IWX issues Area Forecast Discussion (AFD) at Jun 17, 1:20 PM EDT348
FXUS63 KIWX 171720
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
120 PM EDT Mon Jun 17 2024
.KEY MESSAGES...
- There are nearly daily low chances for showers and
thunderstorms this week, especially in the afternoon and
evening hours. Expect coverage to be widely scattered.
- Excessive Heat Warnings and Advisories are in effect due to
an extended period of hot and humid conditions. Max heat
indices this week will range from around 95 to 105, with the
highest values in the warning area and in urban locations.
Overnight lows in the 70s will provide little relief from the
heat. High temperatures will be in the upper 80s and 90s.
&&
.DISCUSSION...
Issued at 522 AM EDT Mon Jun 17 2024
Bottom line up front: Dangerous heat and humidity is expected.
Excessive Heat warnings and advisories are in effect. Heat indices
through at least Friday look to be in the 95 to 100 degree range for
most, with values of 100 to 105 looking more likely in the
warning area (and we have higher confidence). The warning
continues through Friday at 8 pm EDT, and the advisory will
expire at 8 pm tonight. We will re-evaluate day by day for the
advisories.
Here is our local heat advisory/warning criteria for reference:
Heat Advisory criteria is: "A heat index (HI) around 100 degrees or
higher. Consider a warning if the maximum HI is going to be between
95 to 100 degrees for more than 4 consecutive days."
Warning criteria is HI >105 and a minimum HI >75F for a 48 hour
period (or if the advisory criteria continues for more than 4
consecutive days). This is because heat impacts are cumulative-
particularly for more vulnerable urban populations.
Why Warning vs Advisory?
Given that we will have heat indices varying between 95 to 105
degrees Monday through Friday (5 consecutive days), with
potential for Saturday to also have HI's around 100 degrees, I
thought it best to upgrade NW Ohio and portions of Northern
Indiana to an excessive heat warning. It's rare to get prolonged
heat like this in June, and its the first of the year. I also
have the greatest confidence in these locations (around 80% in
us getting HI's in the 100-105 degree range). Even if we don't
get true warning criteria in terms of specific heat index values
and it lingers closer to 100 degrees, we'll have possibly 6
consecutive days of advisory level heat (which verifies our
local warning criteria). To make things worse, overnight
temperatures largely stay in the upper 60s, low to mid 70s which
will provide little to no relief from the heat.
I decided on an advisory for the remainder of the area through this
evening. I am confident we'll see HI values around 100 degrees
today. However, I am less confident through the remainder of the
week how things will work out, particularly Tuesday when we have
more potential for precipitation/cloud cover west and central
(though pick your model-they are all over the place). I suspect that
we will need additional advisories as we go into Wed-Fri, but my
confidence wasn't high enough to continue the advisory all the way
through. It will be better to tackle the borderline areas day by
day.
Flies in the ointment: So what would make this not happen?
There are nearly daily chances for showers and thunderstorms
because we are in a hot, humid, and unstable atmosphere. Models,
even the ECMWF which previously kept us mostly dry, has come to
agree that the western edge of the ridge will continue to break
down, which makes us susceptible to any little shortwave that
moves through. We don't have a firm handle on the exact
placement of convection-as it's largely impossible to narrow
down due to the widely scattered nature of these patterns. We
don't expect any severe weather right now (no SPC Outlooks
either), but I'd monitor for updates on that because it could
quickly change (as we saw last night with the MCV overperforming
all the way across Michiana). With any precipitation chances
comes extra cloud cover-which could limit high temps.
The other issue could be that models the past couple days have been
overdoing the amount of moisture we'll have. We've been mixing
out in the afternoon a little better than what was depicted for
the past couple of days in earlier forecasts. To account for
this possibility in the upcoming week, when doing the forecast
I made the assumption that the dewpoints would be closer to the
mid-upper 60s to around 71. This might very well be too low, as
some of the models have dewpoints as high as 72-75! If we get
the higher dewpoints instead of mixing out, we may need to
upgrade some of the advisories to warnings, and we'll be more
likely to see those 105 HI's in the warning area (depending on
storm/cloud coverage). If not, we should be in decent shape to
verify the warning as long as we get to around a 95-100 HI each
day.
&&
.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 120 PM EDT Mon Jun 17 2024
Expansion of a scattered cu field is readily apparent across
much of Indiana as convective temperatures have been met in an
unstable atmosphere. While VFR conditions are expected to
dominate most, if not all, of the forecast there are a few
challenges that need to be monitored that are not necessarily
captured in the TAFs.
Any subtle features passing through the area could easily pop a
shower or thunderstorms at either site this afternoon and early
evening. The greatest concern at this point is an area of
persistent convection across east central IL, approaching KRZL.
While trends suggest this may stay just west of KSBN, outflow
boundaries from the convection could easily move east and
provide lift for additional showers/storms. A few of the models
actually key in on the KSBN area for one or more rounds of
storms late afternoon into the early evening, but no consistent
signals remain. For now have opted to go VCSH at both sites with
timing and duration of any shower/storm difficult to capture at
this point.
While loss of heating will bring any precip chances to an end
this evening, a weak disturbance, currently producing widely
scattered showers across portions of Kentucky, will drift north
late tonight into Tuesday morning. Think there will be at least
an increase in mid/high clouds but some question as to chances
for showers with this feature. Given the low confidence, have
introduced a broken high cloud deck for the time being late
tonight and defer to later forecasts for possible inclusion of
showers/storms.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Tuesday for INZ005>008-
012>015-020-022-023-103-104-116-203-204-216.
Excessive Heat Warning until 8 PM EDT Friday for INZ009-017-
018-024>027-032>034.
OH...Excessive Heat Warning until 8 PM EDT Friday for OHZ001-002-
004-005-015-016-024-025.
MI...Heat Advisory until 8 PM EDT Tuesday for MIZ078>081-177-277.
Air Quality Alert until midnight EDT tonight for MIZ078-177-
277.
MARINE...None.
&&
$$
DISCUSSION...MCD
AVIATION...Fisher
Source:
IWX issues Area Forecast Discussion (AFD) at Jun 17, 1:20 PM EDT---------------
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