Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Thu Sep 12 2019

Valid Sep 12/1200 UTC thru Sep 16/0000 UTC

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

12Z Model Evaluation with Preferences and Forecast Confidence
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...Northern Plains/Upper Midwest Closed Low and Approaching
Shortwave...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

19z update: The 12z CMC shifted toward the 12z ECMWF which is
typical of a slower breakdown/shearing into the flattening flow,
but still timed well enough with the GFS/NAM and UKMET to support
going to a general model blend at above average confidence

---Prior Discussion---
Closed mature cyclone over the Northern Plains will continue to
lift into Ontario/Quebec by late Friday into Sat.  The
initialization of the 00z ECMWF seems to underplay the imbalance,
stronger internal shortwave across MN (currently), but this does
not seem to have a negative influence on the mass fields much
after the first 12z (which would prefer Hi-res CAMs anyway). 
However, the 00z CMC is a bit slower exiting Ontario, with a
lingering trof/deformation zone that appears to negatively affect
the evolution of the upstream shortwave currently just NW of
Vancouver Island.  Much of the deterministic/ensemble solutions
favor a stronger flow/flatter jet streak across the northern tier,
while the CMC remains compact and drops south.  This makes the CMC
unfavorable in the Northern Plains.


...Closed Low Approaching Pacific Northwest Day 3...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General Model blend
Confidence: Above average

19z update: The 12z UKMET shifted slower to match with the
remaining guidance.  Therefore, will support an above average
confidence general model blend for the Northwest as well.

---Prior Discussion---
Further upstream a broad,strong closed low drops southeast along
the AK coast in the Northeast Gulf of AK.  Similar evolution occur
within the guidance suite, though the UKMET is particularly fast
with drawing the warm conveyor out of the southwest leading it to
hedge east/faster than the growing consensus.  As such will not
favor the UKMET in the Northwest into day 3 at slightly above
average confidence. 


...Tropical System Nearing the Bahamas then Toward FL by the
Weekend...
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Preference: 12z CMC/ECMWF blend (weight to CMC)
Confidence: Below Average

19z update: The 12z CMC continued a favorable trend, inside the
island chain and along FL. The 12z ECMWF trended this direction
being slightly weaker and therefore toward the east, so more
favorable.  The UKMET remains strongest and northeast.  So will
continue to favor the 12z CMC over the ECMWF but both included in
the blend. Confidence remains below average.

---Prior Discussion---
GOES-WV suite detects a strong broad, Tropical Upper Tropospheric
Trough (TUTT) cell over the eastern Gulf with broad southwesterly
flow with modest diffluent axis northeast of the Bahamas.  A more
consistent, convective pulsing area northeast of the Southern
Bahamas is the point of interest, with a growing increase in
global guidance suggesting the wave to spur a weak surface low in
the Southern Bahamas today into tomorrow.  Thereafter, there is a
large spread, particularly between the NAM/GFS and the non-NCEP
solutions.  The 00z ECMWF/UKMET are particularly robust in
developing the mid-level wave and stretching toward the surface
but along the eastern Bahamian chain eventually deepening it over
Grand Bahama midday Sat, and turning it northward parallel to the
FL coast.  The 12z NAM/GFS are much weaker and therefore further
southwest moving through the southern tip of the FL peninsula
before turning, north toward the central FL panhandle.  This is a
slight strengthening trend with the 12z run but still well weaker
than the ECMWF/UKMET.  The 00z CMC while favoring the northern
development, is much weaker and SW of the ECMWF/UKMET, and
clustered well within the CMCE solutions to have some confidence. 
This is also the most middle ground, in line with forecast
preference from NHC (via coordination).  The spread both in
track/placement and intensity makes this a very uncertain forecast
with below average confidence. 

To muddy the situation further, the same wave and TUTT cell will
continue to progress west into the western Gulf by the end of the
weekend; there are some suggestions of a weaker surface wave to
develop toward Day 3.  Guidance is more flat here, and WPC prefers
something closer to an ECMWF/CMC blend solution though more toward
the ECMWF in QPF.


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

Gallina