344 
FXUS63 KIND 071651
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1151 AM EST Thu Feb 7 2019

.UPDATE...
The AVIATION Section has been updated below.

&&

.SYNOPSIS...
Issued at 321 AM EST Thu Feb 7 2019

Heavy rain and the potential for a few severe storms are expected 
today ahead of a powerful cold front set to pass through the region 
late today into this evening. Gusty winds and rapidly falling 
temperatures will follow tonight in the wake of the frontal passage. 
Dry and much colder weather returns Friday through the first part of 
the weekend...but a storm system will bring the return of an 
unsettled pattern Sunday into the middle of next week. The first 
system will bring the potential for light snow accumulations Sunday 
and Monday. A second and stronger storm system will impact the Ohio 
Valley for Tuesday with rain...possibly changing back to snow 
Tuesday night and Wednesday as colder air returns.

&&

.NEAR TERM /Rest of Today/...
Issued at 1000 AM EST Thu Feb 7 2019

Some fog remains across central Indiana as well as lower 
visibilities due to rain and storms, but dense fog has ended and 
allowed the dense fog advisory to expire at 8 am. Heavy rain threat 
will continue through the day, with some isolated severe storms 
possible but widespread threat for severe looks less likely given 
all the rain inhibiting development of instability. Previous 
discussion follows...

Central Indiana set to see a bit of everything over the next 18 
hours or so in what very much feels like the proverbial kitchen sink 
getting chucked at us. Amplified upper level trough will swing east 
out of the northern Rockies eventually intensifying by tonight as it 
tracks into the upper Midwest. At the surface...the warm front over 
the lower Ohio Valley this morning will lift north through the area 
in response to a strengthening low pressure system lifting out of 
the southern PLains this morning. This system will track into the 
Great Lakes by late day and pull a powerful cold front through the 
area by early evening. Heavy rain and flooding...storms and a 
conditional severe threat...fog...gusty winds and the onset of a 
rapid temperature tumble will all occur between now and early this 
evening courtesy of this strong and multifaceted storm system.

We will start with the fog which was touched on above. Fog will be 
most prevalent over the next 4 to 6 hours with the light wind 
flow...sharp and shallow inversion with warm air advecting into the 
region above the inversion. All in all...a classic setup for 
widespread dense fog. The most widespread fog exists over the 
southern half of the forecast area with more isolated pockets of 
dense fog further north. Decided however to issue the advisory for 
the entire area as expect more widespread dense fog will spread 
north over the next few hours as the warm front lifts out of the 
lower Ohio Valley. Suspect as more substantial rainfall and 
convection arrive from the southwest in the 09-11Z timeframe... 
should begin to see improvements in visibilities. Can always cancel 
the advisory early if need be but feel comfortable running the 
headline through 13Z.

Shifting to primary concerns for today...another round of heavy 
rainfall poised to impact the region which will only further 
exacerbate ongoing flooding and potentially worsen it in some areas. 
In addition...the convective threat has grown with a narrow window 
for severe storms from late morning through mid/late afternoon.

Model consensus developing a dual moisture fetch of Pacific and Gulf 
origin over the next few hours as the surface wave pulls out of 
north Texas and intensifies as it interacts with the strong jet 
aloft. This low level jet will only strengthen through the day and 
serve to advect a rich moisture surge into the Ohio Valley with the 
potential for precip water values to peak near 1.50 inches by this 
afternoon...above climatological peaks for early February and at 400-
500% of normal. With the unusually high moisture levels...deep low 
level convergence and diffluence aloft as the upper jet 
approaches...all signs point to waves of heavy and efficient 
rainfall until the cold front can pass late day. The addition of a 
convective component as well only accentuates the heavy rainfall 
concern.

Much of central Indiana is likely to see another 1-2 inches of rain 
in addition to what has already fallen since Tuesday evening. The 
heaviest axis of rainfall should focus near and south of I-70 and 
think some areas may see locally higher amounts than 2 inches if any 
convective training sets up. Already plenty of flooding ongoing and 
with our ground absolutely saturated...runoff rates will increase 
and only worsen ongoing flooding. Flood Watch will continue in its 
current state today.

There are also growing signals that central Indiana will have to 
contend with the first round of strong to severe convection for 2019 
later today as parameters align for about a 3-5 hour window where 
severe potential is amplified. As the warm front surges through the 
area this morning...an area of elevated instability will follow with 
ongoing convection over the lower Ohio Valley increasing across the 
forecast area. As the region slides into the warm sector near and 
south of the warm front by mid to late morning...the arrival of the 
strengthening low level jet into the region will set the stage for 
an increasing damaging wind threat with any low topped convection 
continuing through about 20-21Z. Soundings noting wind speeds 
increasing to near 70kts if not a touch higher as low as 3kft by 
which even shallow convection should be able to tap into and pull a 
fair amount to the surface. While the damaging winds remain the 
primary severe weather concern...presence of the retreating warm 
front interacting with weak instability and a surge of higher 
helicities supports a small tornado threat into the afternoon as 
well. Best potential looks to be primarily over the Wabash Valley 
where there parameters align the best with the core of the strongest 
low level winds overhead. 

Think any conditional severe potential is over by 21Z or so as 
better moisture and stronger part of the low level jet shift off to 
the northeast. Cold front will lag the stronger convection by 1-3 
hours...passing across the area closer to sunset and just after. 
Once the front passes...the onset of immediate and intense cold 
advection will lead to rapidly falling temperatures and an uptick in 
wind gusts.

Temps...expect much of the area to make a run at the lower 60s with 
possibly mid 60s south prior to the cold front passing late 
afternoon and early evening. The warmer MAVMOS was largely a better 
fit for highs in most areas.

&&

.SHORT TERM /Tonight through Saturday Night/...
Issued at 321 AM EST Thu Feb 7 2019

Forecast challenges turn to gusty winds and falling temperatures 
tonight and a return of much colder air throughout the short term.

The front will be clear of the forecast area by 00Z tonight with 
focus shifting to the falling temperatures and gusty gradient flow 
as mentioned above courtesy of cold advection and as strong surface 
pressure gradient. Equivalent potential temp profiles go vertical 
for into the early overnight as well. which would aid in getting 
stronger boundary layer flow to ground level for several hours. 
Expect peak wind gusts to periodically approach 40 mph for several 
hours Thursday night. Will highlight this threat in the HWO and pass 
onto the dayshift to determine if a Wind Advisory may be warranted 
tonight. 

One impact that may need to be considered for the Friday morning 
commute is the potential for a flash freeze as temperatures drop 40-
45 degrees from cold frontal passage late Thursday to daybreak 
Friday. The gusty winds may help to mitigate this concern somewhat 
and dry surfaces...but this will be something to monitor. May also 
see a very brief period with a few snowflakes on the back end of the 
precip shield this evening but this will have no impact.

A strong area of high pressure approaching 1050mb will expand across 
the area from the Canadian prairies for Friday and Saturday with dry 
and much colder air returning. Expect plenty of sunshine and a 
continuation of gusty flow as the pressure gradient remains tight. 
Winds will relax Friday night as the core of the high moves into the 
region. Mainly clear conditions will continue through much of the 
rest of the forecast period as the Ohio Valley remains under the 
influence of the strong high pressure. Clouds will increase late 
Saturday night ahead of the next system poised to impact the region. 

Temps...the temperature changes from late Thursday afternoon to 
daybreak Friday are very impressive. Locations that warm into the 
lower 60s will be in the teens barely 12-15 hours later. Much colder 
air remains for the rest of the period with highs Friday and 
Saturday mainly in the 20s. Lows Friday night will drop into the 
single digits. An overall model blend worked reasonable through the 
short term. Subzero wind chills are possible for some late tonight 
through Friday night.

&&

.LONG TERM /Sunday through Wednesday/...
Issued at 227 AM EST Thu Feb 7 2019

Ensembles are in good agreement overall on the main weather
pattern during this period. Upper ridging initially over the area
will shift off to the east, as a significant trough ejects into
the Great Lakes towards the middle of next week.

Pattern looks unsettled for most of the extended, as periodic
waves eject out of approaching western trough, with the strongest
wave progged to arrive around Tuesday. Ensembles continue to 
suggest precipitation chances from Sunday through Tuesday in 
association with these waves, but differences in timing continue.
Will continue with PoPs from Sunday through Tuesday to cover 
these potential waves. 

Ensembles still indicate potential for mixed precipitation and light
accumulations of snow over parts of the area at times from Sunday
through Monday night. By Tuesday, it appears enough warm air will
get drawn into the area ahead of the strong short wave trough for
rain by Tuesday.

&&

.AVIATION /Discussion for 071800z TAF issuance/...

Issued at 1141 AM EST Thu Feb 7 2019

Thunderstorm potential will continue at all but KLAF for the next
few hours before moving off to the east. LLWS potential during
that time as well due to a low level jet. Ceilings will continue
to be low and then do some bouncing before dropping again. Cold
front (currently across central Illinois at 17z) will pass 
through from northwest to southeast this evening. Winds behind the
front will pick up out of the northwest and could see gusts of 30
to 35 kts with that. Could see VFR late during the overnight
hours, but not high confidence. 

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for INZ039>042-045>049-051>057-
060>065-067>072.

&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan/CP 
SHORT TERM...Ryan 
LONG TERM....JAS 
AVIATION...CP