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FXUS10 KWNH 180429
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1228 AM EDT Sun Mar 18 2018

Valid Mar 18/0000 UTC thru Mar 21/1200 UTC

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

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

...Persistent W-NW flow aloft over Northeast US through Monday...
...Weak surface low tracking from Lake Superior today to Long
Island Sunday associated with digging shortwave aloft...
Preference: Non-CMC blend
Confidence: Above Average

Models remain in excellent agreement in mass fields from Hudson
Bay through the Northeast US and Southeast Canada in the early
periods of the forecast with the main cold air advection surges
and associated shortwaves crossing through.  This includes the
last stronger wave on Monday that is responsible for the weak
surface wave moving from Lake Superior to the Hudson Canyon region
off NY/NJ by early Monday.  The model break from consensus occurs
with the remaining baggy trof over the E Ontario to Nova Scotia
through Tues/Wed.  The 12z CMC is generally weaker and quicker to
shift the weaker wave out with the main closed low, which is
further SW over the Gulf of St. Lawrence towards NB) by Tuesday
than the ensemble solutions support.  The 12z UKMET which is very
strong with the inner core of the same closed low rotates back
through the Gulf of St. Lawrence  Tuesday as well... though it
handles the remaining sharpening of the lingering baggy trof over
northern New England by Wed matching the 00z NAM, 12z ECMWF and
ensemble suite, potentially making it useful over the US domain.
The 00z GFS is a bit weaker with the eventual baggy trof
sharpening presented by the NAM/ECMWF/UKMET on WED, as it shifts
the energy downstream a bit quicker but has trended toward the
NAM/ECMWF/UKMET.  All considered a non-CMC blend is preferred for
the US domain in the Northeast...though larger scale less of the
UKMET would be preferred in SE Canada.  Confidence is above
average given the overall differences are small and the impacts on
sensible weather are low.

...Deamplifying wave becoming increasingly sheared Out in central
Appalachians and through Carolinas later today with associated
surface low...
Preference: General model blend
Confidence: Above average

There is very little model spread with the mass fields for this
system...and no significant differences were noted with the 00z
NAM/GFS runs to break from a general model blend preference at
above average confidence.

...Potent Shortwave at the Base of Devolving Western Closed
Low...closes off while ejecting into Plains late Sunday with
strong lee cyclogenesis.  Low will press through to TN Valley by
early Tues and transfer to coastal low late Tues...
Preference: 00z GFS/12z ECMWF blend
Confidence: Average

GOES-WV mosaic depict in S CA a very strong shortwave at the base
of closed low that dominates the West, as well a low amplitude
shortwave emerging out of the Subtropical Eastern Pacific at Baja
CA at this time.  The Subtropical wave will lift north into TX and
aid negative tilt to the main northern stream shortwave out of NM
and rapidly develop a strong lee cyclone late Sunday into Monday
and start to shift with good pace across the southern Plains
toward the MS River Valley by Monday.  It is at this point,
deterministic guidance begins to differ in handling the system as
it moves into generally confluent upper level flow and weakens.
The first to depart is the 12z CMC which is a bit slower and
stronger through the TN valley and very slow/weak in the transfer
of energy to the developing coastal low midday Tues.  The UKMET
shows some typical bias of being too strong/slow to spin down the
wave into the confluent flow though this counteracts its tendency
to be fast and so paces extremely well with the remaining guidance
and ensemble suite but is a clear stronger wave (nearly 4mb for
the surface low by 18z Tues.

The 00z GFS is well timed with the 00z NAM with respect to the
lead 5H wave/sfc low but the 7H mass fields belay some
strength/upstream interaction influences that suggest the 00z NAM
may be a bit too aggressive. While there are some smaller scale
detail differences by day 3 with the 12z ECMWF and the GFS but
this seems minor and within reason/ensemble spread.  Additionally,
the strengthening upstream shortwave across the Gulf coast as well
as sharpening of the larger scale trof (as energy from the
northern plains slides southeast) by day 3 lead to further
reduction in predictability/certainty... please see sections below
for these specific waves.   As such  00z GFS and 12z ECMWF blend
is preferred at average confidence.

...Low amplitude shortwave reaches California Sunday amplifying
into secondary trough Mon-Tue over the Mid-South reaching the
Southeast late Tuesday supporting secondary coastal low early Wed
off SC Coast
Preference: 00z GFS/12z ECMWF blend
Confidence: Slightly below average

In the wake of the main shortwave in the southwest today, a piece
of shortwave energy is shed from the closed low in the central
Pacific through the ridge that separates the larger features and
enters CA on early Monday with little fanfare and continues to be
low amplitude through early Tuesday into the Red River valley.
Here the larger scale weak trof exists and with stronger
northwesterly  jet flow aloft, it can amplify across the MS Valley
and slow through the Eastern Gulf states.   Along with northern
stream shortwave energy descending from the northern Plains (see
section below) on Tuesday/Wed, generate a stronger neutral to weak
negative tilt and support a secondary surface low off the SC coast
that lifts northeast by the end of the forecast.  The 12z CMC
having lagged with a stronger more closed mid-level circulation
through the TN valley is out of place from the onset and shows a
deeper latitude response to troughing as a whole.  The 00z NAM
similarly has a deeper latitudinal trof having greater
amplification of the wave through the MS Valley and northern
stream support to lead to a closed 5H feature in the TN valley by
Wed...a bit too fast/too strong.  The 00z GFS gets to a similar
point but is much softer and weaker with the wave and its binary
interactions with the other upstream features.  While the 12z
ECMWF is slower than the GFS, its evolution is quite similar.
The 12z UKMET is a bit faster/stronger with this wave typical of
its bias but not dramatically so to completely discount it; it is
discounted mainly due to the lack of upstream digging trof.  All
other guidance/ensemble support the northern Plains system to
descend, but it does not in the 12z UKMET.  As such a 00z GFS/12z
ECMWF blend is preferred.  Confidence is good, but given the
moving parts/influences of downstream setup as well as upstream
timing leads to slightly below average confidence due to high

...Large Closed Low rapidly breaks down Sunday into smaller-scale
distinct waves with several becoming increasingly embedded in NW
Flow over the Northwest and Northern Plains as northern stream jet
sags south, eventually expanding in broad negative tilt trof over
southern third of US Wed...
Preference: 00z GFS/12z ECMWF blend
Confidence: Slightly below Average

GOES-WV mosaic depicts shortwave emerging from the Beaverhead
range into SW MT and a elongated wave over the SW Prairies of
Canada, these two waves will interact across S Canada and as the
mean Arctic jet streak shifts east over Manitoba; the northern
stream is able to buckle and meld with the dancing shortwave pair
leading to weak broadening troughing across S Saskatchewan by late
Monday into Tuesday.  Additional lingering energy through the
Rockies and the subtle Pacific shortwave (see section above) will
allow for this wave to descend into the central Northern Plains
and stretch southeast eventually carving out a larger scale trough
over the eastern US by Wed.

The 12z UKMET is the main outlier with this interaction having
been a bit stronger and shifting east more.  This enables the tail
end of the trof over Hudson bay to influence it dragging the weak
wave eastward and not descend into the central Plains...breaking
significantly from run to run ensemble trend/continuity and other
deterministic guidance.  The 00z NAM shows stronger
amplification/winding up the inner core of the trof over Canada
slipping into N MN by 12z Wed, falling into a typical Day 3
negative bias for the model.  In this flow regime strong
northwesterly jet from the Pacific should keep the shortwave
energy stretched out in carving out the western portion of the
trof in the MS Valley, but the 12z CMC does not do this preferring
to focus on more consolidated waves in the northern Plains/MN by here too the model falls into negative bias.   This
leaves the 00z GFS which also shows typical fast bias descending
into the western portion of the trof by Wed, but the 12z ECMWF is
generally equally here a nice typical compromise between
the 00z GFS/12z ECMWF is likely a solid bet.  Confidence is
slightly below average given high sensitivity of the environment
and likely run to run variability even though the spread between
the GFS/ECMWF is small and agreeable in the ensemble suite.

...Closed Low settling south in the Pacific to around 35N/145W by
Sunday afternoon and slowly drifting east through Tuesday
increasing southwesterly flow with possible atmospheric river
approaching California by Tuesday night...
Preference: 12z ECMWF/UKMET blend
Confidence: Average

Stationary closed low remains between 130-140W and remains until
about Tuesday when an imbalance within the inner core of the low
manifests.  This in combination with weak remaining smaller-scale
remnant shortwave energy from the weakening Western US upper low
along the OR/WA coast, appear to draw out some model differences.
The 00z GFS has backed off the stronger internal wave in the
closed low but remains most amplified/influential with the s/w
along the Pacific coast leading to the closed low to shift east by
the end of the forecast period.  While this is supported by the
NAM and GEFS members, this looks less likely compared to more
concentric and weaker waves presented by the 12z ECMWF/UKMET.  The
12z CMC may be too far elongated...shifted SW with the closed low
on Wed to support it in the blend.  Either way...strong
southwesterly flow ahead of the closed low and associated surface
wave should support strong atmospheric river directed toward
central or southern CA toward the end of day 3 into 4.  Given the
issues with the GFS...WPC prefers the slower ECMWF/UKMET timing
with respect to the rainfall as well.  Confidence is average.

Model trends at
500 mb forecasts at


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