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Area Forecast Discussion (AFD)

Issued by Wilmington, OH (ILN)

FXUS61 KILN 180531

Area Forecast Discussion
National Weather Service Wilmington OH
131 AM EDT Thu Oct 18 2018

High pressure will keep cool and dry conditions over the Ohio 
Valley tonight through Thursday. Rain enters the weather picture
on Friday when a cold front is forecast to cross the area from 
the west. Brisk winds, chilly temperatures, and a chance of 
showers will follow the cold front on Saturday.


The gusty northwest winds from earlier today are quickly
diminishing post-sunset, leaving generally 5-10 knot flow near
the surface heading into the overnight period. It is interesting
that NAM/GFS surface wind projections keep at least some wind
in place all the way through the early morning hours, while
HRRR/RAP projections suggest nearly calm conditions will envelop
the CWA by that time. That may have an impact on temperatures,
on a night in which the exact temperature value (with relation
to 32F) is crucial. In addition, a shield of cirrus is starting
to encroach on the southern sections of the forecast area.
Although these are high clouds, they may have a small impact on
temperatures as well.

Based on current temperature trends, the location of the 
clouds, and the fact that the northwestern sections of the CWA
appear most likely to have winds become calm, some adjustments
were made to temperatures overnight -- essentially tightening
the gradient. This means slightly cooler conditions in the
northern and northwestern sections of the CWA, and slightly
warmer conditions in the far south, as compared to the previous
forecast. As such, temperatures might only make it to the mid
30s in the Kentucky counties, though there is always the
consideration to be made that valleys and hollows could get a
little cooler than their surroundings. No real benefit to
shaving a freeze warning at 930PM, so will leave it in place 

Previous discussion >
Weak and moisture starved cold front is crossing the FA to be
followed by high pressure, with gusty northwest winds bringing 
drier air behind the front. As the 1035 mb high moves closer 
tonight, winds will subside while skies remain clear. Under 
close to ideal conditions for radiational cooling, this will a 
very favorable scenario for freezing temperatures toward 6 am.

Though lows have been landing above guidance in recent nights,
that may not be the case tonight under a regime that will be 
more favorable for radiational cooling. Went close to model 
blend showing low 30s, cold enough to keep freeze warning across
the FA.


There will be little to no sky cover on Thursday as surface high
pressure becomes centered near Portsmouth OH. Increasing high
clouds will be observed Thursday night as the high moves east
across the Appalachians allowing slightly more humid air to 
start spilling in from the west ahead of the next cold front.

Temperatures remaining below normal will reach the lower 50s
Thursday, before falling back to the mid 30s Thursday night.


This period will be characterized by below normal temperatures, 
mostly dry weather, and periodic chances for frost as the cool 
pattern remains locked in over the Ohio Valley. Only chances for 
precipitation comes Friday night and Saturday. 

At 12Z on Friday morning, broad/strong subtropical ridging will 
remain over Florida/Gulf Coast, and ridging over WA/OR atop a weak 
closed low over CA/Baja Peninsula. And atop these features, a rather 
fast/amplified polar jet with longwave troughs over the nrn Pacific 
and nern US/sern Canada.  The only real change in this pattern 
through the middle of next week will be slow eastward drift of the 
weak closed low across the swrn CONUS and nrn Pacific longwave 
trough, with the nern CONUS/sern Canada longwave trough sharpening 
considerably and gaining amplitude.  This is a dry synoptic pattern 
for the Ohio Valley, featuring multiple days of negative PWAT 
anomalies and cool temperatures via the northwest flow through the 
Great Lakes and influence of the longwave trough nearby.

In the details, Friday begins weak warm advection/southerly return 
flow ahead of a very strong shortwave trough digging southeast 
through the Upper Mississippi River Valley, and more subtle/weak 
vort maxes rotating northeast from the lower/mid Mississippi River 
Valley around the large anticyclone over Florida. Well ahead of the 
surface cold front tied to larger height falls spreading through the 
Great Lakes, an axis of moderate-strong low level theta-e advection 
and increased moisture transport will move/develop across the Ohio 
Valley Friday afternoon and evening as large scale /300-200mb/ wind 
fields strengthen over the Great Lakes, placing KY/srn Ohio in the 
right entrance region of a developing jet streak. The result will be 
a developing/broadening band of light rain particularly Friday 
evening across Kentucky into south-central Ohio. Areas north/west 
will be in a slightly less favorable forcing region, so kept rain 
chances below 50% across west central Ohio and mainly west of I-71.

First cold front attendant to the strong shortwave trough dropping 
through the Great Lakes will cross the forecast area late Friday 
night through Saturday morning, followed by a secondary boundary 
Saturday evening as the compact area of height falls swings through 
Ohio. Good synoptic scale lift will work an otherwise moisture-
starved airmass to generate scattered showers and areas of rain 
through this time period. Still a handful of ECMWF ensemble members 
indicating enough overlap of strong low-level cold advection and 
delay in shortwave-induced forcing/moisture to finish this off in a 
bit of wet snow in central Ohio late Saturday night, but not nearly 
enough indication in deterministic models to put into forecast 
right now, but will continue to watch in case a spate of wet snow 
needs to be inserted for a bit late Saturday night. 

Other story on Saturday will be steepening of the lower tropospheric 
lapse rates Saturday afternoon as temps aloft crash.  Mixing will 
deepen considerably but thankfully winds in the lowest couple of km 
will remain moderate, so don't expect hazardous wind gusts at this 
time, but could still see localized gusts to 30-35mph per momentum 
transfer techniques off forecast GFS/NAM soundings.

Beyond Saturday night, several dominant Canadian-sourced high 
pressure systems will drop down through the Great Lakes as negative 
anomaly is maintained in sern Canada. Frost is possible on multiple 
nights through next week, but most likely on Monday morning when 
clear skies and light winds will be juxtaposed with a still-cool 
airmass in place.  Growing season may quickly come to an end most 
locations at some point between the potential tonight...Sunday 
night...or Tuesday night.  Several sunny and cool days on tap Mon-
Wed (and perhaps beyond) as the pattern remains cool and very 


A fair amount of cirrus has overspread the area early this 
morning and it looks like it will hang around at least for the 
next few hours. This will likely help limit fog development at 
KLUK through daybreak. Otherwise, high pressure will build into
the area through the day, with mostly clear skies through the 
rest of the TAF period.

OUTLOOK...MVFR conditions are possible from Friday evening into


OH...Freeze Warning until 10 AM EDT this morning for OHZ026-034-035-
KY...Freeze Warning until 10 AM EDT this morning for KYZ089>100.
IN...Freeze Warning until 10 AM EDT this morning for INZ050-058-059-


NEAR TERM...Coniglio/Hatzos
SHORT TERM...Coniglio