Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Wed Mar 20 2019
Valid Mar 20/1200 UTC thru Mar 24/0000 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...
12Z Model Evaluation Including Preferences and Confidence
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...Trough(s) and Developing Low in the East...
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Preference: Non-CMC blend (weight to ECMWF/UKMET)
Confidence: Average
19z update: The 12z UKMET shifted faster (having weakened the
first digging shortwave through the Great Lakes) and has come into
much better agreement with the initial preference. The 12z CMC
while trending west with the surface cyclone, it remains well east
of the cluster and is shunted northeast faster as well by Sat.
The 12z ECMWF trended stronger and as such favored a slightly
eastward surface cyclone track...keeping it quite close to the
initial preference. The tightening agreement of the UKMET with
the ECMWF provides slightly increased confidence in a non-CMC
blend though weighted more ECMWF/UKMET over the GFS followed by
the lowest weighting in the NAM.
---Prior Discussion---
The synoptic variability is starting to reduce given the main
pieces of energy that combine to develop the East Coast cyclone in
the late week have reached the more data rich network for
assimilation. Complexity is driven by two phasing events: (1) on
Thursday with a southern stream shortwave and another wave digging
sharply through the Great Lakes, and (2) on Friday when more
substantial cyclogenesis will begin over the Northeast. At that
point the initial shortwave will be ejecting in a negatively
tilted fashion up the coast and will phase with yet another
sharply digging shortwave from the Great Lakes.
The 00z CMC is the lone deterministic guidance member (along with
a bulk of CMCE members) that is slow with the northern stream
digging wave through the Great Lakes, allowing for the southern
stream wave to not shear and support surface cyclogenesis well off
shore, starting it off away from the trends/growing consensus. The
00z UKMET, while more like the remaining guidance, is also very
strong with the northern stream digging, delaying the deepening
upper low's track and sfc development well south of the overall
cluster. As the upstream shortwave digging across the Great Lakes
the southern wave is kicked a bit further east and as they both
phase across New England into Saturday, the UKMET is well east and
then shifts northward.
Ensemble solutions from the GEFS/ECENS mean have been coming into
better agreement with traditional biases manifesting in the day
2/3 time period, ie GEFS/GFS fast, and the ECMWF/ECENS mean slow.
The 00z ECMWF however, continued the overall EC trend faster, but
has sped up to be faster than the bulk of ECENS members and is
about mid-point to the the 06z GEFS mean and 00z ECENS
mean...which may be a good thing in a blend that has issues mainly
with timing. The 12z NAM/GFS and 00z ECMWF, depict a very similar
evolution to both phasing systems. However, perhaps the ECMWF a
bit stronger/concentric with the lead wave (surface cyclone
developing wave), reducing shearing and leading to a broader
weaker upper low on Sat into Sun relative to the NAM/GFS. The 12z
NAM continues to be very strong and deep but also tad faster
ejecting Northeast by the end of day 3, which seems less likely
given the magnitude of expected binary interaction (ie, synoptic
wobble/surface redevelopment expected in New England/Gulf of
Maine). All in all, a blend of the 12z NAM/GFS and 00z ECMWF
blend is preferred but weighting toward the ECMWF is suggested.
Confidence is average.
...Remainder of the CONUS...
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Preference: General model blend through 60hrs (23/00z)
Trending toward 12z UKMET by 84hrs (24/00z)
Confidence: Above Average up to 23/00z, average after
19z update: The 12z ECMWF/CMC both trended a bit slower but more
importantly more weighting toward the base of the elongating trof
similar to the 12z NAM across the Plains on day 3. The UKMET
still has a stronger small scale vort center but has shifted a bit
faster than the GFS, putting the trof axis in line with the
NAM/ECMWF/CMC and leaving the GFS uncharacteristically slow. The
timing/orientation of the base of the trof through the Southern
Plains provides the most uncertainty while the remainder of the
trof and coastal shortwave are agreeable enough to support a
general model blend though would favor something close to the
UKMET further south across OK/TX (esp. QPF). Confidence remains
average after 23/00z.
---Prior Discussion---
A strong southeast to northwest ridge dominates the Plains early
in the forecast period. The elongated trof along the West Coast
will split, sending a bulk of energy into the Southwest
consolidating into a closed low over UT by Fri with very strong
model agreement. Model spread starts to increase as the wave
reaches the weakening ridge as well as, the spine of the Central
Rockies. Additionally a secondary mid-range wavelength trof
enters the Pacific Coast more toward the Northwest. The
combination of all 3 factors, continues to stretch the wave from
the Southern Plains to Snake River Plain, with vortical stretching
in Eastern CO/NM. The 12z NAM and 00z ECMWF are a bit more
progressive and favor weakening the wave northward and back toward
WY, falling into line with the last few runs of the ensemble
suite. However, the trend has been toward slowing but also
depicting a highly elongated trof. The 00z UKMET and 12z GFS both
trended a bit slower supporting a a compact shortwave at the
highest diffluence to the more W-E 250mb jet. This helps to tap
increased moisture/instability out of the western Gulf. In
reality the spread is small compared to average/normal but there
is modest spread in the QPF axis/magnitude. So while, not highly
confident, will follow the overall ensemble trends and favor a 12z
GFS/00z UKMET blend at average confidence. This will also
suffice for the blend of the second broader coastal trof (which is
a bit further north) compared to the ECMWF/NAM.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
Gallina