Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 200142

Area Forecast Discussion
National Weather Service Indianapolis IN
942 PM EDT Fri May 19 2017

The NEAR TERM Section has been updated below.


Issued at 205 PM EDT Fri May 19 2017

A low pressure system will lift northeast from Colorado to
northwestern Ontario by late Sunday. As it does, a cold front will
sweep east across central Indiana Saturday night and Sunday. Warm,
moist and unstable air will interact with a stationary front and
later the cold front and result in scattered to numerous
thunderstorms through the weekend. After a brief dry period early
next week, an upper low will drop southeast across the lower Great
Lakes and Ohio Valley and result in cooler and wet weather for the
middle of next week.


.NEAR TERM /Tonight/...
Issued at 940 PM EDT Fri May 19 2017

A couple of thunderstorms are lingering over parts of the east
central zones along a frontal zone that is sagging through those
areas. These should diminish or exit the local area over the next
few hours.

Shower and storm threat may increase again during the pre dawn
hours of Saturday as an upper wave lifts out of Missouri and 850mb
flow backs and increases over frontal zone. Will keep some PoPs
going tonight to allow for present convection and potential for
convection later tonight.

Previous discussion follows.

Models agree that an upper low over eastern Colorado will move
northeast to south central Nebraska by 12z Saturday. In the
meantime, this upper low will spin up a wave along a cold front.
This surface low will move to near extreme northwestern Missouri
Saturday morning. Also, a quasi-stationary front is expected to
remain near south central Indiana providing a decent surface
temperature gradient across the forecast area.

This afternoon, strong to severe storms were devloping and moving
over the eastern half of Indiana ahead of an upper disturbance.
Golfball size hail was reported in Spencer from the storm that was
currently moving toward Bloomington. Behind this area, a lull is
expected in the activity. That said, with warm, moist and unstable
in place and a stationary front nearby, thunderstorms are certainly
a possibility tonight. However, models were not showing any
significant short waves oer upper jet dynamics, and a modest low
level jet does not move across west central Indiana until after 12z.
So, will go with small chance pops tonight. Plenty of instability
around to include thunder and with 30 to 40 knot bulk shear, a few
severe storms can not be ruled out, mainly during the evening per
the DAY1 SPC Outlook.

Northeasterb sections of central Indiana could drop into the 50s
overnight with light northeast winds there to the north of the quasi-
stationary front. The location of this front will have a big impact
on temperatures and it is difficult to know exactly where it will
settle overnight.


.SHORT TERM /Saturday through Monday/...
Issued at 205 PM EDT Fri May 19 2017

Timing and coverage of thunderstorms will be the main focus for the
short term.

The 00z ECMWF and Ensemble Means and 12z GFS were pretty close in
moving the upper low to northwestern Ontario late Sunday. The ECWMF
is just slightly faster. They also agree, a broad weak surface low
will be over the Great Lakes and an associated cold front will move
to west central Indiana 12z Sunday and into western Ohio at 18z
Sunday. With good agreement, a blend of model features should do.

The warm front is expected to lift northeast across central Indiana
on Saturday, allowing the area to destablize with model CAPES to
2000 J/kg during the afternoon. In addition, a modest 30 knot low
level jet will be moving in from the west and a 100 knot upper level
jet will be moving into the Mississippi Valley. As the latter
feature gets closer, along with a cold front, thunderstorm activity
should become more widespread by Saturday afternoon and night and
into Sunday morning. Model precipitabloe water values 1.60 inches or
more are near the max moving curve of climatological soundings from
Wilmington and Lincoln. Thus, could see some heavy rain. In
addition, 30-40 knot bulk shear combined with the instability
supports a severe weather threat including supercells and bows,
especially during the afternoon and early evening. Instability dies
off overnight, but still will be enough for below severe limit
thunderstorms. Damaging winds and large hail will be the main
threats, although a tornado or two can not be ruled out with the
warm front in the area per the DAY2 SPC Outlook which has all of
central Indiana under a Slight Risk for severe weather.

The storms will be ending from west to east early Sunday and into
the afternoon, as high pressure builds in. The break will be short-
lived, however with an upper low bringing more convection to the
area for the long term.

Blended temperatures look reasonable with most areas expected to see
the lower 80s on Saturday in the warm sector. Then, temperatures
will return to near normal by Sunday in the wake of the cold front
with highs in the lower and mid 70s Sunday and Monday. Overnight
lows in the lower and mid 60s Saturday night will drop to the lower
50s Sunday night.


.LONG TERM (Monday night through Friday)...

Issued at 150 PM EDT Fri May 19 2017

Confidence in this forecast is rather high.

For the next several days, the weather will be dominated by large,
powerful systems.  These are relatively easy to forecast. Models
have reached similar solutions.

All this suggests the consensus from Region can be used with possible
temperature errors of 1-3 degrees, and possible POP errors of around
10 percent.


.AVIATION /Discussion for the 200000Z TAFS/...
Issued at 643 PM EDT Fri May 19 2017

Appears frontal system should be southeast of the terminals by
issuance time, so convective threat should diminish by then. May
see an organized convective threat increase after about 200800Z as
remnants of convection currently over Missouri may begin moving
into the local area.

Large area of IFR ceilings in the post frontal zone over northern
Illinois and northwest Indiana has been creeping southeast.
Confidence in low in how far southeast these ceilings will get
later tonight, give the progress of these ceilings may slow down
later tonight in response to next wave approaching. Will monitor
the progress of these ceilings until issuance time, and make a
decision on how to handle things at that time.

Northwest to north surface winds expected to veer around to
030-060 degrees by late evening, and to 090-110 degrees by sunrise
Saturday. Speeds generally in the 8-13 kt range.




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