Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
145 PM EST Sat Jan 05 2019

Valid Jan 05/1200 UTC thru Jan 09/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Overall Pattern Across the CONUS...
Preference: Non-NAM blend
Confidence: Above average

Overall, the guidance is in rather good agreement with the large
scale pattern across the CONUS for the next few days, which will
feature multiple longwave troughs progressing west to east
generally from the Pacific Ocean. The initial closed low impacting
the Mid-Atlantic region will quickly exit offshore by tonight, and
the models show good agreement with northern stream troughing
digging in across the Northeast through Sunday. Meanwhile, broad
longwave troughing will eject inland from the Pacific Ocean across
the Western U.S. through Sunday and then eject east across the
Plains on Monday along with a cold front. By Tuesday, this energy
will progressively cross the Midwest and be moving into the
Eastern U.S. The guidance is generally in good agreement with the
timing and depth of the trough, although by Tuesday, there is some
exception noted to the 12Z GFS and more so the 12Z NAM.

The 12Z GFS has trended a tad deeper overall with the evolution of
troughing crossing the Great Lakes region and appears to be
perhaps a tad too strong with its surface low pressure over the
Great Lakes at the end of the period. Its surface cold front also
tends to lag the non-NCEP guidance owing to a bit slower height
fall evolution. It should be noted though, that at least aloft,
the 12Z CMC and 12Z ECMWF are actually now stronger than the GFS
by the end of the period and have trended this way with the
current cycle. Regarding the 12Z NAM, it is the most out of
tolerance with respect to the timing and depth of a couple of
shortwaves embedded within this larger scale trough. Most notably
the NAM is sharper than the global models with energy crossing the
lower Great Lakes and also entering the upper Midwest by late
Tuesday. This is at the expense of its longwave trough depth which
is weaker now than all of the global models. The NAM is also
farther north than the model consensus with the evolution of low
pressure across southern Canada and toward James Bay. The guidance
by the end of the period also is favoring some cyclogenesis near
the coast of southern and eastern New England. The NAM and GFS are
more ill-defined with this wave development compared to the
non-NCEP models.

Elsewhere, the models all agree in having a rather formidable
large scale ridge over the Western U.S. by the end of the period
as a deep closed low and associated longwave trough evolves
offshore of the West Coast. All of the models show a rather strong
surface low center as well with these strong height falls, but the
NAM is south of the global model consensus with the positioning of
its low center surface and aloft. The global models are well
clustered with the depth and timing of this energy and have
excellent ensemble support from the GEFS and ECENS suites.

Based on the above mentioned comparisons, a non-NAM blend will be
preferred across the CONUS at least for the larger scale mass
field evolution with above average confidence. This will at least
also mitigate some of the modest concerns of the GFS regarding the
Great Lakes and Northeast by the end of the period.


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

Orrison