IND issues Area Forecast Discussion (AFD) at Aug 27, 12:00 PM EDT
737
FXUS63 KIND 271600
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
1200 PM EDT Tue Aug 27 2024
.KEY MESSAGES...
- Above normal heat is expected for the next several days with
a Heat Advisory for Tuesday
- Record highs will be threatened Tuesday and Wednesday
- There are chances for thunderstorms on Tuesday evening through
Wednesday; some could produce isolated severe wind gusts
- There is higher confidence for thunderstorms between late Thursday
through Friday night, with high uncertainty on timing.
- Cooler weather expected for the upcoming holiday weekend
&&
.FORECAST UPDATE...
Issued at 930 AM EDT Tue Aug 27 2024
- Heat Advisory Continues; Sunny and Hot
- Heat Index values 100-107
Surface analysis late this morning shows a large area of high
pressure in place across the east coast. Low pressure was found
near eastern Lake Superior, with a cool front that extended
southwest across MN to NB and western KS. This was resulting in very
warm southerly flow across Central Indiana. Aloft, water vapor
showed a strong ridge of high pressure in place over the middle
Mississippi valley with a ridge axis stretching northeast across NW
Indiana to Ontario and the eastern Great Lakes. Subsidence was shown
across Indiana under this ridge. Meanwhile some ridge riding
convection was found across MN/WI and MI, which will remain well
north of Central Indiana. Dew points across central Indiana were in
the middle 60s, with light southerly surface flow in place. GOES16
shows mainly clear skies across Central Indiana.
Models suggest 850mb temps this afternoon will approach 24C with a
thermal axis set up across Central Illinois to Central and Northern
Indiana amid the southwest flow in place. This will set up a hot
day across Central Indiana. Given these very warm temperatures
across Central Indiana max heating is expected with highs expected
to reach the middle 90s. Subsidence and dry air mixing downward may
allow for dew points to remain in the middle 60s. However, this will
still result in heat index values near 100 or in the lower 100s.
Given this, current heat advisory will continue and appears on
track. Forecast soundings and time heights this afternoon continue
to show a dry column, with subsidence. Thus a mostly sunny sky will
be expected throughout the afternoon.
&&
.SHORT TERM (Today and tonight)...
Issued at 249 AM EDT Tue Aug 27 2024
The ridge axis has now reached the western Ohio Valley, placing
central Indiana within hot and muggy conditions. 850mb temperatures
will be well above normal (highlighted by peak 850 values in the
99th percentile for late August). With mostly clear skies expected
through the late afternoon, this anomalous 850mb temperature will
likely translate to mid 90 surface temperatures across the region.
Also of note will be the approach of weak low level waves on the
northern edge of the ridge, of which should increase southerly flow
and push low 70 dew points over N/W portions of central Indiana. A
few areas in far NW central Indiana could reach dew points in the
mid 70s this afternoon, thanks to a combination of moisture
convergence and agricultural evapotranspiration. The combination of
temperatures between 94-98 and dew points in the low to mid 70s will
create dangerous heat across the state (Heat Indices between 100 and
110). For this reason, the heat advisory has been kept for late this
morning through the early evening. A few areas over SE central
Indiana may fail to reach the typical 105 heat index criteria, but
cumulative effects of recent heat and high overnight lows have lead
to the decision to issue the advisory over these areas.
The aforementioned low level waves will also aid in initiating
thunderstorms across MI, IL and N IN late this afternoon through
tonight. CAMs are signaling at a pronounced subsidence cap this
afternoon, of which should delay CI to the late afternoon and early
evening (6-8PM). As these thunderstorms develop, they will likely
quickly develop dense cold pools and push S/SE towards the greater
instability. There is still high uncertainty on where CI will occur
and the exact MCS storm motion, but in general N/NE portions of
central Indiana will have the greatest likelihood of see some rain
late today through tonight.
Central Indiana will likely be positioned far enough away from
expected CI to lower the severe potential for our CWA. Weak shear
through the entire atmospheric column will inhibit convective
organization leading to predominately outflow dominant MCSs and
quickly decaying systems as they reach central Indiana. With that
said, there is still enough volatility in the mid to low levels that
any thunderstorm that develops along an outflow boundary could
develop isolated damaging microbursts.
&&
.LONG TERM (Wednesday through Monday)...
Issued at 249 AM EDT Tue Aug 27 2024
A broad but steadily flattening upper level ridge will keep hot
temperatures over central Indiana for the second half of the work
week while also introducing increasing convective risks as the band
of westerlies aloft gradually shift south. A strong upper level low
will traverse the U S-Canadian border through late week before
lifting northeast towards James Bay by the weekend. A seasonally
strong cold front will accompany the upper low and move across the
Ohio Valley Friday into early Saturday. This will shove the hot and
humid airmass south of the region for the weekend and into early
next week with a return to comfortable conditions to open up
September.
Wednesday through Friday Night
The Wednesday forecast is low confidence and highly dependent on
potential convection that may impact parts of the forecast area late
today into tonight. There is a plausible scenario where lingering
isolated convection and high level cloud debris could hamper warming
over the region through midday Wednesday with possible convective
redevelopment late Wednesday into Wednesday night with any lingering
outflow boundaries serving as triggers for storms. The overall
synoptic setup is supportive for showers and storms Wednesday
afternoon and evening as the core of the upper level ridge shifts
into the lower Tennessee Valley and the band of westerlies sags
south into the region. Plenty of instability and deep low level
moisture will be present for storms to produce gusty winds and
locally heavy rainfall...with an isolated risk for damaging winds as
well.
Low level thermals are supportive of low to mid 90s for highs over
much of the forecast area Wednesday but again...leftover cloud
debris in the morning and potential convective redevelopment later
in the day could keep temperatures lower. For that reason...feel
that any consideration for an extension of the Heat Advisory into
Wednesday is premature at this time even with dewpoints largely
remaining in the 70s. Later shifts today into this evening can
reevaluate max heat indices for Wednesday as confidence increases.
Ridging aloft will briefly reestablish Thursday in advance of the
deep upper low moving east through the Canadian Prairies and while
the airmass over the Ohio Valley will remain moist and unstable...
the lack of more appreciable forcing aloft should keep afternoon and
evening convection more isolated across the forecast area. Highs
will range from the upper 80s north to the low and mid 90s along and
south of I-70.
The best threat for widespread convection still appears to be set
for Friday afternoon and night ahead of the approach of a stronger
cold front. The addition of slightly deeper BL shear ahead of the
boundary will combine with the moist and unstable airmass over the
region to produce at least the potential for more robust storms to
impact the forecast area into Friday night. Friday will be the last
day in the 90s for the region.
Saturday through Monday
Relief from the heat and humidity is on the way just in time for the
holiday weekend. The cold front will push across the region late
Friday night into Saturday then should gradually settle south into
the Tennessee Valley by late Saturday. Cannot rule out lingering
isolated to scattered convection Saturday over the southeast half of
the forecast area but the overall trend will be for a decreasing
threat for rain as the day progresses. A secondary frontal boundary
will move through the area late Sunday but with limited moisture...
will not likely produce any showers. High pressure will then take
over into early next week with another extended stretch of dry
weather expected.
Humidity levels will remain up on Saturday with the drop in
dewpoints lagging the main cold front. Progressively drier air will
filter into the boundary layer into Sunday with further
reinforcement in cooler and drier air with the passage of the
secondary boundary. Highs will be in the 80s Saturday and Sunday
before slipping back into the mid and upper 70s for Labor Day and
Tuesday.
&&
.AVIATION (18Z TAF Issuance)...
Issued at 1200 PM EDT Tue Aug 27 2024
Impacts:
- Mainly VFR this TAF period.
Discussion:
The TAF sites will remain protected by a large upper level ridge of
high pressure over Indiana. Forecast soundings show a very warm and
dry column with unreachable convective temperatures. Thus VFR will
continue this afternoon and tonight.
The upper ridge axis is suggested to flatten somewhat on Wednesday.
This will allow a short wave within the flow to push across northern
Indiana on Wednesday. This could result in some scattered
thunderstorms at that time.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 9 PM EDT this evening for INZ021-028>031-
035>049-051>057-060>065-067>072.
&&
$$
UPDATE...Puma
SHORT TERM...Updike
LONG TERM...Ryan
AVIATION...Puma
Source: IND issues Area Forecast Discussion (AFD) at Aug 27, 12:00 PM EDT (https://mesonet.agron.iastate.edu/p.php?pid=202408271600-KIND-FXUS63-AFDIND)
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