956
FXUS63 KIWX 170623
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
123 AM EST Fri Jan 17 2025

.KEY MESSAGES...

- Above freezing temperatures allow snowmelt between this
  afternoon and Friday.

- Any initial rain Friday evening changes over to snow Friday
  night. Minimal accumulation is expected.

- Dangerous cold begins filtering into the area Sunday and lasts
  into the middle of next week. Wind chills of 20 below zero
  expected especially Sunday night through Tuesday night. Lake
  effect snow is expected to occur during this time as well.

&&

.DISCUSSION...
Issued at 217 PM EST Thu Jan 16 2025

A main shortwave that moved through this morning and caused a brief
impulse of lake effect snow continues to move away and that allows
the sputtering and starting of snow to shut off. With southwest
winds taking over and warm advection on the back of surface high
pressure taking over, expect high temperatures to warm back into the
30s and reaching the mid 30s, which is warmer than today`s low 30s.
Expect a dry day Friday as well.

Friday night, after sunset, two areas of low pressure approach the
area with one passing by to our north and another passing by to our
south. Given that we are starting the afternoon with mid 30
temperatures, we`ll be starting above freezing. However, it does
appear that wetbulbing will be able to commence to take us down to
snow tonight fairly quickly. Still can`t completely rule out slick
spots either from where rain lingers longer to allow for some icing
or from where snow starts to stick on roads. Given the marginal
temperatures, this should act to restrict the amount of snow
accumulation though to less than 1 inch. The main system snow ends
Saturday morning into Midday time frame, but a period of lake effect
snow on the back side is possible. Given the dry column, lake effect
snow accumulations should stay less than an inch.

Saturday into Sunday, an upper low pressure system dives southward
into Canada. As it does so, it`ll allow some of its arctic cold air
to seep into the region. 850 mb temperatures drop from -10C at this
time to around -28C or so during the day Tuesday. These 850 mb
temperatures are 2 to 3 SDs below zero which normalized to
climatology. These are readings seen on a return interval of 1 day
every 10 years or so when compared to GEFS M-climate. A common
indication of cold air is the high pressure system`s central
pressure that accompanies it as it enters the CONUS and that is 1050
mb, which is above the normal 1040 mb or so range one looks for. By
the time we reach 12z on Sunday, Lake MI water temperatures around
32 or 33 degrees allow for lake effect snow to take shape. Inversion
heights do slowly rise to around 7000 ft or around 775 mb and delta
T values reach 30 degrees Sunday afternoon. One issue may be times
of low level dryness and times of neutral or positive theta-e lapse
rates that could cut into lake effect snow efficiency, but low res
models may also be underdoing these as well. As this cold air seeps
in, we`ll need to parse out which period may be most conducive to
hazardous travel conditions as a result. One initial period where
more intense snow could be observed is Tuesday as some of the
strongest cold arrives in conjunction with a shortwave moving
through Lake MI.

As far as the cold air that looks to arrive between Sunday and early
next week goes... Cold surface temperatures down in the teens on
Sunday, will become single digits above and below zero on Monday and
Tuesday. Combined with wind gusts between 20 and 30 mph at times,
this creates wind chill values that surpass cold weather advisory
threshold (-15F) and approach cold weather warning (-25) threshold
Sunday night through Tuesday morning. Another period of cold weather
advisory temperatures will be possible Tuesday night too. This is
around 50-55 hrs of cold weather advisory criteria or worse
temperatures. There are also periods of cloudless nights forecast
south of the lake effect snow where radiational cooling may allow
for even colder temperatures. Temperatures begin to moderate mid to
late week next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 122 AM EST Fri Jan 17 2025

An expansive low level anticyclone departing the Ohio Valley
will allow for stronger southerly flow to develop today. This
moisture transport will take some time to track northward with
strongest moisture transport affecting northwest Indiana this
evening. This southerly low level jet should produce some
marginal LLWS conditions from mid afternoon through tonight with
numerical guidance suggesting 2000 foot winds approaching 45-50
knots. For tonight, a complex scenario in terms of systems to
monitor with a southern stream trough working across the Lower
MS Valley and a broader northern stream trough working across
the western Great Lakes. Initial dry air could support some
brief snow via evaporative cooling before advective trends take
precip types to rain/snow or perhaps a period of just rain
before changing to snow on back end of this system late Friday
night into early Saturday. VFR conditions will persist through
most of this period, with deteriorating conditions expected
mainly just after this forecast valid period.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory until 4 AM EST early this morning for
     LMZ043-046.

&&

$$

DISCUSSION...Roller
AVIATION...Marsili