Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1249 PM EDT Sat Apr 14 2018

Valid Apr 14/1200 UTC thru Apr 18/0000 UTC

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

12Z NAM/GFS  evaluation with preliminary preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Longwave trough over the Central Plains early today, becoming
negatively tilted tonight and reaching the eastern CONUS Mon...
...Deep surface low over the Plains early today, occluding and
tracking into the Ohio Valley and Great Lakes into Mon...
...Secondary surface low impacting the Northeast on Mon...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/00Z ECMWF blend through 00Z/17
            00Z ECMWF 00Z/17-00Z/18 (12Z NAM secondary option)
Confidence: Slightly above average

The 12Z NAM is slightly slower with the frontal progression into
the East, and given some of the slower biases seen with the
ongoing system across the South and the fact that the upper trough
axis will be swinging negatively tilted through the South and
East, prefer not to be slowest. However, the 12Z GFS seems to go
astray with the development of a secondary, triple point low near
Washington D.C. Monday morning, tracking into upstate New York
Monday night. The 12Z GFS also differs from the consensus
regarding its 500 mb low progression into the Northeast on Monday,
positioned more to the west. Between 00Z/17 and 00Z/18, there is
decent ensemble consensus for a non 12Z GFS blend with the surface
low and 500 mb low center, led by the 00Z ECMWF with the 00Z NAM a
secondary option for New England.


...Longwave trough slowly dropping into the Pacific Northwest this
weekend...
...Surface cyclone emerging into the Plains during the day on
Tue...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z NAM / 00Z ECMWF blend
Confidence: Average

Timing differences appear with the timing of the upper trough into
the West Coast beginning on Monday with the 12Z GFS a bit faster
than the consensus. This is reflected in the ensembles as well
with the GEFS on the leading edge of the guidance while the 00Z
ECMWF is the slowest of the deterministic guidance. The amplified
nature of the flow pattern across the Pacific supports the idea
for future slowing, but given the 00Z ECMWF mean is ahead of the
00Z ECMWF, prefer to stay somewhat conservative on the timing of
the upper trough. A blend of the 00Z ECMWF with the 12Z NAM is
preferred at this time. This blend also proves true for the
surface low across the Plains on Tuesday, with the middle of the
ensemble clustering supportive of a blend between the slower 00Z
ECMWF and faster 12Z NAM.

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

Otto