Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 171736

Area Forecast Discussion
National Weather Service Northern Indiana
136 PM EDT Wed May 17 2017

Issued at 358 AM EDT Wed May 17 2017

Another warm and windy day is expected today. Highs will be
generally in the mid 80s and southwest winds will gust up to 40 mph
at times. There is a low chance of thunderstorms tonight and again
late Thursday. However...much better chances for rain will arrive
late Saturday into Sunday. Cooler conditions are also expected
Friday through Tuesday.


Issued at 131 PM EDT Wed May 17 2017

Wind advisory issued for far western tier of counties with
marginal setup for wind gusts approaching or exceeding 45 mph to
blend with surrounding offices. Will monitor for possible
extension eastward, but threat appears greatest west for the time


.SHORT TERM...(Today and Tonight)
Issued at 358 AM EDT Wed May 17 2017

Main story for today will be the continued warmth and windy
conditions. Initial, moderate amplitude wave now ejecting out of the
high Plains with downstream tightening of low level pressure/height
gradient generating a 40-50 kt LLJ just 2 kft off the surface per
KIWX VWP. Some mix out will occur after sunrise of course but NAM
and GFS both show further gradient tightening by late this afternoon
as vort lobe enters Iowa with 850mb winds climbing near 50 kts.
Expect some increased cirrus blowoff and a few midlevel clouds
possible but this should do little to prevent us from mixing to
850mb given May sun angle and relatively dry surface conditions.
Peak gusts guaranteed to reach 35 mph but question is will we reach
advisory gust criteria of 45 mph (or sustained to 30 mph for 1
hour). Though certainly not unheard of, it is usually difficult to
get those speeds in SW flow/warm sector with a general propensity
for ascending motion, not descent. Efficiency of downward momentum
transport is somewhat limited given turbulent mixing alone.
Furthermore, time/height cross-sections show best winds not arriving
until after 21Z, when peak superadiabatic heating/mixing will be
waning. Yesterday`s highs also underperformed a bit, suggesting
mammoth subsidence inversion may be curtailing mixing heights a tad.
Given these factors, have decided to hold off on an advisory for
now. Day shift can monitor trends and issue one if it appears mixing
will be more efficient than anticipated. Timing of strongest winds
looks to be in a fairly narrow window of 19-22Z with best chances in
our SW. Headline or not, bottomline is that it will be a windy day.
Thermal profiles for today are very similar to yesterday. Still
inclined to lean on the warmer side of guidance but yesterday`s
highs do give some pause. Only minor tweaks to inherited forecast
with generally mid 80s and a few sporadic 87/88`s possible.


.LONG TERM...(Thursday through Tuesday)
Issued at 358 AM EDT Wed May 17 2017

Story for tonight and Thursday remains largely unchanged, although
now with some more hi-res guidance to buoy the case against
overzealous GFS QPF. Lead wave increasingly sheared/dampened by
prevailing SW flow and upstream Canadian shortwave. Weak prefrontal
trough tries to make a run at our CWA tonight but encounters
increasingly hostile environment with fairly dry and still somewhat
capped profile. With primary HFC and differential CVA bullseye
passing northwest of our area, it will be difficult to generate any
precip. Most of latest hi-res CAM`s show convection decaying by the
time it reaches our area with just a few residual showers. Did leave
in a slight chance PoP given these solutions and in deference to
modest 800mb moisture surge. Similarly, also left in thunder mention
but no strong or severe weather is expected. Actual cold front, such
as it is, will not pass through the area until late Thursday,
leading to one more warm day. Parent shortwave virtually gone by
then with local midlevel heights actually rising late in the day.
Still, there is some broad 1000-850mb convergence with modest
instability (500-1000 J/kg 0-1km MLCAPE) that could touch off a few
storms late in the day. Marginal lapse rates/instability with
residual capping concerns and lack of coherent forcing would portend
a low chance of any strong storms. However, if a more robust updraft
does manage to develop, damaging winds will be possible given strong
wind/shear profiles. Actually, will probably gust to 30-35 mph
in the afternoon with diurnal mixing alone.

As for Friday, 00Z deterministic GFS is finally onboard with drier
solution championed by ECMWF. This solution has been preferred for
days given the strength of ridging/subsidence over Great Lakes and
effect of relatively cold lakes this time of year. Surface front is
shunted to the Ohio River with potent 850mb theta-e trough for most
of our area. Will still watch potential for convectively enhanced
waves to eject out of approaching trough but LLJ and associated
warm/moist air advection are virtually nonexistent until Saturday.
Best rain and thunderstorm chances will finally arrive late Saturday
into Sunday when primary upper low ejects and sends healthy cold
front into the area. Increasing isentropic ascent could lead to some
prefrontal convection but suspect best chances for widespread
moderate rain will be along cold front on Sunday. Still a bit early
to discuss details but this prefrontal precip and a relatively early
frontal passage will likely limit instability. With best height
falls well to the NW, severe weather chances are low but will
continue to keep an eye on it. Cool/dry/tranquil weather returns for


.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
Issued at 131 PM EDT Wed May 17 2017

Main story will be strong winds across the area with southwesterly
winds of 20 to 25 kts with gusts to 35 kts. Winds will diminish
somewhat tonight, resulting in an increasing threat for LLWS at
both sites until stronger mixing of winds takes place again by mid
morning Thursday.

With regards to cigs/vsby...VFR conditions expected with little
more than high clouds and some passing cu.


IN...Wind Advisory until 7 PM EDT /6 PM CDT/ this evening for INZ003-

LM...Small Craft Advisory until 4 PM EDT Thursday for LMZ043-046.




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