Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1109 AM EST Sun Feb 17 2019

Valid Feb 17/1200 UTC thru Feb 21/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidences
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...Exiting shortwave/Coastal low development Today/Tomorrow...
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Preference: Non-NAM blend
Confidence: Above average

Closed low over the Mid-west will continue to shift northeast and
shear with strong 150+kt 250mb jet screaming the southern stream
eastward supporting a coastal low development later tonight/early
tomorrow morning.  The 12z NAM has trended much stronger is about
4mb deeper than the consensus. While it is on track initially, it
cyclonically curves northward as it races south of Cape Race. 
Inclusion of the NAM only will tilt the wave toward this deeper
solution, so would favor going non-NAM for this portion of the
CONUS/Western Atlantic.


...Remaining CONUS/Western Trof(s) ejection into Midwest Wed...
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Preference: General model blend through 19/18z
            00z CMC/UKMET and ECENS mean thereafter
Confidence: Average to below average

The western trof continues to develop today into the Desert
Southwest, with the pivot/fulcrum of the larger scale trof
enhancing as well W WY by Mon/early Tues.  Guidance is fairly
agreeable with timing and shape of the trof even through Day 2
(18z Tues) into the Central Rockies; however, it is after this
time period model spread, mainly driven by timing differences,
begin to manifest as it ejects into the Plains into the Midwest,
as well as intersecting with strengthening Gulf/Caribbean moisture
stream into the Lower Red River/Mississippi River Valley setting
up a clear heavy rainfall signal for Day 3 (Wed).   This spread is
affected both by magnitude of downstream ridge amplification
(westward extent of the exiting 150+kt 250mb jet) but also the
timing of the nose of Gulf of AK jet digging the new trof by late
in the forecast period.  It is the leading edge of the jet and the
diffluence ahead of it that determines the placement and
orientation of Rockies trof, as it either delays the ejection as
presented most dramatically by the 00z ECMWF (faster upstream
shortwave, more neutral tilt to the elongated trof into NW
Mexico/Eastern Pacific), or like the 12z NAM/GFS that are much
further west allowing for the lead wave to eject a bit faster with
a less connected but greater positive tilt to the trof across N
Mexico. 

The operational ECMWF is supported by the ECENS mean, but is the
most westward of the guidance with convective development/warm
conveyor belt axis and appears to be too far to be considered
ideal/acceptable at this time even though the shape/evolution of
the mass fields wave as a whole looks appropriate/favorable just a
bit too far west.  The 00z UKMET is more like the ECMWF and may be
even a bit slower, suggesting stronger warm conveyor low level jet
enhancing convection later over the MS River Basin, but at least
has a more favorable placement/axis to the QPF as a whole.  The
00z CMC appears to be the greatest compromise/middle ground
solution at this time and may be greatest favored in the mass
fields; though there is some concern the moisture return stream
from the Caribbean is a bit slow and therefore east, allowing for
increased QPF/intersection in the warm air advection regime into
the terrain of the southern Appalachians which is not supported in
the remaining guidance.

As noted before, the 12z NAM trended away from the ECMWF and
toward the faster GFS; however, it is very strong and as the
Southwest wave ejects it is kicked into a small scale negative
tilt orientation by the fulcrum shortwave energy digging well
south toward the Four Corners before kicking into the Plains too. 
This falls in line with the known over-amplification, negative
bias associated with it.  Opposed to this is the 12z GFS, that is
fast, but is not as aggressive with the kicker "fulcrum" wave and
so in this flow regime, is a bit more sensible with a
flatter/weaker solution across the Midwest than the NAM.   The 06z
FV3-GFS was between the ECMWF and NAM but like each were much
stronger with the wave and deeper with the surface solution;
leading to more uncertainty/confidence.   Overall will follow a
00z CMC preference for the mass fields after 00z Wed, which show
some similarity to the 00z ECENS mean/UKMET solutions.  
Confidence is average to below average in the mass fields and most
uncertain with the orientation/eastward extent of the eastern
branch of the warm conveyor belt (aligned with the QPF axis)
across the upper Ohio or Tennessee River valley Wed. 


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

Gallina