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Area Forecast Discussion (AFD)

Issued by Wilmington, OH (ILN)

FXUS61 KILN 170538

National Weather Service Wilmington OH
138 AM EDT Tue Sep 17 2019

High pressure and a dry airmass will build across the region
through the week with temperatures remaining above normal.


Strong mid level ridge over the mid/upper Mississippi River 
Valley. This has resulted in a northwest flow aloft over the 
the Ohio Valley. Surface cold front which has pushed south of 
ILN/s FA will continue south thru the TN Valley overnight. 
Surface high pressure will build across the Northern Great 
Lakes, causing the low level flow to veer to the northeast. 

Weak low level CAA will occur thru the night with an advection
of low level moisture into the area from the northeast
overnight. Surface dewpoints are still in the lower and middle 
60s. This scenario should lead to the development of stratus. 
Question revolves around how far south the stratus will make it 
by sunrise. Have brought low clouds into about the northern half
of ILN/s FA but confidence in exactly how far south these 
clouds develop is not high. Further south where clouds do not 
make it some valley fog will be possible in the protected 

Lows tonight are expected to be in the low to mid 60s.


Some possible clouds across the area Tuesday morning should
scatter out into a cu field as high pressure and a drier airmass
builds into the region. Highs on Tuesday will be in the low to
mid 80s with lows Tuesday night in the upper 50s to lower 60s.


For the start of the long term period, the strong shortwave energy 
currently over the Pacific Northwest will be ejecting northeast into 
Manitoba over the high amplitude ridge, whose axis is forecast to be 
centered over the central Great Lakes.  Meanwhile, a closed low will 
be coming onshore coastal Washington State.  East of the ridge, a 
persistent downstream trof will linger over New England.

The period begins in the local area with surface high pressure in 
place over New England, and dry easterly flow across the CWA. 
This surface high shifts to the south heading late into the work 
week and into the start of the weekend as low level flow swings 
around to the southeast and south. Under the strong mid level ridge, 
high temperatures will likely continue to run 10-12 degrees above 
seasonal normals for mid September.

As the Pacific Northwest closed low opens into a trof and slowly 
ejects eastward, the ridge shifts eastward and begins to flatten on 
Sunday and Monday as the trof slides across the Great Lakes. As 
heights lower, expect a cold front to drop southeast across Ohio 
late Sunday night into Monday, with clouds and precipitation 
persisting through the day Monday. This front will also lower the 
anomalously high maximum temperatures in place since the second week 
of September.


Concern through the remainder of the night will be on stratus
development across northern Ohio, and expected expansion and
movement to the southwest on increasing northeasterly flow off
Lake Erie. Cold advection amidst a still moist low level airmass
should continue to encourage stratus formation over northern
Ohio where recent observations indicate moistening in the
boundary layer /light fog/ and GOES satellite imagery now shows
pockets of stratus developing along Highway 30. This will expand
as it sinks south-southwest toward CMH/DAY/LCK, but confidence
has decreased in where this stratus will slow down - with latest
indications it may be just north of the DAY/CMH/LCK corridor. So
have removed prevailing MVFR ceilings at these terminals later
tonight and through the sunrise hours, and replaced with a TEMPO
of MVFR and light fog. Will be very close whether the MVFR cigs
will be a transient feature or lock in for the better part of
the late night and Tuesday morning after sunrise.  

Once daytime heating cycle commences, the fringes of this
stratus area will begin to erode and break into a SCT/BKN
cumulus field as it lifts into VFR category by late morning or
early afternoon. Northeasterly winds will be modest below 10kts.
Cumulus should become quite cellular in nature and be quick to
diminish this evening with the loss of daytime heating.  

Expect some valley fog through sunrise as well for KLUK where
ongoing forecast lowers visibility to IFR and this still looks
good given calm winds and very low surface temp/dewpoint spreads
as of 04Z. Should see fog form soon in the Little Miami River
Valley with northeast winds advecting this fog over the
airfield at LUK.  

OUTLOOK...No significant weather expected.