IND issues Area Forecast Discussion (AFD) at Dec 10, 3:24 AM EST909
FXUS63 KIND 100824
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
324 AM EST Tue Dec 10 2024
.KEY MESSAGES...
- Areas of fog possible early this morning
- Scattered high impact heavy snow showers and squalls possible
Wednesday afternoon and evening
- Briefly turning much colder Wednesday Night and Thursday
- Rain chances and near normal temperatures return for this weekend
&&
.SHORT TERM (Today through Wednesday Night)...
Issued at 323 AM EST Tue Dec 10 2024
Mid level clouds remain prevalent across the region early this
morning...with an axis of lower stratus expanding north into the
forecast area. Just north of the stratus deck...pockets of fog have
developed. Webcams suggest this has not become widespread to this
point but it does appear to be steadily expanding to the west. Temps
at 08Z ranged from the upper 30s in the northern Wabash Valley to
the lower and mid 40s over the southern half of central Indiana.
The next 48 hours will see a transition back to winter weather as a
meridional upper level trough pivots out of the northern and central
Plains and into the region by Wednesday strengthening as it moves
east. There will be a couple narrow windows for light precipitation
later this morning and again late tonight focused largely over the
southeast half of the forecast area in closest proximity to a nearby
frontal boundary and baroclinic zone. Then as the trough axis
amplifies and builds into the region on Wednesday...potential
continues to grow for a period with a locally high impact snow
showers/squalls during the afternoon and early evening.
Today and Tonight
The initial issue is the fog potential over the next few hours
mentioned above. Fog is most pronounced across western and central
Ohio but there has been a gradual spread to the west into the
forecast area over the last few hours just north of the expanding
stratus deck. There remains uncertainty in just how far northwest
the stratus will come especially with an axis of drier air within
the boundary layer noted from southeast lower Michigan back to the
southwest into eastern Illinois. Considering the light to calm winds
in the near surface layer and the potential for the mid level clouds
to remain the prevalent deck...have nudged up fog coverage for the
next several hours. Should coverage of lower visibilities become
more widespread may consider an SPS.
The expansion of the lower clouds is in response to a surface wave
riding along the frontal boundary to our southeast. The low will
lift out of the western Tennessee Valley northeast into Ohio by mid
afternoon. Stratus should eventually encompass the southeast half of
central Indiana later this morning with the potential for a few
light showers as well into the afternoon. To the northwest...mid
level clouds will remain prevalent and there may be brief periods of
filtered sunshine at times.
Skies will remain mostly cloudy tonight with RH progs showing
continued low and mid level moisture streaming across the region
from the southwest. An axis of mid level frontogenesis will lift
northeast into the Ohio Valley quickly overnight and may generate a
narrow area of light precipitation focused across southeast portions
of the forecast area between 06Z and 12Z Wednesday. Cold advection
will increase as the night progresses with low level thermal
profiles becoming supportive for snow as the precip type. At this
point...precipitation looks light with perhaps a tenth of an inch at
most as the axis of frontogenesis moves through the region.
Wednesday and Wednesday Night
A narrow surface ridge will follow in the wake of the mid level
frontogenesis that moves through the Ohio Valley in the predawn
hours Wednesday. Surface flow will back to the west-southwest as
well which will diminish the cold advection at least temporarily
along with setting up a relatively quiet start to the day.
By midday Wednesday however focus will shift to the amplifying upper
level trough that even appears to take on a bit of a negative tilt
as a deep upper level low dives south out of the Canadian prairies.
At the surface...a sharp cold front will rapidly approach from the
northwest Wednesday afternoon and a sharply colder Arctic
airmass will follow with its passage.
The approach of the front and intensifying mid and upper level
forcing with the amplifying trough will combine with very steep
lapse rates likely to peak near 550mb and weak instability to
produce a convective snow response ahead of the front. While the
overall setup is not the classic snow shower/squall scenario...the
intensity to the steepening lapse rates and the presence of weak
instability in the vicinity of a strong Arctic boundary supports the
potential for scattered but potentially intense snow bursts and/or
squalls across the forecast area focused on the mid afternoon to
early evening timeframe. With temperatures in the near surface and
ground layer hovering near freezing then dropping by early
evening...concerns are increasing for a high impact/low snowfall
event that will coincide with the evening rush hour when traffic
volume is higher.
The strength of the front and the intensity of the cold advection
immediately behind it will produce strong wind gusts that may
approach Wind Advisory criteria for a short period of time across
the northern half of the forecast area. The gusty winds will create
drastically reduced visibilities under the heavier bursts of snow
which will further cause problems on area roads. Once snow ends
during the evening...could see a few locations that remain under
snow bursts the longest make a run at an inch...but most areas will
see less than a half inch. Will continue to ramp up our messaging
for the impact concerns from snow bursts/squalls Wednesday afternoon
and evening and further refine details in future forecasts today and
tonight.
The strongest wind gusts will drop back quickly by late evening but
it will remain brisk Wednesday night as the much colder air
encompasses the region. Skies will clear Wednesday night with high
pressure building into the region.
Temps...the initial surge of cold advection today will produce a
modest gradient in the 40s form northwest to southeast across the
forecast area. Lows tonight will fall back into the mid and upper
20s. On Wednesday...temperatures will rise during the first half of
the day into the low and mid 30s before falling back as the Arctic
front arrives in the afternoon. Lows will fall into the teens
Wednesday night and with the brisk northwest winds continuing all
night...subzero wind chills are possible over the northern half of
the forecast area.
&&
.LONG TERM (Thursday through Monday)...
Issued at 323 AM EST Tue Dec 10 2024
The long term period will see multiple passing shortwaves, bringing
a couple chances for rain. Frigid temperatures on Thursday, with
highs in the 20s to near 30, will start off the period, but will
quickly warm back to near normal for Friday and beyond as more
southerly winds dominate. By Friday, quick upper ridging will move
through, aiding in the warmer temperatures and a dry start to the
weekend.
While models lack agreement on timing and precipitation amounts for
the weekend and beyond, there is at least agreement on a short wave
moving through over the weekend and again early next week.
The first wave should bring rain across central Indiana sometimes
between Friday night through Sunday, depending on the timing of the
associated front. It is also unclear on the track the surface low
will take. Global models do show that it should track north of the
forecast area but unsure just how far north which will impact where
the highest precip totals fall. Currently leaning towards central
Indiana falling in the quarter inch to three quarter inch range for
the weekend. Should the precip fall either Friday or Saturday
nights, it is also possible that snow could mix in for some areas
across the north.
Another quick ridge and surface high will then move through,
allowing for dry weather and temperatures to stay steady, before the
region is hit with another upper wave. Again, it is unclear the
timing or detail with this wave, but should bring a line of showers
through early next week.
&&
.AVIATION (06Z TAF Issuance)...
Issued at 1218 AM EST Tue Dec 10 2024
Impacts:
- MVFR ceilings expand north overnight with pockets of IFR and lower
in fog
- Greatest threat for patchy fog through daybreak at KBMG and KIND
- VFR conditions return by midday
Discussion:
Lower stratus gradually expanding northeast into southern parts of
Indiana early this morning in advance of a surface wave tracking out
of the western Tennessee Valley. This feature may bring a few light
showers as far north as KBMG over the next few hours otherwise rain
should remain southeast of the terminals. While confidence is medium
to high in MVFR ceilings returning...there is much lower confidence
in the potential for fog and perhaps patchy dense fog to form through
daybreak. Already seeing IFR and lower visibilities develop across
southwest Ohio and southeast Indiana and there is some potential for
that to expand northwest towards KBMG and KIND over the next few
hours. Will monitor and adjust as needed.
Low level moisture will linger through midday with an inversion
persisting in the near surface layer. As the day progresses
however...lower stratus will become more scattered with a mid level
deck becoming predominant. Expect VFR conditions for all four
terminals from mid afternoon through tonight.
Light and variable winds through daybreak will gradually become
northwest at around 10kts this afternoon.
&&
.IND WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
SHORT TERM...Ryan
LONG TERM...KF
AVIATION...Ryan
Source:
IND issues Area Forecast Discussion (AFD) at Dec 10, 3:24 AM EST---------------
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