Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/27/23

Area Forecast Discussion

National Weather Service Indianapolis IN

 1035 PM EST Tue Dec 26 2023

.Key Messages...

- Fog Possible tonight

- Chances for light rain and snow Wednesday night through Friday

- Best chances (10-25%) for accumulating snow (0.1-0.5in) will be
  over SW central Indiana late Wednesday night into Thursday
  morning on elevated surfaces

- Seasonable temperatures return by Wednesday and persist

&&

.Forecast Update...
Issued at 1035 PM EST Tue Dec 26 2023

No major changes made to the forecast with this evening`s update.

An elongated area of high pressure is centered over the region
tonight while a stacked area of low pressure spins over the Central
Plains. Subsidence under the high and low level dry air advection
around the occluded low have resulted in clearing low level clouds
this evening across all of Central Indiana. Latest IND ACARs
sounding does show a saturated layer developing around 500mb due to
mid and upper level moisture advection increasing ahead of the low.
These high clouds appear to have minimal effects on radiational
cooling so far at this hour as dew point depressions continue to
drop at all sites and RH values rise into the 90s to near 100
percent. Will be watching for fog developing during the overnight
hours and towards sunrise for portions of the Wabash Valley. These
higher clouds may limit how widespread fog is later tonight. Looks
like the best chance for fog will be from Kokomo to Lafayette, down
to Terre Haute, and any low lying wind sheltered areas, the usual
foggy locations.

Mainly quiet weather expected during the day tomorrow with highs in
the mid to upper 40s across the state. Clouds will be on the
increase ahead of the approaching system to the west. Refer to the
Long Term portion of the discussion for more information on
potential impacts Wednesday night into Thursday from this system.

&&

.Short Term...(This evening through Wednesday)
Issued at 302 PM EST Tue Dec 26 2023

Surface Analysis early this afternoon shows an area of deep low
pressure along the KS/NB state line. The deep low was providing
broad cyclonic flow aloft across the region, including mild
southerly flow across the Ohio Valley and Central Indiana. An area
of high pressure was found over the lower Mississippi Valley with a
ridge axis extending northward into western KY and the Wabash
Valley. Water vapor shows the deep, also nearly vertically stacked
low over the center of the United States. The water vapor imagery
shows the moist conveyor belt within this system has shifted to
points east of Central Indiana, approaching the Appalachians.
Meanwhile, dry air on southwesterly flow aloft was intruding into
the system aloft, with subsidence building near the surface ridge
axis over southern IL, W TN and W KY. This dry air was surging
northeastward toward Central Indiana. The back edge of cloud cover
was found over SW central Indiana.

Tonight...

The upper low to the west is not expected to make much progress
eastward overnight as the models suggest it only to drift to NW MO
as it remains cut-off from the faster steering flows aloft. This
will allow for the continued steady progress of the dry intrusion
across Central Indiana through the evening. This will lead to a SW
to northeast progression of the clearing line across Central
Indiana, with skies eventually becoming partly cloudy to mostly
clear. Forecast soundings show a dry column arriving across the
region and the 925 RH fields also show decreasing values in
conjunction with the arriving dry air intrusion. Thus we will trend
toward mostly clear tonight. Models show a meso surface high
pressure system setting up over Central Indiana, an evolution of the
arriving surface ridge. The combination of clear skies and light
winds will make for good radiational cooling effects, leading to fog
development. Current projected dew point depressions are suggested
to fall to 2F or less, thus some patchy fog with isolated dense fog
overnight will be possible toward diurnal minimums. Lows in the
upper 20s and lower 30s will be expected.

Wednesday...

Models suggest the upper low will make some slow progress toward
Central Indiana reaching southwest parts of the state by late
afternoon. The previously mentioned surface high will dissipate and
get nudged eastward as the surface low approaches and the dry inflow
to the broad cyclone begins to get cut off. This will mean a return
of cloud cover to Central Indiana as the lower level flow becomes
more cyclonic. Forecast soundings through the day fail to become
deeply saturated as we see mainly a return of upper level moisture
as the low approaches. Dry air remains present within the lower
levels, as moisture remains limited. Thus some morning fog will be
expected early, but as heating resumes through the day, this fog
should burn off leading to some peaks of sunshine. However, that
should be short lived as the upper low begins to arrive in the
afternoon, leading to a return of high clouds. Thus a true mix of
clouds and sun. Cold air advection will be in place, leading to
highs in the middle to upper 40s.

&&

.Long Term...(Wednesday night through Tuesday)
Issued at 302 PM EST Tue Dec 26 2023

The beginning of the long term will pose the greatest impacts to
central Indiana through the next 7 days with a broad deep closed low
passage Wednesday night through Friday. Many uncertainties still
remain, including chances for measurable snowfall, of which is
outlined below.

Wednesday Night into Thursday:

The first chance for precipitation will be over this time period as
a 900-800mb low pressure centroid passes over SW Indiana. This
should create a modest deformation zone, with enough lift to tap
into the low level saturated profile. The low will have a slight
negative tile aloft by the time it reaches Indiana, but will be
stacked enough for dendrite growth within 850-700mb layer as 6-6.5
C/km lapse rates push this saturated layer below -8C. As visualized
in the text above, the forcing and dendritic growth layers are not
exactly aligned leading to lower QPF, but should still be enough for
light to moderate precipitation rates.

Latest ensemble guidance is less variable on low track with most
members placing the deformation zone within a 50-100mile radius over
SW Indiana. This still leaves enough uncertainty though to
complicate the forecast as the deformation zone will be narrow
leading to a sharp cutoff on QPF amounts. For now, this will be
broadened to accommodate the variability, but keep in mind that QPF
could shift as confidence increases.

The other variable increasing uncertainty is near to just above
freezing surface temperatures. Temperatures aloft will be sufficient
for dendrite growth (as explained earlier), but with precipitation
onset near to just after sunset on Wednesday, surface temperatures
may be too high for ice crystals, resulting initially in rain. With
that said, wet bulb temperatures near the freezing point will aid in
increasing snow flake to rain drop ratios as precipitation continues
and surface temperatures evaporatively cool.

In totality, this leads to a rather complicated forecast for SW
central Indiana Wednesday night into Thursday morning. The current
expectation is for precipitation to start as rain, and transition
towards a rain/snow mix overnight, but the band of precipitation may
end up further south, avoiding our SW counties. Rates may be
moderate at times as frontogenetical forcings overlap the growing
saturated layer, but in general should remain light. Ground temps
above freezing will lower overall impacts, but if rates maintain
intensity, pockets of light accumulations could occur. 10th to 50th
Snowfall Percentile still points towards a null event with most
areas 0-T (most likely scenario), but the 90th Percentile showcases
the upper end potential with max values near 1 inch.

Thursday PM through Friday:

This time period will also pose a marginal snowfall threat, but in
this instance, over all of central Indiana. As the closed low moves
eastward, the WCB will wrap around the low and tap into a higher
moisture source over the Mid-Atlantic region. The will create a
quasi-TROWAL north of the low. Vertical lift will be rather weak
initially on Thursday, but by Thursday night, narrowing of the
upstream ridge will intensify the NE/E mid level jet creating
pockets of vertical lift over central Indiana. With both forcing and
moisture occurring on the mesoscale, it is difficult to pin point
when and where precipitation will occur, but there is enough of each
for small bands to potentially initiate.

Once again, surface temperatures will be a factor with daytime highs
on Thursday in the upper 30s and overnight lows Thursday night near
freezing. Any precipitation that does fall during daylight will fall
as rain, but after dusk a mixed precipitation phase is likely. Web
bulb temperatures Thursday night also support the transition to snow
within areas that experience prolonged evaporative cooling. Ground
temperatures will likely be above freezing, and rates should be weak
enough to avoid impacts with any snowfall only accumulating on
elevated and grassy surfaces.

Saturday through Monday:

High pressure will build in over the weekend as ridging develops
aloft. this will end the chances of precipitation. With minimal
temperatures advection behind the closed low, temperatures should be
near seasonal with current forecast highs in the upper 30s to low
40s across the region.

&&

.Aviation...(00Z TAF Issuance)
Issued at 649 PM EST Tue Dec 26 2023

Impacts:

- Chance for MVFR/IFR Fog overnight

Discussion:

Latest satellite imagery shows a large dry slot, associated with an
occluded low over the Plains, pushing through Central Indiana this
evening. Dry air has resulted in MVFR cigs to break up with all
sites in Central Indiana now reporting VFR cigs as clouds continue
to break up. Expect the clearing trend to continue over the next few
hours.

Elongated area of high pressure directly over the region may create
optimal conditions for radiational cooling and fog development
overnight and towards sunrise. Latest IND ACARS soundings already
shows a strong inversion just under 1 km agl, which should work to
keep low level moisture trapped near the surface. This set up can
lead to patchy fog development across the region, especially in
known fog prone areas, such as the Wabash River Valley, KLAF, KHUF,
and KBMG. Included Tempos for reduced visibility down to 2 to 3
miles towards sunrise. Confidence on where and when fog develops is
somewhat low; however hi-res short term guidance suggests the best
time for fog development after 08z and through around 13z Wednesday
morning. Fog is expected to lift and dissipate through the late
morning hours Wednesday with VFR conditions expected for the
remainder of the day.

Winds will remain light through the period becoming almost calm
overnight then northeasterly under 8-10 kts tomorrow afternoon.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Update...CM
Short Term...Puma
Long Term...Updike
Aviation...CM