#BOTS! Bloomington Indiana's first snow is likely tonight and tomorrow.


.Short Term...(Today through Sunday)
Issued at 305 AM EST Sat Nov 13 2021

Many locations across central Indiana have seen their first
snowflakes of the season over the last several hours. Early this
morning...pockets of flurries continue to drift across the northeast
half of the forecast area in tandem with an upper trough axis. 08Z
temperatures were in the 30s with continued brisk westerly winds
making it feel even colder.

The warm pleasant weather enjoyed through much of the first ten days
of the month now feel like a distant memory and will feel even
further away as the weekend progresses. The first true winter-like
weekend of the season is upon us with cold gray weather expected
through Sunday with the added potential for a dusting of snow late
tonight into Sunday morning as a surface wave tracks through the
lower Great Lakes.

A deep upper level low was centered over James Bay this morning with
a sharp trough axis currently swinging through the region. This
will be replaced by subtle ridging aloft later today before another
piece of energy on the back side of the upper low pivots across the
upper Midwest and into the Ohio Valley for Sunday. At the surface...
ridging will follow in tandem for later today with the upper ridge
ahead of the aforementioned surface wave dropping out of the
northern Plains late today and across the lower Great Lakes for
Sunday. The combination of these features will be a continued
unsettled weather pattern for much of the weekend.

Scattered flurries continue early this morning but have generally
decreased in coverage as drier air aloft begins to advect into the
region. Despite the drier air expanding with the ridging as the day
progresses...model soundings and RH progs show a stout low level
inversion that will persist all day and into this evening. With
moisture trapped beneath the inversion...a low stratus deck will
remain through this evening and while the moisture remains shallow
with little to no forcing aloft...cannot rule out a few flurries or
sprinkles lingering into the afternoon. Steepening lapse rates
beneath the inversion also support this potential which should
mainly focus over the northern half of the forecast area. Brisk W/SW
winds will continue today as well though not be anywhere near as
gusty as on Friday.

The approach of the surface wave from the northwest later tonight
will bring the first measurable snowfall to much of central Indiana
focused predominantly during the predawn hours through Sunday
morning before warming through the boundary layer gradually
transitions snow back to rain. Nice mid and upper level diffluence
will develop ahead of the approaching surface low and upper trough
late tonight which in tandem with a surge of isentropic lift aided
by strengthening 850mb flow will spread precip into the region from
the west after 05-06Z. Critical thicknesses support the precip type
to be snow for much of the forecast area through daybreak Sunday
with any mixing with rain likely confined to the lower Wabash Valley
where the boundary layer will be slightly warmer. Precip type will
gradually switch to rain through midday Sunday from southwest to
northeast before snow mixes back in late day with renewed cold
advection behind a cold frontal passage.

As mentioned above...confidence continues to grow in seeing the
first measurable snowfall for much of the forecast area early Sunday
with a dusting up to perhaps around an inch with the highest amounts
across northern counties. With road temperatures still quite
warm...expect most accums will be confined to grassy and elevated
surfaces with little to no impacts to travelers. This will likely
melt as temperatures warm on Sunday. Precip will be coming to an end
by sunset Sunday as the low pulls away and high pressure begins to
build in from the west.

Temps...well below normal temperatures are expected through the
weekend. Low level thermals support undercutting guidance with highs
later today likely to struggle to 40 degrees. Sunday will end up
only subtly warmer by a couple degrees. Brisk conditions today with
the potential for higher gusts Sunday will bring a raw chilly feel
to the air. Lows tonight will drop into the upper 20s and lower 30s.

&&

.Long Term...(Sunday Night through Friday)
Issued at 305 AM EST Sat Nov 13 2021

The beginning of the long term period will be dry but chilly after a
trough exits the area and ridging takes its place. Tight pressure
gradients will continue winds gusting to around 25 kts for Sunday
night, but as the axis of the surface high settles in, winds will
drop back down to 5 to 10 kts for the start of the work week.

The airmass origin will change quickly during the first half of
the week from a chilly, westerly airmass to a warm, southerly one.
This shift in the flow comes ahead of another trough, allowing
for strong warm air advection as temperatures Tuesday and
Wednesday are expected to briefly return to the 60s. A surface
cold front associated with the trough will allow lift for the
moisture also being advected into the area. A line of rain showers
is expected to track from NW to SE through the day Wednesday and
possibly into the day Thursday. The exact timing of this system
has yet to be ironed out, but models are trending more towards
Wednesday to Wednesday night than Thursday at this time. There is
agreement on a lack of instability so will keep out any mention of
thunder for now.

Behind the front, a northwesterly airmass will return temperatures
back into the 40s and 30s, with lows possibly dipping into the 20s,
for the remainder of the period. Friday looks dry due to subsidence
passing through, but central Indiana could see additional rain and
mixed precipitation over the weekend.

&&

.Aviation...(12Z TAF Issuance)
Issued at 539 AM EST Sat Nov 13 2021

IMPACTS:
- MVFR ceilings through early evening...then returning towards
daybreak Sunday
- Periodic westerly wind gusts approaching 20kt through early
afternoon
- Snow showers impacting the terminals predawn hours Sunday

DISCUSSION: A few flurries continue to drift across central Indiana
early this morning underneath a thick MVFR stratus deck. As drier
air aloft expands into the region...the flurries should gradually
diminish through the day. However...a stout inversion will keep the
stratus deck over central Indiana all day with only modest
improvement towards sunset. Westerly winds will be in excess of
10kts into the afternoon with periodic gusts to near 20kts before
falling back by late afternoon.

Model soundings show the inversion relaxing this evening which
should enable a return to VFR ceilings into the early overnight.
Clouds will remain though as low pressure approaches from the
northwest. Precipitation will expand across central Indiana after
06Z with low level thermals supporting the precip type as snow.
Could see visibility restrictions within the light snow through
daybreak Sunday with ceilings once again returning to MVFR and
perhaps dropping to IFR or lower Sunday morning. Winds will back to
southerly towards daybreak Sunday.

No comments:

Post a Comment