Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 281732
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
132 PM EDT Fri Apr 28 2017

.UPDATE...
The AVIATION Section has been updated below.

&&

.SYNOPSIS...
Issued at 100 PM EDT Fri Apr 28 2017

A warm front will lift north across central Indiana on Saturday.
Then, a cold front will sweep through from the west Sunday night.
Record moisture for this time of year combined with the fronts,
upper level support and instability will result in numerous strong
thunderstorms through the weekend with a potential of heavy rain and
flooding. After a brief break, more showery weather is possible for
the middle of next week.

After a muggy weekend, cooler weather will follow next week.

&&

.NEAR TERM /Late this afternoon, tonight and Saturday morning/...
Issued at 100 PM EDT Fri Apr 28 2017

Latest water vapor imagery was showing an upper circulation over
eastern Utah and western Colorado within a broader Rockies trough.
Meanwhile, a front was over the Tennessee Valley.

Main focus late this afternoon and tonight will be on heavy rain and
flooding. Very good setup for heavy rain with record precipitable
water values, an advancing warm front and a strong upper jet over
the Great Lakes. Best focus for convergence will be over south
central Indiana near the nose of a 40-50 knot low level jet and in
exit region of 130 knot 250 millibar jet. Frontal forcing combined
with strong isentropic lift on the 305K level, precipitable values
to over 1.75 inches and CAPE to over 1000 j/kg will result in
widespread thunderstorms. The combination of shear and instability
will also support severe storms, especially over our southwestern
counties, where SPC has us in a Day 1 Enhanced Risk. Slight and
Marginal Risks extend further to the north over the county warning
area. The thunderstorm coverage and intensity may lull a little bit
Saturday morning as the front lifts slowly northward and the low
level jet subsides. However, by afternoon, the best chances for
thunderstorms will shift to our northwestern counties, which will
also be closer to a 45 knot plus jet moving in from the Ozarks. In
addition, model instabilities are much higher Saturday afternoon
then this afternoon and evening. Thus, more severe weather is
possible Saturday afternoon and evening. SPC has a Day2 Slight Risk
over west central Indiana flanked by a Marginal Risk elsewhere.

With such good combination of synoptic forcing, moisture transport,
instability, slow moving front and upper jet dynamics, should see
numerous storms over south central Indiana late this afternoon
through tonight and could see enough for flash flooding. Thus, after
coordinating with adjacent offices, went with a Flash Flood Watch
from Sullivan to Decatur counties and south through noon Saturday.
More flooding concerns are possible if not likely the remainder of
the weekend. So, stay tuned to further updates.

With southerly flow and clouds around, temperatures will not fall
much tonight. All but the far north should likely to be in the warm
sector by Saturday afternoon, so should see a strong southeast to
northwest gradient and highs close to a MOS blend in the mid 60s far
north to lower 80s south.

&&

.SHORT TERM /Tonight tHrough Sunday Night/...

Issued at 100 PM EDT Fri Apr 28 2017

A very active weekend ahead. GFS and NAm push the warm front north
to Northern Indiana and Michigan on Friday night...placing much of
Indiana within the warm sector. Best Isentropic lift is lost after
06Z-09Z as the front and the associated dynamics push northward.
Time heights also trend toward subsidence after 06Z. THus will start
the tonight period with high pops as the front is expected to be
passing and then trend lower as the night progresses as the front
and associated dynamics weaken and push to the north. Given the
expected ongoing warm air advection tonight and cloud cover will
trend lows at or above mavmos.

More rounds of rain will be possible on Saturday and Saturday Night.
GFS and NAM suggest Indiana will be place within the warm and moist
flow within the warm sector. Models suggest that the deep low
pressure over the deep low pressure over the southern plains will
eject several weak embedded short wave toward Missouri...Illinois
and Indiana. However...the warm frontal boundary which should be a
focus for precip should be well to the north by Saturday.
Furthermore...as the day progresses on Saturday...forecast soundings
show some drying within the column.
As the models tend to have difficulties tracking these weak waves
being ejected and the placement of the expected warm front remains
to the north of Central Indiana...confidence is low in this
forecast. Higher confidence for heavy rains appear to be out of the
IND forecast area...farther to the west...within the expected flow.
Thus will keep pops in the forecast for now given the uncertainty of
the evolution of this system...with best pops on Saturday and
Saturday Night at the NW points of the forecast area. Again...with
Indiana within the warm sector...will trend highs and lows at or
above MAVMOS.

Best organization arrives in the area on Sunday afternoon and Sunday
night as the upper low is finally expected to begin to push
northeast. Strong dynamics ahead of the low will push across Indiana
late in the day and into Sunday evening. Forecast soundings again
suggest a saturated column from 21Z monday through 06Z Sunday. The
310K GFS Isentropic surface shows strong upglide along with
favorable specific humidities over 5 g/kg. Forecast soundings show
Pwats are over 1.5 inches as well. Thus confidence for
showers/storms a is High for Sunday afternoon and Sunday night.
Forecast Builder willing...will try to trend pops highest during
that time frame. Will work a blend on Temps.

Confidence in widespread heavy rain event for Central Indiana
remains low. As stated earlier...best forcing appears to be west of
central Indiana...across Illinois and Missouri...within the flow.
Furthermore the the warm frontal position on Saturday remains
uncertain. Don`t get us wrong...we have high confidence in some rain
for central Indiana this weekend...particularly on this
afternoon/tonight and again on Sunday afternoon and Sunday night as
it is at these time when the best forcing will be pushing through
the area. However...the extra forcing and rain to result in a heavy
rain event on Saturday an Saturday Night remains uncertain for
Central Indiana...due to a lack of organized forcing...and models
hinting at a dry column much of Saturday Afternoon and Saturday
Night.

&&

.LONG TERM /Monday through Thursday/...
Issued at 240 AM EDT Fri Apr 28 2017

Ensembles indicate that system that is expected to affect the area
over the weekend will be in the process of lifting off into the
Great Lakes by Monday. Will keep some PoPs in the forecast for
Monday and Monday night to cover potential shower activity in the
post frontal zone.

There is quite a bit of variance among the ensembles as to the
strength and location of the next trough that is expected to move
into the area around the middle of next week. Some of the members
indicate a strong low pressure system moving into the area, while
others are weaker and/or more suppressed. Differences are
probably due to phasing issues. Will go with PoPs for next
Wednesday and Thursday for now and monitor future trends.

&&

.AVIATION /Discussion for 28/1800Z TAF Issuance/...

Issued at 119 PM EDT Fri Apr 28 2017

VFR conditions will erode into MVFR this evening and possibly into
IFR later tonight.

Scattered light showers currently over Central Indiana will
continue over the next several hours with little effect to
visibility. Widespread heavier showers and scattered
thunderstorms will move into Indiana after 21Z and will persist
through the TAF period as an upper level disturbance approaches
the Midwest. Visibilities will drop to 3 miles or less in areas
with heavier showers. Ceilings will lower through the night and
will bottom out around 800-1000 ft by 06Z and should begin to
rise after sunrise as showers begin to dissipate. Winds will
generally be out of the south to southeast at less than 10 kts
through the period with occasional gusts possible with any
convection that moves over.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 8 PM EDT this evening through Saturday
morning for INZ060>065-067>072.

&&

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....JAS
AVIATION...White


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