Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1124 AM EST Mon Feb 25 2019

Valid Feb 25/1200 UTC thru Mar 01/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Deep closed low/trough over the Pacific Northwest...
...Undercutting shortwave arriving over northern CA Wed...
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Preference: Blend of the 00Z GFS/GEFS mean
Confidence: Average

Slow moving, positively titled shortwave over Washington State
will generally remain in place through the entire forecast period.
Chunks of energy will eject out into the Northern Rockies today
and tomorrow then the system will retrograde a bit, reload, and
eventually close off. A secondary piece of energy coming from the
Pacific will undercut the main low and push inland across northern
California Wednesday. By Thursday and Thursday night, the main
closed low will then wobble onshore.

Model agreement is actually fairly high despite the slow moving
and complex nature of the system this week. Some of the typical
model biases were seen, like the GFS being fast with some of the
energy going out into the Rockies, as well as being on the eastern
side of the model spread with respect to the closed low position
as it shifts back offshore. By Thursday evening, the GFS is the
fastest to open up the wave as it gets into the Rockies. The
NAM/UKMET are strongest with the secondary piece of energy coming
into California on Wednesday and also the most removed from the
parent low. The CMC is weakest and barely has a signal with that
feature.

As a result, despite the complex setup, feel a general model
blend, weighted toward the GFS/GEFS mean, will suffice for the
western US this week. No individual operational deterministic run
appeared to be an outlier, but the GFS/GEFS fall in the middle of
the ensemble spread. Including some of the slower guidance should
help alleviate the fast GFS bias too.


...Shearing shortwave energy over the Midwest/OH Valley...
...Weak surface wave crossing the OH Valley/Mid-Atlantic...
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Preference: General model blend
Confidence: Average

A fast moving shortwave will work through the Upper Midwest and
Great Lakes mid-week associated with a larger closed low north in
Canada. Model agreement is about average, with some of the typical
model biases seen. The GFS is faster bringing the energy toward
the Great Lakes while the ECMWF and the NAM to some degree are
slower. The CMC/UKMET are a bit more amplified compared to the
rest of the models. Despite all of this, the QPF amounts and
placements are fairly clustered between the models, so a general
model blend should yield a solution acceptable for this time
period.


...Southern US / Gulf Coast Region...
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Preference: Non-NAM, Non-GFS blend
Confidence: Average

Broad zonal to slightly southwest flow aloft will persist through
the forecast period across the southern US and Gulf Coast region.
Several subtle impulses will eject out into the flow. While in the
big picture the latest guidance is in good agreement, there are
differences in timing and strength of the shortwaves, and this is
impacting the QPF distribution along the coastline. The ECMWF,
UKMET, CMC are in good agreement showing a bi-modal axis, one over
TX/LA and then another off the east coast of FL. Alternatively,
the NAM/GFS have QPF maxima over the western FL Gulf Coast. For
now, a non-NAM, non-GFS blend will be preferred to follow the
signal with better continuity and less run to run variability.


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

Taylor