Prognostic Meteorological Discussion
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FXUS10 KWNH 221850

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
249 PM EDT Thu Mar 22 2018

Valid Mar 22/1200 UTC thru Mar 26/0000 UTC

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

12Z Model Evaluation...Including Model Preferences and Confidence

...Departing Nor`easter Reaching Newfoundland by Friday...
...Broader Trough over the Northeast with Shortwave Digging from
Ontario Today to the Mid Atlantic Coast by Friday...
Preference: General Model Blend
Confidence: Above Average

Models show a high degree of similarity in their mass field
depictions of these systems, even beyond 24 hours. Some slight
detail differences exist, but the preference is for a general
model blend which should create a reasonable consensus.

...Deep Trough / Closed Low Offshore the Northwest thru Friday...
...Opening into a Series of Shortwaves Moving into the
Intermountain West this Weekend...
Preference: Blend of 12Z GFS, UKMET, ECMWF
Confidence: Average

The 12Z NAM appears to have initialized stronger with this low,
and more amplified through the base of the trough from around
40-45N latitude, along 130W longitude. As would be expected with
such an initialization difference, this does have an impact on the
remainder of the forecast as errors grow with time. The other
primary difference exists with the 12Z CMC, which pushes the
trough axis into the West faster than the other global models and
ensemble means. The 00Z CMC presents a reasonable forecast through
48 hours but then begins to diverge on timing. Therefore, the
preference is a blend that does not include the NAM, and leans
toward non-CMC global models.

...Southern Stream Wave Related to Ongoing Atmospheric River in
California Ejecting through the West and into the Plains by Friday
...Lee Cyclogenesis in the Plains; Surface Low Moving to Mid
Mississippi River Valley by Saturday...
...Deamplifying Shortwave as Strong Arctic Shortwave Digs South
into the Northeast and Mid Atlantic this Weekend...
Preference: 50% ECMWF; 25% UKMET, CMC; 25% GFS
Confidence: Average

The 12Z NAM likely has errors with this system emanating from its
different initialization off the West Coast at 12Z. As the
shortwave reaches the Plains and becomes sheared out into the Ohio
Valley, the NAM begins amplifying the upstream ridge much more
than the other available models. It also joins the 12Z GFS in
showing a slightly more amplified shortwave trough in the Plains,
with slightly faster timing. The other global models show a
flatter shortwave, slightly further north and slightly slower. The
effects of the NAM initialization error may have less of an effect
at these earlier time ranges, so in the earlier part of the
forecast the primary difference seems to be between the NCEP and
non-NCEP models. No strong evidence supports completely favoring
one solution over the other, so a blend of the global models will
be utilized. However, greater weight will be placed on the non-GFS
models given there is slightly more support for that scenario.

Model trends at
500 mb forecasts at


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