591 
FXUS63 KIND 241957
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
257 PM EST Tue Nov 24 2020

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

&&

.SYNOPSIS...
Issued at 257 PM EST Tue Nov 24 2020

Rain will become widespread late tonight and continue Wednesday as a 
strong storm system moves across the region. The potential exists 
for a few stronger storms as well ahead of the cold front Wednesday 
afternoon and early evening. Dry weather will then return for 
Thanksgiving into the weekend. Low pressure will track into the Ohio 
Valley late weekend into early next weekend with the potential for 
more precipitation and perhaps the chance for some light snow by 
Monday.

&&

.NEAR TERM /Tonight/...
Issued at 257 PM EST Tue Nov 24 2020

Rain has moved well off to the north and east of the region this 
afternoon...leaving mainly cloudy skies across central Indiana. A 
cirrus shield blanketed much of the southern half of the forecast 
area with filtered sunshine. 19Z temperatures ranged from the lower 
40s north to the lower 50s across south central Indiana.

Much of the next 10 hours or so through the evening will be quiet as 
drier air advects across the Ohio Valley from the south on the back 
side of the surface high pressure off the mid Atlantic coast. 
Expansive cirrus shield referenced above will persist across the 
region and maintain a mainly cloudy sky for the rest of the daylight 
hours and into the evening. Occasional gusts to around 20mph will 
continue through sunset before dropping back a bit.

After midnight...the weather will become quite a bit more active as 
low pressure currently swinging across the Texas panhandle ejects 
northeast towards the Missouri Valley by early Wednesday. The 
expansion of a strong low level jet into the Ohio Valley during the 
predawn hours will transport deep moisture into the region with rain 
becoming widespread across the entire forecast area after 06Z. 
Rainfall rates will steadily increase towards daybreak Wednesday as 
the 850mb jet energy intensifies and lift strengthens. The heaviest 
swath of rainfall should focus along and south of the I-70 corridor 
but the entire region will get in on the rain. Not out of the 
question to see an embedded rumble or two across the lower Wabash 
Valley prior to daybreak as well with model soundings hinting at 
some elevated instability across the lower Ohio Valley.

Temps...lows will be reached this evening with temperatures slowly 
rising overnight as warm advection strengthens with the onset of the 
widespread rainfall. Expect temperatures to bottom out in the lower 
and middle 40s for much of the areas before rising about 5 degrees 
across the board by daybreak.

&&

.SHORT TERM /Wednesday and Wednesday Night/...
Issued at 257 PM EST Tue Nov 24 2020

Forecast challenges focus on the continued high impacts from the 
aforementioned low pressure system...set to track from the Missouri 
Valley across the region by Wednesday evening. Locally heavy wind 
whipped rainfall...scattered convection and a potential for a few 
strong storms ahead of the low and cold front late Wednesday 
afternoon will make for an overall miserable day for the region.

The onset of the widespread and steadier rainfall noted above after 
06Z will continue Wednesday morning with lift peaking over the area 
as the core of a 70kt low level jet tracks through the region. 
Enhancement aloft will develop with a dual upper level jet structure 
placing the region squarely in the sweet spot between the jets while 
providing excellent diffluence in the mid and upper levels that will 
only accentuate rainfall rates through the morning. The presence of 
some elevated instability as well will support embedded thunder 
across the region through early afternoon. While streams are 
generally doing fine...the potential for additional rainfall amounts 
of an inch or greater in spots coming just a few days after the 
weekend rain will likely lead to localized flooding in spots.

Model guidance in general has sped up the approach of the 
intensifying negative tilt upper level low pressure and the 
associated surface wave by a few hours from previous runs. This now 
takes the bulk of the lift with the low level jet east of the region 
by 18Z with the widespread rainfall shifting east as well. The 
arrival of the surface wave now by late day poses an interesting 
scenario that has raised the ceiling subtly but noticeably for 
scattered stronger convection impacting parts of the forecast area 
during the late afternoon and early evening. 

Virtually all of the CAMs today have pinpointed a 4 hour window from 
roughly about 20Z through 00Z where a narrow axis of weak 
instability lifts into the region ahead of the surface wave and 
front. While the instability will be a limiting factor as it tends 
to be this time of year...the amount of available shear and helicity 
present is well within the bounds of that supportive of a low 
CAPE/high shear QLCS setup. In addition...soundings show a short 
period where the thermodynamic structure becomes supportive of 
steeper lapse rates and dry adiabatic flow which would be more 
efficient in pulling stronger boundary layer winds to the surface 
with any stronger cell. Anticipate a thin broken line of low topped 
convection eventually coalescing as it traverses east with the 
potential for localized stronger wind gusts and perhaps a brief 
tornado as well with hodographs showing a 1-2 hour period aligned 
with solid directional shear in the lower levels. The overall 
confidence in this scenario playing out remains low but as mentioned 
above...the low and front coming into the area late day as opposed 
to Wednesday night has raised the ceiling for scattered strong 
convection. Expect the primary area of concern for storms will be 
across the southeast half of central Indiana...generally near and 
south/east of a Terre Haute-Indy Metro-Muncie line.

Any threat for stronger convection will be east of the forecast area 
by 00Z with lingering showers gradually diminishing Wednesday night 
as the system moves away to the east. Gusty winds peaking at 30-
35mph Wednesday afternoon should also gradually drop back Wednesday 
night as the surface pressure gradient relaxes. Model soundings 
indicate a low level inversion strengthening Wednesday night with 
clouds likely to linger into Thanksgiving morning.

Temps...the surge of warm moist air courtesy of strong southerly 
winds Wednesday will push temperatures into the upper 50s and lower 
60s during the afternoon over much of the region. Behind the system 
Wednesday night...lows will fall into the 40s.

&&

.LONG TERM /Wednesday Night through Monday/...
Issued at 355 AM EST Tue Nov 24 2020

Long term will start off with a an occluded front extending from a 
surface low near Rensselaer to a triple point near Kokomo 00z 
Thursday. The associated cold front will be moving east to near the 
Interstate-65 corridor and a sharp upper trough will be following 
near the Illinois border and ready to plow through central Indiana.

Already limited instability will be dying with sunset and support 
from the northeastward marching front entrance region of a 115 
knot 250 millibar jet may only aid eastern or northeastern 
sections of central Indiana through 01z or 02z. But, 1000-500 
millibar bulk shear magnitude of 40 knots and some instability at 
the start suggests a few storms could remain reasonably strong 
over northeastern sections for a couple of hours after 00z Friday.
Otherwise, good confidence convection will end quickly southwest 
to northeast as the system departs. Model blend PoPs look great on
trends with likely PoPs NE and slight chance PoPs SW through 06z 
and then dry after as surface high pressure moves in from the 
southern states and an upper ridge moves into the Great Lakes and 
southern Plains. 

Thanksgiving will be a dry with above normal temperatures in the 
50s, under the influence of surface high pressure. Friday will 
remain relatively mild, but then a cold front with limited moisture 
will serve mainly to bring in slightly cooler temperatures for the 
remainder of the weekend. A strong cold front that will have more 
moisture to work with could bring more precipitation in by Sunday. 
Some snow could mix in early Sunday and Sunday night into Monday, 
but do not expect much if any snow accumulation.

&&

.AVIATION /Discussion for 241800Z TAF Issuance/...
Issued at 1156 AM EST Tue Nov 24 2020

IMPACTS: 
- Southwest winds gusting to 20kts this afternoon. 
- Low level wind shear developing this evening and lingering into
  Wednesday morning. 
- Widespread rain after 06Z through early  Wednesday afternoon 
  with occasional IFR conditions Wednesday afternoon. 
- Scattered convection possible as well...especially Wednesday 
  afternoon. 
- Southerly winds gusting to near 30 kts by late 
  Wednesday morning.

DISCUSSION: 
After a relatively quiet afternoon and evening...active pattern 
developing for aviators tonight into Wednesday as strong low 
pressure tracks through the Ohio Valley. Expect widespread rain 
with embedded thunder after 06Z Wednesday continuing through early
Wednesday afternoon. IFR conditions are possible near and beyond 
daybreak. Scattered convection is possible between 18Z and 00Z 
Wednesday with southerly winds gusting up to 30kts ahead of a cold
front which will pass the terminals early Wednesday evening.

CONFIDENCE:
- High Confidence in low level wind shear concerns from late 
  evening into Wednesday morning.
- High Confidence in widespread rainfall late tonight and
  Wednesday
- Low to Medium Confidence in embedded thunder late tonight into
  Wednesday
- High Confidence in wind gusts this afternoon and again on
  Wednesday.

&&

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

&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan
SHORT TERM...Ryan
LONG TERM...MK
AVIATION...Ryan