Prognostic Meteorological Discussion
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771
FXUS10 KWNH 191834
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
233 PM EDT Thu Apr 19 2018

Valid Apr 19/1200 UTC thru Apr 23/0000 UTC

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

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

...Trough over the Northeast with closed low developing today...
...Secondary surface low development off the Northeast coast...
...Deepening cyclone reaching Newfoundland by Friday evening...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS; 00Z ECMWF; 00Z CMC
Confidence: Above average

As the trough is departing the Northeast, models generally are in
good agreement, but there are increasing differences related to
timing beyond that. The models all show similar intensities for
the surface low as it deepens and heads toward Newfoundland, but
the 00Z UKMET is faster than other deterministic models, while the
12Z NAM is slower. The 00Z ECMWF and CMC, and the 12Z GFS, are
clustered closer together and are also quite close to the timing
of the GEFS and ECMWF Ensemble means. The preference is to avoid
the fastest model forecast in situations with a deepening and
occluding cyclone, so will lean toward a blend of the GFS, CMC,
and ECMWF which are most tightly clustered.


...Strong closed low aloft near the Sierra Nevada today, pushing
to the Four Corners by Friday, and into the South-Central US this
weekend...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Average

Models are beginning to converge on the timing of the eastward
progression of this closed low, with the 12Z NAM showing the
greatest difference -- still on the fast end of model spread.
Given the greater ensemble support for the consensus of the other
deterministic models, the preference is to exclude the NAM for
this system at this time. There are some lingering differences
between the remaining models. Notably, the 00Z UKMET shows more of
a southeastward dig into the Lower Mississippi River Valley later
this weekend, while other models keep the mid-upper level low
focused further north. Although the ensemble support is better for
the CMC, GFS, and ECMWF forecasts, we can`t rule out a low
focusing further south, perhaps partially driven by persistent
convection that may tend to build closer to the Gulf moisture
source. Therefore, the preference is for a blend of the CMC, GFS,
ECMWF and UKMET.


...Strong shortwave approaching the Pacific Northwest and British
Columbia late Friday before shifting north of the US border this
weekend...
...Surface cold front crossing the Pacific Northwest...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General Model Blend
Confidence: Slightly above average

Models show reasonably good agreement with this system, so a
general model blend is preferred.


...Lower amplitude shortwave reaching the Pacific Northwest early
Sunday and pushing into the Intermountain West...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Average

The 12Z GFS has continued a trend toward a more amplified wave
that is a little slower, while the 12Z NAM maintains a flatter,
faster wave. The GFS is now in line with the 00Z runs of the CMC,
UKMET and ECMWF, and the 06Z GEFS which had a trough axis that was
more in line with the other global models. The preference,
therefore, is to trend toward a non-NAM blend.


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

Lamers


$$





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