Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1229 AM EDT Sun May 13 2018

Valid May 13/0000 UTC thru May 16/1200 UTC

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

00Z GFS/NAM evaluation with preliminary model preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GFS/NAM initialization errors do not seem to degrade their short
range forecast.


Upper low approaching CA Tuesday night
Great Basin Upper low which drifts north
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The guidance shows good agreement with these systems.  A
compromise of the 12z ECMWF, 12z Canadian, 12z UKMET, 00z GFS, and
00z NAM is preferred with above average confidence.


Possibly convective/stacked cyclone in the Gulf of Mexico
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: GFS/NAM blend; confidence average

The 12z Canadian turns the system northwest into the Gulf Coast a
bit faster than the remainder of the guidance.  Since the guidance
in general can retrograde mid-level cyclones too much, did not
trust the Canadian solution.  Otherwise, while the guidance has
some convective/gridscale feedback within the atypically moist
(for May) and unstable environment, the 12z ECMWF was the worst of
the bunch, rotating a system near Naples north, then northwest
across FL into GA and AL at 850 hPa.  This distorted the ECMWF's
pressure pattern more than most of the other guidance, washing out
the deep layer cyclone in the Gulf, which was out of sorts with
the 18z GEFS/12z ECMWF ensemble mean solutions.  The 12z UKMET was
weak and east of the ensemble mean guidance.  A compromise of the
00z GFS/00z NAM was preferred here which fit the ensemble mean
guidance best and made the most meteorological sense.  Confidence
is average in this choice.


Base of the eastern Canada upper low/trough
Front in the northern Mid-Atlantic/New England states Tue/Wed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: UKMET/Canadian/ECMWF blend
Confidence: Slightly above average

Aloft, the guidance agrees on rather flat/quasi-zonal flow across
the Great Lakes/Northern Mid-Atlantic states/New England states. 
At the surface, the 00z NAM in two days (before it folds in better
with the other guidance), and then the 00z GFS in three days, end
up farther north with the front near the 40th parallel.  With a
surface high of 1020+ hPa moving from southern Quebec into New
England, a more southerly frontal position towards D.C. makes the
most sense (an old NMC rule of thumb).  This is in the direction
of the 12z UKMET/12z Canadian/12z ECMWF.  A compromise of these
pieces of guidance is preferred with slightly above average
confidence.


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

Roth