IND issues Area Forecast Discussion (AFD) at Nov 27, 3:26 AM EST250
FXUS63 KIND 270826 CCA
AFDIND
Area Forecast Discussion...CORRECTED
National Weather Service Indianapolis IN
326 AM EST Wed Nov 27 2024
.KEY MESSAGES...
- Rain expected today, mixing or changing to snow at times mainly
north of I-70. Light amounts (Trace to 1") with minimal impacts
expected.
- Much colder Friday through Tuesday. Coldest days, Fri-Sat-Sun.
- Chances for light snow across southern Central Indiana Saturday
Night and Sunday. Confidence is low.
&&
.SHORT TERM (Today and tonight)...
Issued at 303 AM EST Wed Nov 27 2024
THROUGH THIS MORNING
Clouds continue to build in ahead of a short wave trough which is
currently ejecting into the Plains from the Intermountain West.
These clouds should continue to thicken and lower with time through
the morning hours. Some light echoes have appeared on radar with
increasing distance from the radome, indicating virga. Indeed, ACARS
soundings out of IND show a deep dry layer below 700mb. It will take
some time to fully saturate this layer before precipitation begins
at the surface later this afternoon. Additionally, these thick
clouds have largely prevented radiational cooling since sunset.
Temperatures remain in the mid to upper 30s across much of the area.
REGARDING RAIN AND SNOW POTENTIAL
Guidance continues to be in good agreement regarding the upcoming
storm system and its evolution. Briefly: the aforementioned
shortwave will induce cyclogenesis across the Plains and the
resulting low will track eastward passing south of Indiana later
today. The low will bring precipitation to much of central and
southern Indiana as it passes by. There has been a slight trend
towards a warmer solution which gives more rain than snow. The trend
is apparent in nearly all guidance as of 06z. Still, most models do
show some snow mixing in despite this.
Global models continue to depict a broad area of baroclinicity
stretching across Indiana from west to east. Additionally, they show
a PV anomaly riding eastward along this zone...which indicates the
potential for frontogenesis and mesoscale precipitation banding.
Since this zone is rather broad, there remains some uncertainty as
to exactly where these bands, if they form, set up. Areas that
experience such a band may see locally higher precipitation totals.
Locations under these bands may also be some of the first to see
precipitation reach the ground, which may complicate onset timing a
bit.
In terms of snowfall amounts...it's tricky. This system has no fresh
source of cold air as low pressure currently resides over eastern
Canada. In fact, low and mid-level flow ahead of the approaching
shortwave is out of the south with a slight warm air advective
component. Snow, should it occur, will be highly dependent on
diabatic and evaporative processes. It is entirely possible that
snow only occurs in the heaviest precipitation bands while rain
occurs elsewhere. Our most likely scenario is a trace to an inch
of accumulation on grassy/elevated surfaces, mainly north of
I-70. Should more intense rates occur within a mesoscale
precipitation band...then locally higher amounts are possible. In
this scenario, up to 2-3 inches is possible in a narrow corridor.
Total liquid equivalent precipitation should range from a quarter to
a half of an inch on average. There may be a sharp northern edge to
precipitation totals as a rather dry air mass is present just to our
north.
&&
.LONG TERM (Thursday through Tuesday)...
Issued at 303 AM EST Wed Nov 27 2024
Thanksgiving -
Thursday will be a transition day as the Wednesday and Wednesday
night storm system departs to the east coast. The upper flow is set
to develop into a cold northwest pattern through the day on
Thursday, which will quickly allow for the arrival of polar
continental air making its way to the Ohio Valley for much of the
next week. This will allow for the arrival of some of the coldest
air of the season thus far.
At the surface on Thursday, the cold front associated departed storm
system will push across the deep south while strong high pressure
settles across the plains. The coldest air will not yet arrive until
Thursday night. This will set up strong cold air advection amid
northerly surface flow across Indiana. Forecast soundings show
drying within the lower levels arriving on Thursday. Thus we will
aim for a partly cloudy sky on Thanksgiving, with highs in the lower
40s. Clouds will more likely be present in the morning hours as the
previous system departs. Northwest winds around 10 mph will make
wind chill temps feel rather cold.
Thursday Night through Sunday Night -
Cold air advection will begin in full swing on Thursday night and
Friday. The deep upper low over Quebec and Ontario will allow for
the continued northwest flow across the plains into the Ohio Valley.
Forecast soundings show subsidence and a dry column through this
period. The coldest air looks to arrive on Saturday into Sunday as a
core of -14C air at 850mb is shown to descend upon Central Indiana
from the northwest. Models suggest a few passing waves within the
northwest flow aloft, but forecast soundings fail to show deep
saturation at any point, with plentiful lower level dry air in place
across Indiana. Confidence is low on timing any such wave at this
point, thus the NBM may give some chc pops to denote these waves,
but any precip with them would be minimal. At this time best chances
for any precipitation would be Saturday and Sunday nights. Again,
confidence is low for precip, but very high for cold temperatures.
Monday and Tuesday -
Overall, little change is seen within the upper pattern on Monday
and Tuesday, with strong ridging still remaining in place over the
west coast and a deep upper low over Ontario. This will bring
continued northwest flow to Central Indiana aloft. Within the lower
levels models begin to shift some of the coldest air off to the
northeast, suggesting a warming trend. Highs may recover to the 30s
on these days, but with arctic surface high pressure settled over
the plains the northerly flow is expected to continue. Thus will try
and continue here with a dry, partly cloudy type forecast, with
below normal temperatures.
&&
.AVIATION (06Z TAF Issuance)...
Issued at 1243 AM EST Wed Nov 27 2024
Impacts:
- VFR Conditions are expected into Wednesday Afternoon
- Rain/snow expected Wednesday evening
- MVFR conditions likely once rain/snow begins, brief IFR possible
in snow.
Discussion:
Mid to high level clouds continue to build in from the west. Some
virga is possible at times, and radar shows some light echoes with
increasing distance from the radome. Expect ceilings to slowly lower
with time through the morning hours.
By 21z, guidance shows light precipitation breaking out across
central Indiana. Most terminals will begin as rain, but could mix
with snow should a heavier band of precipitation set up overhead.
By 00z, a changeover to all snow is possible which could further
deteriorate flying conditions. Ceilings should be pushing into MVFR
territory by this time as well.
Precip likely comes to an end between 06-09z for most terminals,
which may be just after the end of the TAF period for some.
Winds should be light and variable for the first 6 hours of the TAF
period, becoming southeasterly as the day progresses. Winds
gradually gain a northerly component as low pressure passes to our
south, strengthening to around 10kt from the north northwest by
Thursday morning.
&&
.IND WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
SHORT TERM...Eckhoff
LONG TERM...Puma
AVIATION...Eckhoff
Source:
IND issues Area Forecast Discussion (AFD) at Nov 27, 3:26 AM EST---------------
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