Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 170446

Area Forecast Discussion
National Weather Service Indianapolis IN
1146 PM EST Tue Jan 16 2018

The AVIATION Section has been updated below.


Issued at 240 PM EST Tue Jan 16 2018

Cold and dry winter weather is expected to slowly transition to
warmer temperatures later this week. Strong high pressure over the
Central Plains is expected to drift southeast to the deep south by
Friday. This will allow a southwesterly flow of warmer air to arrive
by Friday.

The next chance for precipitation will be Sunday and Monday...
when a frontal system will bring rain to the area.
A mix with snowflakes is possible by Monday.


.NEAR TERM /Rest of Tonight/...

Issued at 1010 PM EST Tue Jan 16 2018

For the update...lowered low temperatures by a couple degrees in
most locations based on current observations, trends, and snow
cover. Increased sky cover for the next few hours especially in the
south and east to better match satellite trends and obs, and
adjusted downward in the northwest where some scattering is
occurring. With winds decreasing during the night, think lowest wind
chill values may be those seen now or over the next couple of hours,
with slightly higher values near daybreak with the light winds.
Chances for flurries will continue overnight, but coverage will be
less than earlier this evening. Previous discussion follows...

Time height sections across the area shows a mostly dry
column tonight with some saturation within lower levels. Subsidence
appears to be present within the column through the period.
Aloft...lee side ridging on the broad ridge in place over the
western United States continues to provide cold NW flow aloft along
with dry air.

GFS continues to hint at the dome of cold air remaining over
Central Indiana tonight as 850mb temps remain around -15C
overnight. Thus will again trend lows at or below the forecast
builder blend, Given the thin cloud cover across the area will
trend skies toward partly cloudy.


.SHORT TERM /Tonight through Friday/...

Issued at 240 PM EST Tue Jan 16 2018

Quiet weather is expected to persist through Friday. GFS and NAM
finally shows the dome of cold air beginning to exit the area on
Wednesday as 850mb temps begin a ridge toward -4c by 00Z Thursday.
Meanwhile at the surface...the arctic surface high begins to push
southeast to the deep south and the lower level flow responds by
becoming more southwesterly. Time heights respond with slightly
more cloud cover within the mid levels on Wednesday. By Thursday
and Friday the upper flow reveals more weak ridging aloft while
mainly a zonal flow of Pacific air persists within the lower
levels. Forecast soundings through the period fail to show much
in the way of saturation...thus will mainly trend toward a partly
cloudy sky during this period. Furthermore with the gradual warm
air advection through the period will trend temps at or above the
forecast builder blend.


.LONG TERM /Friday Night through Tuesday/...
Issued at 240 PM EST Tue Jan 16 2018

The long term portion of the forecast will be significantly warmer
as the west coast ridge breaks down and shifts eastward across the
country. This will allow a trailing trough axis to push into the
Rockies Saturday and induce cyclogenesis in the lee of the Rockies.
Warm advection and isentropic lift out ahead of this developing low
will necessitate low pops for rain showers early in the weekend,
with chances increasing late in the weekend into early next week as
a well developed low pressure system pushes northeast through the
Great Lakes and drags a cold front through the area. The vast
majority of precipitation with this system will fall as rain,
although it may turn just cold enough with the wrap around
precipitation behind the cold front for a few snow flurries.

Blended initialization handled things well and few significant
changes were required.


.AVIATION /Discussion for 170600z TAF issuance/...

Issued at 1139 PM EST Tue Jan 16 2018

Mainly MVFR ceilings at the sites to start the TAF period. Still
seeing scattered flurries around the sites but they are only very
rarely reducing visibilities below 6SM, so generally included them
as a VCSH. These flurries should taper off by daybreak. Ceilings
should rise sometime overnight or near daybreak as more scattering
occurs with high pressure building in. This is still only medium
confidence though as subsidence from the high could keep the low
level moisture trapped and keep ceilings from becoming VFR as
currently expected by around 12z or so. KLAF may start the period
VFR but then could see some bouncing between VFR and MVFR before
maintaining VFR ceilings within the next few hours.

Winds should be from 240 to 280 through the period at 4 to 10 kts.




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