Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8

FXUS10 KWNH 260627
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
226 AM EDT Mon Aug 26 2019

Valid Aug 26/0000 UTC thru Aug 29/1200 UTC

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

00Z Model Evaluation with Preferences and Forecast Confidence

...Overall Pattern Across the CONUS...
Preference: General model blend
Confidence: Above average through 78hrs slightly above average
Exception: Coastal Low: 00z GFS/NAM (slightly below average)

07z update: The mass fields, with a small exception of the
Atlantic Coastal surface low (see 2nd paragraph), are in
remarkable agreement particularly through 72hrs across the CONUS.
Again, by 84hrs, the UKMET and NAM are still a bit too fast with
the second Canadian shortwave but that is minor.

The surface low of the Northeast continues to befuddle the
guidance with a substantial variance noted by the UKMET, CMC and
ECMWF from prior runs.  The interesting point though is they are
very agreeable with each other, so there may be a critical piece
of data added to their assimilation that missed the NAM/GFS (which
are also in strong agreement with each other).  A slightly
vertically deep cyclone in the new non-NCEP suite, suggests the
surface low does a tricordial wobble with the anticyclonic loop of
the 5H low (even manifest by the 00z NAM).  This does not make it
any clearer on which direction to take, but the interests along
coastal New England will see a much different forecast.  At this
point, will not deviate too much from initial
preference/continuity of the 00z GFS/NAM but reduce confidence to
slightly below average for this wave.

---Prior Discussion---
The 00z NAM/GFS continued another cycle of consistency and strong
deterministic guidance agreement with the synoptic
pattern/evolution over the CONUS.   As with any run, there are
some very small scale amplitude differences, as well as,
differences due to convective development/feedback.  The largest
differences in the mass fields/QPF axes, are driven by slight
timing differences with the frontal zone crossing the Northeast
and the interaction with the moisture feed from the the
tropical/subtropical surface wave to the Southeast Wed into Thurs.
The UKMET is a bit faster and it along with the 00z NAM suggest
slightly earlier deep moisture flux for some heavy rainfall across
SE ME.   This is interesting, given the spacing between the UKMET
and NAM is probably the largest spread with the surface wave, and
the UKMET is stronger than the NAM. Overall, the deep closed low
in S Canada and the frontal zone spread is small enough to support
a general model blend, but the surface wave off the coast has
moderate spread.  The ECMWF has once again become fast and strong,
paired closest to the UKMET, while the GFS and NAM are weakest and
draw westward due to the approaching height-falls.  The CMC also
bends left but is very strong by the end of Day 3, which does not
seem a good fit to continuity and the ensemble suite.  Overall
would favor a NAM/GFS solution with some lower weighted UKMET and
some ECENS/GEFS mean in the blend.

Elsewhere, there is a small timing issue with a strong shortwave
across NW Canada on Day 3.  The 00z NAM and 12z UKMET are too
fast, though with little sensible affect to even the northern tier
of the CONUS, but would shade away from those in a larger scale
blend.  So overall a general model blend is supported, with only
exceptions noted above, with an above average confidence through
about 60hrs before reducing to slightly above average afterward.

Model trends at
500 mb forecasts at


$$ is the U.S. government's official web portal to all federal, state and local government web resources and services.