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000
FXUS63 KPAH 200446
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1146 PM CDT Sat Aug 19 2017

.UPDATE...
Issued at 1145 PM CDT Sat Aug 19 2017

Updated for 06Z aviation forecast discussion.

&&

.SHORT TERM...(Tonight through Monday night)
Issued at 249 PM CDT Sat Aug 19 2017

The last of the convection is about to exit Todd county, but we
will have to watch the north again late this afternoon. There is a
weak boundary noted in the wind fields north of I-64 that might
try to touch off some isolated convection as it interacts with the
remnant moisture from this morning`s convection over southeast
Illinois. Also, over the far western portions of southeast
Missouri, lower 90 temperatures and mid 70 dewpoints may be
enough to touch off isolated convection. Neither area should
amount to much, but they are worth keeping an eye.

It has gotten warmer today than expected, and the trend in
guidance through Monday is hotter as well. To go along with the
hotter temperatures, dewpoints have also trended upward, likely
due to the lack of a bonafide frontal passage today. The result is
heat indices over much of southeast Missouri and southern
Illinois climbing into the 100-105 range Sunday afternoon. This
area may see some isolated convection, but confidence is not that
great. Do have the slightest of possible PoPs in the forecast
beginning around midday. Outside of convection, some locations may
very well push Heat Advisory criteria.

...Eclipse Forecast...

The 12Z guidance continues to develop convection over portions of
southeast Missouri by midday Monday and then attempts to build it
northeast into southwest Illinois in the afternoon. Of course,
these models have no clue about the eclipse, so if this is pure
heat of the day stuff, guidance is most likely overdone in the
total eclipse area. Have limited PoPs mainly to slight chances.

Temperatures on Monday may not be quite as hot as Sunday, but the
humidity will likely be worse. As the sun resumes full power
after the eclipse, portions of southeast Missouri and southern
Illinois could see heat indices 100-105 and that could lead to
some isolated to scattered convection. Also wonder if a
differential heating boundary may develop and focus convection along
the edge of the totality area. Lots of things to consider for the
diurnal/convective trends. Will continue the Special Weather
Statement for near Advisory level heat and humidity expected
Sunday and Monday.

At this time, it appears that west Kentucky, especially the Lakes
and Pennyrile regions, will have the best weather (least clouds,
heat and humidity) for viewing the eclipse.

.LONG TERM...(Tuesday through Saturday)
Issued at 249 PM CDT Sat Aug 19 2017

An abrupt change to cooler and less humid weather will occur as a
cold front passes Tuesday night. There is some potential for strong
storms along and ahead of the front. Lots of clear sky and pleasant
weather will follow the front for the rest of the week.

As far as the frontal timing, the models are in fair agreement the
front will cross the Lower Ohio Valley early Tuesday night. The
highest pops will be in the likely category late Tuesday and Tuesday
evening. This appears to be the time frame when the frontal forcing
will coincide with the maximum diurnal instability. Deep-layer wind
fields will be slightly enhanced by an approaching 500 mb shortwave
trough. Some vigorous convection is possible if there is ample
sunshine and destabilization ahead of the front. Any widespread
morning convection or debris cloudiness would tamp down the
potential in the afternoon. High temps will depend on the timing of
convection. The forecast will call for upper 80s, which is in the
middle of an unusually large guidance envelope.

Wednesday will be the first day of the newly arriving Canadian air
mass. After any lingering showers end in southern Kentucky, drier
air should bring clearing skies in the afternoon. A fairly strong
surface high over the upper Mississippi Valley will produce a north
breeze. Highs will be mostly in the lower 80s.

Thursday and Friday, the core of the coolest low-level air will pass
across the Ohio Valley and Great Lakes. 850 mb temps will fall to
around 12C. Highs will be around 80, with lows in the upper 50s. The
surface high will move slowly east across the Great Lakes region,
producing a light northeast wind flow of dry air.

On Saturday, the flow aloft will weaken as the upper trough exits
the eastern USA. The surface high will weaken and move further east,
resulting in very little surface wind flow across our region. Temps
and humidity will be slow to moderate, given the lack of a well-
defined low level return flow. Highs will be mostly in the lower
80s, with lows in the lower 60s.

&&

.AVIATION...
Issued at 1145 PM CDT Sat Aug 19 2017

Clear and calm conditions for most of the night may result in
some fog development, but it should be of the shallow and patchy
variety. MVFR fog is forecast at the moment for a few locations,
but lower conditions are certainly possible. There is some
indication that some lower VFR clouds may increase late tonight
which could hamper development. South winds will pick up with
mixing mid-morning along with some fair weather cu. Any
convection Sunday will be very isolated and therefore is not
likely to impact the TAF sites.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$