Prognostic Meteorological Discussion
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FXUS10 KWNH 200636
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
236 AM EDT Fri Apr 20 2018

Valid Apr 20/0000 UTC thru Apr 23/1200 UTC

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

00Z Model evaluation including preference and forecast confidence

...Exiting Northeast Closed Low with lingering troughiness through
Preference: 00z GFS/ECMWF/CMC blend
Confidence: Slightly above average

Goes-16 WV loops depict strong vort center over the mouth of the
Hudson River starting to swing north-north east... while the
surface reflection is now well out of US waters...southeast of
Nova Scotia.  Strong model agreement remains with this vorticity
center and the lingering trof that remains across New England
through the weekend.  As the trof exits, a shortwave out of the
northern stream drops into New England and reinforces the broad
larger scale troughing into Monday.  The 00z NAM however, appears
to show typical day 3 over amplification producing a very deep
trof that drags south and southwest to elongate the closed low
over the Southeast.  This is not likely given the overall
agreement in the remaining guidance and ensemble suite.  A such a
general model blend is good through Sunday but a Non-NAM blend
should be employed into Monday at above average confidence.

07z update:  While the CMC/UKMET and ECMWF all remain agreeable
with the exiting trof, the UKMET has broken from the tighter
faster solutions and initial preference with respect to the
shortwave crossing New England late Sunday into Monday, in the
same vain as the 00z NAM just not as deep.  Still believe this is
an aberration than a trend, though confidence is slightly reduced
in the day 3 period in a 00z GFS/ECMWF/CMC blend to slightly above

...Strong closed low aloft in Colorado River Valley pushing to the
Four Corners by Friday, and into the South-Central US this weekend
and East South Central US Mon
Preference: Non-NAM blend
Confidence: Slightly above average

Goes-WV mosaic depicts a strong and fairly concentric closed low
centered over the UT/NV/AZ tri-state junction with a fairly broad
warm conveyor shield dominating the US Rockies and high plains
with broad diffluence across NM shifting to TX.  The strong
surface low over the Great Basin will fill across the central
Rockies and shift toward new lee cyclogenesis across the Permian
Basin.  Models remain in good agreement mass-wise through this
transition.  By late Sat into early Sun, the low as opened to an
inverted sfc trof across eastern TX.  It is at this point, the
upper low is starting to break down into its constituent internal
parts/shortwaves.  This leads to larger model variation with the
CMC showing some weakness...the 00z NAM showing some
stronger/tighter vorticity centers, as the closed low migrates
eastward.  Convective feed back leads to differences in
timing/intensity and depth of the internal waves; yet larger scale
mass fields remain remarkably stable which manifests with good
placement/strength and timing of the surface wave/frontal zone
across the East South-Central US states by Monday.    The 00z NAM
and 12z UKMET are a bit broader E to W while the CMC remains
generally shallower with the 00z GFS and 12z ECMWF the closest
guidance members tight to the ensemble suite.  All considered, a
general model blend is supported weighted to the 00z GFS/12z ECMWF
but given internal variation, convective/upscale
influences...confidence remains slightly above average as smaller
scale run to run variation is expected.

07z update:  The 00z ECMWF shifted to even tighter agreement with
the 00z GFS...further supporting a higher weighting to these
common solutions.  However, the UKMET and CMC both, like the NAM,
are well within the tightening ensemble suite to keep at lower
weighting to account for this run to run, smaller scale
inter-variability seen in the deterministic runs.  Generally, the
CMC is further south through the Lower MS into the Central Gulf
states...the UKMET is a shade faster and the NAM is north and a
bit deeper.  Given the tighter agreement of the UKMET and CMC to
the GFS/ECMWF...a Non-NAM blend is now preferred though still
weighted toward the  GFS/ECMWF at slightly above average

...Strong shortwave approaching the Pacific Northwest and British
Columbia late Friday before shifting north of the US border this
weekend...Surface cold front crossing the Pacific Northwest...
Preference: General model blend
Confidence: Slightly above average

Models continue to show strong agreement with this system
especially with the timing/strength of the front crossing the
Pacific Northwest.  There is some model discrepancy much further
north, as the 00z NAM is stronger further north (how the top of
the trof crosses the Northern Canadian Rockies) that lead to
differences in mass/surface low placement across boreal regions of
the Central Providences, but does not affect the CONUS
timing/placement/strength of the mass fields.  As such a general
model blend is preferred.

07z update: No significant change here, especially in the CONUS,
with the 00z ECMWF/GEFS/UKMET or CMC.  As such a general model
blend remains preferred at slightly above average confidence.

...Lower amplitude shortwave reaching the Pacific Northwest early
Sunday...amplifies and pushes into the Northern US Rockies
Preference: 1/3 00z GFS & 2/3 12z ECMWF blend
Confidence: Average

In the wake of the strong shortwave lifting into South-central
Canada...a low amplitude wave emerges from the Pacific and with
upstream ridge strengthening, is also able to amplify as it enters
the Pacific Northwest (SW OR/N CA specifically) late Sunday into
Monday.   Guidance is a bit uncharacteristically ordered with
respect to timing... as the 00z GFS continues to be the slowest
piece of guidance but also is a bit more amplified and narrower
than the slightly faster 12z ECMWF/CMC.  The 12z UKMET is fastest
and is a bit broader over through the wavelength, perhaps
influenced by the prior it is further north
comparatively within the suite.  The 12z NAM is slower than the
ECMWF like the GFS, but is also much narrower and injected a bit
south of the main clustering.  This leads to significant deviation
with respect to QPF (please refer to QPFPFD for additional
details) though lesser in the mass field.  Still the NAM should
not be considered given these factors.  The 12z CMC is a bit
broader and faster than the ECMWF and is connected to the northern
stream wave a bit more.  All considered, will favor a 1/3 GFS to
2/3 ECMWF blend.  Confidence remains average given large variation
in the spread and influence of downstream and upstream stronger

07z update:  The 00z ECMWF showed very little change from the
initial preference, though the UKMET/CMC both trended slightly
narrower, slower and a bit south...there remains enough departure
from the initial preference to keep them excluded from the blend
at this time.  However, their changes provide some additional
confidence in the initial preference and higher weighting toward
the ECMWF.

Model trends at
500 mb forecasts at


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