Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 190025

Area Forecast Discussion
National Weather Service Northern Indiana
825 PM EDT Fri Aug 18 2017

Issued at 805 PM EDT Fri Aug 18 2017

Dry conditions will continue this evening but chances for
scattered showers and isolated thunderstorms will increase later
tonight into Saturday morning. Lows tonight will drop into the
60s, with highs on Saturday near 80 degrees. High pressure then
builds in later Saturday through Monday with dry conditions and
seasonable temperatures. Monday night through Tuesday evening will
feature the next chances for showers and thunderstorms as a cold
front moves through the region.


Issued at 805 PM EDT Fri Aug 18 2017

Convection has developed this evening across west central Illinois
downstream of a fairly strong upper level vort max moving into
central Iowa. Subtle backing of low level flow in advance of this
feature will allow a weak low level theta-e ridge to push into
northern Indiana providing some weak elevated instability.
Previous forecasted PoPs focusing on the 05Z-12Z time period from
west to east still appear to be on track given this setup, with
weak nature of elevated instability keeping thunder more isolated.
Other than some minor tweaks to current sky cover trends, no
appreciable forecast changes are anticipated at this time for the
first period.


.SHORT TERM...(This Evening through Saturday)
Issued at 210 PM EDT Fri Aug 18 2017

A shortwave over the Upper Midwest will sharpen southeast into
leftover longwave troughing over the Great Lakes region later
tonight into early Saturday. Backing low-mid level flow with this
seasonably strong wave will tighten the downstream moisture/thermal
gradient over nrn IN/nw OH with a 3-6 hour window for
chances 5-11z across nrn IN/Lower MI and nw OH 8-14z. Elevated
instability into the developing frontal slope may support a few
thunderstorms and brief heavy downpours. The overall moisture
quality does appear to be somewhat lacking with this system
despite good deep layer dynamics. This may keep precipitation more

Subsident northwest flow in wake of the upper wave will allow for
quick drying and clearing from west to east by later Saturday
morning and afternoon. This clearing should allow afternoon highs to
recover to near 80F.


.LONG TERM...(Saturday Night through Friday)
Issued at 210 PM EDT Fri Aug 18 2017

The synoptic pattern will briefly transition to a more zonal flow
configuration Sunday into Monday. This will promote mainly dry and
warmer conditions with weak low level ridging dominating during this

Upper level longwave trough amplification into the Eastern US mid-
late week then brings a strong cold through around Tuesday/Tuesday
night, followed by cooler/dry wx Wednesday through Friday. A
couple rounds of showers/storms may accompany the frontal passage
Monday night through Tuesday night with severe weather not out of
the question Tuesday aftn/eve given ample pre-frontal moisture
return under increasing wsw flow aloft.


.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
Issued at 805 PM EDT Fri Aug 18 2017

A fairly strong upper level short wave across northwest Iowa will
continue to slide east-southeast through the remainder of the
evening into the overnight. A fairly good low level thermal ribbon
downstream of this feature combined with good dynamics should
allow shower/iso thunderstorms to increase across central Illinois
the remainder of the evening hours. Best chance of
showers/isolated thunderstorms for terminals still appears to be
in the 05Z-12Z period. Elevated instability looks to be on the
weak side but strength of frontal and upper level forcing should
be sufficient for some risk of thunder at terminals. Have limited
mention to VCSH at terminals for the 00Z TAFS but once confidence
in more refined period of greater shower potential increases, may
need to amend to tempo SHRA or possibly tempo TSRA mention. Low
level baroclinic zone will sink south of terminals on Saturday
setting up mainly dry conditions and light northwest winds 10
knots or less.




SHORT TERM...Steinwedel
LONG TERM...Steinwedel

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