Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1112 AM 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 with the 12Z GFS and more so the 12Z NAM. The 12Z
GFS has trended deeper overall with the evolution of troughing
crossing the Great Lakes region and is generally the deepest
solution now with the height falls by Tuesday. This leads to a bit
stronger low pressure over the Great Lakes at the end of the
period. Its surface cold front also tends to lag the non-NCEP
guidance. The 12Z NAM is a bit more out of tolerance with respect
to the timing and depth of a couple of shortwaves embedded within
the 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. 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
rather ill-defined with this wave development compared to non-NCEP
models and are considered questionable considering their upstream
height fall evolution involving the Great Lakes.

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 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