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FXUS10 KWNH 201831

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Fri Apr 20 2018

Valid Apr 20/1200 UTC thru Apr 24/0000 UTC

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

12Z Model evaluation including preference and forecast confidence

18Z UPDATE: No change to the preliminary preferences, as model
distribution and preliminary reasoning largely stands with the
arrival of the 12Z model runs. Overall, the preference trends
toward a blend of the 12Z GFS and 12Z ECMWF, with the greatest
differences coming from the 12Z UKMET (particularly with the
closed low moving across the South) and the 12Z NAM (with the
secondary shortwave moving into the Northwest).

...Closed low exiting through the Canadian maritime provinces,
with lingering trough and reinforcing weak shortwaves through the
Preference: Non-UKMET blend
Confidence: Above average

Models are overall in very good agreement with this system and the
lingering trough across the Northeast US into the weekend. The
primary difference is with the 00Z UKMET, which takes a shortwave
from northern Alberta, and digs it more sharply (track is further
SW) into the Great Lakes this weekend, leading to substantial
differences with the structure of the larger-scale trough. The
preference is to lean toward the multi-model and ensemble

...Strong closed low in the Four Corners steadily pushing east
into the Southeast by Monday...
...Inverted surface trough in the western Gulf Coast region, with
weak surface low developing in the central Gulf Coast states
Preference: 12Z GFS; 00Z ECMWF
Confidence: Slightly above average

The strong closed low over the West will continue to make gradual
progress to the east, steadily weakening with an increasingly
broad trough as it pushes into the Southeast. The 00Z UKMET and
12Z NAM indicate slower weakening, but are on opposite ends of
model spread in terms of eventual latitude of the mid-level center
of circulation by 24.00Z (Monday afternoon). The UKMET is further
south and advances the trough further east than the other models,
while the NAM is furthest north. The ensemble means and other
global models sit in between, and the preference is to lean toward
a blend of the 12Z GFS and 00Z ECMWF which best represents this

...Strong shortwave approaching the Pacific Northwest late Friday
before shifting north of the US border this weekend...
...Surface cold front crossing the Northwest states...
Preference: General model blend
Confidence: Above average

Models are in very good agreement with the mass fields for this
system and a general model blend is preferred with high confidence.

...Secondary shortwave reaching the Pacific Northwest early
Sunday; amplifying and pushing into the Northern Rockies Monday...
Preference: Non-NAM blend
Confidence: Average

In a departure from the usual bias, the 12Z GFS is the slowest
with this wave but is slightly faster than its 00Z run. It is also
the most amplified as the wave approaches the coast, with the 12Z
NAM being the least amplified. The GFS has reasonably strong
support from the GEFS ensemble, while the 00Z ECMWF, CMC and UKMET
are supported by similarity to the ECMWF Ensemble Mean. The
preference is to include all four global models without a strong
reason to discount the GFS. A blend of the four would trend
slightly toward a faster scenario, but still incorporate 25
percent of the slower scenario (GFS). The 12Z NAM is not preferred
given that it has considerable differences from the global models,
including breaking off a piece of the shortwave and pushing it
through the Desert Southwest.

Model trends at
500 mb forecasts at


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