Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 210528

Area Forecast Discussion
National Weather Service Indianapolis IN
1226 AM EST Wed Feb 21 2018

The NEAR TERM Section has been updated below.


Issued at 222 PM EST Tue Feb 20 2018

A cold front will sag southward across Central Indiana
tonight...spreading rain showers across the state as it passes.
High pressure will then build across northern Indiana on
Wednesday as the departing cold front stalls across Kentucky.

Meanwhile...warm southwest flow aloft will continue to allow moist
tropical air to surge into the Ohio Valley. This will result in
daily chances for rain across Indiana for the rest of the week.
Flooding threats will continue through the week with the expected

Look for temperatures to remain above normals the next several


.NEAR TERM /Overnight/...
Issued at 1226 AM EST Wed Feb 21 2018

Pushed back the arrival time for the large shield of rain a couple
of hours based on radar trends.

Previous discussion follows...
Issued at 222 PM EST Tue Feb 20 2018

Surface analysis early this afternoon shows strong low pressure in
place across the Great Lakes with a strong cold front stretching
from SE Wisconsin across Central Illinois to Central Missouri.
Water Vapor imagery shows the story today. A deep trough was found
across the western United States...resulting in a sharp SW flow
across the Mississippi and Ohio River Valleys. Water Vapor shows
the plume of tropical moisture streaming across the Illinois and
Indiana from the tropics. Radar shows showers continuing to stream
across NW Indiana and Central Illinois.

GFS and NAM suggest the cold front to the west will sag southeast
across Central Indiana overnight. Again...favorable conditions
remain in place for precip. Tropical flow aloft...excellent lower
level convergence. A warm and relatively humid air mass ahead of
the front. Forecast soundings show a saturated column arriving by
09Z with Pwats near 1.32. Time height sections show excellent
saturation within the mid and lower levels. 305K GFS Isentropic
surface shows a surge of mixing ratios over 6 g/kg tonight ahead
of the front. Thus 100 pops tonight appear reasonable. Given the
expected progression of the front...will hold off on extending the
flood watch for now...but given possible rain amounts of over
0.50 inches...isolated nuisance flooding cannot be ruled out.
Given the expected cold air advection yet ongoing rain in the wake
of the front...will trend lows very close to the forecast builder


.SHORT TERM /Wednesday through Friday/...

Issued at 222 PM EST Tue Feb 20 2018

For much of Wednesday the GFS and NAM suggest the cold front sags
to Kentucky...taking the bulk of the precipitation with it.
However...aloft the strong...SW tropical flow continues to stream
into the Ohio Valley. Yet another short wave embedded within the
flow is suggested to pass across Central Indiana...mainly late in
the day toward evening. Time heights show good lift aloft...yet
some dry air within the lowest levels as high pressure build
across Wisconsin and the Great Lakes. Thus best chances for precip
should be across the southern parts of Central Indiana on
Wednesday...closer to the lingering front...with lesser chances to
the north where more dry air will have settled in. Will trend pops
in this manner.

On Wednesday night another the GFS and NAm suggest another short
wave approaching Central Indiana with the stalled cold front
across Kentucky. Again excellent saturation will be found within
the lower levels as suggested by the forecast soundings. Again
will trend pops at or above the forecast builder values...mainly
across points across southern Central Indiana...closer to the
stalled front.

THE GFS suggests a break in upper forcing in Thursday with little
in the way of dynamics passing at that time. However the warm and
moist southwest flow continues across Indiana aloft. Meanwhile at
the surface...High pressure moving across the Great Lakes will
begin to drift east of Indiana...which will begin to allow return
flow to begin for Friday. Will only keep some minimal precip
chances across the area on Thursday with little in the way of
dynamics and only the departing short wave early in the day
providing forcing. By late in the day and into Thursday
evening...dry air appears to intrude into the column.

Yet another short wave looks to arrive on Late Thursday Night and
Into Friday. Forecast soundings show deep saturation by Friday
Morning with pwats suggested near 1.20 inches again. Meanwhile the
stalled front over Kentucky begins to return northward as a warm
front...providing yet another focusing mechanism. Thus will again
trend toward very high pops on late thursday night into Friday.

Overall will stick close to the forecast builder blend on temps.


.LONG TERM /Friday night through Tuesday/...
Issued at 232 PM EST Tue Feb 20 2018

Ensembles in good agreement with the main long wave pattern during
this period. Troughing expected to hold over the western parts of
the country, while the eastern ridge flattens out some by early next

Most of the active weather is expected early in this period, as
disturbances periodically eject out of the western trough, with the
last one passing through around Saturday night. Several of the
ensemble members spin up a sub 990mb low associated with this last
wave, while others are weaker and more sheared with this final wave.

Will keep PoPs in the forecast from Friday night into Sunday morning
based on above. Progged precipitable waters still close to
climatological maximums until this final wave passes, so heavy
rainfall remains a threat. If a trend develops towards the stronger
solutions, the thunder threat may need to be pulled farther north.


.AVIATION /Discussion for the 21/806Z TAF Update/...

Issued at 1144 PM EST Tue Feb 20 2018

Rainfall has been slow to arrive at TAF sites tonight, only
impacting KLAF currently. However, it should start spreading into
the remaining TAF sites over the next few hours as a cold front
moves into central Indiana. Conditions will quickly deteriorate
to MVFR and then IFR levels with onset of rain, and there will be
little improvement until late in the TAF period when VFR
conditions return with strengthening high pressure after Wed 18Z.
Meanwhile, winds will remain gusty out of the southwest over the
next several hours with a low level wind shear threat. They will
gradually veer to the west and then northwest this morning with
frontal passage.


Flood Watch until 7 AM EST Wednesday for INZ021-028>031-035>049-



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