540 
FXUS63 KIND 120538
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
138 AM EDT Tue Sep 12 2023

...Updated Aviation Discussion...

.Forecast Update...
Issued at 916 PM EDT Mon Sep 11 2023

A modest low level jet was bringing PWATs to near 1.5 inches, or 
near the 75th percentile, to the upper Wabash Valley. This combined 
with a cold front just northeast of Warren county, a broad upper 
trough, seen on H20 pivoting southeastward from Hudson Bay to the 
upper Midwest, and a 100+ knot upper jet, over Lake Michigan, will 
result in scattered to numerous showers, through the night, with 
activity in general spreading southeast. Raised PoPs over northwest 
sections to likely to start with off, this evening, based on radar 
trends. Also, left thunder in, through 03z, as LAPs and SPC 
mesoanalysis page are both showing weak instability with CAPES to 
400 J/kg. That said, Earth Networks Total Lightning Data was still 
not indicating any nearby strikes. With the best combo of lift and 
moisture over northwest sections and loss of solar heating, kept the 
lower scattered PoPs over areas south of I-70.

&&

.Short Term...(This evening through Tuesday)
Issued at 233 PM EDT Mon Sep 11 2023

An upper trough and cold front will bring scattered to numerous 
showers to central Indiana later this afternoon into Tuesday 
morning. Much cooler air moves in behind the cold front.

Rest of This Afternoon...

Clouds will continue to overspread the area and then thicken and 
lower. Some initial lift ahead of the cold front and approaching 
upper trough will bring some scattered showers to the northwest 
third to half of the area, mainly late this afternoon.

There will be some weak instability around, so an isolated 
thunderstorm can't be ruled out.

Temperatures will remain warm thanks to the sunshine from earlier in 
the day.

Tonight...

A relatively weak low level jet will bring in some additional 
moisture this evening. Forcing from the right rear quadrant of an 
upper jet along with broader forcing from the upper trough will 
impact mainly the northern half of central Indiana. 

This forcing combined with the increased moisture should lead to 
numerous showers across the northern half or so of the forecast 
area. Will go likely PoPs there. Weak instability may lead to a 
rumble of thunder early, mainly north.

To the south where forcing will be weaker, will keep PoPs in the 
chance category.

Clouds and rain will keep low temperatures in the 60s.

Tuesday...

The upper trough will move into the area, providing some forcing. 
The cold front will be off to the south. Best forcing with the upper 
trough will be north of central Indiana, and forcing from the cold 
front will be south of the area. 

Given this, coverage of rain will be highest outside of central 
Indiana. Will keep PoPs in chance category. Outside of the showers, 
some sprinkles or drizzle could be around with a low cloud deck in 
place behind the departing front.

Total rainfall amounts from this event will remain light, with 
amounts less than a quarter of an inch.

Clouds and cold advection will keep temperatures only in the lower 
70s for highs.

&&

.Long Term...(Tuesday night through Monday)
Issued at 233 PM EDT Mon Sep 11 2023

Climatologically normal temperatures for the long term are highs 
in the upper 70s and lows in the upper 50s. Much of the long term 
period we expect to be below normal, but not substantially. Patchy 
fog is possible Wednesday morning, but more conditional Thursday 
morning. Meteorological details and uncertainties are covered below. 

A positively tilted mid-upper trough will be passing overhead 
Tuesday night. Its warm conveyor belt, deep moisture plume and 
associated rain band will have already exited the area, however. The 
upward slope of moisture/clouds with higher latitudes should be 
exiting as well as jet entrance region and associate ascent shift 
eastward. This, along with favorable MSLP pattern and lingering low 
level moisture, should result in favorable radiative cooling 
conditions for some fog later in the night through early Wednesday 
morning. This may be enhanced some depending on the magnitude of the 
preceding rain event and resultant wet ground. Fog should be patchy 
in nature favoring normally colder low-lying terrain. Dense fog is 
possible in this pattern. 

On Wednesday, there should still be some residual low level moisture 
but cooler/drier continental air mass will steadily advect in. Some 
patchy fog potential is expected again early Thursday morning, as 
MSLP pattern supports light/calm winds. One caveat is potential for 
high-level Pacific moisture plume arriving as the upper jet buckles 
northward on the eastern end of amplifying Plains ridge.

With good radiational cooling conditions both Wednesday night and 
Thursday night, we will stray away from any bias corrected 
temperatures and favor raw/statistical model guidance. 

A warming trend will occur as ridging builds in late week. 
Uncertainty in the forecast arises by Saturday as ensemble spaghetti 
plots show increasing spread and noteworthy differences in timing 
and amplitude. In either case, temperatures won't be significantly 
impacted as this particular synoptic pattern isn't favorable for 
high-latitude cold air. Thus, the cold advection phase won't be 
substantial in magnitude. However, modest moisture return 
interacting with minor-moderate forcing with the shortwave trough 
should result in a band of rain passing through central Indiana 
sometime between late Saturday and early Monday. Once model spread 
decreases we will be able to narrow that window of opportunity for 
precipitation. Given the moisture and forcing magnitude, ensemble 
mean is generally near or below 0.10" for this precipitation event 
with few outliers. 

Later in the week of the 18th preliminary signal in the medium-range 
models is for a warming trend but no indication of a significant 
precipitation event at this time.

&&

.Aviation...(06Z TAF Issuance)
Issued at 138 AM EDT Tue Sep 12 2023

Impacts:

- MVFR to IFR cigs and vis developing overnight 
- Gradually improving vis and cigs to VFR this afternoon
- Fog likely late tonight 

Discussion:

Current satellite and radar imagery shows lower clouds and rain 
showers approaching from the west and northwest. Expect ceilings and 
vis to lower to MVFR to IFR levels over the next few hours as rain 
overspreads the region. Low clouds and vis will likely persist 
through around 12-14z as the front pushes through the region. Expect 
a gradual improving trend  with rising cigs and vis late morning 
through the afternoon as drier air advects into the region.

Winds shift to the NW this morning then to the north later today and 
tonight. Speeds of around 10 kts expected during the day, then 
diminishing tonight. High pressure settling into the region will 
likely create good conditions for fog development tonight; therefore 
Have already included fog and and MVFR conditions in KLAF, KHUF, and 
KBMG TAFs for the very end of the TAF period. Potential is there for 
IFR or LIFR conditions to develop due to patchy areas of dense fog.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Update...MK
Short Term...50
Long Term...BRB 
Aviation...CM