Area Forecast Discussion
Issued by NWS Indianapolis, IN

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905
FXUS63 KIND 171508
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1008 AM EST Sat Feb 17 2018

.UPDATE...
Near Term and Aviation Sections have been updated below.

&&

.SYNOPSIS...
Issued at 321 AM EST Sat Feb 17 2018

Scattered light rain...possibly mixed with snow...will impact
central Indiana later today and this evening as a quick moving
system passes through the Tennessee Valley. High pressure will bring
a brief return to dry weather tonight and Sunday. Several factors
coming together for the potential for an extended period of heavy
rain and flooding beginning early Monday and continuing through at
least the middle of the upcoming week as a frontal boundary becomes
stalled over the region. Low pressure tracking into the region next
weekend may bring another round of heavier rain.

&&

.NEAR TERM /Rest of Today/...
Issued at 1008 AM EST Sat Feb 17 2018

.Update...Mixed rain and show over southern Illinois will spread
east across southern Indiana next few hours.  Precipitation farther
north will move in with the arrival of the northern stream wave later
this afternoon.

Main changes with this update were to tweak temperatures cooler
some areas this afternoon and to add mixed rain snow to those areas
with a surface temperature in the mid 30s or lower.   Also raised
POPS slightly some areas this afternoon and eastern sections this
evening per high res model.

Portion of previous near term discussion follows... The combination
of a surface wave tracking through the Tennessee Valley later today
followed by a northern stream upper wave rotating through the lower
Great Lakes will bring a threat for light precip for much of the day
over the southern half of the forecast area and later this afternoon
further north.

Bulk of the more substantial forcing associated with the surface
wave passing through the Tennessee Valley will remain south of the
forecast area...with precip largely dependent weak isentropic lift
and forcing from the upper level energy traversing the region late
day. Initially...a fairly stout dry layer in the lower levels will
need to be overcome and the presence of the drier air and lingering
subsidence is likely to be a limiting factor in northward extension
of precip for late morning and early afternoon. Evaporative cooling
processes will serve to saturate the boundary layer and consequently
will support light snow as the initial and primary precip type over
southern counties this morning based on model sounding analysis. As
the near surface layer warms through the day though...should see a
transition to light rain as the precip shield expands north and
east.

The arrival of the northern stream wave aloft by late afternoon will
bring additional light precip into northern counties which will last
into the evening. Considering expected temps through the near
surface layer and antecedent ground temps...any snow that occurs
throughout the day will have no impact across the region.

Temps...an overall model blend looked very reasonable for high temps
later today based on low level thermals. Most locations will rise
into the upper 30s to lower 40s this afternoon.

&&

.SHORT TERM /Tonight through Monday Night/...
Issued at 321 AM EST Sat Feb 17 2018

Forecast challenges primarily will focus on the beginning of an
extended and potentially significant period of heavy rain and
flooding set to impact the Ohio Valley late Sunday night through the
middle of the upcoming week.

The aforementioned upper wave should be east of the region by mid
evening with any lingering precipitation wrapping up by midnight.
Rain will remain the primary precip type based on low level thermals
but as the boundary layer cools during the evening...the potential
for light snow to mix in will increase before ending...especially
over northeast counties. Again...no impacts are anticipated.

Skies will clear from west to east overnight as high pressure
reestablishes over the Ohio Valley. The high will track east to the
Mid Atlantic region on Sunday but deep subsidence will linger for
much of the day as return flow begins to establish in wake of the
departing high. The result will be the best weather day of the next
several days highlighted by mostly sunny skies and milder
temperatures. Mid and high level clouds will increase towards late
day but more substantial cloud cover will hold off until after dark.

The upper level flow regime across the country will transition back
to more amplified pattern by the end of the weekend as a strong
upper low over the Pacific Northwest establishes a strengthening
upper trough over the western U S which will only amplify further
through midweek. On the flip side...an anomalously strong upper
ridge for this time of year will move west and entrench off the
southeast coast by late Monday into Tuesday. In between these two
features...a broad southwest flow energized by a juiced subtropical
jet will align from Baja California northeast across the southern
Plains...Arklatex region and up into the Ohio Valley. With moisture
origin from the equatorial Pacific in addition to the Gulf of
Mexico...growing confidence exists in the potential for an
exceptionally rich moisture plume to advect into the Ohio Valley by
Monday and continue into the middle of the week.

The deep moisture fetch will interact with a slowly eastward
progressing cold front that will eventually stall or become nearly
stationary just to the northwest of the region by late day Monday as
its eastward movement is effectively blocked by the strong upper
ridge off the southeast coast. With a strong low level jet in excess
of 50-60kts aligning on the warm side of the boundary by Monday
afternoon...expect rain to become widespread on Monday and Monday
night with the potential for locally heavy rainfall developing late
Monday into Monday night and potentially beyond as well. Precip
water values by late Monday will be around 1.4 inches...in the
vicinity of the climatological max for this time of year and in
excess of 300% of normal.

High pops are warranted beginning early Monday and continuing
through the end of the period...with highest pops focused over the
northwest half of the forecast area in closer proximity to the
positioning of the low level jet. The presence of weak instability
aloft warrants an embedded thunder mention as well...especially by
Monday night.

For more on the developing heavy rain and flooding threat...see the
hydrology section below.

Temps...Developing warm advection on Sunday warrants pushing highs
above guidance by a few degrees as many locations will warm into the
lower 50s. Even warmer air will transport into the region Monday
courtesy of the strong low level jet with highs surging into the
60s. A model blend looked reasonable for low temperatures through
the period.

&&

.LONG TERM /Tuesday through Friday/...
Issued at 142 AM EST Sat Feb 17 2018

ECMWF suggests a very active...warm and wet period of weather next
week. THE ECMWF suggests SW flow in place across Indiana aloft for
much of next week. Strong HIgh pressure is expected to be parked
across east of the Carolinas while a deep trough remains in place
over the western United States. This may even allow this SW flow to
develop tropical plumes...enhancing water vapor transport into the
Ohio Valley for much of next week. ECMWF then also ejects a few weak
short waves at Indiana embedded within the SW flow next week. Timing
and and confidence in these individual waves remains low at this
time...however...more features still remain in play for precip
development. Looking in the lower levels...A cold front is suggested
to sag across Indian a on Tuesday night into Wednesday...before
stalling near the Ohio Valley. This frontal Boundary will remain a
focus for precip development next week...providing good lower level
convergence while moisture and short wave stream aloft. Thus with a
favorable set-up for rain for much of next week...pops insert by
forecast build are quite high and this appears reasonable. However
as stated earlier...confidence in specific timing remains low.

Give the Warm and Moist SW flow...mainly above normal temperatures
will be expected this period.

&&

.AVIATION /Discussion for the 171500Z KIND Taf Update/...
Issued at 1008 AM EST Sat Feb 17 2018

Minor tweaks made to KIND TAF to better match current conditions.
No other changes are needed at this time.  Previous aviation
discussion follows...

VFR Conditions are expected to deteriorate to MVFR this afternoon
and then IFR tonight.

A Short wave over the Central Plains is expected to push into the
Ohio Valley this afternoon and exit the area tonight. Ample
forcing should provide sct showers as the wave passes after 18Z.
Forecast soundings show marginal saturation and Time heights show
best saturation limited to the lower levels. Confidence for
specific time is still not present...however a period of VCSH
after 18Z appears very reasonable as the forcing passes. Cooler
high pressure is expected to arrive in the area after 00Z with
some cold air advection leading to some lingering trapped stratocu
this evening.

Forecast soundings go dry this overnight as the surface high
builds across the area...and a return to VFR will be possible.


&&

.HYDROLOGY...
Issued at 321 AM EST Sat Feb 17 2018

Confidence continues to grow in a high impact period with heavy
rain producing areal and river flooding for much of the first
half of the upcoming week and possibly beyond. Several factors
support the potential for heavy rainfall including broad southwest
flow aloft transporting deep moisture into the Ohio Valley...a
strong low level jet over the region...a nearly stationary frontal
boundary and precipitable water values in excess of the
climatological max and peaking at nearly 400% of normal.

Rain will begin early Monday and continue through Wednesday.
Potential exists for rainfall amounts up to 3 to 4 inches in
spots during this timeframe. Will convey the threat via the
HWO...ESF and social media graphics at this time...but these
amounts and the potential flooding concerns will likely warrant a
need for flood watches eventually over parts or all of central
Indiana.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan/JH
SHORT TERM...Ryan
LONG TERM....Puma
AVIATION...JP
HYDROLOGY...RYAN/JH



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