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Model Diagnostics Discussion
(Latest Discussion - Issued 0631Z Apr 04, 2020)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
231 AM EDT Sat Apr 04 2020

Valid Apr 04/0000 UTC thru Apr 07/1200 UTC

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

12Z Model Evaluation with Preferences and Confidence

...Elongated frontal zone in Great Lakes today and through
Northeast/Mid-Atlantic Sunday/early Monday...
Preference: General model blend
Confidence: Slightly above average

07z update: No significant changes were noted to break from
initial preference/ a general model blend remains

Very narrow, elongated frontal zone currently across WI to W AR
will continue to press eastward but with main shortwave already
angled into Canada, low level flow and mass field responses become
weak over the Mid-MS valley into the Ohio Valley.  The upstream
kicker shortwave in MT will swing through and stretch across much
of Canada.  So while there may be sizable mass differences across
the border to the north, the timing of the front south of it
across the Great Lakes into the Northeast/Mid-Atlantic is solid to
support a general model blend. 

By late Sunday/Monday, the shortwave energy in north-central
Canada will press across Interior New England/Rooftop of Maine
with some small timing differences and surface low placement
differences.  The ECMWF seems most out of place and
uncharacteristically fast, while the strength in flow is stronger
with the 00z NAM/GFS (being a tad south with the shortwave within
the overall suite). Thinking a compromise it best here, to support
a general model blend at slightly above average confidence.

...Frontal zone across TX/Lower MS valley, subtropical jet/weak
shortwaves lingering through Tues...
Preference: Non-NAM, weight toward GFS then ECMWF
Confidence: Below average

07z update: The 00z UKMET trended a bit faster than the 00z GFS
and with the CMC/ECMWF both trending a bit faster further east
with the 850-7H frontal zone/QPF axis will support a Non-NAM blend
hedging toward the GFS/ECMWF compromise in timing/placement. 
Still given the uncertainty remains with mesoscale convergence
fields and moisture/instability axes alignment, it remains below

---Prior Discussion---
The tail end of the elongated/narrowing frontal zone across the
Mississippi Valley currently resides in the Coastal Plains of TX. 
This front will remain in place and influenced by subtle shortwave
features emerging from the subtropical jet stream and eastern
Pacific moisture stream.  The 00z NAM is now particular fast and
supports pressing low level confluent flow eastward with less
overall forcing to lead to increased organization.   The 12z
ECMWF/CMC both continue to suggest greater moisture streaming
through the Mexican Plateau allowing for reduced EML/capping
bleeding off the terrain (though cap is very strong over the
Plateau itself).  With greater backed low level flow off the Gulf,
better low level ascent/convergence both are more aggressive
keeping convection west, which may be a bit too aggressive in
coverage/magnitude.  The UKMET/GFS are a nice compromise...and
while the mass fields are only slightly different, the affects on
QPF remain sizable.  As such, a 00z GFS and 12z UKMET solution is
favored but at below average confidence, given importance on
meso/miso scale features/interactions which are too hard to lock
down in a more chaotic flow regime.  

...West coast shortwave(s) expanding to deep closed low by Sun off
OR/CA coast, slowly wobbles to Central CA coast by Tues...
Preference: Non-GFS blend
Confidence: Slightly above average

07z update: The 00z GEFS mean trended toward the operational GFS,
but is still more in line with the NAM and the non-NCEP solutions.
 The UKMET trended back toward the ECMWF/CMC enough to support a
non-GFS blend overall at this point.

---Prior Discussion---
Lead shortwave is well defined in WV suite from GOES-W just off
the OR/CA coast.  This wave remains well defined and agreed upon
through the guidance suite as it devolves/shears into the flow
across OR/ID into W MT by Sunday. 

The next wave will continue to amplify and become a closed low by
later today just west of Vancouver Island and will drop into a
position just of the NW coast of California by Sunday morning,
with fairly good agreement in placement/depth of the associated
surface low.  It is expected the internal dynamics within the
closed low will allow for a tricordial wobble, slowing its
southward parent motion toward the central CA coast by Monday,
again still in solid model agreement; however, this will change as
models start to show slightly different magnitudes of internal
instability with respect to the binary interaction.  To stabilize
this, the ensemble suite/means should help to push toward higher
probabilities of outcome.  This helps to eliminate the UKMET which
has shown continued westward bias in the suite over the last few
days.  This also helps to eliminate the GFS which continues to
further trend being fast, losing some energy northward, then
eventually the remaining energy accelerating south and east by the
end of day 3, generally weaker. This is not supported in the GEFS
suite and matches typical negative bias.  The 00z NAM is a bit
tighter with the overall internal structure but looks solid with
the GEFS/ECENS means as well as the ECMWF/CMC.   As such a 00z NAM
and 12z ECMWF/CMC blend will be preferred at slightly above
average confidence. 

...Shortwave/northwest Pacific jet crossing Northern Rockies
Mon/Tues with associated northern Plains surface low...
Preference: General model blend
Confidence: Slightly above average

07z update: The CMC and UKMET both trended slower and overall the
evolution of this portion of the trof and Pacific trof have a
solid agreement to have increased confidence overall.  There
remains some uncertainty to the strength/sharpness of the
downstream ridging by Day 3, but is small enough to support a
general model blend at slightly above average confidence.

---Prior Discussion---
As the closed low digs and the eastern portion of the Canadian
gyre slides under the deep blocking ridge over the northern Hudson
Bay, the western portion of the gyre slide south across the
Canadian Rockies.  Additionally, strong Pacific jet tightens the
trof and surges it east-southeast into northern MT supporting lee
cyclogenesis in the High Plains by Monday afternoon.  This
wave/surface cyclone will press into the Northern Plains by the
end of the short-term forecast.  The 00z GFS depicted a sizable
slowing trend and now appears comparable to the 00z NAM and 12z
ECMWF in timing, though the 00z NAM appears to have some typical
late Day 3 over-amplification suggestions perhaps with greater
southern stream influences (broad southwesterly flow ahead of deep
closed low).  The 12z CMC seems uncharacteristically fast, though
also has a more amplified downstream ridge given even greater
blocking further downstream in eastern will shift away
from its influence.  The UKMET also shows greater negative
tilting/swinging the base of the trof through faster, severing the
connection to the southern stream/closed low earlier...allowing
for a deeper surface wave and mass/wind field response.  As such
will favor the ECMWF within a 00z GFS/NAM and 12z ECMWF blend. 
Confidence is average in this blend. 

Model trends at
500 mb forecasts at