Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1239 PM EDT Tue Apr 17 2018

Valid Apr 17/1200 UTC thru Apr 21/0000 UTC

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

12Z Model Evaluation...With Preliminary Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Deep occluded cyclone over the Northeast today, gradually
exiting northeast through the Canadian Maritimes by Thursday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-CMC blend
Confidence: Above average

Models continue to show very good agreement with this system, with
limited spread in many of the mass fields. The primary exception
appears to be the 00Z CMC, which shows more significant height
rises in the wake of the departing low, with a weaker low overall
aloft. This affects the overall cyclonic circulation and a non-CMC
blend is preferred.


...Shortwave ejecting into the central Plains Tuesday Night...
...Moving east into the Ohio Valley by Wednesday Night, and
phasing with a digging Arctic shortwave over the Northeast on
Thursday...
...Surface low moving across the northern tier of states...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend; weighted toward 00Z ECMWF
Confidence: Slightly above average

Models have generally continue to move into better agreement with
this system, and despite some differences are fairly similar
overall. The 12Z GFS and 06Z GEFS Mean continue to show a surface
low track a little further north through the Midwest and into the
Mid Atlantic. They also continue to be slightly faster with the
trough aloft, with the 00Z CMC and UKMET on the slow end, and the
00Z ECMWF, ECMWF Ensemble, and 12Z NAM more in the middle. At this
point, the differences are generally small enough that a general
model blend is preferred to account for lingering uncertainty, but
the preference is also to weight the blend more to the 00Z ECMWF,
which maintains continuity with the overnight model diagnostic
preference and also has reasonable run to run continuity.


...Potent Pacific shortwave digging into the West Coast Wed...
...Closing off a strong upper level low in the Great Basin on
Thursday with surface low development...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00Z ECMWF; 00Z ECMWF Ensemble Mean
Confidence: Average

Differences with this system are relatively minor through 19.12Z
(Thursday morning), but grow more substantial as a mid-upper level
low closes off over the Great Basin and Intermountain West. Most
models show multiple closed height contours at 500mb (at 60m
intervals), which tends to support a forecast with a slower
progression. This is best exemplified by the operational 00Z ECMWF
and the 00Z ECMWF Ensemble at this point. The 00Z CMC is even
slower, but is more of an extreme scenario relative to the
ECMWF+GEFS ensemble members, and is considerably slower than the
00Z CMC Ensemble Mean. The 12Z GFS and 12Z NAM are on the faster
end of model spread. The 00Z UKMET is less expansive with its
upper level low and structurally seems difference than many of the
other deterministic models. Therefore, the preference is to lean
toward the 00Z ECMWF and its ensemble mean.


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

Lamers