Area forecast discussion
National Weather Service Wilmington Ohio
640 am EDT Wednesday may 22 2013
Synopsis...
low pressure will move east across the Great Lakes today...with a
cold front moving into the Ohio Valley tonight. Showers and
thunderstorms will be possible ahead of this front. A secondary
trough will bring a chance of rain again on Thursday. Much colder
air will move into the region on Friday...as high pressure builds
into the Ohio Valley through Saturday.
&&
Near term /until 6 PM this evening/...
a somewhat-disorganized surface low is currently located in Iowa
and northern Illinois. This low will slowly move to the east
today...strengthening slightly as it crosses the southern Great
Lakes. South of the low...a weak cold front will move into the iln
County Warning Area by evening. This front is tough to pick out on surface
temperature fields...but is a bit more evident when examining
Theta-E slightly aloft...and also looking for slight wind shift
from south-southwest to west-southwest. At the same time...a closed upper low over the
northern Mississippi Valley will begin its move to the east...with
a shortwave rotating northeast through Indiana and Michigan. In
the right entrance region of an upper jet...the setup over Ohio
will be favorable for large-scale lift.
The Standard suite of models (gfs/NAM/CMC/ecmwf) has thus far
appeared much too aggressive with the overnight precipitation
forecast over Illinois and Indiana. This is admittedly a region
where lift is forecast to continue to enhance through morning...in
a region of falling heights and a 30-40 knot low level jet. Dewpoints north
of the current Tennessee convection are about 3-6 degrees lower than
in the southern states...but the air mass could not be described
as particularly dry. As was the case yesterday...the hrrr has the
closest approximation to the current activity. Several other
convective-allowing models depict quite a bit of convection moving
into the iln County Warning Area as early as 12z...which does not appear realistic
given the lack of instability and current radar trends. The
overall chances for rain in the iln County Warning Area do not appear particularly
high through the morning hours...though slightly more so in the
western part of the County Warning Area.
The main concern for the forecast today is the potential for
severe weather this afternoon. Given the wide variance in the
handling of the current weather situation...and the fact that
these mesoscale features will be very important in convective
forecasting for this afternoon...it is tough to trust any of the
specific quantitative precipitation forecast/sim-radar depictions for the afternoon hours.
To begin with the biggest limiting factor for today...instability
will be in question. Given the expected cloud cover...and even
precipitation in some places...the surface temperature forecast is on the
low side of guidance. Given the thermal profile...MLCAPE will
struggle to reach 1000 j/kg...with the most instability forecast
to be in the southeastern parts of the County Warning Area. In the
northwest...where temperatures will be the coolest...instability will be
even less (500 j/kg or so). Soundings are relatively moist through
their depth...with rather low local heights (around 2000-3000 feet
agl) forecast on the GFS/NAM (the rap13 appears a bit drier).
Because of the moist conditions...the cape profile is narrow.
Unlike yesterday...the overall forcing environment will be more
favorable for convective development...even if instability is
weak. Wind shear is also somewhat of a strength for
today...although model-to-model assessment on shear values is not
in as good of consensus as is usually the case. Most of the shear
will be fairly unidirectional (all in the top-right quadrant of
the hodograph)...but with a decent change in speed. 30-40 knots of
deep layer shear will be present...with 15-25 knots in the 0-1km
layer. 0-3km srh values will get into the 100-200 range...so there
will certainly be some potential for rotating storms. This could
be enhanced by boundary interactions or areas of backing surface
winds (perhaps further north and closer to the surface low).
The lack of instability...especially in the hail growth
zone...will limit the potential for large hail (perhaps outside of
a stronger / rotating updraft). The threat for strong winds and
tornadoes does appear to exist...but the thermodynamic environment
seems to limit this from being a higher-end severe event. A
cloudier/grungier morning would support an isolated severe threat
with clusters of storms capable of strong winds and maybe a bit of
rotation. The outside shot for today would be a greater deal of
boundary layer destabilization (between insolation and moisture
advection)...which would bring a greater risk of all the severe
modes coming to The Table. However...given all the evidence...this
forecast will lean toward the former solution. The severe weather potential statement will follow
suit with a relatively typical-sounding severe threat wording.
&&
Short term /6 PM this evening through Friday night/...
the initial cold front will be crossing the iln County Warning Area during the
evening hours...producing a relatively small change in the
temperature and wind fields. Behind the front...precipitation will
become less likely...especially in the south and southwest
portions of the County Warning Area.
A secondary trough (both aloft and at the surface) will be
rotating southeast through the County Warning Area on Thursday. This will bring a
renewed chance of precipitation...especially in the northern and
eastern parts of the County Warning Area. Instability will be very weak...but just
enough for some risk of thunder. The air mass aloft will be
increasingly cold...helping to lead to the possibility of
convective showers.
Along with this activity will be a secondary cold front...much
better defined than the first. A north-northwest wind shift will allow for
cold advection to begin on Thursday (and especially Thursday
night). As a result...the maximum temperature gradient for Thursday is
somewhat sharp (about 10 degrees from SW to ne) and Thursday
night min temperatures may get into the 30s in the northern portions of
the County Warning Area.
With dry high pressure in place for Friday...relatively tranquil
conditions are expected.
&&
Long term /Saturday through Tuesday/...
dry weather should continue on Saturday for most of the area
except for far southeast counties where a developing frontal
boundary may cause a few showers.
For Sunday through Tuesday...that frontal boundary is forecast to
linger across Kentucky. This pattern will provide a chance of
showers each day to the southern part of the forecast area...with
northern locations enjoying a period of dry weather.
Slightly below normal highs averaging in the lower 70s on Thursday
will be followed by below normal 60s on Friday under clouds and
showers. Temperatures will then gradually rebound from Saturday
Onward...with highs reaching the middle and upper 70s by Tuesday.
&&
Aviation /12z Wednesday through Sunday/...
region will remain in a moist south to southwest flow at the
surface and aloft as the center of an upper level low pivots
southeast toward the western Great Lakes. As the low moves
southeast...a surface cold front will push east through our area
tonight.
Latest analysis and models indicate that our area is still in an
instability minimum this morning. As such...the low level jet
that is over the area may produce some scattered showers this
morning.
For this afternoon into this evening...the prospects of showers
and thunderstorms will increase as daytime heating interacts with
large scale synoptic lift. Best chance for widespread showers and
thunderstorms will be from middle afternoon into the evening hours.
Have placed this in the tafs with a predominant MVFR -shra and a
vcts/cb. As storms become more apparent and can be tracked on
radar...tafs will be fined tuned accordingly. With a moist
airmass...some local MVFR ceilings and IFR visibilities will be possible in
thunderstorms. Daytime heating/mixing will produce south/southwest
winds of 12-16 knots with gusts up to 30 knots.
For tonight...the cold front is forecast to push east across the
region. Large scale lift decreases as does the instability.
However...models indicate that there could be a band of showers
and a few embedded thunderstorms along and ahead of the front.
Have indicated this with a vcsh as thunderstorms chances will wane
between 01z and 06z. After frontal passage...there should be a
reprieve in precipitation but low level moisture will remain. It is unclear
at this time if MVFR ceilings will develop behind the front so
have left them at VFR for now.
Outlook...MVFR conditions possible with showers and embedded
thunderstorms Thursday into Thursday evening.
&&
Iln watches/warnings/advisories...
Ohio...none.
Kentucky...none.
In...none.
&&
$$
Synopsis...hatzos
near term...hatzos
short term...hatzos
long term...coniglio
aviation...Hickman
