Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1211 PM EST Sat Mar 02 2019
Valid Mar 02/1200 UTC thru Mar 06/0000 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...
12Z NAM/GFS Evaluation Including Preference and Forecast Confidence
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...Surface low offshore New England and Nova Scotia tonight/early
Sun...
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Preference: General model blend
Confidence: Above average
The 12z NAM remains on the western edge of the tightly clustered
low center track from east of Nantucket Island to a position east
of the northern tip of Nova Scotia through 03/12z. However, the
differences in strength and positioning are no longer too large to
keep the 12z NAM out of the general model blend. Otherwise, there
is very good model agreement with the track and strength of the
surface low, so a general model blend is preferred, with above
average confidence.
...Phasing short waves and resulting surface low tracking from the
Mid Atlantic to Newfoundland...
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Preference: General model blend (with less weight toward the 00z
ECMWF)
Confidence: Average
The 12z NAM has shifted to the east with its surface low track
(when compared to its previous two model cycles), a result of
better agreement concerning the timing of the phasing of the
northern and southern stream short wave into a broad cyclonic flow
over the northern part of the US. The 12z GFS has remained
consistent with its previous two model cycles in keeping the
surface low further east. Though it has been trending slowly
eastward with its depiction of the surface low track, the 00z
ECMWF remains the westernmost member of the guidance envelope
(joined with the 00z ECMWF ensemble mean). The general model trend
has been to phase the northern and southern stream a bit later,
resulting on a more easterly surface low track.
Given this trend, a general model blend is preferred, though at
this point less weight should be given the 00z ECMWF because of
its surface low track (and precipitation swath). Because there is
still some variability concerning when the phasing occurs, and
considering the sensitivity of the forecast to that variability,
confidence remains average.
...Closed mid-level low near WA coast Monday...
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Preference: General model blend
Confidence: Average
The 12z NAM is west of the consensus with the closed mid level low
that crosses Vancouver Island by 04/00z, then wobbles over
northwest WA through the end of the period. The 12z GFS is a bit
further southeast than the 00z ECMWF/00z ECMWF ensemble mean with
the placement of the closed mid level low by 06/00z. Given that
the system remains more or less in place after crossing Vancouver
Island, a general model blend is preferred. However, the model
spread results in an average forecast confidence.
...Trough approaching the CA coast Tue...
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Preference: 12z GFS/00z ECMWF/00z CMC blend
Confidence: Average
The 12z NAM remains close to the model consensus early it takes a
closed low from 32N 142W at 04/12z to a position near 30N 133W by
05/12z. After that tine, it becomes faster than the consensus
(mirroring the timing of the 00z UKMET). By contrast, opens the
closed low at the bottom of the low wave trough more slowly, which
is more consistent with the 00z ECMWF/00z CMC and the GEFS and
ECMWF ensemble mean positions. The timing issues will have an
effect on how quickly tropical moisture is picked up and
transported to the CA coast by the end of the period. At this
point, a slower timing is preferred, though the spread in the
guidance results in only average confidence.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
Hayes