Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
133 AM EDT Tue Sep 03 2019

Valid Sep 03/0000 UTC thru Sep 06/1200 UTC

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

00Z Preliminary Model Evaluation, Confidence, and Preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Overall Pattern Across the CONUS...
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Preference: General model blend through 48hrs
            00Z GFS/12Z ECMWF thereafter
Confidence: Above average through 48hrs
            Average thereafter esp in northern stream


Ridge in the west and trough in the east will persist through the
forecast period. Outside of Hurricane Dorian (see preferences
below), the evolution will be determined by shortwaves lifting
into the Pacific Northwest and then ejecting across the northern
tier. The first of these will move onshore Washington State
Tuesday night and then quickly deamplify as it encounters the
pinched and fast flow atop the Southwest mid-level ridge. Although
the guidance is initially in good agreement in both amplitude and
speed of this feature, as it weakens and shifts southeast across
the Northern Plains and towards the Great Lakes, the CMC and UKMET
both begin to outpace the remaining global suite including the
ensemble means. The 00Z/NAM takes the opposite approach, and
begins to lag the the ensemble consensus, and this slower solution
seems unlikely in the fast flow. While a faster solution may be
correct, the better clustering and consistency of the
GFS/ECMWF/GEFS makes this preferred for this leading shortwave.

On day 3, a second and more robust impulse digs into the Pac NW
and begins to deamplify atop the ridge. Once again, discrepancies
in timing and amplitude develop as the system begins to shear to
the east, and the more consistent GFS/ECMWF are favored for this
region late in the forecast period as well.


...Tropical Wave nearing Tamaulipas, Mexico...
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Preference: Non-NCEP blend
Confidence: Slightly above average

A weak wave and surface reflection will continue to drift
westward, likely coming onshore south of the mouth of the Rio
Grande Wednesday. There is quite a bit of spread in timing and
placement, with the NAM being north/fast, and the GFS being quite
slow, compared to the better-clustering of the Non-NCEP guidance.
Heavy rainfall is likely as this moves onshore, but uncertainty
remains into how far north the heavy QPF will spread. For now, a
Non-NCEP blend for placement and QPF seems most reasonable.


...Hurricane Dorian...
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Please refer to the official National Hurricane Center forecast
track

Early on, through about 48 hours, the UKMET/ECMWF/GFS are well
clustered to the 03Z NHC advisory positions. However, as Dorian
lifts along the South Carolina/North Carolina coasts, the global
suite diverges considerably. The ECMWF becomes slow and south of
the track, while the GFS continues to be a fast and west outlier.
The CMC actually becomes quite well aligned by day 3, but is fast
and west prior to that. This leaves the UKMET as again the most
reasonable proxy to the track, but with some weight on the ECMWF
and GFS for their consistency and relative proximity.

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

Weiss