Prognostic Meteorological Discussion
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FXUS10 KWNH 201903

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
203 PM EST Tue Feb 20 2018

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

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

...12Z model evaluation with final preferences...

***Note that due to data flow issues at WPC/NCWCP, the 12Z
ECMWF/UKMET/CMC were not available for comparison and the
preliminary discussion below will serve as the last discussion for
this data cycle***

...Positively tilted mid-upper level trough over the western U.S.
with strong cold front passing through the Northeast on
Wednesday...extending southwestward into the southern Plains...
Preference: 12Z GFS / 00Z ECMWF Blend
Confidence: Slightly Above Average

The 12Z GFS was noted to be slightly quicker with the progression
of the leading edge of the mid-upper level trough into a potent
ridge situated in the western Atlantic, when compared to the
remaining model guidance. This is noted early on Thursday from the
northern Mid-Atlantic into the lower MS valley. Ensemble means
support their respective deterministic runs with a blend of the
faster GFS and slower ECMWF appearing reasonable at this time. At
the surface, the 12Z NAM looks to take a weak surface low along
the MS valley too far north and west when compared to the
remaining consensus.

...Shortwave amplifying southward along the West Coast on
Thursday...possibly closing off in the mid-levels before reaching
the Four Corners region on Friday...
Preference: General Model Blend
Confidence: Average

There are relatively minor differences with this wave...the 12Z
NAM shows stronger/more closed off with the shortwave, the 12Z GFS
is slightly faster and the 00Z ECMWF less amplified and slower.
The 00Z UKMET/CMC seem to split these differences with varying
degrees of spread depending on the forecast hour noted through
00Z/24. Overall, a general model blend is preferred as no single
model appears to be best at all forecast hours, with a blend
appearing to smooth out the differences and result in a solution
nearest to the ensemble means.

...Lower amplitude shortwave nearing the Pacific Northwest on
Friday with associated cold front...
Preference: Non-00Z ECMWF Blend
Confidence: Below Average

The 00Z ECMWF is flatter and faster with this shortwave as it
approaches the Pacific Northwest, while a non-00Z ECMWF consensus
supports closer to the more amplified 12Z NAM/GFS. Given the
amplified nature to the upper flow over the central and eastern
Pacific Ocean, more amplification downstream of a building ridge
over Alaska by Friday makes sense. However, confidence is below
average given larger than average ensemble spread with no clear
trend and a wide range of possibilities seen in the ensemble
spaghetti data.

Model trends at
500 mb forecasts at


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