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Model Diagnostics Discussion
(Caution: Version displayed is not the latest version. - Issued 0655Z Mar 19, 2018)
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Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Mon Mar 19 2018

Valid Mar 19/0000 UTC thru Mar 22/1200 UTC

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

00Z Model Evaluation...Including Model Preferences and Confidence

...Potent shortwave entering Southern Plains crossing to the
Mid-Atlantic Coast by Tuesday...
...Surface low developing in the Southern Plains, moving into
Tennessee Valley Mon night, transferring to Coastal Low Tuesday...
Preference: 00z GFS/CMC/ECMWF blend
Confidence: Average

The 00z NAM once again shifts toward a slower initial compact
shortwave moving out of the Plains in the Ohio valley possibly
under some increased downstream ridging but more importantly a
much faster secondary shortwave (see section below) that leads to
the initial wave stalling over the central Ohio valley, allowing
for ideal warm sector upscale growth of the coastal wave as it
lifts northeast. For these reasons, the NAM is not preferred at
this time  The 00z GFS continues it trend northwest crossing the
Ohio Valley through the Mid-Atlantic, and as such the surface wave
likewise shifts and is on the northwest side of the modest
clustering.  The 12z UKMET/CMC are a bit slower than the GFS/GEFS
members and while paced well with the ECMWF are both a bit further
south, with the ECENS mean appearing to a be the most middle
ground.  At this point, the differences are small enough for this
initial wave to be within a preferred blend, a non-NAM blend. 
However, as the secondary wave interacts very late Tuesday into
Wed...preferences will change (please see section below). 
Confidence is average in this initial wave's blend.   

07z update: The 00z UKMET shifted a bit southeast of the other
guidance...when much of the guidance has been shifting
northwest...this leads to the weakening surface reflection/frontal
zone to be displaced from an otherwise modest agreement.  The CMC
tightened toward the GFS solution.  This is while the ECWMF slows
a bit particularly with the inner core of the upper low stalling
in central OH by late Tuesday, and begins to be affected earlier
by a slightly faster approach of the western descending branch of
the upper jet.  At this point, the preference is toward a 00z
GFS/CMC/ECMWF blend at average confidence.

...Low amplitude shortwave entering California now then amplifying
over southern states Monday-Tuesday...
...Secondary Surface Cyclogenesis Along Coastal Carolinas by
Tuesday Night and Possibly up the East Coast...
Preference: 00z GFS/CMC blend
Confidence: Slightly below average

Initial setup across the eastern third of the US plays a critical
role in handling this wave and its secondary coastal cyclogenesis
late Tuesday into Wed.  Given the large departure, both in
stronger depth and further west for the anchoring pivot of the
deepening eastern trof over the Ohio Valley, the 00z NAM is not
preferred here as well.

The remaining guidance shows fairly solid agreement in
timing/orientation of the wave as it cross the MS Valley and
orients more positively crossing the Gulf states late Tuesday into
Wed.  After this point the depth/snap acceleration of the base of
the trof across S GA/N FL and feedback from Gulf Stream energy
start to split the guidance.  An additional player in the
evolution is the strength and placement of the tail end of
northeastern trof/shortwave energy that sets up in SW Quebec and
potentially acts as the fulcrum to accurate the wave northeast
while amplifying/consolidating the inner core.  This is most
evident in the 12z ECMWF operational run...which shows a very
strong/compact solution that is further south than much of the
ensemble suite, leading to a rapid northeast acceleration and
surface low in the Gulf of ME by Thurs.  This is a bit too
aggressive at this time.  The 12z CMC and UKMET were both set up
by the initial wave a bit further south and east than the
GFS/ECMWF and as the wave swings around the base and opens a
modestly diffluent pattern aloft allowing for a strong deepening
off the Mid-Atlantic coast and slower northeast shift, than the
ensemble suite.  The CMC may also be a bit too close to the coast
initial given the slower mid to upper level trof over the Ohio
valley with the initial wave; however it is still bit more
favorable in mass than the UKMET which hangs back and is slow to
lift out but may be still slow to the overall trend set in the
ensemble suite.    The 00z GFS having shifted a bit NW with the
initial wave starts closer to the coast but given the height falls
from this wave, lift quickly and match quite well to the 12z ECENS
mean.  Additionally, there is a bit more agreement toward a
stronger pivot/fulcrum wave in Quebec like the ECMWF but not as
drastic/influential, leading to a more off-shore track off New
England Thurs.   All considered the 00z GFS best represents the
favorable 18z GEFS and 12z ECENS means.  Confidence is slightly
below average in this blend given fairly high spread and
sensitivity to small run to run fluctuations.

07z update: The 00z ECMWF shows remarkable inconsistency within
its own deterministic and ensemble member suite with the evolution
of the secondary wave, favoring a slower and much more wound up
solution ushering in a much colder solution that allows for the
surface wave to be tucked closer to the coast in a wobble off the
Delmarva...this is not favored at this time. The 00z CMC which was
a little south of the preferred 00z GFS for the initial wave,
develops wave further east with the inner core sliding up the
coast matching well within the GEFS/ECENS/CMCE ensemble suite. 
The 00z UKMET, like the ECMWF and NAM favors much stronger
upstream ridging in New England reinforced by that stronger/slower
exiting trof through SE Canada.   As such a 00z GFS/CMC blend is
preferred to best represent the the GEFS/ECENS mean.  Still
confidence remains slightly below average given high model
sensitivity to small scale detail/timing.

...Large closed low over the Northwest rapidly breaking down today
and tonight into small distinct waves with limited motion of the
waves into collapsing shear Axis Mon-Tue with remnant vort maxes
feeding into East Coast trough Wednesday...
Preference: Non-ECMWF blend
Confidence: Average

The semblance of the old upper low has faded into its constituent
shortwave elements with the strongest being located along the
Alberta/Montana boarder at this time and a few weak swirls
associated with the old northwest remnants of the sheared closed
low earlier this weekend, that linger from the Black Hills to
eastern Sand Hills of NEB on the upwind side of the sharp ridge
from ND to the Ohio Valley.  These features along with an
amplifying shortwave over N Saskatchewan form into a weak loosely
organized shear axis from S Canada stretching southeast into the
western side of the building eastern US trof by late Tues/Wed.  As
expected with such weak features, there is modest variation in
locations but the overall influence on sensible weather and flow
regime appear to come to a general consensus and therefore a
general model blend is supported.  Lower scale influence may play
a highly sensitive role if ideal situated to grow upscale, and
while unlikely this uncertainty makes confidence in a general
model blend only average overall.

07z update: The 00z ECMWF shows a stronger more concentric portion
of the energy sliding into the western side of the developing
large scale trof Wed and leading to a more wavy appearance, which
given the strong shearing environment seems a bit unlikely.  The
UKMET/CMC are a bit smoother like the GFS/NAM though the UKMET
generally shears out a bit quicker across Canada.   All
considered, a non-ECMWF blend would be supported.

...Closed Pacific low near 35N/145W drifting east and eventually
opening into a wave near 135W by Wednesday morning with increasing
deep SW flow and atmospheric river into Southern CA...
Preference: 00z NAM/ECMWF/CMC blend
Confidence: Slightly above average

Goes-W WV depicts a consolidated closed low near 35N145W with a
warm conveyor belt east of 140W pointing toward triple point
further north.  A compact shortwave is elongating eastward along
45N with a strong vort center on the western periphery helping to
influence a slight northeastward imbalance to the closed low.  The
00z GFS and 12z UKMET are most aggressive in accentuating this
northeastward shift but 00z analysis would suggest slight
displacements of strongest internal shortwave features aligning
better with the ECMWF/CMC and 00z NAM.  The latter all of which do
eventually show northeastward shearing but more influenced by the
strengthening/digging shortwave and eventual deep layered cyclone
off BC extending south to 40-45N by late Wed (see section below). 
At this point continuity favors the ECMWF/CMC and NAM...though the
NAM having a deeper/faster digging northern system ejects the
surface wave northeast a bit faster than the ECMWF/CMC.  This is
minor in the blend that a ECMWF/CMC and 00z NAM blend is preferred
at slightly above average. 

Of note: SW flow/AR event is favored in all deterministic guidance
to have fairly high confidence in a very dangerous heavy
rainfall/snowfall event in S California/Sierras.  Please refer to
WPC QPFPFD and QPFHSD for additional details for QPF/Snowfall
amounts and confidence.

07z update: The 00z UKMET trended a bit slower but not
significantly so to even reach the timing of even the faster 00z
NAM.  With only minor difference from the 12z run to the 00z run,
the ECMWF and CMC continue to be favored in a blend with the NAM
for this system at slightly above average confidence.

...Shortwave Rounding Pacific Ridge over near Alaska by Tuesday
digging sharply south and developing a closed low over the Gulf of
Alaska by Wednesday with possible surface low development near
Vancouver Island...
Preference: General Model Blend
Confidence: Slightly above average

Deterministic and ensemble suite show very strong agreement in the
development of a strong/latitudinal deep closed low over the
eastern Gulf of Alaska that will extend into the northeastern
Pacific by late Wed into Thurs.  Of course there are differences
but most of it is related to interaction/timing difference with
other features particularly the closed low near 35N/145W and
how/when it is absorbed into this deepening trof.   The 00z GFS
continues to be fast lifting the southern wave and is absorbed
about the time the jet reaches the base of the trof slinging the
surface low up coastal WA toward Vancouver Island by Thurs.
likewise the UKMET is faster too and is a bit narrower with the
overall trof shape in the Eastern Gulf of AK.  The 00z NAM digs a
bit deeper to eventually draw the surface low north, and is also
generally narrower with the overall trof shape.  The 12z ECMWF/CMC
both are a bit southeast initially with the developing closed low
centroid and by Thursday are a bit broader.  Still, the
differences seem very small given the otherwise good
timing/strength.  As such a general model blend is preferred but
given the preference toward the NAM/ECMWF and CMC for the southern
wave...likewise weighting will favor these in the blend for this
system as well.  Confidence is slightly above average.

07z update: No significant changes were noted as the ECMWF/CMC
continue to be a bit broader and favor the shortwave sliding down
the AK panhandle and BC Coasts over the upstream digging
shortwave.  Timing and evolution of the large scale deep cyclone
remain good enough to continue initial preference of a general
model blend.

Model trends at
500 mb forecasts at