Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
323 PM EDT Thu May 17 2018

Valid May 17/1200 UTC thru May 21/0000 UTC

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

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

...Upper troughing/cold front crossing the Northeast
today/Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
Preference: General model blend 
Confidence: Above average 

The models show similarly with this system.


...Broad upper trough over the Southeast third of the CONUS...
...Upstream MCVs digging into the Southeast U.S. trough...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
Preference: near the 12Z ECMWF/CMC
Confidence: Slightly above average

The 12Z NAM/GFS are quicker to move the base of the trough axis
toward the east for Saturday/Saturday night, but with a blocking
ridge to the east and biases in the GFS to be too quick to slide
upper trough axes to the east, it seems more reasonable to stay
toward the slower side of the guidance. The 12Z ECMWF/CMC may have
room to speed up a bit, but ensemble data is weighted near the
ECMWF at this time, with the 12Z UKMET a bit slower. No major
adjustments were noted with the 12Z ECMWF/UKMET/CMC relative to
their previous cycles concerning this system.


...Large upper trough/closed lows over the West...
...Ejecting east across the central Plains this weekend...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: non 12Z UKMET blend
Confidence: Below average

Ensemble and deterministic trends have been stronger with a closed
low forming over the central High Plains Friday night as a large
western trough moves east. While this is likely a result of
convective enhancement, the stronger 12Z NAM/GFS may be showing
some elements of feedback, but the 12Z UKMET is on the weaker side
of the latest deterministic guidance.

Regarding the surface evolution, the ensemble scatter low plots
show a large degree of spread with the GEFS members clustered more
to the west and ECMWF members farther east. Toward the middle of
this spread is what will be preferred at this time but with
reduced confidence, with this being closest to a non 12Z UKMET
compromise..


...Upper trough crossing the upper Midwest this weekend...
...Cold front/surface wave crossing the region...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non 12Z NAM blend
Confidence: Slightly above average

The 12Z NAM stands out in the ensemble guidance as a near outlier
with a slower/deeper trough axis over the northern Plains. Outside
of the 12Z NAM, the models show reasonable agreement.


...Upper trough digging into the West on Sunday...
...Energy lifting north out of the northern Rockies...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

Overall trends in the models have moved toward a deeper trough
axis across the West Coast by Sunday morning. The 12Z
ECMWF/UKMET/CMC made adjustments such that there is broad
consensus regarding the trough's timing and strength as it reaches
the west coast this weekend. A general model blend is recommended
at this time supported by clustering within the latest ensemble
spaghetti plots.


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

Otto