Prognostic Meteorological Discussion
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FXUS10 KWNH 171707

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
107 PM EDT Sat Mar 17 2018

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

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

12Z 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 are generally in excellent agreement with the mass fields
from the Hudson Bay Region into the Northeast US, associated with
continued W-NW flow on the southwest periphery of the broad upper
level low over Eastern Canada. The most notable differences are
with the 00Z CMC, which amplifies the upper level pattern more
than other global models: a deeper trough along 60W and higher
heights upstream in response along 80-90W. As a result, it also
builds a slightly stronger surface ridge in faster across the
Northeast. The preference is to stay closer to the global model
consensus, and thus a non-CMC blend.

...Deamplifying Wave Becoming Increasingly Sheared Out as it moves
from Ohio Valley into Carolinas This Weekend...
...Associated Weak Surface Low Tracking On Similar Path...
Preference: General Model Blend
Confidence: Above Average

There is very little model spread with this system, both aloft and
at the surface, and minor model differences don`t seem to make a
significant difference in their sensible weather impacts.
Therefore, a general model blend is preferred with high confidence.

...Large Closed Low near Oregon Rapidly Breaking Down by Sunday
into Small-Scale Distinct Waves...
...Several Distinct Shortwaves Becoming Increasingly Embedded in
NW Flow over the Northwest and Northern Plains as Northern Stream
Jet Sags South...
Preference: General Model Blend, Weighted to 12Z GFS
Confidence: Below Average

As the upper level low begins to rapidly break down over the next
24 hours, the distribution, motion, and strength of the resulting
waves should be somewhat chaotic. That being said, the models do
seem to have a decent handle on several primary shortwaves that
should emerge -- one lingering near the coast of Oregon and
Washington through Monday before moving into the Intermountain
west, and another near southern Alberta eventually moving into the
Northern Plains. As would be expected in a complex flow pattern,
models do offer somewhat different forecasts, and confidence is
not high enough to select a particular model. However, the 12Z GFS
does appear to be reasonably situated in the middle of the
distribution in terms of timing of the waves as they begin to push
east, so will place a slightly greater weight on the GFS. The
messy breakdown of the upper level low will also likely contribute
to uncertainty downstream and at later forecast hours elsewhere in
the CONUS.

...Potent Shortwave at the Base of Devolving Western Closed Low...
...Strong Trough Ejecting into the Plains later on Sunday...
...Lee Cyclogenesis over the Southern Plains...
...Low Pressure System Reaching the Mid Atlantic Coast Tuesday...
Preference: 65% GFS (12Z); 35% ECMWF (00Z)
Confidence: Slightly Below Average

Models continue to show a fair amount of uncertainty with this
system, particularly as the trough reaches the Mississippi River
on Monday Night and begins to enter a region of accelerating,
confluent flow on the south side of the persistent upper level low
over eastern Canada. Ensemble sensitivity analysis from the GEFS,
CMC, and ECMWF ensembles all show that the forecast of the surface
low off the Mid Atlantic coast on Tuesday is very sensitive to the
ridge and associated upper level height patterns from the Upper
Mississippi River Valley into western Ontario on Sunday and
Monday. Higher heights (and thus a "faster erosion" of the upper
level low over eastern Canada) tend to correspond to a slower
system, and vice versa. The 00Z CMC most aggressively builds the
heights in this ridge, notably more than any of the other global
models. The 00Z UKMET also shows this signal, but to a lesser
extent. Given that higher heights have been a bias occasionally
observed in the CMC lately, the preference is to lean away from
that scenario at this time. Both the UKMET and CMC have the
surface low furthest to the west (slowest) by 21/00Z (Tuesday

The 12Z NAM offers a surface low track that is further northwest
and slower than most other models. As is its tendency, it has a
stronger trough as it emerges into the Plains, and that has
downstream implications over the next 24-36 hours with slightly
more downstream ridging along the coast, more meridional flow
downstream of the trough, and a stronger vorticity max. Given a
lack of support from other models (it is situated northwest of
most of the 00Z ensemble members), the preference is also to lean
away from the NAM at this time.

The 12Z GFS and 00Z ECMWF seem to be more plausible scenarios
right now. The overall flow pattern (more zonal, with less
blocking) would tend to favor flatter and more progressive low
tracks. By 21/00Z, the GFS low position is situated almost exactly
in the middle of the other operational models. It is slightly
slower than its previous run, but over the past several runs has
offered decent consistency on the track if not the exact timing.
However, the 00Z ECMWF can also not be discounted. It is faster,
but still situated well within the envelope of most of the
ensemble members (GEFS and ECMWF). Therefore, the overall
preference is to lean more toward the GFS, but incorporate a
decent amount of the ECMWF as well.

...Low Amplitude Shortwave Reaching California on Sunday and
Amplifying into Secondary Trough Mon-Tue over the Mid-South...
...Reaching the Southeast Atlantic Coast late Tuesday with
Secondary Coastal Surface Cyclogenesis...
Preference: Non-NAM Blend
Confidence: Average

The 12Z NAM has a different configuration to this trough as it
begins to amplify across the Southern US, partially due to
differences with how it handles the lead wave, and differences
with how quickly it amplifies the wave over the Plains. The NAM
does this earlier, which creates more substantial height
differences aloft rapidly on Monday. The remaining models do show
some differences, but are actually fairly similar in the position
and strength of the upper level shortwave, vorticity max, and
surface low position as cyclogenesis begins late on Tuesday.
Therefore, the preference is for a Non-NAM blend.

...Closed Low Settling South in the Pacific to around 35N/145W by
Sunday Afternoon and Slowly Drifting East through Tuesday...
...Increasing Southwesterly Flow and Possible Atmospheric River
Approaching California by Tuesday Night...
Preference: Non-NAM Blend; Weighted to the 12Z GFS
Confidence: Average

Overall model differences with this system are not extremely
large, as is typically the case with slow-moving closed lows that
are initialized quite close. By Tuesday, some variance does emerge
that may have an impact on the sensible weather. The 12Z NAM has a
more amplified downstream ridge along the West Coast, more than
the global models and it is excluded from the preference at this
time as a result. Otherwise, the global models are reasonably
close. The 12Z GFS is situated in the middle of the other models
in terms of timing of the low position and strength. It is very
close to the 00Z ECMWF Ensemble Mean, and sits between the 06Z
GEFS Mean and 00Z operational ECMWF. Therefore, a general Non-NAM
blend is preferred, with slightly greater weight placed on the GFS
as the model closest to a consensus scenario at this time.

Model trends at
500 mb forecasts at


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