Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
100 AM EDT Thu Jul 18 2019

Valid Jul 18/0000 UTC thru Jul 21/1200 UTC

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

00Z Preliminary Model Evaluation Including Preference and Forecast
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Overall Pattern Across the CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend 00Z GFS/18Z GEFS/12Z UKMET (Northwest)
            General model blend (rest of the CONUS)
Confidence: Below average (Northwest)
            Above average (rest of the CONUS)

Guidance suite remains in very good agreement across the eastern
2/3 of the CONUS and into the Desert Southwest. An expansive ridge
will expand from the Desert Southwest all the way into New
England, producing intense heat for a large portion of the
country. The expansion of this ridge will drive the remnant
shortwave from previously dissipated T.D. Barry off the Southern
New England coast, and there is very good model agreement on the
timing and placement of this feature. Additionally, the ensemble
envelope for height anomalies within the expanding ridge is also
small, suggesting high confidence in the ridge placement, and this
will drive any of the sensible weather impacts far north across
the extreme northern tier of the CONUS or into southern Canada.

Less confidence exists in how the Northwest will evolve, and how
energy will begin to break down the ridge in the Upper Midwest
into day 3. A closed low over Hudson Bay will re-orient itself
south of St James Bay. At the same time, a broad closed low over
western Canada will shear out into a series of shortwaves rotating
around the St James Bay low. The evolution of these features will
impact how the ridge breaks down, but guidance differs
considerably in timing. The NAM is quite strong, and is an
outlier, breaking down the ridge likely too quickly, especially
noting the tendency of models to break down large ridges much more
quickly than typically occurs. The CMC develops a secondary jet
maximum across the Northern Plains on day 3, likely in response to
a modest shortwave ejecting within the mean flow, but this creates
displaced/high QPF and is not agreed by any other guidance. The
best consensus involves the GFS/GEFS/UKMET which are all in decent
agreement with ridge breakdown, shortwave positioning, and
evolution of a closed low off the Pac NW coast late. The ECMWF,
while not too far from this blend, appears a bit strung out into
the ridge, which seems unlikely based on the amplification of the
synoptic flow.


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

Weiss