Prognostic Meteorological Discussion
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835
FXUS10 KWNH 251857
PMDHMD

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Sat May 25 2019

Valid May 25/1200 UTC thru May 29/0000 UTC

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

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

...Overall CONUS Overview...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM days 1-2, ECMWF/GFS/UKMET on day 3
Confidence: Average

19Z Update: Most significant difference with the 12Z Non-NCEP
suite was a trend upward in strength of the day 2 shortwave
lifting across the middle of the country near 12Z Monday morning.
ECMWF trended stronger, which matched the GFS and UKMET. The CMC
also is a bit sharper with this feature than previous runs,
leaving the NAM as the sole weak solution at this time. By day 3,
the important difference in the CMC is how it handles the
evolution of the closed low opening into the middle of the
country. While uncertainty remains, the UKMET/GFS/ECMWF remain
tightly clustered with a slower eastward translation of this
feature.


Previous Discussion:
Flow across the CONUS remains anchored by an anomalously strong
subtropical ridge across the Southeast downstream of a deep
longwave trough over the West. While the guidance remains tightly
clustered in the overall synoptic pattern, differences in
mesoscale features as shortwave energy is repeatedly shedded to
the northeast through the middle of the country, are beginning to
arise.

This is evident on day 1 /Sunday/ as the 12Z/25 NAM is stronger
and further south with the shortwave lifting out of the Southwest.
This is well south of the remaining guidance, and at the edge of
the envelope. With the strength of the subtropical ridge across
the Southeast, it seems unlikely this feature would traverse into
the ridge as sharply as the NAM suggests. While this is a
relatively minor feature overall, it plays significantly on the
sensible weather and potential heavy rainfall in the middle part
of the country. For this reason the NAM should be used with
caution on day 1, but the remaining global suite is reasonable.

By day 2, a secondary impulse sheds from the western trough and
across the middle of the country once again as it gets caught up
around the periphery of the dome of high pressure to the east. A
more widely spaced envelope of solutions exists into Monday, but
the NAM again is the model left out of the blend as it is much
weaker than the rest of the guidance, and displaced from the
consensus, likely in response to its difference on day 1 breaking
down the ridge. The UKMET is on the strong edge of the solution
envelope, but recent 12Z/25 GFS has trended stronger, and is in
line with the CMC/ECMWF so can remain in the blend.

By day 3, spread becomes more significant as the closed low across
the west opens and begins to lift eastward towards the persistent
ridge over the Southeast. The 00z/25 ECMWF remains a very
consistent solution with respect to its prior two runs, and
matches well with the 00z/25 UKMET and 00z/25 GEFS mean, and the
new slower 12Z/25 GFS. Initially, the movement of this closed low
will force downstream amplification of the ridge, which leads to
jet-streak enhancement north of the Great Lakes in response to
intensifying confluent mid-level flow. Uncertainty abounds into
how this low will eventually break down the ridge, noting that
continuous shedding of energy will be progressively sheared
eastward in the strengthening flow to the northeast, potentially
aiding in weakening the ridge. However, guidance tends to be too
aggressive with breakdown of these strong ridges, so the
consistent and slower ECMWF/UKMET/GEFS solutions become preferred
by the end of this forecast period.


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

Weiss

$$





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