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FXUS10 KWNH 240629
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
228 AM EDT Thu May 24 2018

Valid May 24/0000 UTC thru May 27/1200 UTC

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

00Z Model evaluation including preferences and forecast confidence

...Surface low development over the Gulf of Mexico by Saturday...
Preference: 00z ECMWF/CMC/NAM blend
Confidence: Average

A tropical easterly wave is currently located in the Gulf of
Honduras up the Yucatan Peninsula with a weak upper low just north
of it with a strengthening ridge over the Western Gulf/Eastern
Mexico.  Cycle to cycle ensemble trends show an increasing
suggestion of the surface wave to lift northward and develop
toward Saturday as the upper low broadens over the Gulf.  The 12z
ECMWF continues to develop off the NE tip of the Yucatan and lift
north through the central Gulf supported by a bulk of ECENS
members that are also clustering tighter with time.  The 00z NAM
continues to track close to the ECMWF with some typical Day 3 bias
toward a convectively influenced deeper solution.  The 12z UKMET
is most out of phase suggesting the wave/surface low remain south
in the NW Caribbean with convective response over Cuba; which is
becoming further from growing consensus.  The GFS which has been
also well distant (east along the FL coast, due to greater
convective response in that direction) continues to slowly shift
toward the ECMWF/NAM but remains on the eastern fringe of the
solution suite (even compared to the bulk of GEFS members...those
that is lessening too).  The 12z CMC shows typical tropical
cyclone bias of strong cyclogenesis and shows greatest upper level
deepening/support as well.  As such will favor a 12z ECWMF/00z NAM
blend at this time.  While the ECMWF is consolidating on a general
track solution there remains large model/ensemble spread, quite
contingent on convective response to have slightly below average
confidence in this blend.

07z update: A slight eastward shift by the ECMWF and a slight
slowing and westward shift by the CMC shows fairly tight agreement
with the 12z ECMWF and the 00z NAM.  The 00z GEFS continues to
show modest spread in solutions and remains well east.  The UKMET
while trending toward a stronger and northward shifting surface
low, it remains a day or so behind even the GFS and still favors a
Cuba to FL West Coast track.  As such will continue a ECMWF/NAM
blend but include the slightly deeper CMC.  Solutions seem to
stabilizing around this compromised blend, so confidence is
increased to average.

...Shortwave and associated surface wave clipping northern New
England late Fri/Sat...
Preference: General model blend
Confidence: Above average confidence

After the exit of the shortwave crossing northern New England
today, tightly packed height-lines usher in strong northwest flow
with a broader longwave cyclonic curvature noted.  This curvature
starts to buckle and sharpen a bit toward late Friday, and as such
a broad but sharpening surface pressure falls will occur across
northern New England and SE Canada.  Model guidance shows
mesoscale variability but overall the mass fields are strongly
aligned with good timing/shape to consider a general model blend
at above average confidence as the wave passes Sat connecting back
to the wave over the Great Lakes (see section below).

...Multiple shortwave features across Northern US Rockies that
will slide eastward along US/Canadian border with associated low
into the western Great Lakes by Sat/Sun...
Preference: General model blend (along US/Canadian Border)
            Non-NAM blend across Lower MO valley/Central MS Valley
Confidence: Slightly above average

Goes-WV suite depict three compact shortwave features over the
Northwest US into Southern Canadian Rockies with
strongest/tightest circulation over MT lifting north.  A surface
low has developed in response across the MT/Dakota`s border
region.  The vorticity centers will continue to rotate around each
other as they over-top the larger scale ridge and start to slide
eastward into confluent flow along the Central US/Canadian
boarder.  Generally, there is west to east stretching of the
centers through this time period...but there is strong model
agreement through this time period.  The only significant
departure is more related southward across E KS/MO into the
Central MS river valley with the 00z NAM.  Here significant
convective development (not supported well by other global
guidance), leads to mass field feedback with an eventual 1006mb
surface low early Sat in MO.  Further north a general model blend
is likely sufficient, but the NAM needs to be removed across the
central US.

07z update: The 00z ECMWF trended west and slower than the main
cluster but still well within the fairly tight cluster.  Likewise
the UKMET/CMC and 00z GEFS mean support the initial preference of
a general model blend with the main shortwave/surface low  but
still favor a non-NAM solution along the weak surface cold front
in the MO valley Sat/Sun.

...Development of closed low off Central West Coast that reaches
CA by late Thurs/early Fri before moving into Great Basin by Sun...
Preference: General model blend
Confidence: Above average

Goes-W WV shows the incipient stages of the developing closed low
at the base of the Northeastern Pacific/Gulf of AK trof slowly
advancing eastward.  Strong model agreement exists within the
ensemble and deterministic suite with small differences in timing
remaining.  The 00z NAM and GFS both are a bit faster and stronger
with an internal shortwave feature rotating through the base of
the parent circulation on late Fri through Sat as the low crosses
CA.  This leads to a slightly more elongated closed low entering
the Great Basin while showing typical faster bias and both are are
paired with the faster 12z UKMET.  The 12z ECMWF is slower but
more concentric as it does not enhance the internal shortwave as
dramatically as the GFS/NAM.  All in all, a general model blend is
likely a good compromise and more likely to verify,  Confidence is
Slightly above average.

07z update: The 00z CMC trended toward the NAM/GFS/UKMET solution
suggesting greater N-S Elongation due to the internal shortwave
rounding the base of the closed low late Sat into Sun, but still
remains slower than the GFS/NAM or UKMET in the process.  The 00z
ECMWF trended a bit faster while remaining a bit more
concentric/symmetric.  As such confidence is above average in a
general model blend.

Model trends at
500 mb forecasts at


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