off to the east
through Wednesday, which allows for phasing to occur a bit farther
east over New England Wednesday night and Thursday. The 12Z CMC,
12Z ECMWF and especially the 12Z UKMET are all slower, and
consequently allow for phased low pressure to linger a bit farther
back to the west over the interior of the Northeast. The GFS and
ECMWF tend to show the strongest phasing and actually by Thursday
night tend to come into somewhat better spatial/timing agreement
along with the CMC solution, but the UKMET remains a slow outlier
post-phasing. For now, an attempt to compromise between camps will
be preferred given the ensemble spread, while also attempting to
approximate the model consensus. Thus a GFS/ECMWF blend will be
preferred for now.


...Elongated trough with embedded shortwaves crossing the
Rockies/Black Hills...
...Energy dropping southeast to the lower MS Valley Thursday
before weakening through the Southeast by Friday...
...Low pressure near the Mid-Atlantic by late Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z NAM, 12Z GFS and 12Z ECMWF
Confidence: Average...becoming below average

Energy crossing the northern Rockies and High Plains will drop
southeast over the next couple of days and dig in toward the
southeast U.S. by Friday. The 12Z UKMET becomes generally a weaker
and faster outlier in digging the energy across the South, with
the best model clustering generally residing with the 12Z NAM, 12Z
GFS and 12Z ECMWF. All of this energy is expected to drive an area
of low pressure across the interior Southeast by Thursday and then
northeast up toward the Mid-Atlantic region by Friday. However,
there is some notable spread with respect to the placement of low
pressure at the end of the period as the guidance differs with
degree of possible northern stream interaction at that time. The
NAM and GFS show more northern stream interaction and focus low
pressure more over the interior of the Mid-Atlantic on Friday. The
UKMET, CMC and ECMWF are farther east although there has been some
trend to focus the low pressure at least closer to the
Mid-Atlantic coastal plain by the end of the period. The
preference for now is to lean toward a compromise solution that is
close to a blend of the NAM, GFS and ECMWF, which is also
generally favored by the 12Z GEFS mean.


...Northern stream portion of Gulf of Alaska trough breaking off
and sliding southeast through Northern Rockies early Thursday then
starting to anchor the Global scale trough from the Great Lakes to
the lower MS Valley late Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

Energy emanating from the northern Gulf of Alaska will cross
through western Canada and drop southeast through the northern
Rockies and Plains states through Thursday and help lead the way
for more of a longer wave trough evolution over the Eastern U.S.
by late Friday. The guidance tends to agree on the larger scale
with the details of the height falls evolution and agree with
focusing the bulk of the energy farther north closer to the Great
Lakes on Friday along with an area of low pressure. However, there
remains some modest spread toward the base of the trough with the
details of how much energy separation occurs and the timing of it.
Most of the models are hinting at at least a weak closed low by
Friday over the Gulf Coast states, but the UKMET tends to be the
most emphatic with this at this time. Based on the latest spread
of guidance and the ensemble means, a general model blend will be
preferred with all the details of this height fall evolution.


...Closed low evolution toward the West Coast by Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Slightly above average

The southern portion of an elongated trough axis offshore the West
Coast and extending north through the Gulf of Alaska will separate
out and advance east toward the West Coast by Friday as a
well-defined closed low. On the larger scale the model differences
are rather small with respect to timing and depth, and so a
general model blend will be preferred for now.


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

Orrison

$$





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