Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 211419
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
1019 AM EDT THU JUL 21 2016

.SYNOPSIS...
A large ridge of high pressure over the central portion of the
United States will bring hot and humid conditions to the Ohio Valley
through the weekend. Temperatures will be at their warmest from
Friday through Sunday, with at least some chance of showers and
storms as well. A cold front is forecast to bring a better chance of
storms on Monday, along with a slight drop in temperatures through
the first half of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Mainly sunny skies will continue across the region through the
day. 12z soundings is slightly warmer than 24 hours ago. So highs
a bit warmer than yesterday seems reasonable. Pretty good gradient
in dew points across the forecast area. So higher heat indices
will be in western counties with some locations reaching the upper
90s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/...
Heat and humidity will be the main focus through Friday and the
weekend, but a challenging forecast for potential convection is
also a part of the situation.

With surface high pressure moving into the southeastern states,
and the plains ridge continuing to strengthen and expand, the
pattern over the Ohio Valley will favor increasing boundary layer
theta-e advection on 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. What has changed in this forecast cycle is that
confidence in reaching advisory criteria has diminished just
slightly from before, thanks to a number of factors. Looking at
several sources of information -- direct model projections,
anomaly analysis, ILN sounding and dewpoint climatologies -- it
should be noted that while this will be an anomalously warm
period, it will not be at a level approaching the extreme end of
the scale. By Friday afternoon, model projections suggest 850mb
temperatures of around 21C-22C. The local ILN sounding
climatology would suggest that this is around 2 standard
deviations above normal. 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. When factoring in a little bit of boundary layer
mixing, forecasting dewpoints this high does not seem like a
prudent choice -- instead, the forecast will allow dewpoints to
peak out in the mid 70s on Friday, close to SREF mean values. As
for temperatures, even assuming an unimpeded path to heating, it
would be difficult to fight the model consensus too much in a
pattern like this one (anomalous but not extreme). Guidance
numbers for Cincinnati are 93 (MAV/MET) with an SREF mean of 90.
Even the warmest member of the SREF at CVG reaches only 93
degrees. Further complicating things is the chance of convection
and clouds on Friday, which remains a low-confidence forecast, as
will be discussed below. This forecast will stay at the high end
of the SREF plumes and near the guidance numbers, allowing for max
temps on Friday that will range from 90 (far northern CWA) to 93
(northern Kentucky). 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.

In terms of convective potential for Friday, the larger-scale
models are of no help with regard to storm timing and placement,
simply printing QPF across wide swaths of the region.
High-resolution models (WRF-ARW/WRF-NMM/NSSL-WRF Realtime) appear
mostly lost once beyond their depictions of Thursday evening /
Friday morning convection upstream, and even those forecasts have
been incredibly jumpy from run to run. All in all, the convective
forecast for Friday is extremely low in confidence, and at this
point the best forecast may be just to focus on the pattern --
weak boundary layer convergence ahead of a front, with the
possibility of a shortwave rounding the northeastern periphery of
the ridge, within a very warm/moist but somewhat capped air mass.
Hazards from storms that develop on Friday will likely be limited
to very heavy rainfall (with a very moist air mass and slow storm
motions) and gusty winds (either from downbursts or cold pool
development). Nonetheless, the overall severe threat appears
fairly low. On Saturday, the convergent zone ahead of the front
will shift to the southwest, and precipitation chances will be
limited to the southwestern half of the ILN forecast area (with
some slightly drier air advecting into central and northern Ohio).
Convective chances will increase again on Sunday for the entire
CWA.

Potential headlines for heat are the final item to discuss. With
the slight forecast reduction in max temps for Friday, heat
indices are only expected to reach 100 degrees for a little less
than half of the ILN forecast area. Confidence in a CWA-wide heat
advisory for Friday is not high at all. Given the potential for
upstream convection impacting the situation on Friday, confidence
is only moderate even for the expected-to-be-warmest southwestern
corner of the forecast area. After collaborating with neighboring
offices, and noting that Friday remains the third period of the
forecast, it was decided to wait one more forecast cycle before
making a decision on initial headline placement. Regardless, this
will still be an abnormally hot period, wherever advisory
headlines end up becoming necessary. As mentioned earlier, a
Special Weather Statement will be issued to discuss the heat on
Thursday, and also to indicate the concerns for Friday through
Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
On Sunday night/Monday...the MT shortwave mentioned above should
be crossing to the north of the Great Lakes and will likely be the
impetus for better storm coverage/development but timing is
concerning which may bring better rain chances Sunday night. A
little better flow with the wave may allow for a little better
storm coverage/organization/severity if a Monday frontal passage
occurs at peak heating.

On Tues/Wed...some relief though height falls will be rather modest
and cooling too. Expect temps/dwpts to fall back to more seasonable
summertime values during this time.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
The center of surface high pressure will settle over the mid
Atlantic region today, then it will drift off the coast tonight.

For today, return flow around the high will bring an increase in
surface moisture. This will result in some FEW-SCT diurnal
cumulus clouds along with southerly winds in the 7 to 10 knot
range.

For tonight, models are still having difficulty in discerning
where upstream convective complexes may head toward as they dive
south/southeast around the periphery of an expansive mid level
ridge from the upper Mississippi River Valley/upper Midwest.
Certainly, there will be some debris clouds from the convection,
but it is unclear how much of this convection will make it into
our forecast area by 12Z Friday. So although there may be a risk
of a thunderstorm late tonight, low confidence in location and
coverage support keeping the terminals dry.

OUTLOOK...Thunderstorms possible Friday through Monday.

&&

.ILN WATCHES/WARNINGS/ADVISORIES...
OH...None.
KY...None.
IN...None.

&&

$$
SYNOPSIS...Hatzos
NEAR TERM...
SHORT TERM...Hatzos
LONG TERM...Binau
AVIATION...Hickman


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