Severe Weather Threat for Saturday


A large part of the mid part of the US could be under the gun for thunderstorms, even a moderate risk of severe weather in Minnesota and surrounding areas. Below is the convective outlook for today as issued by the Storm Prediction Center.

Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
1251 am CDT Sat Apr 09 2011

Valid 091200z – 101200z

..there is a MDT risk of severe thunderstorms from extreme southeastern South Dakota across southern
Minnesota and northern Iowa into southwestern WI…

..there is a slight risk of severe thunderstorms surrounding the MDT risk from
central MN/WI…southwestward across southeastern Nebraska into northwestern Texas…

..there is a slight risk of severe thunderstorms from the lower Ohio Valley into
SC…

A potentially significant weather pattern is developing across
portions of the country east of The Rockies over the next few days.
Of particular interest Saturday will be a corridor of the upper MS
valley during the late afternoon into the evening hours where very
large hail and even a few strong tornadoes are possible. Another
area of organized severe is possible from portions of the lower Ohio
Valley…southeastward into SC.

..upper MS valley…

Upper ridging will gradually break down across the upper MS valley
during the latter half of the period as height falls begin to
overspread the plains in response to a strong and progressive upper
trough over The Rockies. Prior to large scale forcing affecting
this region…an early morning mesoscale convective system or region of thunderstorm
clusters will be shifting east across MO/IL. In the wake of this
activity…moisture will surge northward across eastern Kansas into eastern Nebraska/Iowa as
low level jet begins to respond and increase in excess of 40kt during the
afternoon hours. At this time it appears capping will prevent
meaningful daytime thunderstorm development across the MDT risk
region and boundary layer destabilization should be maximized near
the MO river across western Iowa by 10/00z. Forecast soundings across
this region suggest inhibition will weaken considerably within
confluent surface flow and thunderstorms should develop initially across
southeastern South Dakota/eastern Nebraska where low level lapse rates will be maximized. With
the potential for SBCAPE in excess of 3000 j/kg vertical shear
profiles will prove very favorable for supercell structures.
Additionally…low level hodographs strongly suggest the potential
for tornadoes as surface-3km helicity is expected to be in excess of 300
m2/s2. For this reason a few strong tornadoes could be noted across
the warm sector immediately south of the retreating warm front.
Once thunderstorms initiate across this region it appears an upward
evolving area of severe thunderstorms should converge/evolve in an
east-west fashion along the warm front. With very steep lapse rates and
MUCAPE values exceeding 2000 j/kg atop the cooler airmass severe
elevated supercells capable of producing very large hail will exist
across southern Minnesota in addition to the large hail threat with warm sector
supercells.

Activity should also develop southwestward during the overnight hours along
the advancing surface boundary across western Iowa into southeastern Nebraska.

Farther south along the dryline…strong boundary layer heating
across the Southern Plains should easily remove convective inhibition
across western OK into northwestern Texas. Scattered thunderstorms are expected to
develop across this region as temperatures soar into the 90s where
surface dew points hold in the upper 50s/lower-mid 60s. Shear profiles
will support supercells and large hail…although an isolated
tornado can not be ruled out near dusk as low level winds back and
before convection decouples from boundary layer.

..lower Ohio Valley southeastward to SC…

An early morning mesoscale convective system should be ongoing at the beginning of the
period across portions of MO/IL…an extension of ongoing activity
that is evolving/spreading across Kansas/MO region. Latest model
guidance suggest strong boundary layer heating will develop ahead of
this convection across Kentucky into southern WV and an upward evolving thunderstorm
threat will ensue by 18z where northwesterly mean shear vectors on the order
of 50-60kt will enhance organizational potential. Although
supercells would seem possible in this environment…it/S possible a
more organized mesoscale convective system will be the convective Mode as early morning
activity should not completely die off before boundary layer
destabilization occurs. Large hail and damaging winds seem likely
with this activity.

.Darrow/Hurlbut.. 04/09/2011

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