Weekly Severe Weather Discussion

Severe Weather Discussion for April 14 – 20, 2008.
Valid: 04/13/2008.

Highlights:

– A freeze is likely for much of Oklahoma tonight.

– A quiet few days as temperatures warm through Thursday. A brief
cool down on Friday, with a warming trend into the weekend.
Temperatures will be in the mid 60s on Monday warming to 70s Wed –
Fri, and then upper 70s for next weekend.

– Isolated showers and thunderstorms are possible Wednesday and Thursday.

– Isolated severe thunderstorms are possible on Sunday.

Probabilities:

– Severe Thunderstorms in Oklahoma: 15%
– Storm Chase: 10%
– Winter Precip: < 5%

Discussion:

Amazingly active period last week, which will be followed by
relatively little in the way of sensible weather much of this week.
Some interesting notes from last week. Significant severe weather
occurred Monday, Wednesday, and Thursday across Texas and Oklahoma,
with much more across the U.S. High-end impacts occurred for several
days. The upper system responsible for this event produced very
strong winds Friday and Saturday, with unseasonably cold temperatures
these two days. A little global warming is looking good about now.

The large and very cold (500mb temps -34C) longwave trof will slowly
progress east the next few days. The slow progression and deep
latitude penetration of this system will result in a cold Canadian
surface high intrusion into the Gulf through Tuesday. This will push
deep and tropical moisture to the Yucatan/Cuba latitude. Models
indicate that the center of the surface high will progress across the
central U.S. and eventually to Florida as the southern stream system
ends up out of phase and hangs back as a closed low over the eastern
Atlantic. This occurs on Wednesday as western U.s. system moves
towards the area and splits with the northern stream out running the
southern stream system. The 12z GFS/ECMWF are in unusually good
agreement regarding this pattern.

In a typical April the system for Wed/Thu would be favorable for
severe thunderstorms or even a medium-end event. However, due to the
prior cold intrusion, quality moisture will be difficult to find. GFS
does indicate strongly modified moisture moving north as Gulf
trajectories remain 090, thereby recirculating cool air into the Gulf.
12z NAM is a little more robust on surface dewpoints in the mid 50s
with 850mb dewpoints around 10C than the GFS's 50F/8C respectively.
Either way, given that the upper system will still be west, believe
that storm chances are rather low. Storm chances improve late
Wednesday into Thursday as the upper system moves across Oklahoma.
The best chance of precipitation appears to be central/east Oklahoma
and Texas. I'm not convinced of severe weather potential, however
given that the upper system will pass directly over Oklahoma, there
will be plenty of lift for sustained updrafts. Mid level wind will be
strong, but directional shear appears to be at a minimum given the
closed nature of the low and the 850mb-500mb lows being vertically
stacked. There may be some 0-1km helicity, but 0-3kmb won't be that
good.

Upper system moves by and weakens as it gets east of the region. The
surface high with this system will be weak and shouldn't go much
farther south than southern Texas. This is near climatology.

12z GFS/ECMWF agree that another western U.S. longwave trof will
develop on Saturday and start east by late in the weekend. As this
occurs the Canadian high should be gone, allowing Gulf trajectories to
turn more 130-150. GFS develops a large warm sector by Sunday with
deep layer moisture in-place across the region. Speed shear will be
decent, but enough for rotating updrafts. Directional shear will be
very good. Initial GFS CAPE values are 2500 J/Kg and LI -8, with a
dryline located along the TX/OK border. Deep layer moisture will be
in-place per 850mb dewpoints of 14C and surface dewpoints 65-70F. GFS
develops convection along the dryline towards 7pm next Sunday. The
setup looks prototypical of a plains Day 1/Day 2 outbreak situation.
However, given that the event is 7.5 days out, I'm very hesitant to
go any higher on probabilities and will cover a medium-end event with
special discussions. The general setup certainly favors one or two
medium-end events around the 20th-22nd.

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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