Area Forecast Discussion
Issued by NWS Wilmington, OH-- Highlight Changed Discussion --
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FXUS61 KILN 201024
Area Forecast Discussion
National Weather Service Wilmington OH
624 AM EDT WED JUL 20 2016
High pressure will extend across the Ohio Valley and mid-Atlantic
states today, with warm and generally dry conditions expected. As a
large ridge builds over the central part of the country, heat and
humidity will continue to increase through the weekend, with the
warmest conditions expected on Friday and Saturday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A large ridge of high pressure aloft has become well established
over the central plains. At the surface, high pressure currently
extends from the mid-Atlantic through the Ohio Valley, and will
grow as it shifts gradually southward over the next day or two.
Near-surface flow over the ILN forecast area remains quite weak,
and this will remain the case through the day today, with only
light southeasterly winds expected. The air mass has changed only
very slightly from yesterday, allowing for a slightly-adjusted
persistence forecast for max temperatures today, with no
significant change in surface dewpoints either.
The only significant challenge for the forecast today is for any
convective potential. First and foremost, the cluster of storms
currently over southwestern Indiana has continued to defy model
projections by maintaining a slow easterly progression. This
cluster split off from an earlier MCS, which had been propagating
southeast along the 925mb theta-e gradient. The storms, as they
currently exist and propagate, are expected to soon outrun their
favorable environment. There is a notable dewpoint gradient from
southwest Indiana to southeast Indiana, and less favorable thermal
conditions just off the surface will also mean that any elevated
instability feeding the storms will become weaker with eastward
extent. SPC mesoanalysis indicates about 1000 J/kg of MUCAPE where
the storms are currently located, but this diminishes to well
under 500 J/kg at the border of the IND/ILN forecast areas. If
they were to survive, at their current rate, it would take until
7AM for the storms to move into the ILN CWA.
Though a few models suggest a chance for some precipitation in the
western half of the forecast area this afternoon -- and a very,
very isolated shower/storm or two may well occur -- the vast
majority of locations should remain dry. The forecast will do the
same as of now.
.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
Increasing heat and humidity will be the main focus through the
rest of the week, but a challenging forecast for potential
convection is also a part of the situation.
By Thursday, the center of the surface high will be moving south
from the mid-Atlantic region, and the pressure gradient over the
Ohio Valley will begin to tighten. As this occurs, heights will
rise, as the plains ridge continues to strengthen and expand.
This pattern will favor a warming and moistening of the boundary
layer on Thursday and Friday. There will be little additional
change going into the weekend, as the ridge will flatten somewhat,
and 925mb-850mb winds will take a slight northerly component
(limiting additional warm advection).
Confidence remains high in a very warm period from Friday through
Sunday, and the HWO will maintain its mention of possible
advisory-criteria heat (heat indices over 100 degrees) for all
three days. After a careful analysis of all available information
sources -- direct model projections, anomaly analysis, ILN
sounding and dewpoint climatologies -- it should be noted that
this will be an anomalously warm period, but not at a level
approaching the extreme end of the scale. By Friday afternoon,
model projections suggest 850mb temperatures of around 22C. ILN
sounding climatology would suggest that this is around 2 standard
deviations above normal, and GFS/NAM anomaly analyses suggest
similar values. Surface dewpoints are forecast to increase
significantly between Thursday afternoon and Friday afternoon,
with raw model projections reaching as high as the upper 70s to
around 80 degrees. It is exceptionally rare to have dewpoints
reach 80 degrees -- it has happened on only a few days in the
entire CVG period of record. Even dewpoints in the upper 70s are
rare, even in mid to late July. With this air mass not looking
quite so historic, the dewpoint forecast will be allowed to peak
out in the mid 70s on Friday, with the understanding that if
mixing is a little deeper than expected, these values could even
drop a little more during the afternoon. Further complicating
things is the chance of some convection (or at least clouds) on
Friday, which (as will be discussed below) appears slightly
greater than in previous forecasts. With the above factors
considered, the temperature forecast for Friday was actually
lowered slightly, allowing for a range from around 91 degrees
(central Ohio) to 97 degrees (north-central Kentucky) on Friday.
Record highs (well above 100 degrees for all three climate sites
on Fri-Sat-Sun) are easily safe, as records like that typically
require a drier air mass to challenge.
The end result is that Friday -- the day most likely to have the
highest heat index values -- appears likely to meet heat advisory
criteria for most (if not all) of the ILN forecast area. Warning
criteria of 105 degrees or higher will be possible in the
southwestern sections of the CWA, especially for the Cincinnati
metropolitan area and northern Kentucky. Similar conditions are
expected for Saturday and Sunday, but dewpoints may come down by a
couple degrees over the weekend.
Analyzing the convective potential through the rest of the short
term forecast period continues to be a challenge. The air mass is
expected to remain quite unstable, but somewhat capped under the
ridge, meaning that widespread developmental convection will be
unlikely. However, virtually all available computer models develop
scattered and generally-disorganized convection across the Great
Lakes and Ohio Valley at times through the next few days. In the
absence of any larger-scale or well-defined lifting mechanisms,
this activity will likely be tied to indeterminate mesoscale
features or weak shortwaves rounding the northeastern periphery of
the ridge. Through this forecast cycle, precipitation chances will
be kept very low through Friday morning.
By Friday, a cold front will be moving south into the Great Lakes.
Ahead of the front, as was described earlier significant heat and
humidity will build into the Ohio Valley. Weak convergence in
between (mainly across northern Indiana and Ohio) is expected to
lead to a little bit more of an organized chance for showers and
storms. PoPs will again be kept very low until the degree of
capping (or any signs of further support for lift) can be
determined more clearly. As the cold front moves south and pivots,
a slightly drier and more stable air mass will enter the southern
Great Lakes, shifting the chances for additional convection a
little further to the southwest. However, the entire ILN forecast
area will still be allowed a low-end precipitation chance in the
.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
On Sunday, greater short wave energy entering the northern plains
is forecast to develop a surface low that will drag cold front
toward the ILN area. A better chance for thunderstorms will exist
in increasing moisture and forcing ahead of the front. The chance
for convection will continue Monday as the front makes its way
across the area. Look for drier conditions Tuesday under weak high
pressure behind the front.
Temperatures will be hot. Somewhat cooler readings in the mid and
upper 80s are expected Monday and Tuesday due to modest cold
.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --A dying MCS over south central Indiana is not expected to make it
to the southwestern terminals of KCVG/KLUK this morning.
Otherwise, surface high pressure centered over the eastern Great
Lakes this morning will continue to extended southwest into the
mid and upper Ohio Valley through tonight.
Some cirrus can be expected from time to time in the northwest
flow aloft along with some scattered diurnal cumulus clouds. Can
not completely rule out an isolated pop up shower/storm in the
heat of the day near the southwestern terminals where instability
will be the highest, but this is obviously too small to place in
Winds will be calm or light easterly this morning, light and
variable this afternoon, then back to a light east or southeast
OUTLOOK...Thunderstorms possible Sunday.
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