Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
107 PM EDT Tue May 22 2018

Valid May 22/1200 UTC thru May 26/0000 UTC

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

12Z Model Evaluation...with Preliminary Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Inverted trough and possible surface low development in the
Gulf of Mexico from Thursday into Friday...

Preference: Blend of 12Z GFS; 00Z UKMET
Confidence: Slightly below average

Models still show quite a bit of variability with respect to the
pace of development of a surface low in the western Caribbean and
central-eastern Gulf of Mexico, as well as the eventual track of
this low. Many of the specific questions related to the track
exist beyond the forecast period of this discussion (ending Friday
afternoon, 26.00Z), so the focus here is specifically on near-term
development and the location of this development.

The 12Z NAM and 00Z ECMWF still appear to develop a surface low
much faster and further to the northwest of all other models on
Friday. The 00Z CMC also develops a surface low over the Gulf in
the same time frame, but is closer to the Yucatan Peninsula. All
three models show surface pressure falling to around 1005 mb by
26.00Z, which would be at least several standard deviations below
normal and represent considerable growth of a low pressure system.
This appears to be far too fast given current satellite trends of
the system, and the National Hurricane Center (as of 12Z) has a
near 0 percent chance of tropical cyclone development in the next
48 hours. Given this, the preference is to lean toward models with
a slower development in the short term. This preference also
extends to the surrounding mass field patterns in the Gulf Coast
region and the Florida Peninsula.

Note that this preference does not necessarily pertain to the
eventual development and track of a potential low beyond the first
three days of the forecast. For more details, please refer to the
NHC Tropical Weather Outlook and the WPC Medium Range Forecast
Discussion.


...All other areas and systems of interest...

Preference: Blend of 12Z GFS; 12Z NAM; 00Z ECMWF
Confidence: Slightly above average

The pattern across the CONUS will continue to be characterized by
relatively weak flow over the next few days, with the primary
exception being a more amplified trough and closed upper level low
approaching the West Coast. In general, model mass field agreement
is relatively good in this large-scale pattern, and sensible
weather impacts should largely be driven by smaller shortwaves and
MCVs embedded in the weak flow and perhaps convective outflows.
Given the potential for convectively-driven boundaries and forcing
mechanisms for future convection, there is not a strong model
preference in any particular area outside of the Gulf Coast (see
the previous section). However, the 00Z CMC and UKMET do have some
noteworthy differences. They both amplify the ridge over the
south-central U.S. more than other models, and amplify the
upstream trough along the West coast more as a result as well.
With the UKMET, the amplification of the ridge extends up into the
Northern Rockies. This may affect the overall sensible weather
impacts such that the preference is to lean toward a blend of the
remaining models -- the NAM, GFS, and ECMWF.


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

Lamers