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Area Forecast Discussion (AFD)

Issued by Wilmington, OH (ILN)

FXUS61 KILN 191752

National Weather Service Wilmington OH
152 PM EDT Thu Sep 19 2019

High pressure over New England will slowly migrate south 
towards the Mid Atlantic and the Carolinas tonight into Friday.
Surface flow around this high will turn southerly, bringing an
increase in moisture. The high will then remained anchored 
across the Carolinas this weekend as a slow-moving cold front 
approaches the Ohio Valley Sunday night, and crosses early 


Forecast this afternoon and evening is well in hand and no
changes were made. Generally sunny skies in the north with
partly cloudy skies in the southern reaches of the CWA will
persist. Models show increasing potential for isolated showers
moving ssw-nne overnight, which would at least keep some cu over
the area in the evening hours. 

Highs will remain in the mid to upper 80s today with dry air in


It should become mostly clear this evening. Models then indicate
that the low level flow above the surface, or around 850 mb, 
will veer to the south/southwest. Moisture near this level may 
receive enough weak lift/ascent to result in a stray shower or 
sprinkle, spreading from southwest to northeast after midnight. 
Have included a 20 PoP for measurable precipitation. Otherwise,
skies will become partly cloudy. Lows will be in the lower to 
mid 60s.

On Friday, models indicate that deeper moisture will slowly lift
north through the day. Again, a small chance of a stray shower
or sprinkle will be possible, with that chance lifting northward
during the day. Skies will be partly cloudy. Highs will mostly 
be in the mid 80s.


Extended forecast period still characterized by warm to very 
warm temperatures and overall questions about what kind of rain 
we can squeeze out of a very unimpressive/weakening upper trough
trying to blast over and through the massive subtropical ridge 
which has been camped over the eastern United States. As 
detailed below - we may need to start thinking fire weather 
concerns by later in the weekend. 

Flow pattern on Friday evening will still feature a deep 
longwave trough in the Intermountain West ejecting northeast, 
and a large subtropical ridge downstream over the eastern US, 
with the axis of highest heights relative to climatology in the 
Ohio Valley into southeast Canada. As the upper trough ejects 
into and atop the ridge over the weekend, heights will slowly 
fall in the Ohio Valley allowing a weakening cold front getting 
strung out more and more southwest-to-northeast with time as the
main height fall center pushes away to shift through the area. 
Height rises begin again in the days following weak frontal 
passage /Tuesday and Wednesday/ thus above normal temperatures 
are going nowhere fast. Earlier hints at a more significant 
pattern change for next week are long gone - although admittedly
the spread/noise in the ensemble at Day 7 is pretty significant
suggesting there is still some detail to work out. Regardless -
the signal for above normal temperatures all the way through 
next week is quite strong. 

A signal that continues to slowly gain support in the ensembles
/especially EPS/ is for a weaker/slower frontal passage 
attendant to the trough passing through the area later in the 
weekend into early next week. In fact, if 19.00Z deterministic 
ECMWF is any representation of how things evolve - we've just 
tacked another very warm day - perhaps even the warmest day of 
September - to the end of the weekend. Jury is still out on this
trend - but given ECMWF boundary layer and larger scale flow 
performance in this very dry / drought-developing September in 
comparison to GFS/GEFS - we are apt to latch on more 
significantly to what we are seeing in the ECMWF/EPS. 

The upswing of that was to adjust Saturday /and especially 
Sunday/ to a warmer solution with max temps around 90F in the 
major metro areas with a drier boundary layer /dewpoints in the 
50s/ than what was previously forecast. Also - delaying rain 
chances until later in the night on Sunday per ECMWF/EPS trends 
which are notably slower/drier/hotter for the weekend. 
Considering deterministic ECMWF has +26C at 925mb lurking for 
both weekend days in/near the ILN CWA - further upward 
adjustments may be needed to the max temp forecasts - as these 
925mb temps earlier this month yielded widespread low-mid 90s - 
and soils now are much drier than they were 10 days ago. Sunday 
could turn out to be a downright hot day given the forecast T/Td
profile we are seeing in many EPS members.

This is concerning for the potential for elevated fire weather 
conditions both days, but particularly on Sunday when ECMWF 
brings the driest boundary layer airmass into the ILN CWA, and 
combines with southwesterly winds of 10-20mph on an increasing 
pressure gradient ahead of the slowly advancing front. We may 
need to start ramping up fire weather hazards if this signal 
continues for the weekend /mostly Sunday/. 

Front shifts through late Sunday night or likely Monday with a 
yet- to-be-seen-with-confidence band of showers and isolated 
storms. EPS trends do not suggest rainfall will do much to dent 
developing drought conditions - with basically 0% of the members
showing anything more than 0.50", and only 40-60% of the 
members showing > 0.10". Some brief cooling /in a relative 
sense/ and dry weather for Tues, before returning theta-e 
advection and warming aloft induce another small chance of 
showers for Wednesday.


VFR conditions will be found over the TAF sites this forecast
period with fair wx cu this afternoon gradually migrating nne
and overspreading northern TAF sites this evening as a scattered
deck. Models are hinting at isolated showers passing nne
overnight, potentially hitting any TAF site but more likely no
TAF site overnight. The better expectation of this shower threat
would be an occasional bkn deck that would remain VFR if it does
eventually constitute a ceiling. 

Do not have ceilings in the forecast nor precipitation tonight.
Subsequent forecasts will need to amend if the isolated showers
develop. Currently it is not being shown during a favorable 
diurnal time and there is no h5 s/w or vorticity maxima noted on
any model that does happen to show precip. The most telling 
driver overnight appears to be the h7 upward motion after 
possible initiation late today in the vicinity of a 
dissipating/lifting surface trough.

OUTLOOK...Thunderstorms possible on Sunday night into Monday.




NEAR TERM...Franks
SHORT TERM...Hickman