Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 232007

Area Forecast Discussion
National Weather Service Indianapolis IN
307 PM EST Thu Feb 23 2017

The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated


Issued at 307 PM EST Thu Feb 23 2017

The unseasonably warm weather of the last week will come to an
abrupt end Friday evening as a powerful cold front sweeps across the
region.  Scattered thunderstorms and the threat for severe weather
will accompany the passage of the front Friday afternoon and
evening. Much colder conditions more typical of late February will
return for the weekend along with the potential for snow showers
late Friday night and Saturday. After a dry Sunday, wet and
unsettled weather will continue through the first half of next week.


.NEAR TERM /Tonight/...
Issued at 307 PM EST Thu Feb 23 2017

Plenty of stratocu present across the region this afternoon as a
warm front sets up over northern portions of the forecast area. Sun
was peaking through the cloud deck sporadically...and the strong warm
advection developing had enabled temps to surge into the mid and
upper 60s over most areas despite the cloud cover.

Focus for the rest of the afternoon and evening is on potential for
isolated to scattered convective development in the vicinity of the
boundary and a wave traversing it moving into Ohio. Current
mesoanalysis indicating an axis of MLCAPE values from 500-1000 J/KG
extending east-west across the southern half of Ohio back into far
eastern Indiana. These areas saw more sunshine and consequently
better warming earlier. Despite the clouds over the area...appears
further weak destabilization may develop back west into the forecast
area beneath a subtle inversion. This is likely to be just enough to
generate isolated convection for the rest of the afternoon...focused
over the eastern half of central Indiana. Additional showers and
perhaps a storm or two could extend back into the northern Wabash
Valley into the evening with low level convergence in the vicinity
of the warm front.

Scattered showers and a few storms will focus over northern portions
of central Indiana through the evening...gradually shifting north
with the warm front overnight as the forecast area works solidly
into the warm sector. Despite the presence of the nocturnal
inversion...increasing flow through the boundary layer as a low
level jet expands into the area will translate to increasing gusts
at the surface late tonight into early Friday. will feel downright balmy tonight as the warm advection
and increasing winds keep temps from falling much at all. MOS
guidance looked reasonable as lows fall only into the upper 50s over
much of the forecast area.


.SHORT TERM /Friday and Friday Night/...
Issued at 307 PM EST Thu Feb 23 2017

The primary focus for the period will be the threat for severe
weather as storms impact the forecast area mainly during the
afternoon and evening ahead of a strong cold front.

Friday morning will start windy and warm as the aforementioned warm
front lifts into northern Indiana. Fair amount of moisture noted in
the lower levels on both model soundings and RH progs and will
likely see stratocu across the region.  With that being said...the
moisture is shallow enough that some sunshine is anticipated into
the early afternoon. Expect gusts to kick up rather fast during the
morning as the initial surge of the low level jet noses into the
area with the surface low tracking from northern Missouri into
northern Illinois.

Focus then shifts to the severe weather threat which will begin to
grown from 19-20Z into the evening. Model consensus has generally
come into line on timing of the greatest threat for severe storms
impacting the forecast area from mid afternoon through mid evening.
Moisture return is a bit problematic and may serve as a mitigating
factor on amount and intensity of storms. With that being said...
overall dynamics associated with the system are strong enough to
overcompensate for the limited moisture fetch.

While the severe weather threat is present over the entire forecast
area...the greatest risk will likely set up over the eastern half of
central Indiana east into western Ohio. Potential exists in this
area for an initial round of potentially discrete convection
developing in the vicinity of the prefrontal trough followed by the
squall line anticipated to organize along the cold front. The low
level jet will transition across the region into the afternoon hours
with the back side of the jet aligned over eastern Indiana and
western Ohio.  Mid and upper level diffluence is pronounced with the
exit regions of both jets over the Ohio Valley.  The upper jet
structure in particular favors a splitting of the northern and
southern jets over the Ohio and Tennessee Valleys...further
accentuating the diffluence aloft. Wind analysis showing best
potential for directional shear through the column will develop from
Indy east around 21Z...maximizing over eastern Indiana and western
Ohio into the early evening. The instability axis will align in this
area too immediately along and ahead of the trough...with MLCAPEs
around 1000 J/KG which is impressive for late February.

Taking all of this into consideration...plausible scenario
developing where local and brief backing of the near surface layer
along and immediately ahead of the prefrontal trough will present a
brief enhanced threat for rotating storms and possibly tornadoes.
Again...this looks to be focused over the eastern half of the
forecast area in the 21Z to 00Z time period. The cold front itself
will arrive during the evening and provide another round of severe
weather in the mode of a thin...low topped and fast moving squall
line with damaging winds as the primary threat. There will exist a
small tornado potential as weak and short lived couplets develop
along the line...a typical occurrence with Ohio Valley QLCSs. The
presence of the stronger dynamics over eastern Indiana may allow for
a strengthening of the convective line as it moves east across the
forecast area.

The cold front and squall line will be east of the forecast area by
03-04Z. Passage of the front will bring an abrupt end to the
unseasonably warm weather enjoyed over the last week or so. Much
colder air will spill into the region immediately behind the front
Friday night with light precipitation redeveloping as wraparound
moisture rotates into the area predawn Saturday. The boundary layer
will become sufficiently cold enough to support rain mixing with or
completely changing over to light snow by Saturday morning.

Temps...generally took a split of the MOS guidance for highs Friday.
Should the sun appear for an extended period of time...could easily
see highs approach MAVMOS levels in the 70-75 degree range. Went
slightly cooler than this in the upper 60s and lower 70s with the
potential for the stratocu. Record high watch once again for Indy as
the high of 71 from 2000 definitely could be threatened.
Temperatures will tumble behind the front Friday night...falling
into the 30s. Continued gusty winds will make it feel even colder.


.LONG TERM /Saturday through Thursday/...
Issued at 235 PM EST Thu Feb 23 2017

Temperatures tumble back to seasonal on Saturday in the wake of
the strong frontal system. With cyclonic flow and a little upper wave
moving through, could see enough forcing to produce brief snow
showers (or a mix of rain and snow in the south) across the area
during the morning hours. With the dry air also advecting in think
the initialization slight chance pops work well. By afternoon
enough dry air should be in place and the trough axis will be east
of the area so should see dry weather. Would not rule out a stray
sprinkle or flurry, but low enough chance to leave out at this
time. Brief ridging should allow for dry weather through Sunday
afternoon. For Saturday night this could mean mostly clear skies
and lows down into the low to mid 20s. Some return flow develops
on Sunday allowing highs to climb back into the mid 40s and Sunday
night lows should be near or just above the freezing mark with
increasing clouds and chances for snow or a rain snow mix as a
warm front forms south of the area and overrunning develops.

After that expect rain chances to continue until late Wednesday
night with the upper ridge off to the east and upper waves moving
through a more progressive upper flow with a trough to the west
and a ridge to the east. Model differences with temperatures and
precipitation chances past Monday night are pretty large and thus
fairly low confidence forecast from Tuesday on. For that reason
did not add any thunder Tuesday/Wednesday, as GFS and ECM differ
on availability and timing of instability. There is a signal that
the warmup will continue though, so should see warmer than normal
temperatures from Monday on.


.AVIATION /Discussion for 231800Z TAF Issuance/...
Issued at 1215 PM EST Thu Feb 23 2017

MVFR stratus will continue to impact the area at times this
afternoon and again Friday morning.

Frontal boundary draped across the area this afternoon with
stratus in the vicinity of the boundary and cu developing further
south. Ceilings have generally been below 2kft on the stratus with
the cu developing at a higher level between 3 and 4kft. The
boundary will transition to a warm front this afternoon and begin
to lift north...with the lower stratus likely maintaining over
northern portions of central Indiana. Further south...expect
continued cu development within a weakly unstable airmass.
Potential for isolated convective development during the second
half of the afternoon as well...especially at KIND and KLAF.
Confidence levels in convective impacts are too low to consider
any more than a VCSH mention at KLAF but will continue to monitor.

Scattered showers or perhaps a few rumbles of thunder remain
possible this evening over northern portions of central Indiana
before the front shifts north of the region late tonight. Model
soundings indicate potential for a reestablishment of MVFR
ceilings during the predawn hours and into Friday morning. Winds
will begin to increase substantially Friday morning as an intense
low pressure system tracks into the mid Mississippi Valley and
eventually the Great Lakes. Expect southwest winds to become gusty
early Friday morning and continue for much of the day...with max
gusts in excess of 30-35kts becoming likely.

Threat for stronger convection ahead of the cold front will impact
the terminals late afternoon into the evening Friday.





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