Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1249 PM EDT Sat Aug 23 2014
the aviation section has been updated below.
issued at 207 am EDT Sat Aug 23 2014
High pressure over the Tennessee River valley will continue to
keep a very warm and humid air mass over central Indiana the rest
of the weekend. Showers and thunderstorms will be possible each of
the next few afternoons as the warm and humid air mass remains in
place across the area.
Rain chances will diminish at the beginning of next week as the
high pressure system further establishes across the Ohio Valley.
Rain chances will return during the middle of the next work week
as a cold front sags into the Ohio Valley from the upper Midwest.
Near term /rest of today/...
issued at 1015 am EDT Sat Aug 23 2014
Steamy morning ongoing with localized fog burning off. Have seen a
few spots with locally dense fog but observation indicating steadily
improving visibilities. Temperatures at 14z generally ranged from 75 to
Current forecast in really good shape with only minor adjustments
necessary. Predominant easterly flow across a good portion of the
forecast area this morning as a weak surface wave noted in
mesoanalysis over western Illinois with the remnant frontal
boundary extending east/southeast to near a khuf to ksdf line. Convective
cluster over eastern Iowa to the north of the surface wave moving
slowly E/NE. While the instability axis and Theta-E ridge both
follow the boundary into the forecast area...all short range
guidance suggesting convection will continue to move east/NE
following the wave aloft while gradually weakening through the
early afternoon as it moves away from the low level jet.
For central Indiana...anticipate scattered convection developing
during the afternoon in the vicinity of the frontal boundary as
the airmass becomes moderately unstable. Storm development will
be largely dependent on instability...deep moisture and the front
and any additional outflows with little to no appreciable forcing
aloft. Soaking rains and frequent lightning will be the main issues
with convection today as with storm motions generally 10kts or
less and precipitable waters remaining around 2 inches. Certainly any stronger
cell will have the potential to produce gusty winds.
Zone and grid updates out.
Short term /tonight through Monday night/...
Issued at 207 am EDT Sat Aug 23 2014
Main forecast issue during this period will be probability of precipitation.
GFS and NAM continue to suggest that the strong ridging in place
aloft over the Mississippi River valley will amplify and move east
building across Indiana and the Great Lakes. This will keep all
ridge riding short waves well to the north and east of Indiana and
keep Indiana under the protection of the ridge axis. Meanwhile
forecast soundings show some warm air arriving in the middle
levels as 700mb temperatures rise to at or slightly above 10c. By Sunday
night and Monday...subsidence begins to take shape within the
column underneath the ridge as forecast soundings begin to show
dry air infiltrating the column.
By Monday afternoon and Monday night...the upper ridge axis is
expected to pivot east to New England...but Ohio Valley remains
protected aloft by a large closed upper high pressure
system...providing subsidence. Thus will continue the trend toward
a dry forecast through Monday night. Given the little change in
850mb temperatures overall...will trend highs at or slightly above
persistence...with warmest temperatures coming on Monday and into Tuesday
as subsidence allows for better heating of drier air.
Long term /Tuesday through Friday/...
Issued at 207 am EDT Sat Aug 23 2014
Ensembles suggest upper ridge initially over the area at the
beginning of the period will get suppressed back to the south as an
upper trough moves into the Great Lakes by the end of next week.
Ensembles suggest precipitation potential will move back into the
area starting Tuesday as upper ridge begins to flatten out...with
the best chances next Wednesday and Thursday as a frontal system
associated with the approaching upper trough moves in. Will keep
chance probability of precipitation going in the forecast from Tuesday through Thursday.
Some of the ensemble members hang the southern end of the upper
trough back across the area into Friday...with an attendant
precipitation threat at that time. Will add some probability of precipitation for Friday as
well to cover this possibility.
Aviation /discussion for the 231800z taf issuance/...
issued at 1249 PM EDT Sat Aug 23 2014
MVFR or worse fog possible overnight. Otherwise VFR expected outside
Convection across northern Illinois at the moment may move into the
taf sites within the first few hours of valid time. Still lots of
uncertainty in how far southeast the storms will turn and whether
middle level warm air will inhibit development.
At any rate klaf looks to have the best chance so will go tempo
there. Will go thunderstorms in the vicinity elsewhere. Of note also is a wind shift behind
an old outflow. This will cause winds to become west. This should
occur about the time convection moves in.
Convection will likely end this evening but more is possible
overnight. Confidence too low to include. Have put MVFR fog in at
all but kind overnight...but worse conditions are possible if clouds
Visit US at http://www.Weather.Gov/ind
Follow US on facebook...twitter and youtube at: