Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
130 AM EST Wed Dec 18 2019
Valid Dec 18/0000 UTC thru Dec 21/1200 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...
00Z Model Evaluation with Final Preference and Forecast Confidence
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...Closed mid-level low crossing New England on Thursday...
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Preference: General model blend
Confidence: Above average
The models show similarly with this system. No significant changes
were noted with the 00Z ECMWF/UKMET/CMC relative to their previous
cycles.
...Trough off West Coast splits through to north-central and
southern Plains...
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Preference: 12Z or 00Z ECMWF/12Z CMC blend
Confidence: Average
...07Z update...
Timing changes continued with the 00Z non-NCEP suite compared to
their previous 12Z cycles. The 00Z ECMWF slowed down slightly with
the closed low progressing eastward into the Southern Plains on
Friday. The change was small enough to allow it as part of the
preference. However, the 00Z CMC slowed down more significantly
and is now slower than the 00Z NAM. Ensemble spaghetti plots and
trends do not support such a slow solution as depicted by the 00Z
NAM/CMC. The 00Z UKMET on the other hand looks reasonable with
timing until after 12Z/20 when it speeds up closer to the 00Z GFS,
but continues to lack more of a symmetric closed off as seen in
the remaining guidance. The final preference will be a blend of
the 12Z CMC with the ECMWF (either 12Z or 00Z).
...previous discussion follows...
Timing differences show up as an upper trough crosses into the
Great Basin with the 00Z NAM slowest and 00Z GFS fastest. The
timing differences continue into Friday but the 12Z UKMET ends up
with less of a closed mid-level low over the Southern Plains and
more of an elongated positively tilted trough axis.
Ensembles support their deterministic counterparts with the ECENS
members slower than the GEFS members. Canadian members are in the
middle but closer to the ECMWF members. There has been gradual
trending toward a middle ground over the past 3 12/00Z ensemble
cycles. The preference is to go toward the middle ground, near the
12Z ECMWF/CMC but with only average confidence given the remaining
ensemble spread and potential for future timing adjustments.
The 12Z ECMWF/CMC preference also holds true with the northern
portion of the upper trough that tracks toward the Great Lakes. By
Saturday morning, ensembles favor a middle ground, between that of
the slower 12Z ECMWF and faster 12Z CMC which happens to be near
the 00Z NAM/GFS.
...Persistent onshore flow into the Pacific Northwest......
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Preference: 00Z NAM/GFS/ECMWF blend, less GFS weighting by 12Z/21
Confidence: Slightly above average
...07Z update...
The latest 00Z non-NCEP suite is similar with the surface wave
west of the Olympic Peninsula but the 00Z UKMET is a bit faster
Thursday night. Otherwise changes with the frontal timing were
small with the preference continuing to point toward a blend of
the 00Z NAM/GFS/ECMWF, with less weighting on the faster 00Z GFS
by 12Z/21.
...previous discussion follows...
Onshore flow into the Pacific Northwest on Thursday begins to
alter as a large mid-upper level trough axis approaches the region
and steady/moist southwest flow aims into Washington and Oregon.
The models have come into fairly good agreement regarding a
frontal wave forecast to be about 250-300 miles west of the
Olympic Peninsula 00Z/20. Afterward, timing differences appear
with the base of the longwave trough axis as it begins to
approach, with the 00Z GFS fastest compared to the remaining
deterministic and ensemble consensus. Given the relatively
consistent run to run guidance from the ensembles, and the 00Z GFS
placement ahead of the ensemble clustering, less weight on the 00Z
GFS is recommended toward 12Z/21, with a 00Z NAM/12Z ECMWF blend
more greatly preferred.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
Otto