Prognostic Meteorological Discussion
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079
FXUS10 KWNH 221718
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1217 PM EST Thu Feb 22 2018

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

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

12Z model evaluation...with latest model preferences and confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Front stretching from the Mid-Atlantic to southeast Texas...
...Surface wave crossing the upper MS Valley/Great Lakes...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

The models are in reasonably good agreement with the orientation
of the surface front across the South this period, and also with
the details of the surface wave lifting up across upper
Mississippi Valley and Great Lakes. A general model blend will be
preferred.


...Shortwave initially over the Pacific Northwest...
...Dropping into the Four Corners region on Friday...
...Cyclogenesis over the Plains and lifting to the Midwest...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z GFS and 00Z ECMWF
Confidence: Average

The models take rather strong shortwave trough digging down across
the Pacific Northwest and drop it quickly into the Four Corners
region on Friday. Model spread is minimal until the system ejects
out into the Plains by Saturday where well-defined cyclogenesis
will occur and the system lifts northeast toward the Midwest. The
12Z NAM ends up stronger aloft with its energy traversing the
upper Midwest. The 12Z GFS is also quite energetic, especially
with its degree of negative tilt and strong vort energy, but does
not have quite the degree of height falls as the NAM. The 00Z
UKMET, 00Z CMC and 00Z ECMWF all show a somewhat weaker system.
However, there is some degree of ensemble support from the GEFS
and the ECENS for a somewhat stronger low center to evolve with a
negatively tilted trough that lifts northeast out of the central
Plains and across the upper Midwest. For now, will suggest a
compromise in mass fields from the GFS and ECMWF, and discounting
the NAM, UKMET and CMC at this time.


...Shortwave arriving over the Pacific Northwest on Saturday...
...Crossing the Four Corners/High Plains on Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-GFS consensus
Confidence: Average

The next in a series of shortwaves to drop southeast down across
western North America and into the Pacific Northwest will arrive
on Saturday and then rapidly drop down into the Four Corners
region on Sunday. The models are in good agreement with the mass
field details of this system until Sunday when the 00Z UKMET
begins to lag the model consensus, and the 12Z GFS begins to
outrun it. The 00Z CMC and 00Z ECMWF tend to favor the slower
UKMET, with the 12Z NAM more in between the GFS and NAM. There is
a bit better ensemble support for the slower consensus including
the GEFS mean and ECENS mean as the energy begins to eject out
into the southern High Plains by late Sunday. Will prefer the
slower consensus away from the GFS for now, led by the non-NCEP
guidance.


...Shortwave arriving over the Pacific Northwest on Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z GFS and 00Z ECMWF
Confidence: Average

The guidance brings another shortwave into the Pacific Northwest
on Sunday along with a progressive cold front. The guidance is in
relatively good agreement although the 12Z NAM appears to be
perhaps just a tad more progressive. A blend of the 12Z GFS and
00Z ECMWF best approximates the model consensus at this time, and
so a blend of these solutions will be preferred for now.


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

Orrison

$$





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