Prognostic Meteorological Discussion
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FXUS10 KWNH 211914

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
213 PM EST Tue Jan 21 2020

Valid Jan 21/1200 UTC thru Jan 25/0000 UTC

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

12Z Model Evaluation with latest Preferences and Forecast

...Closed low off the Southeast Coast...
Preference: Non-NAM blend...for height fall timing/low placement
            12Z NAM/NAM-conest...for surface low strength
Confidence: Slightly above average

The upper low dropping across the Southeast will continue
east-southeast and offshore tonight before crossing just north of
the Bahamas on Wednesday. Relatively strong cyclogenesis will
occur over the next 12 to 24 hours, with low pressure then pulling
east and well away from the U.S. by later Wednesday and Thursday.
The 12Z NAM appears to be tucking its low center development a bit
too far to the west and southwest (closer to the northern Bahamas)
through 36 hours as the global models have come into a strong and
consolidated agreement on a solution that is farther east by
comparison. However, the global models may be collectively a tad
too weak with the low center given the degree of sharp height
falls/PV anomaly strength (suggested in Airmass RGB imagery) and
potential for added latent heat/convective processes fostering
lower surface pressures. The hires models (HREF) support the
global model timing/positioning through 36 hours, and tend to be a
little stronger than the global models but still may not even be
strong enough. The relatively stronger NAM/NAM-conest strength
appears more plausible, but with a global model blend for

...Subtropical shortwave over the Southwest...
...Energy riding northeast toward the Upper Midwest...
Preference: Non-NAM blend
Confidence: Above average

The models take a well-defined subtropical stream shortwave trough
and vort center over AZ quickly off to the east-northeast tonight
which will bring it over the central High Plains by early
Wednesday. The energy will then lift up toward the Upper Midwest
by late Wednesday while gradually weakening, and the remnants of
this system should ultimately become absorbed by the stronger
northern stream height falls amplifying out across the High Plains
by Wednesday night. The 12Z NAM tends to hang onto a stronger vort
center/700 mb reflection compared to the hires model suite (HREF)
and the global models as the system rides up toward the Upper
Midwest. A non-NAM blend is preferred for the mass fields with
this system.

...Current upper low/trough crossing into the Pacific Northwest...
Preference: General model blend
Confidence: Above average

The deep layer closed low just offshore the Pacific Northwest will
weaken into a progressive trough as it crosses British Columbia
through tonight, but is forecast to then reorganize and deepen
over southern Alberta on Wednesday before dropping southeast
across the northern High Plains. At least through Wednesday, the
models are in good agreement with this system.

...Binary interaction of closed lows over the Plains on Thursday...
...Consolidated deep layer closed low over the OH Valley Friday...
Preference: Non-NAM blend... on Thursday
            GFS/CMC/ECMWF blend...on Friday
Confidence: Slightly below average

Guidance continues to suggest binary interaction of closed lows
across the Plains on Thursday, with one closed low dropping
southeast across the northern Plains, and another developing
farther south over the central Plains/lower MO Valley by late
Thursday. At least through Thursday, the 12Z NAM appears to be a
deep outlier with the collective mass field evolution. By early
Friday, the two closed low begin to consolidate and move through
the OH Valley as one area of surface low pressure deepens over the
Midwest, and a second one develops over the lower MS Valley and
moves east across the Gulf Coast states. The 12Z GFS and 12Z NAM
are farther north with the merged system relative to the 12Z
non-NCEP solutions, with the NAM notably slower/deeper. Of the
non-NCEP models, the UKMET is still the farthest south solution.
The 12Z GEFS mean is actually a tad north of the GFS, with the 12Z
CMCE and 00Z ECENS a little farther south, but not to the extent
of the UKMET. The deterministic CMC/ECMWF solutions are well
supported by their respective ensemble means.

Based on the latest model spread/clustering, the stronger/slower
NAM and the farther south UKMET appear to be the bigger outliers.
The CMCE/ECENS suites tend to give better support to a consensus
of the GFS/CMC and ECMWF solutions, as the GEFS suite may be
overall too far north. So, a blend of the GFS/CMC and ECMWF will
be preferred at this point for the Friday time frame, with a
non-NAM blend for Thursday as the energy initially evolves over
the Plains.

...Weak shortwave impulse skirting Vancouver Island early
Preference: Non-UKMET blend
Confidence: Above average

The models bring a highly progressive and low-amplitude shortwave
embedded in deep layer southwest flow toward Vancouver Island
early Thursday. The energy should clip at least northwest WA
before weakening as it moves across mainland British Columbia. The
12Z UKMET appears to be a deeper outlier with the system, with the
remaining guidance well clustered. Thus, a non-UKMET blend will be
preferred at this point.

...Shortwave crossing the Pacific Northwest on Friday...
Preference: Blend of the 12Z GFS/CMC/ECMWF
Confidence: Above average

The models bring a progressive, low-amplitude shortwave in across
the Pacific Northwest on Friday. The 12Z UKMET appears to be too
progressive, with the 12Z NAM likely being too slow. The 12Z
GFS/CMC/ECMWF solutions are clustered in between and more closely
represent the model consensus, so a blend of these will be

Model trends at
500 mb forecasts at


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