All the models continue to show a large meridional trough developing
in the western US for May 21 to May 25. The ECMWF/Canadian
have been reasonably consistent for the past several days. If anything
there has been a slight westward trend in the models in the upper
trough and dryline. The models are also showing a weak upper
trough over Texas out ahead of the main upper trough. This little
feature seems to happen a lot. I think this pattern looks phenomenal from
a chasing viewpoint. Unlike progressive shortwave troughs that provide 1
shot deals, this system will likely provide several days of chasing. Typically
with such systems, supercells are most common along the eastern edge
of the strong flow and sometimes in the weaker flow on the periphery of
the large gyre. Four cases immediately come to mind,  namely, May 5, 1993,
May 10-11, 1991, April 21, 1957,  and June 18, 1975. June 14-17, 1965
also featured a large meridional trough with several days of tornadoes along the
front range, but this system was further west than the one we are dealing with.

One thing to watch for by May 23 and 24 is outflow from overnight
storm clusters.The cap should be sufficiently weak by then so that
outflow could run amuck across the central plains, leading to a
panhandle surprise. I really like that large upper trough the ECMWF
is progging over the northeast. This will lead to a warm front over the
central plains on Thurday with upslope flow in central and western NE.
The best place could be just northeast of the  intersection of the pacific cold
front, dryline and warm front in southwest Nebraska.But this is sure to change.
Actually, if that warm front verifies, it could be good from eastern WY into
northern KS. Friday and Saturday look favorable too, with outflow boundaries
possibly playing a role.

One reason why meridional flow leads to great chase weather in the plains
compared to the midwest is the cap. If you shift this pattern further
east about 10 deg longitude, then there would be little capping and
widespread storms would probably be the result. Mid level flow would be
from the gulf instead of the elevated terrain of Old Mexico and Rockies.

Attached is my rendition of the high resolution EC for those who dont get
this product. I actually messed up the 500mb heights. The 570 height
line should be further east. The EC is usually conservative with
sfc dewpoint.

http://bangladeshtornadoes.org/UScases/051091/05109112zh5.gif
large tornadoes west Texas, southwest SD moving north

http://bangladeshtornadoes.org/UScases/050593/05059312zh5.gif
http://bangladeshtornadoes.org/UScases/050593/05069300zh5.gif
southwest Kansas and Nebraska panhandle wedges moving north

http://bangladeshtornadoes.org/UScases/042157/04225703zh5.gif
west Texas wedges moving nnw/northwest

http://bangladeshtornadoes.org/UScases/061875/06197500zh5.gif
family of strong tornadoes moving north in southwest Nebraska

Jonathan Finch