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Area Forecast Discussion (AFD)

Issued by Wilmington, OH (ILN)

                            
000
FXUS61 KILN 191528
AFDILN

AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
1128 AM EDT Wed Jun 19 2019

.SYNOPSIS...
An approaching area of low pressure will induce more showers and
thunderstorms today and especially tonight as it swings through
the Great Lakes and Ohio Valley. Temperatures will remain below
normal during the day, and near or above normal at night owing
to the large amounts of moisture in the air over the area. The
low will swing a cold front through the Ohio Valley on Thursday
with more showers and thunderstorms. The threat for flooding and
flash flooding will continue over the next couple of days. Some
dry weather is expected on Friday, but more building heat and
humidity are on tap for the weekend with increasing chances for
several episodes of thunderstorms.

&&

.NEAR TERM /THROUGH TONIGHT/...
Already starting to see a few showers re-develop across the
Tri-State area at 15Z. This trend will continue into the
afternoon as the atmosphere destabilizes. Environment remains
moist, and the ground is already so saturated, so primary
concern this afternoon will be heavy rainfall that could produce 
flash flooding. Later this afternoon, cant rule out localized 
gusty winds within isolated stronger thunderstorms, but again 
the primary hazard right through the afternoon will be the 
rainfall and potential for flooding.

Updated grids for today, mainly adjusting PoPs/Wx/QPF to be in
line with latest radar trends and high resolution guidance. High
temperatures today will rise into the upper 70s to around 80. 

Prev...

GOES East water vapor and RAP analysis show strong shortwave
trough moving through IA/MO at this hour with flat zonal flow
across the Ohio Valley. Low level boundary which has been
stalled across central Indiana and Ohio for days remains - and
continues to be a source for very slow moving showers and
thunderstorms this morning /reference recent Pickaway County
flooding/. A weak impulse and slight enhancement to the
southwesterly low level flow is driving a northeast-advancing
line of showers across the lower Scioto Valley. Moisture remains
anomalously high across the entire area - with the 19.00Z KILN
RAOB sampling PWATS of 1.60" amidst weak /moist adiabatic/ lapse
rates through a large chunk of the troposphere.  

Through about 17Z or 18Z, activity will continue to be rather
loosely focused and isolated - much like what we've seen
overnight thus far. Activity over the lower Scioto Valley will
continue to lift slowly north/northeast and may expand in
coverage somewhat through the sunrise hours. Further north, the
area along/south of I-70 may continue to foster the development
of an isolated slow moving shower or cluster of showers owing to
the weakly convergent low level flow in vicinity of the old
boundary draped across this region. All of this activity will
have the potential for very heavy/warm-rain dominated/low
centroid cores with rainfall rates in excess of 1"/hour. 

As convective temperatures are reached early in the afternoon,
expect to see the moisture-laden airmass ignite with scattered
showers and storms - again many of these will be slowly moving
and produce torrential localized rainfall rates and flooding.
Coverage of these will grow with time owing to increased large
scale ascent from the approaching shortwave trough and approach
of left-exit ageostrophic enhancement to the vertical motion
field with the 250mb jet - from west to east across the 
forecast area. Forecast soundings across the west half of the 
forecast area /CVG...DAY/ late in the afternoon and evening 
feature effective shear around 30kts - not a very impressive 
number but enough that loosely organized multicell or brief 
supercell structures will be possible with any storm or storm 
cluster. Amidst MLCAPE of only 750-1250 J/KG (again owing to 
meager lapse rates aloft), low level shear will still be very 
weak during these hours. As the afternoon turns to evening, 
storm motions will begin to pick up as the stronger mid/high 
level flow overspreads the forecast area very slowly. Thus, 
severe chances probably peak in the evening hours when the best 
juxtaposition of storm coverage and slowly increasing flow are 
overlapped. Wind /primary/ and hail /much less so/ are the 
primary threats during the afternoon into the mid evening, 
especially with any storm/cluster which can begin to accelerate 
a little owing to the slowly increasing flow aloft. But given 
how saturated most areas are - the true primary threat with any 
storm is likely going to be torrential rain rates and flash 
flooding - especially during the afternoon when storm motions 
will still be quite slow. 

From mid-evening through the night - low pressure gradually
deepens and moves through the forecast area. It is looking
pretty clear that a wing of rain/storms will be developing and
pushing northeast across the forecast area and begin to pivot in
an area of enhanced low level convergence on the north/east
track of the deepening low. This could foster a considerable
flood/flash flood threat - and while the area which is impacted
most is still lower confidence - do feel this will set up a
little further south than multi-model blends /HREF/ closer to
better low level convergence and instability. For the ILN CWA,
this means areas along I-70 may be in for a several hour period
of very heavy rainfall as this band slowly translates northeast
and begins to pivot on the north side of the low level
circulation which traverses the CWA. Collaboration with WPC on
the Day 1 ERO which places a MOD risk for flash flooding across
this area.  

Severe threat overnight - while overall may not be as high as
the late afternoon to mid-evening time period - is certainly not
zero. In fact, low level shear will be on a marked rise through
the night as the surface flow backs and weakens while 1-2km flow
increases as a reflection of the low level jet tied to the
shortwave trough moves through. So, despite instability 
weakening with time, low level shear will increase based on the
path of the surface low. So any lines of convection or discrete
cells developing overnight may have the tendency to produce 
marginally severe wind gusts and of course we cannot rule out a 
brief/weak tornado or two in these setups of increasing low 
level shear with such a moist/convectively active environment. 
As mentioned above - severe threats are likely going to pale in 
comparison to the threats from flooding and flash flooding.

As surface trough works into the forecast area toward sunrise,
expect convection to become more scattered and winds to turn
westerly.

&&

.SHORT TERM /THURSDAY/...
Cold front tied to the shortwave trough will drag southeast
across the forecast on Thursday morning and afternoon. This will
most-certainly ignite more showers and thunderstorms during the
day, so keeping the Flash Flood Watch through Thursday. Better
severe threat will be off to the east where the deepening
boundary layer will have more opportunity to destabilize
sufficiently. A cool day - highs only in the low/mid 70s owing
to the frontal passage and clouds/showers.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
The unsettled and active weather pattern that has been entrenched 
across the region for nearly a week now will extend right into the 
long term period -- but potentially due to a slightly different 
reason/setup than has been the case since last weekend. 

With the sheared-out S/W energy pulling east of the Ohio Valley 
Thursday night, we will welcome a much needed dry period -- but even 
this could be very brief from Thursday night through the first part 
of Friday afternoon. Aloft, weak midlevel ridging will build east 
into the region early Friday, with a broad and weak surface high 
briefly progressing through the Ohio Valley and Great Lakes region 
Friday morning. However, even by Friday evening, the pattern will 
become a bit more unsettled once again as midlevel energy from a 
closed low over the upper Midwest ejects southeast toward the mid-
Mississippi River Valley and potentially into the western Ohio 
Valley. There is fairly good model agreement even at these time 
ranges regarding this disturbance progressing southeast late Friday 
into early Saturday, but there are still inconsistencies regarding 
the depicted track/path and timing of this disturbance progressing 
southeast from an origin point somewhere in the vicinity of 
Iowa. There are several things to highlight regarding the 
upcoming pattern for this weekend which are discussed a bit 
further below:

For one, there is very good model consensus on an increasingly warm 
and humid airmass building northeast into at least the southern and 
western portions of the Ohio Valley starting as early as Friday 
afternoon. This is an environment that perhaps has not yet been felt 
locally this season thus far -- with the prospect of dewpoints in 
the mid 70s arriving by Saturday with highs in the mid to upper 80s -
- pushing heat indices potentially close to the 90 degree mark for 
locations west of I-75 and near/south of the Ohio River. However, 
this may in fact not be the biggest weather story going into this 
weekend -- but rather it is likely to be the potential for one or 
more strong MCSs tracking through at least a portion of the Ohio 
Valley. 

The first real chance at a mature MCS moving close to the ILN FA 
will occur sometime Friday evening or Friday night. However, upon 
closer inspection, it appears as of right now, that the favored 
corridor for this storm activity may end up being just to the 
west/southwest of the ILN FA. This may occur for several reasons -- 
namely the orientation of the low level instby gradient axis, which 
depending on your model of choice, is likely to set up from 
approximately northern IL southeastward through central KY. Where 
this instability gradient sets up, which will be largely dependent 
on how quickly low level moisture is able to advect back northeast 
Friday afternoon, will ultimately dictate the favored corridor for 
potentially intense storm activity Friday evening/night. The GFS 
continues to be aggressive with the advection of low level moisture 
northeast into far southwest portions of the local area by Friday 
evening, while the ECMWF and NAM are admittedly a bit more reserved 
(especially the ECMWF). A slower return of higher DPs (i.e. 
mid/upper 60s) would ultimately favor a MCS track potential more 
across the western Ohio Valley and mid-Missouri Valley. This in fact 
may end up being the favored solution -- especially as any storm 
complex is likely to trend toward the better instby, which would act 
to shift the overall storm movement further south than east with 
time. This favored further west solution would leave most of the 
local area mainly dry through Friday night, except perhaps for the 
Tri-State area and points south/west which stand a better chance at 
seeing some isolated storm activity. However, with all of this being 
said, there is considerable uncertainty in the orientation of the 
instby axis as it relates to the best forcing for large scale ascent 
courtesy of the aforementioned S/W energy poised to track southeast 
from Iowa late Friday. 

Even with all of this being said, it appears as of right now that 
the better setup for more widespread storm activity locally 
could be Saturday into Saturday night for the ILN FA. Many of 
the same ingredients will be in place across the region as will 
be the case late Friday, except for the fact that an abundance 
of low level moisture /and therefore high to extreme instby/ 
could be in place locally by Saturday. This will occur near and 
on the lee side of a midlevel trof axis centered across the 
center of the Ohio Valley, which certainly could act to stunt 
organization potential a little (with winds aloft a bit lighter 
than would otherwise be the case in classic MCS setups). 
However, with this being said, the low level wind fields (i.e. 
850mb and 925mb) begin to strengthen a bit more Saturday opposed
to Friday across the local area, with the nosing of a 
seasonably strong LLJ northeast into the heart of the Ohio 
Valley. This setup, with high to potentially extreme instby on 
the order of 3000j/kg+ setting up across the western/southern 
Ohio Valleys, combined with S/W energy diving southeast from the
western Great Lakes, as well as a gradual increase in the deep-
layer shear profiles Saturday, certainly all lend themselves to 
an environment potentially conducive to MCS development and 
propagation along the NW-SE oriented instby gradient. 

Still, there remains considerable uncertainty in the exact 
orientation and placement of best low level moisture and instability 
as well as the exact track of an embedded disturbances within the 
midlevel flow. Moreover, any activity is likely to be moving into or 
near a midlevel ridge axis, which would suggest that weak midlevel 
winds could end up lessening the potential severity or impact a tad. 
But certainly the setup for one or more complexes of storms will be 
watched carefully as we progress into the weekend.

Into early next week, quasi-zonal flow will continue before a more 
progressive S/W tracks east across the region by Tuesday. The stormy 
pattern may continue into early next week before signs for a 
slightly drier pattern emerge towards the middle of next week. 

Seasonable warmth and humidity -- as well as the daily potential for 
showers and storms -- will be the norm for the long term period, 
with no sign of any cooler/drier air moving in through early next 
week. Friday will undoubtedly be the coolest day of the long term 
period, with temperatures reaching into the lower 80s.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
IFR conditions have again become rather widespread this morning
with stratus/fog impacting much of the area. This has been in
and out of several of the TAF sites this morning and expect
stratus to peak in coverage at 12Z and then begin to slowly
lift/break up but it will take some time. Have prevailing or
TEMPO for fog/low ceilings at just about every TAF this morning.
Improvement in conditions but showers expected to begin moving
in/developing by later in the morning or early afternoon. These
will be scattered and if the potential for brief IFR conditions
in torrential downpours. Coverage of showers and storms
increases later this afternoon and this evening - have this
covered with VCTS given low confidence in timing at different
locations. Feel with time that the higher coverage of storms
will shift north into DAY/CMH corridor and perhaps somewhat
lesser coverage at CVG/LUK/ILN but confidence not terribly high.
Ceilings should be VFR or MVFR in the precipitation from this
afternoon through tonight with light southerly flow which turns
southwest late in the TAF forecast period. 

OUTLOOK...Thunderstorms will be possible again Thursday, and 
again Friday night through Sunday.

&&

.HYDROLOGY...
As mentioned above, MOD risk for flash flooding through sunrise
on Thursday especially along/just north of existing stalled
boundary and anticipated track of slowly pivoting band of
rain/storms over central Ohio tonight. Could see another 1-3" of
rain in this area in a short amount of time - and given the
amount of impact recent slow moving storms have had - we simply
cannot take any more widespread heavy rainfalls without serious
flooding/flash flooding potential. Radar estimated rainfall for
the past 7 days is really bullseyed on the ILN CWA into southern
Indiana. Widespread 2-3" amounts, with many counties having seen
4-7" in the past 7 days. June streamflows are running way above
normal, and reports into the office with recent events suggest
runoff is substantial.

&&

.ILN WATCHES/WARNINGS/ADVISORIES...
OH...Flash Flood Watch through Thursday evening for OHZ026-034-035-
     042>046-051>056-060>065-070>074-077>082-088.
KY...Flash Flood Watch through Thursday evening for KYZ089>100.
IN...Flash Flood Watch through Thursday evening for INZ050-058-059-
     066-073>075-080.

&&

$$
SYNOPSIS...Binau
NEAR TERM...BPP/Binau
SHORT TERM...Binau
LONG TERM...KC
AVIATION...Binau
HYDROLOGY...