IND issues Area Forecast Discussion (AFD) at Jul 14, 9:53 AM EDT908
FXUS63 KIND 141353
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
953 AM EDT Sun Jul 14 2024
.KEY MESSAGES...
- Showers and thunderstorms possible today and tonight, damaging
winds and heavy rainfall are the primary hazards.
- Periodic shower and thunderstorm chances through Wednesday, with
low confidence in one or more windows of potential severe weather
- Hot and humid Sunday through Tuesday, possibly nearing Heat
Advisory criteria, particularly Monday
- Milder and less humid late this week into the weekend
&&
.MESOSCALE UPDATE...
Issued at 953 AM EDT Sun Jul 14 2024
Sunny skies with oppressive humidity is present across the forecast
area this morning...with the airmass already quickly becoming
moderately unstable. 1330Z temperatures ranged from the mid 70s to
the lower 80s with dewpoints well into the 70s.
Complex forecast for the rest of the day into tonight but the
atmosphere is starting to show its hand with respect to convective
risks across the forecast area. In summary...confidence has grown
with respect to impacts from the ongoing cluster over northern
Illinois which will arrive in the northern Wabash Valley around or
shortly after 15Z. We will first look at expectations with respect
to this first cluster for late morning into the afternoon...then
discuss potential for an additional cluster of storms this evening
into the overnight.
Convective Cluster #1 (Late Morning through Afternoon)
The ongoing convection over northern Illinois has weakened from
earlier this morning and has largely been producing subsevere winds
over the last few hours. The strongest of the cells have shifted to
the western part of the line over northwest Illinois in closer
proximity to the higher instability. That being said...the strong
surface heating ongoing this morning downstream of the complex over
the region is enabling a quick destabilization with the KIND ACARS
sounding already showing CAPE values around 2000 j/kg. The airmass
will only further destabilize over the next several hours and the
result will be a gradual strengthening to the convection as it
approaches from the northwest.
A focal point for the storms is likely to be a remnant outflow
boundary that earlier storms laid down across northern Indiana. The
boundary appears to have retreated back north slightly over the last
hour or two but roughly extends from north central Illinois E/SE
into far northern portions of the forecast area. While convection
will be possible southwest of that boundary...especially if a mature
cold pool can organize...high confidence exists in storms riding that
boundary into the afternoon in tandem with the instability and theta-
e gradients which will be nearby. Have raised pops as a result
through mid afternoon...with highest pops over the northeast part of
the forecast area. Damaging winds will be the primary threat from
these storms with the high levels of instability and increasing
presence of stronger DCAPE values as well in addition to very heavy
rainfall and lightning.
Nudged highs down across northern counties as well this afternoon
with the expected increase in clouds and convection. It will remain
oppressive with dewpoints peaking in the mid 70s but max heat
indices approaching 100 degrees are likely to remain particularly
confined to the lower Wabash Valley.
Convective Cluster #2 (Tonight)
Questions abound with respect to how additional convection may
evolve into the evening and overnight with growing model consensus
in at least a second cluster forming over the upper Mississippi
Valley this evening and migrating southeast into the region
overnight. Model soundings show the airmass recovering in wake of
the convection over the next few hours with moderate instability
present and the potential for another line producing damaging winds.
As the night progresses though and storms become increasingly
elevated...heavy rainfall and localized flooding may become the
primary concerns with the redevelopment of a westerly nocturnal low
level jet supporting back building cells as the line pushes S/SE
into the area along the flank of the theta-e ridge.
At this point...likely not going to have a good feel on intensity
and track of a second cluster tonight until the storms approaching
from the northwest move through later this afternoon. The main
takeaway point is that the ceiling appears to have raised on
potentially a second round of storms impacting parts of the
forecast area late evening into the overnight.
Additional mesoAFDs will be issued over the next several hours.
&&
.SHORT TERM (Today and tonight)...
Issued at 309 AM EDT Sun Jul 14 2024
Two main forecast challenges exist in the short term period. Namely,
possible convection and associated hazards along with excessive heat
potential.
CONVECTIVE POTENTIAL AND HAZARDS
First, we'll discuss the convective potential today. As of this
writing (230am) we have two upstream convective systems. The nearest
is located in the Chicagoland area and the further one is near
Minneapolis. Both of these systems have mature cold pools and are
generally propagating southeastward towards Indiana. Timing places
the first MCS near Lafayette early this morning, around
sunrise...and the second (should it survive) during the afternoon
hours today.
Our main challenges regarding these are: how far do they propagate
before dissipation, timing and hazards, and subsequent convective
redevelopment (if any). These challenges are particularly tricky
since, when dealing with cold pools, convective allowing models can
struggle. The HRRR, for instance, has been having a very difficult
time with these two complexes. Tossing CAMS to the side for now, we
can look at our mesoscale convective system (MCS) conceptual models
to get an idea on what may happen.
Downstream across Indiana, ACARS soundings out of IND show abundant
MUCAPE along with a 20-25kt west-southwesterly low-level jet (LLJ).
Aloft, we see rather little in the way of upper-level flow with only
a 25kt 250mb wind. A plane-view look at current conditions, we see a
diffuse theta-e gradient oriented north-south across western
Indiana. Models generally take this gradient and lift it
northeastward with time.
So what does this tell us? For one, there is plenty of instability
for these convective systems to work with. Second, the presence of
an instability gradient/advection along with a LLJ and lack of shear
tells us it may propagate preferentially southward more than
eastward (greater influence from upshear propagation than downshear
propagation). The lack of shear may also allow the system to become
cold pool dominant quickly which may mitigate severe potential. The
system may weaken through the morning hours as the low-level jet
diminishes after sunrise.
The first MCS, should it propagate southward enough, may strengthen
the theta-e gradient. This may allow the second MCS, near
Minneapolis, to propagate a bit further southwest than one would
expect. Again, this depends on how the first MCS evolves. Any
lingering boundaries/cold pools may also serve as a focal point for
later convective initiation. Model soundings show much more
instability developing by the afternoon along with large amounts of
DCAPE, and convection may gain the potential to produce damaging
winds along with marginal hail.
There are a lot of moving parts regarding today's convective
threat...and a variety of things can happen. One or both MCSs
propagate further than modeled...one or both could dissipate
early...scattered afternoon convection develops independently of the
morning MCSs...or develops on their remnant outflow boundaries.
Today's forecast will likely need updates as the evolution of
upstream convection becomes clear.
TEMPERATURES AND HEAT POTENTIAL
As mentioned above in the convective section, we have a 25kt low-
level jet which is coming from the southwest. The theta-e gradient
is associated with the advection of warm moist air from the
southwest. This air mass will allow for temperatures a few degrees
higher than yesterday, with most locations seeing highs in the lower
90s. Dew points will be high as well, with readings in the lower 70s
being common. Still, heat indices top out near 100 for the most part
which is below advisory criteria. Additionally, the convective
potential and uncertainty described above makes advisory-level heat
far from a given. The highest temperatures and heat indices look to
be in our southwestern counties, which are deeper into the warm air
mass and further from the convection approaching from the north.
&&
.LONG TERM (Monday through Saturday)...
Issued at 309 AM EDT Sun Jul 14 2024
Monday through Wednesday...
Hot and humid mid-summer will peak through the early portions of the
long term as southwesterly breezes gusting up to 20 mph maintain low-
70s dewpoints Monday, with perhaps mid-70s for many locations on
Tuesday. These oppressive humidity values will hold morning lows
mainly in the mid-70s and combine with the warm advective flow to
bring afternoon highs in the 92-96F range Monday...with the same
maximums expected for at least the southern half of the region
Tuesday. Less certainty surrounds the corresponding convective
threat that will be focused north of Indianapolis, which should
allow for only marginally hot conditions across the north on
Tuesday. The high humidity will boost heat index values at least 5-
10 degrees above the temperature...with afternoon max heat indices
expected to range from generally 100-105F Monday, and 95-105F
Tuesday...although criteria heat indices will be more easy to reach
in southern river valleys.
Convective showers and thunderstorms will certainly be possible amid
what should be abundant PM instability, albeit with little wind
shear to help realize this potential. Best shot at organized
strong/severe storms will be possible arrival of upstream MCS-type
cells forming along CAPE gradient near a line from Twin Cities to
Chicagoland. This conditional threat would be greatest across
northern counties with stronger winds from collapsing cells the main
hazard. Greater certainty in numerous to widespread showers and
thunderstorms crossing the region from northwest to southeast with a
solid cold frontal passage through PM hours Tuesday. Less-
impressive, yet still ample CAPE will combine with mediocre mid-
level lapse rates and what will probably be inconsistent wind shear.
Expect a few strong storms, but severe potential for this time is
less certain and also conditional. Expect lingering showers from
north to south into Wednesday under the passing frontal zone.
Thursday through Saturday...
Confidence is increasing in noticeably milder and less humid
conditions during the climatologically hottest week of the year. The
amplified western ridge will induce cooler surface high pressure to
plunge southeastward from south-central Canada through the mid-week.
What now appears may be a quasi-omega block across most of Canada,
should serve to slow the west to east progression of these lower
heights...with high pressure taking all three days to slowly cross
the Great Lakes, with northerly/northeasterly low-level flow
prevailing into central Indiana.
This will translate into at least 2 days with temperatures 5-10
degrees below normal amid low humidity levels for most locations. A
modest north-south gradient in both values is expected...with
morning lows on the coolest overnight likely ranging from the mid-
50s in far northern zones to around 60 towards the Ohio Valley.
Expect rain-free conditions to be the rule under a column sporting
precipitable water values almost 50 percent below normal for mid-
July.
&&
.AVIATION (12Z TAF Issuance)...
Issued at 635 AM EDT Sun Jul 14 2024
Impacts:
- Low confidence in a few showers or storms today
- Southwesterly wind gusts 20-25kt this afternoon.
Discussion:
A convective complex over northern Indiana is weakening as of 630am.
A few showers and storms are possible this afternoon along the
system's remnant outflow boundary. The best chance of this is from
LAF to IND. Additional showers or storms may form elsewhere
regardless of morning convection or remnant outflow boundaries.
Location and timing of showers/storms will be difficult to
predict today, so confidence is low.
Besides shower and storm chances, we'll see gustier winds compared
to recent days. Southwesterly winds gusting to 20-25kt are possible
after sunrise.
Cloud cover is also tricky as it depends on the convective situation
mentioned above. At the very least, diurnal cumulus is expected to
develop along with high cirrus clouds from upstream convective
debris. Mid-level clouds are possible at times especially LAF to IND.
&&
.IND WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
MESOSCALE...Ryan
SHORT TERM...Eckhoff
LONG TERM...AGM
AVIATION...Eckhoff
Source:
IND issues Area Forecast Discussion (AFD) at Jul 14, 9:53 AM EDT---------------
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