Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1020 PM EDT Sat Jul 12 2014
the aviation section has been updated below.
issued at 146 PM EDT Sat Jul 12 2014
Active weather is expected the rest of the weekend as a slow
moving frontal boundary over Wisconsin and Iowa slowly sags
southward through central Indiana on Sunday. This will result in
shower and thunderstorm chances on tonight with best chances for
precipitation on Sunday.
A secondary cold front will pass through the region on Sunday
night and Monday...once again resulting in more chances for
showers and isolated storms.
Cool...Canadian high pressure will arrive in the Ohio Valley in
the wake of that cold front...providing temperatures well below
seasonal normals. Highs through much of the next work week will
only reach the 70s...making temperatures feel like late September
rather than middle July.
Near term /rest of tonight/...
Issued at 951 PM EDT Sat Jul 12 2014
widespread convection has developed ahead of cold front over Iowa
and northern Illinois. Trended precipitation movement from northwest to
southeast across central Indiana tonight with activity really
increasing over the northwest quadrant after sun 06z. Southern
portions of forecast area will most likely not see any precipitation
until closer to sunrise tomorrow morning. Current temperatures across the
area are generally in the middle to upper 70s with southwesterly
winds of 5 to 10 miles per hour. Overnight lows are still expected to be in
the low 70s. Updated grids have been sent.
main forecast challenge tonight will be probability of precipitation.
GFS and NAM suggest a short wave over the upper Midwest will
eject east...over the Great Lakes and northern parts of Indiana
overnight. Good convergence is expected over northern Indiana and
northwest Illinois ahead of the front and propagation suggests these
features will sag southeast toward central Indiana. A few timing
differences here as the GFS is a bit quicker than NAM. Thus will
trend probability of precipitation overnight highest across the northwest parts of the
forecast area where proximity is best and trend toward the dryer
NAM across the southern forecast as in part for the same reasons.
Given the expected rain and dewpoints...a blend of mavmos and
metmos works fine.
Short term /Sunday through Tuesday/...
Issued at 146 PM EDT Sat Jul 12 2014
Its gonna rain!
GFS and NAM both agree that the frontal boundary over the upper
Midwest will be sagging south across central Indiana on
Sunday...resulting in precipitation. 310k GFS isentropic surface even
shows good upglide along with specific humidities over 9.9 g/kg.
Time height sections show deep saturation through the
column...mainly during the morning hours. Forecast soundings also
show a saturated column. Meanwhile lower level q vectors show good
convergence. Thus with all of these factors pointing toward precipitation
will trend probability of precipitation higher on Sunday as these features pass. Given the
expected clouds and rain...will trend high temperatures lower than the mav.
Best dynamics and forcing appear to exit the area on Sunday night
as isentropic forcing is lost and dryer air begins to move into
the area from the west. Forecast soundings and time heights also
dry out considerably by Sunday night. May carry a small chance
during the evening hours in the case the system is a bit slower to
progress...but will trend toward a dry forecast after 06z Monday.
Given the Star of cold air advection in the wake of the frontal
boundary will trend lows at or below mavmos.
On Monday a secondary cold front will push across the area as much
cooler temperatures begin to push into central Indiana front the
northwest. Moisture will be more limited with this
boundary...however with a strong change in air mass beginning will
carry some low chance probability of precipitation for light precipitation on Monday as these
features pass. Precipitation not expected to be heavy.
Dry and colder weather expected on Monday night and Tuesday as
Canadian high pressure noses into the Ohio Valley. Strong cold air
advection is set to build across the area by Monday night as 850mb
temperatures fall toward 8c by 12z Tuesday. Forecast soundings dry out
dramatically amid associated subsidence. Thus will trend toward
partly cloudy Monday night and Tuesday with temperatures at or below
Long term /Tuesday night through Saturday/...
Issued at 224 PM EDT Sat Jul 12 2014
Models are close enough that the allblend initialization was
accepted for most forecast parameters.
The upper low that will bring the colder air will lift to the
northeast during the long term...but the area will remain in a
larger trough aloft. This will allow temperatures to moderate...but
still remain below average for most of the period.
Models continue to hint at some instability showers on Wednesday but
still feel the odds of these are too low to mention at this time.
Some also show some rain on Saturday in warm advection pattern...but
uncertainty is too high to mention this either. Thus kept long term
Aviation /discussion for the 130300z ind taf update/...
issued at 1020 PM EDT Sat Jul 12 2014
No need to make changes to the taf.
Previous discussion follows...
Would not rule out brief MVFR conditions this evening or an isolated
shower or storm...but best chance will be after 09z with current
Iowa convection settling southeast across the area ahead of a cold
front. Went with 6 hour prob30 MVFR groups to handle that activity
and MVFR thunderstorms in the vicinity after the front moves through. The front should reach
laf after 15z and bmg after 19z with southwest winds 10 knots or
more switching to west and northwest.
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