187 
FXUS63 KIND 292032
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
332 PM EST Tue Dec 29 2020

.UPDATE...
The SYNOPSIS...NEAR TERM...SHORT TERM...LONG TERM and AVIATION Sections
have been updated below.

&&

.SYNOPSIS...
Issued at 332 PM EST Tue Dec 29 2020

A large two part storm system will impact central Indiana beginning 
late tonight and continuing into the first part of the weekend. The 
first part will be associated with a frontal boundary moving through 
the area...bringing rain early Wednesday into Thursday morning with 
perhaps a light wintry mix to end. After a lull in precipitation 
Thursday...the second part of the system will impact the region 
Thursday night and Friday with locally heavy rainfall and an 
increased threat for flooding. There is also potential for light 
icing across northern parts of central Indiana Thursday night before 
temperatures warm. Precipitation chances will continue into Saturday 
with drier weather returning late weekend into next week.

&&

.NEAR TERM /Rest of This Afternoon and Tonight/...
Issued at 332 PM EST Tue Dec 29 2020

Clouds continue to thicken this afternoon as moisture increases 
ahead of an approaching warm front. Still a few peaks of sun left in 
eastern counties but even there expect clouds will take over by 21-
22Z. Afternoon temperatures were in the 30s.

Much of the near term will focus on table setting ahead of the first 
part of the multi-wave storm system set to impact the region into 
the first part of the weekend. The approach and passage of the warm 
front over the next several hours into this evening will bring a 
small threat for very light precip largely as sprinkles and/or 
flurries as a narrow band of moisture aligns with a stronger axis of 
isentropic lift at 290K and 295K levels grazing the northern Wabash 
Valley. Model soundings continue to show a broad layer of very dry 
air present below 10kft which should keep most precip developing 
near that axis of isentropic lift from reaching the ground which 
radar returns currently is ongoing over central illinois. Will carry 
slight chance pops for a few hours over far northern counties 
through about mid evening with dry weather returning thereafter as 
the warm front and associated lift shift north of the forecast area.

The rest of the tonight will largely remain dry until the predawn 
hours as the deeper moisture plume associated with the cold front 
approaches from the west and northwest. Stronger isentropic lift 
will begin to overspread central Indiana after 09Z with a strong low 
level jet moving into the region. Will see rain increase 
substantially over the northwest half of the forecast area in the 09-
12Z time period as a result. Winds will become gusty later tonight 
as well with the approach of the front.

Temps...temperatures will drop back briefly this evening before 
rising overnight as warm advection takes over. Expect many locations 
in the 40-45 range by daybreak Wednesday.

&&

.SHORT TERM /Wednesday through Friday/...
Issued at 332 PM EST Tue Dec 29 2020

Forecast challenges focus on the large two part storm system set to 
impact the region into the first part of the upcoming weekend.

Model guidance remains in relatively good agreement on the main 
features over the next few days...although there has been a subtle 
shift back to the southeast with the second system set to track 
through the area on Friday. More on that below.

An amplifying upper low will track across the northern Great Lakes 
on Wednesday before shearing out further to the northeast Wednesday 
night. This feature has come in stronger over the last day or two 
and will aid in pushing the cold front across the region late 
Wednesday and Wednesday night in tandem with a surface low tracking 
into the Great Lakes. 

Widespread rainfall will be ongoing across the northwest half of 
central Indiana at daybreak Wednesday and should encompass the rest 
of the area through midday. A combination of strong isentropic 
lift...broad diffluence aloft and a 60kt+ low level jet will 
transport deep moisture into the region will aid in widespread 
coverage of steady rainfall all day as the front moves into the 
forecast area from the west. The front will continue to progress 
southeast slowly Wednesday night with precipitation continuing along 
it. As colder air advects into the region from the north behind the 
front...there remains concern at areas of light mixing of precip 
types on the northern flank of the rain...most likely in the form of 
light snow. Any amounts that accumulate will be minimal...at 
generally just a few tenths of an inch and likely focused over the 
southern half of the forecast area in closer proximity to the 
retreating rainfall and deeper moisture. Expect little to no issues 
from a wintry mix through Thursday morning.

The front will shift south of the Ohio River on Thursday with a 
brief but stronger area of high pressure spreading into the area. 
Now anticipating an extended period of mainly dry weather across all 
of central Indiana for much of Thursday  Model soundings and RH 
progs continue do keep plenty of mid level moisture over the region 
and even as lower clouds depart in the morning...they are likely to 
be replaced by thicker clouds aloft as the second part of the system 
begins to organize over east Texas. The result will be a relatively 
cloudy and chilly New Years Eve for most with at least some 
potential for scattered light precip focused over the southern half 
of the forecast area.

The second surface wave will ride northeast from the northwest Gulf 
of Mexico in tandem with a strengthening upper wave lifting 
northeast into the Missouri Valley early Friday and then into the 
Great Lakes Friday night. With a strong Gulf fetch developing on the 
lee side of the surface wave...widespread precipitation will spread 
north quickly Thursday night. Initially...soundings show a deep 
layer of subsidence and drier air through the lowest 10-15kft which 
will take time to saturate...likely needing stronger lift coming 
north with an intense low level jet that will peak over the Ohio 
Valley early Friday. Surface temperatures are likely to slip back to 
just below freezing levels Thursday evening...especially over the 
northern half of central Indiana. While a brief period of sleet near 
onset is quite plausible considering the evaporative cooling that 
will be ongoing to saturate the boundary layer...expect precip type 
to transition relatively quickly to liquid...with freezing rain 
becoming a concern for the northern counties as temperatures will 
likely be at 30-32. Warm advection will steadily strengthen 
overnight through daybreak Friday with the approach of the low level 
jet...but potential is there for some light icing generally north of 
an Attica-northern Indy suburbs-New Castle/Muncie line. Considering 
temps just below freezing and saturated ground from rains late 
tonight into Wednesday...icing impacts are likely to be somewhat 
mitigated.

Precipitation will fall as all rain Friday with the surface wave 
tracking up into the region. Expect continued widespread rainfall 
with the 70+kt low level jet traversing the area. There has been 
a subtle increase in elevated instability across the area Friday that 
may eventually warrant the inclusion of isolated thunder into the 
forecast. For now...will keep thunder out and continuing to monitor 
trends. Rainfall coverage will become more scattered by late day 
Friday as the low lifts through the area. By Friday evening...am 
expecting 1-2 inches of rain with locally higher amounts total from 
the two part system.

Temps...low level thermals support leaning towards the warmer MAV 
guidance for highs Wednesday ranging from the mid 40s to the lower 
50s. As the front moves into the area from the west during the 
afternoon temps will begin to fall. Much cooler on Thursday with 
temperatures likely to hold in the mid and upper 30s. Undercut 
guidance with expected cloud cover all day. Friday has now come in 
warmer with low level thermals supporting highs for the first day of 
2021 in the 50s and maybe even threatening 60 degrees in southern 
counties. Expect lows mainly from the upper 20s to the mid 30s 
through the period. 

&&

.LONG TERM /Friday Night through Tuesday/...
Issued at 332 PM EST Tue Dec 29 2020

The period should start as the long-duration storm system's second 
part is passing central Indiana...transitioning steady rain to only 
slight chances of light rain/drizzle.  At least moderate breezes 
should continue Friday night as the gradient surrounding the 
system's occlusion moves off into the eastern Great Lakes. 
Precipitation should remain all rain as the main event tapers off 
Friday night with temperatures staying above freezing.  

Despite the main storm system's departure to our northeast, broad 
upstream troughiness over the southern Plains will then slowly 
cross the Mid-West through the remainder of the weekend. The broad
remnant H700 dry conveyor from the strong occluded front of 
Friday's system should cut off any further Gulf moisture from 
subsequently reaching Indiana...and moderate confidence exists 
best mid-level PVA should stay to the south side of the CWA. While
these features appear to be guiding NBM POPs <10%...decided to 
introduce slight chances of precipitation across much of the 
region Saturday from pre- dawn to evening hours. The remnant cold 
conveyor will include a broad ribbon of mid-level RH over 
90%...and it appears the rearrangement of the atmosphere into a 
weak open-wave pattern should have just enough surface convergence
to shake out a few light rain/snow showers...as temperatures 
likely stay above 32F.

Weak high pressure will build back into the region Sunday night, 
finally bringing a return to partly/mostly clear skies. Light to 
moderate south to southwesterly breezes and generally partly cloudy 
skies early next week will then allow a slow moderation into above 
normal readings, as dry conditions prevail.  High temperatures 
should be in the upper 30s to low 40s this weekend...and the 40s 
early next week. Overnight lows are expected to be in the mid to 
upper 30's Friday night...and the mid 20's to low 30's Saturday 
night through Monday night.

&&

.AVIATION /Discussion for 292100Z TAF Update/... 
Issued at 332 PM EST Tue Dec 29 2020

Only minor adjustments needed for mid afternoon update. 18Z
discussion follows.

IMPACTS:
-LLWS 40-55 kts 6-18z
-MVFR ceilings by 10-12z
-Rain begins 10-14z
-IFR ceilings with steadier rain arriving by 14-17z

DISCUSSION:
Significant rain event on tap for Wednesday across central Indiana
will be preceded by thickening clouds today...although MVFR
ceilings should not arrive until pre-dawn. Rain coverage and
intensity will increase through the morning, deteriorating flight
rules to IFR ceilings (and MVFR visibility), which should last
though 18z. LLWS developing 6-11z, should then drop below 40kt 
during 15-18z. KIND is expected to return to MVFR after 18z.

Winds will continue at generally 7-13 kts with gusts as high as 
20 kts along/north of I-70 through 12z...while continuing to veer 
from the current ESE/SE direction to southerly by 9z. Winds will 
increase Wednesday morning to 10-15 kts with gusts 20-25
kts...while further veering to SSW/SW.

CONFIDENCE:
-High confidence in winds shifting to southerly late tonight
-High confidence in MVFR ceilings arriving with rain by 12z
-Moderate confidence in IFR ceilings beginning 14z-17z
-Moderate confidence in 45-50 kt LLWS 11-18z at KIND...and in
 40-50 kt LLWS 8z through at least 15z at all other TAF sites

&&

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

&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan
SHORT TERM...Ryan
LONG TERM....AGM
AVIATION...AGM/Ryan