Prognostic Meteorological Discussion
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FXUS10 KWNH 251719
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
118 PM EDT Wed Apr 25 2018

Valid Apr 25/1200 UTC thru Apr 29/0000 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 central Appalachians today...
...Energy lifting into the Mid-Atlantic and Northeast by
Thursday...
...Phasing with northern stream shortwave energy crossing the
Great Lakes...
...Surface low lifting up across the Northeast...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Slightly above average

The models again advertise the phasing of southern stream
troughing/closed low energy over the central Appalachians region
with the arrival of a northern stream shortwave dropping southeast
across the Great Lakes region. This will allow low pressure
initially over the Mid-Atlantic to lift up across the interior of
the Northeast where it will deepen further with the expected
phasing of streams. The models are in reasonably good agreement
now with the details of the phasing, and so a general model blend
will be preferred at this time.


...Energy dropping southeast to the lower MS Valley Thursday
before crossing the Southeast by Friday...
...Low pressure lifting from the Southeast to the Northeast Friday
and Saturday...
...Phasing with northern stream energy over the Northeast by
Saturday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z NAM, 12Z GFS and 00Z ECMWF
Confidence: Average

Energy crossing the central Plains today will drop southeast and
cross the Gulf Coast states through Friday before then lifting up
across the Northeast on Saturday as it then phases with northern
stream troughing crossing the Great Lakes region. This will allow
low pressure developing over the Southeast by Friday to lift up
across the Mid-Atlantic and into the Northeast by Saturday with
some gradual deepening of the low center. The models are coming
into better agreement gradually with the evolution of the low
center in particular, but the 12Z NAM and 12Z GFS do again take
their low centers a little left of the non-NCEP guidance. Although
there has been some gradual westward trending of the ECMWF and CMC
solutions. The GEFS mean favors the westward track of the low
center in association with a bit quicker and stronger northern
stream phasing, and there is some ensemble support for this as
well from the European and Canadian ensemble camps. Will prefer a
blend of the NAM, GFS and ECMWF at this time.


...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: Slightly above average

Energy crossing western Canada will 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
differences are more related to timing as the 12Z NAM and 00Z
ECMWF are a bit slower than the 12Z GFS and 00Z UKMET. The 00Z CMC
splits the difference. The preference will be to compromise with
the details of the troughing/energy from the Great Lakes region
south down toward the Gulf Coast. Thus a general model blend to
resolve the smaller scale and more modest differences.


...Closed low evolution toward the West Coast by Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Slightly above average

A well-defined closed low will be approaching the West Coast by
Friday as energy south of the Gulf of Alaska advances east. The
closed low should begin moving inland on Saturday toward the Great
Basin. The models are in reasonably good agreement on the large
scale with the height falls, however the 12Z NAM becomes a bit of
a stronger outlier with the system by Saturday as the energy moves
inland. Will prefer a non-NAM blend with this system.

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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