Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1235 PM EDT Mon Mar 19 2018

Valid Mar 19/1200 UTC thru Mar 23/0000 UTC

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

12Z Model Evaluation...Including Model Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Strong shortwave crossing the Mid Mississippi Valley today...
...Surface low crossing the Tennessee/Ohio Valleys tonight...
...Evolving Mid-Atlantic/New England nor'easter thru Wednesday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z NAM, 12Z GFS, and 00Z ECMWF
Confidence: Average

The models take a strong and compact mid level closed low and
associated trough axis currently over the middle MS valley off to
east today and through the TN/OH Valleys tonight along with a
well-defined area of low pressure. By early Tuesday, this energy
will be moving into the central Appalachians, but there will be
additional upstream shortwave energy digging southeast across the
Midwest that arrives and captures the lead energy. This will lead
to an amplifying trough/closed 500/700 mb low center that crosses
the central Appalachians and southern Mid-Atlantic region Tuesday
night through early Wednesday. Multiple low centers will be
redeveloping near the East Coast as energy transfer commences,
with one low crossing SC on Tuesday and one developing just east
of the Delmarva. These lows should consolidate east of the
Delmarva by Wednesday morning before then lifting northeast
offshore southern New England through early Thursday. The models
show decent agreement at least on the larger scale with this
coastal low transition and evolution, although the 00Z UKMET does
look to be too weak with the deepening closed low and its surface
reflection along the East Coast. The 00Z CMC also looks to be
perhaps a tad too progressive in lifting the system off to the
northeast and away from New England by Thursday. The 12Z NAM, 12Z
GFS and the 00Z ECMWF cluster together a bit better and have
stronger ensemble support. So, will be preferring a blend of NAM,
GFS and ECMWF at this point with the overall model mass fields.


...Closed low over the Northwest rapidly breaking down today...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

The models take the closed low just offshore the Pacific Northwest
and dampen it out rather rapidly over the next 24 hours. Some of
the remnant vort energy is then expected to shear out downstream
over the Plains through Wednesday. A general model blend will be
preferred with this given the good model mass field agreement.


...Closed Pacific low near 34N/140W drifting east...
...Opening into a progressive wave that crosses the West Coast...
...Deep layer southwest flow and atmospheric river into CA...
...Deep trough/closed low offshore the Northwest by Wednesday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-UKMET blend
Confidence: Slightly above average

GOES-W WV imagery and satellite derived winds depict a
consolidated closed low near 34N140W which should begin to
gradually shift off to the east over the next couple of days and
weaken as it comes under the influence of a much stronger northern
stream trough and associated closed low that will be digging south
offshore British Columbia and the Pacific Northwest. The remnants
of the initial closed low will get caught up in deep layer
southwest flow off the West Coast and will be slingshotted inland
ahead of the mean layer trough. This will set the stage for a
strong atmospheric river to impact central and southern CA with
enhanced moisture transport from the subtropics. The better model
clustering tends to favor the non-UKMET solutions as the 00Z UKMET
brings a somewhat stronger lead shortwave impulse/surface low
closer into the West Coast through late Wednesday. The UKMET is
also more out of tolerance with the details of the large
trough/closed low digging south offshore the Northwest by
Wednesday. All of the models though show strong atmospheric river
conditions setting up and arriving across central/southern CA by
either Tuesday night or early Wednesday and continuing through
Thursday. A non-UKMET blend will be preferred for the model mass
fields at this time.


Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Orrison