Area forecast discussion
National Weather Service Wilmington Ohio
605 am EDT Tuesday may 21 2013
Synopsis...
a warm and humid air mass will remain in place through Wednesday.
Occasional periods of showers and thunderstorms will be possible.
A cold front will move through the region on Thursday...before
cooler air settles in on Friday and into the weekend.
&&
Near term /until 6 PM this evening/...
significant changes were made to the forecast in the near
term...as a ragged mesoscale convective system currently moving across central Indiana
will continue its way into the iln forecast area this morning. All
models...including convective allowing models...forecast a gradual
weakening of this feature as it continues moving to the east.
Mesoscale analysis shows the near-surface environment is very
unfavorable for this line to maintain its strength...with much more
inhibition (up to 100-150 j/kg) than when it was in central
Indiana...and a near-surface inversion over the iln County Warning Area.
However...as some elevated instability will remain in place...the
convection is not expected to dissipate entirely. There is some
shear in place in the low levels (30-40 knots from 0-1km...which
is somewhat impressive). Winds ahead of the line are generally out
of the south...and winds at 925mb are out of the southwest and up
to 40 knots. This has led to some isolated pockets of stronger
winds on the line. However...it is unlikely that this shear will
be realized over the iln County Warning Area...given the more hostile
thermodynamic environment. Thus...the severe threat this morning
appears low...though some gusty winds will certainly be possible.
The focus for the rest of the day is on the potential for
additional storms this afternoon and evening. The first challenge
is a near complete lack of proper modeling of the current
convective complex. The hrrr appears to be the closest...though it
is a couple hours behind. The NAM solution...which
features precipitation moving through Indiana and Ohio during the
late morning and early afternoon (3-6 hours behind the current
activity) does not appear reliable. With a lack of synoptic scale
forcing...mesoscale features will play a big role in the potential
for storms...and these features will be highly dependent on the
evolution of the morning convection (placement of outflow / cloud
debris limiting destabilization).
In general...the pattern will favor the greatest instability in
the southern portions of the forecast area...perhaps as high as
1500-2500 j/kg of mixed layer cape. Still a ways removed from the
brunt of the jet...the iln County Warning Area will not get into a great deal of
wind shear today...perhaps up to 30 knots in the deep layer and
not much at all in the lowest levels. The instability should be
surface based and uncapped (rap13 forecasts show a 3 degree c drop in
700mb temperatures by afternoon)...and this appears to be enough to
support some severe threat. The lack of stronger shear and
organized forcing should be a limiting factor. It is possible that
the morning convection may end up leaving a boundary that could
act as the focus for convective development in the afternoon
hours.
The temperature forecast is also significantly dependent on the
above factors...though the air mass is certainly capable of
supporting temperatures as high as the middle to upper 80s. Raw model temperatures
are in wide variance from model to model...with intense heating
before precipitation forces drops in the surface temperature fields (and
at different times and places). While a general consensus was
used...in the interest of being a bit conservative given the
uncertain convective forecast...the maximum temperatures were knocked down a
couple degrees from there.
&&
Short term /6 PM this evening through Thursday night/...
convection will likely be ongoing for at least part of the
forecast area during the Tuesday evening hours...with a diurnal
min leading to more tranquil conditions overnight. However...by
Wednesday morning...this may begin to change again. A shortwave
moving out of the Southern Plains will eventually make its way
just east of the iln County Warning Area by morning...with model agreement on an
associated convective complex spreading into Indiana by then.
Meanwhile...surface low pressure will strengthen as it moves
through Illinois and into Michigan...with general surface
troughing and height falls aloft spreading into the iln County Warning Area. This
is a favorable environment for lift...and should support a likely pop
forecast through most of the day Wednesday. Although generally
unidirectional...the increasing pressure gradient and
juxtaposition with the right entrance region of the upper jet will
also support an increased bit of shear and forcing (respectively).
The limiting factor for severe potential will be
instability...which does not appear to be particularly strong (and
this is not surprising given the lack of expected surface
heating). Temperatures aloft will continue to drop...which should help
generate 500-1000 j/kg if some sun can be realized. An alternate
possibility would keep precipitation over the County Warning Area through the
morning and early afternoon...essentially limiting destabilization
to near-zero. The soundings look rather moist...leading to thin
cape and low LCLs.
It is an interesting contrast in severe potential from today
(tuesday)...with different elements of the mesoscale analysis
appearing favorable on either day.
The large northern plains upper low will finally be on the move by
late Wednesday...crossing the iln County Warning Area on Thursday. This will bring
another high chance of probability of precipitation on Thursday...mainly in the north and
generally with diurnal timing. Behind the axis of this
trough...rapid drying is expected.
Temperatures will begin a downward trend after the poorly-defined
cold front (associated with the Wednesday surface low) moves
through the region. A stronger change in near-surface air mass is
expected behind the trough on Thursday night...with northerly
winds advecting colder and drier air into the region.
&&
Long term /Friday through Monday/...
surface high pressure will build down from the north and into our
area Friday into Saturday...keeping a seasonably cool airmass in
place through the start of the weekend. Highs on Friday will be in
the middle to upper 60s and with the surface high overhead...lows
Saturday morning will dip into the low to middle 40s. The surface
high will begin to shift off to the east through the remainder of
the weekend as middle level ridging begins to establish itself aloft.
This will result in a gradual return to warmer temperatures
through the rest of the Holiday weekend. Models are still trying
to suggest the possibility of some energy dropping down from the
northwest late in the weekend. However...timing and placement are
highly variable so will maintain a dry forecast at this point.
&&
Aviation /10z Tuesday through Saturday/...
large scale upper level low centered over the northern plains will
make little eastward progression over the next 24 hours. Region
will be located in a warm...moist south/southwest flow at the surface
and aloft.
First item of concern will be a cluster of convection ongoing
across southwest Indiana and western Kentucky. It is unclear how
much of this will hold in tact as it pushes toward the kcvg and kluk
terminals between 12z and 15z so have included a thunderstorms in the vicinity/cumulonimbus at
theses taf sites but will make a last minute decision based on
radar trends if a tempo group is needed. As our area is in a moist
southwest flow aloft...additional shower and thunderstorm activity
will be dependent on any linger outflow boundaries from early
morning convection...embedded disturbances aloft and how much
sunshine the area receives for instability. Models are not
offering much in terms of specific timing except to favor the
diurnal cycle and have include only thunderstorms in the vicinity/cumulonimbus at the taf sites as
such. This may significantly change depending on previous
mentioned factors if thunderstorms get going on any mesoscale
feature.
For tonight...models suggest a lull in the convective activity in
terms of coverage. Coverage should increase on Wednesday as a more
significant disturbance rotates from southwest to northeast across
the region out ahead of a cold front. Showers and thunderstorms
look like a good bet for Wednesday into Wednesday evening.
Outlook...thunderstorms likely Wednesday into Wednesday evening.
MVFR conditions possible Thursday into Thursday night.
&&
Iln watches/warnings/advisories...
Ohio...none.
Kentucky...none.
In...none.
&&
$$
Synopsis...hatzos
near term...hatzos
short term...hatzos
long term...jgl
aviation...Hickman
