See the full forecast discussion below for all the details.
National Weather Service Indianapolis IN
1230 PM EDT Fri Jul 16 2021
...Updated Aviation Discussion...
.Forecast Update...
Issued at 929 AM EDT Fri Jul 16 2021
Short term models suggest a decent amount of lift is expected
across the forecast are today, courtesy of 850 mb flow of 25-30 kts.
This should result in periods of rain and thunderstorms across the
area for the rest of the day. Another convectively induced mid level
area of vorticity may be located over southwest Missouri at this
time. This feature may help to enhance the thunderstorm threat by
mid to late afternoon, as it drifts towards southwest Indiana. Will
continue with high PoPs today.
Precipitable waters analyzed around 1.75 inches this morning will
result in a continued locally heavy rain threat today. Despite lapse
rates close to moist adiabatic levels, marginal deep layer shear
exists over the local area, which may result in high wind threat
with any stronger convection that may develop. This maybe especially
true towards mid to late afternoon, with the approach of the mid
level feature over southern Indiana.
Given the expected clouds and precipitation today, the highs may be
too warm. However, they don`t have to rise too much to reach the
expected highs. Will leave the temperatures alone for now, and
monitor the trends.
&&
.Short Term...(Today through Saturday)
Issued at 304 AM EDT Fri Jul 16 2021
Surface analysis early this morning shows a weak frontal boundary in
place from northern MO...across Illinois to Central
Indiana...extending northeast to Lake Erie and southern Ontario. A
very warm and humid air mass was in place along and south of the
boundary with dew point temperatures in the upper 60s and lower 70s.
Radar shows ongoing convection...along and ahead of the front...from
Central Indiana upstream to southern IL and southern MO. Water vapor
shows a plume of moisture in place aloft from the American southwest
to MO...Indiana and southern Ontario.
Today the weak upper trough in place over the upper
midwest...sagging toward the Ohio Valley will be the main trigger
for ongoing convection across Central Indiana. Models suggest that
as the weak upper tough pushes across Indiana today...favorable
dynamics aloft will be in place through the day...before finally
exiting the area tonight. Forecast soundings ahead of this upper
trough reveal a very saturated column through the day with pwats
that are very moist...over 2.00 inches. Forecast soundings also
reveal attainable convective temperatures...but minimal CAPE. HRRR
suggests convection developing through the day as the boundary and
dynamics continue to linger across the area. Thus given the ongoing
returns on radar along with the expected development through the day
will trend pops near 100 today. As the system pushes to the east
through the day...will try to diminish pops from NW toward SE late
today...but best drying looks to arrive after 00Z.
Isolated Flash flooding could be a concern today given our high
pwats. However storm motion seems quick enough that storms should be
moving quickly across the area. Flash flooding will be more likely
in areas that are repeatedly struck with heavy rains. Will continue
to monitor for that.
Tonight...the models show the weak upper trough and associated
forcing pushing east of Indiana...allowing a dry NW flow to arrive
aloft. The dry NW flow looks to arrive near 03Z as that is when deep
moisture within the forecast soundings begin to be lost as pwats
fall to less than 1.5 inches. This drying within the column is an
indication of subsidence. Meanwhile within the lower levels the slow
moving frontal boundary is expected to sag to the Ohio
Valley...allowing the arrival of slightly cooler and less humid air.
Thus will keep the high pops in place after 00Z particularly across
the southeast where drying will arrive last. Will try to trend
toward a dry forecast by 12Z as any coverage at that point should be
isolated or less with arrival of less humid air and subsidence.
On Saturday...weak surface high pressure over the upper midwest and
Great Lakes looks to provide a cooler NE surface flow to Central
Indiana. However aloft a weak upper trough looks to be sagging south
out of the Great Lakes...providing cyclonic flow aloft. Forecast
soundings show plenty of dry air within the mid levels due to
ongoing subsidence...but do reveal some CAPE near 2000 J/KG and
attainable convective temperatures. Thus may include some slight chc
pops on Saturday afternoon for an isolated diurnal shower/storm.
Confidence is low for that.
&&
.Long Term...(Saturday night through Thursday)
Issued at 304 AM EDT Fri Jul 16 2021
The long term period will transition from generally slight chances
of showers/t-storms late this weekend...to 2-3 summer-like but rain-
free days to start the work week...before a return of slight chances
of storms mid/late week. The threat of convection will have
diminished by Saturday night-Sunday, when the second part of this
weekend`s upper wave crosses the region...with quite dry mid-levels
bringing PWATs into the 1.00-1.50 inch realm in the wake of the cold
frontal passage. Nevertheless the small, but potent vort max could
shake out a few showers, especially over southeastern counties
Sunday afternoon. Low chances that any additional rains
will further flash flood potential.
Past isolated showers over southeastern counties on Monday, as the
last of the trough finally departs Indiana, the work week should
start rain-free. Light northerly breezes, courtesy of surface high
pressure ridged along the western Great Lakes, will bring slightly
more comfortable dewpoints in the mid 60s. The western ridge`s
eastern extent will be nudging into central Indiana, providing ample
sun and seasonable warmth. The prevailing mid-level northwesterly
flow coming around the eastern side of the ridge will present the
potential of ridge-riding convection spilling into the Mid-West...
however kept POPs to slight chance or less to end the period given
low confidence. Temperatures will not deviate far from normal for
the long term, with the CWA situated on the gradient between the hot
western US and milder Northeast.
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