Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
237 PM EDT Sun Oct 21 2018

Valid Oct 21/1200 UTC thru Oct 25/0000 UTC

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

12Z model evaluation...with final preferences and confidence
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...Overall Pattern Across the CONUS...
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Preference: 12z GFS/ECMWF
Confidence: Above average through 48 hours; average 48-84 hrs

19z update: The 12z UKMET trended faster than consensus with the
mid-level energy moving across central/northern Plains by
Wednesday, meanwhile the ECMWF and GFS now are in similar
agreement with timing. However, the ECMWF is not as deep/strong as
what the GFS advertised. For the other parts of the CONUS
(northeast US, northwest US), the GFS/ECMWF are in reasonable
agreement. As a result, a general blend of the GFS/ECMWF will
suffice across the CONUS.

---previous discussion---
In general, the synoptic pattern across the CONUS through
Wednesday evening will be characterized by troughing over the
Great Lakes / eastern US while a ridge across potions of the
western US break down with the approach of a shortwave trough in
the northwest US. Eventually the remnants of TS Willa will spread
into the southern US (see section below).

Through 48 hours, the models are in decent agreement in the large
scale pattern (troughing over northeast US; approach of trough in
the northwest US). However, differences become more substantial
between 48-84 hours, especially across the northern Plains and
northern Rockies as pieces of energy break off with the western US
trough. Between 500 and 700 mb, the differences are notable, as
the GFS is much deeper/stronger compared to the 00z ECMWF.
Meanwhile, the CMC is displaced further to the north. Finally, the
NAM is also stronger across the northern Plains. The 12z GFS is
more amplified than the 06z GEFS, ECENS, and ECMWF...so whether
this is a trend or an outlier will be evaluated more with the rest
of the 12z guidance.

With this in mind, outside of the northern Plains on Day 3, a
general blend of the 12z GFS with the 00z ECMWF/UKMET/CMC will be
preferred for this cycle.

...Tropical Storm Willa...
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Preference: 12Z GFS/12Z ECMWF blend closest to 15Z NHC advisory
Confidence: See NHC products

19z update: The 12z ECMWF lies well with the current (15z)
advisory, especially in later forecast hours (>48 hrs) when the
GFS was too fast (NAM way too fast, CMC slow). So while the ECMWF
is a touch slower compared to the advisory, it represents the best
proxy for the latest advisory at this time.

---previous discussion---
The guidance is in reasonable agreement that TS Willa will
continue moving north then northeastward into Mexico over the next
several days. Differences continue to lie in the forward motion.
The 12z NAM is considerably faster and is an outlier not
considered. Meanwhile, the UKMET/CMC remain on the slower side of
the envelope. The 12z GFS matches well with the 15z advisory
through about 48 hours, but then accelerates too quickly. As a
result, a blend of the 12z GFS with the 00z ECMWF seems to match
well with the current advisory, although it might be a tad on the
faster side of that blend. For the latest information on Tropical
Storm Willa, see the official NHC advisory.

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

Taylor