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Area Forecast Discussion (AFD)

Issued by Wilmington, OH (ILN)

FXUS61 KILN 151802

National Weather Service Wilmington OH
202 PM EDT Sun Sep 15 2019

Surface high pressure will gradually move east of the region 
today. A weak cold front will move south through the Great Lakes
and Ohio Valley tonight and Monday, perhaps bringing a few 
light showers or sprinkles. Dry conditions are then expected 
through much of the upcoming week with temperatures remaining 
above normal.


Convective activity is continuing to push east across northern 
Illinois and northern Indiana this morning. Hi res models
including the latest 12Z HRRR continue to suggest this activity 
will weaken and dissipate as it moves into the ridge and drier 
airmass in place across our area. Suppose a few showers may 
still sneak into our far northwest, but given the expected 
dissipation, will maintain a dry forecast at the moment and 
continue to monitor over the next hour or two. Otherwise, debris
cloudiness from this will likely affect at least the northwest 
half of our area into early afternoon before beginning to 
dissipate through mid to late afternoon. With the assumption 
that the clouds will dissipate, will continue with highs 
ranging from the lower 80s in the north to the mid/upper 80s 
across the south.


As a mid level ridge builds northward into the middle and upper
Mississippi River Valley, mid level northwest flow will develop
across the Great Lakes. This will push a weak cold front south 
across the region tonight and Monday. Models suggest that a few 
showers or sprinkles may occur with this boundary, mainly across
the northern sections. Skies will range from mostly clear to
partly cloudy tonight with mainly partly cloudy skies expected
on Monday. After lows in the lower to mid 60s, highs on Monday
will range from the lower 80s north to the upper 80s along and
south of the Ohio River.


This period will be characterized by persistent anomalous 
mid/upper level ridging over the eastern U.S. downstream of a 
deep upper trough in the western U.S. This will lead to a high 
confidence period of well above normal temperatures and mostly 
below normal precipitable water /PWAT/ which should lead to 
large diurnal swings in temperature, very little or no 
precipitation, and comfortable humidity unlike the most recent 
spell of very warm weather. 

In the details, at 12Z on Tuesday, a closed 590dm 500mb 
anticyclone will be over Arkansas with the mid level ridge axis 
extending well north into Canada. Longwave trough over the 
western U.S.will have a potent shortwave trough digging through 
NV/ID. This initial shortwave will eject northeast atop the 
ridge axis and weaken as it moves north of Lake Superior with no
impacts on sensible weather in the Ohio Valley. A deep closed 
low will then develop in the longwave trough on Thursday with 
the mid/upper level ridge axis building further and then 
extending northeast into the Ohio Valley. The entire longwave 
trough will begin to shift east on Friday and Saturday as the 
closed low ejects as a potent shortwave trough into MT/WY on 
Friday and then the Dakotas on Saturday. This process will 
slowly lower heights in the Ohio Valley next Saturday/Sunday and
suppress the ridge by shifting it eastward.

Sensible weather/impacts - very little to none expected the 
entire period. A very quiet period of very warm weather is 
expected with strong agreement in the EPS/GEFS of a very large 
bubble of mean 5- day 850mb positive temp anomalies centered to 
our northwest and fully encompassing the Ohio Valley and eastern
U.S. Expect daily highs to be some 7-10 degrees above normal 
each day. Like with the warmth last week - the GFS and its 
statistical MOS are likely way too cool. And while deterministic
ECMWF and its statistical MOS seem warm, this data verified 
much better in the stretch of 90s last week. So have pushed 
temperatures on the very high end of the model/MOS envelope 
considering we've been so dry in September, and dry soils are no
doubt leading to 1) lower afternoon dewpoints and 2) more 
efficient heating of the boundary layer. Thus, forecast maximum 
temperatures in the Tues-Sat period will be well into the 80s 
each day, with afternoon dewpoints likely mixing back into the 
50s with low level trajectories having some component of 
easterly fetch through most of this period. Further still, 
background tropospheric PWAT will be below normal on every day 
except Saturday, enhancing the potential for drying/mixing each 
day. Would not be surprised to see daily diurnal temperature 
differences approach 25F given very warm afternoons and still 
somewhat cool nights. 

Deeper moisture finally starts to edge into the Ohio Valley 
Friday/Saturday. Can't completely rule out a pop up storm on 
Friday in the far west - but think the lightly-precipitating GFS
is too moist in the boundary layer and thus too aggressive with
destabilization and shower coverage. Forecast GFS soundings 
Friday afternoon indicate upwards of 750 J/kg of MLCAPE, while 
ECMWF EPS data shows none of the 50 members having as much as 
500 J/kg of CAPE, with a good 5-7F lower dewpoint in the ECMWF. 
So will side with a drier ECMWF on Friday, and Saturday as well 
given agreement in the 15.00Z CMC. The approach of the upper 
trough on Saturday night into Sunday will finally return an 
appreciable rain chance to the forecast area.


Still a few remnant showers pushing southeast toward our area 
but these should continue to weaken and not affect the TAF 
sites through mid afternoon. However, a fair amount of higher 
level debris clouds will continue to spill southeast across our 
area through the afternoon hours. These clouds should then 
dissipate heading into this evening. However, a weak cold front
will drop down from the northwest later tonight and into Monday
morning. This could lead to a few showers at the northern TAF 
sites, but the chance appears low enough to leave mention out 
of the TAFs. The main result will be a lowering of cigs across 
mainly our north through the later part of the TAF period. 
These cigs should remain mainly VFR, but some MVFR cigs may be 
possible at times later Monday morning. 

OUTLOOK...No significant weather expected. 




SHORT TERM...Hickman