Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/18/24
Area Forecast Discussion
National Weather Service Indianapolis IN
956 PM EST Wed Jan 17 2024
.Key Messages...
- Light snow Thursday morning for North Central Indiana, minor
accumulations under an inch expected
- Another round of light snow Thursday night into Friday morning
with minor additional accumulations expected (1 to 3 inches total)
- Arctic air returns this weekend, lows near zero Friday and
Saturday night with below zero wind chills
&&
.Forecast Update...
Issued at 956 PM EST Wed Jan 17 2024
Observations (including ACARS sounding at KIND) show a very dry
lower atmosphere this evening. More moisture is in the mid and high
levels, as could be seen on satellite imagery.
Strong winds in the mid levels are already creating some lift across
northern portions of the area and across northern Indiana, but this
has just created some of the mid clouds thanks to the dry atmosphere
below.
Coupled upper jet streaks along with a decent low level jet will
impact the area later tonight, producing lift aloft. It will take a
while to moisten up the lower atmosphere though, resulting in virga
for a while. Eventually, moisture will be enough so that some snow
may reach the surface, mainly in the northwestern forecast area.
Will keep some chance category PoPs northwest, with dry conditions
southeast. Little if any accumulation of snow is expected.
Southerly winds and increasing clouds have kept temperatures in the
20s this evening. With thickening clouds overnight, nudged up low
temperatures given recent trends.
For past 12Z Thursday, uncertainty remains in the forcing and
moisture profiles. However, with the coupled jet streaks around
early, this may provide enough forcing for the likely PoPs in the
far northwest 12-15Z or so. Thus, have made no changes, and will
continue to monitor.
&&
.Short Term...(This evening through Thursday)
Issued at 302 PM EST Wed Jan 17 2024
Focus shifts to the next chance for light snow across the region
late tonight into Thursday morning. Little change made to the
overall forecast for the tonight and tomorrow period. An elongated
area of low pressure in the Plains shifts eastward tonight extending
northeastward into the Great Lakes. At the same time, Indiana will
be placed in an area of enhanced lift within the right entrance
region of a departing jet streak and in the left exit region of a
small jet streak developing over the Lower Ohio Valley. An
increasing low level jet (LLJ) ahead of this system will work to pump
in lower level moisture northward into the state through the
overnight hours. BUFKIT soundings indicate cloud bases gradually
lowering through the night as saturation through the column
increases. Rising motion at the nose of the low level jet and within
the upper jet streak couplet will promote deeper saturation through
the column and in the dendritic growth zone. By 3-6 AM tomorrow
morning, precipitation processes within a growing iso-thermal layer
will lead to light snowfall over northern portions of central
Indiana. This forcing will slowly move to the SE along the LLJ axis
but will weaken as diurnal LLJ strength wanes. Efficient dendritic
growth will likely lead to high SLRs and some accumulation even with
only a few hundredths of QPF. The current expectation with this
round is 0.5 to 1.0 inches, with the highest snow totals over
northern portions. With a jet couplet setting up over the region in
addition to a strong LLJ, potential is there for some isolated areas
of heavier snowfall to set up in bands. The most likely area for
this to set up is across far north Central Indiana. Expect slick
travel conditions Thursday morning, especially north of I-70.
The best forcing for ascent weakens during the afternoon hours as
the LLJ wanes and the jet streak couplet pushes off to the east.
Soundings still indicate the potential for enough left over moisture
behind the departing system for scattered flurries and/or drizzle
through the rest of the afternoon. With southerly flow working to
scour out the retreating arctic airmass, Thursday may be the
"warmest" day of the week. While the pressure gradient and LLJ
weaken, weak southerly flow will in the lower levels will result in
highs in the mid 20s across the north and highs trying to reach the
freezing mark in South Central Indiana.
Another, stronger, system will be approaching Thursday evening into
Friday giving another chance for minor snow accumulations. See the
Long Term Forecast for more details on that next system.
&&
.Long Term...(Thursday night through Wednesday)
Issued at 302 PM EST Wed Jan 17 2024
The primary forecast challenge through the long range continues to
be cold temperatures, potential snowfall, and a pattern flip towards
the end of the period.
SYNOPTIC OVERVIEW
Quasizonal upper-level flow exists across the CONUS today, with
slight cyclonic curvature east of the Rockies. A large upper-level
low pressure system is currently spinning across eastern Canada as
well. Strongly negative teleconnections (NAO and AO) have lead to a
blocking pattern which has caused this upper low and flow pattern to
remain in place for the past several days. With the primary axis of
the polar jet just to our south, arctic air has flowed freely
southward contributing to the very cold weather we have seen.
Continued blocking is shown by most guidance, so no relief from the
very cold weather is expected through the weekend.
A potent vort max embedded in the mean flow has made it onshore over
the Pacific Northwest yesterday evening, and is expected to dive
southeastward towards Indiana tomorrow. Favorable jet dynamics aloft
(left exit), vorticity advection, and isentropic lift in the lower
levels all point toward precipitation formation ahead of this
system. Model soundings show saturation in through the column
beginning Thursday evening continuing into Friday. Additionally, the
profile through the dendritic growth zone is deep and contains a
bulk of the upward motion. Dendrite production could be quite
efficient, which may lead to higher ratios and thus snowfall rates.
However, given the fast-nature of the system, maximum snowfall
potential is likely limited to some extent.
The various teleconnections do turn positive, however, by early next
week. As such, a pattern shift is beginning to appear more likely.
Developing split flow over the west coast is likely to initiate the
shift, as a branch of the jet stream heads northeast into Canada and
another dives southward into the southern Plains. Ridging builds in
response across the southeastern US. Given our position relative to
these features, southwesterly flow through the column is to prevail
much of next week. With multiple waves rounding the southern stream
trough over the Plains and then lifting northeastward within the
southwesterly flow...Indiana is in a good spot for repeated rounds
of precipitation throughout the coming week. Temperature profiles
look warm, so mainly rain is expected though a few hours of freezing
rain or sleet is possible at onset Monday night if the retreating
cold is a bit too stubborn.
WEATHER HAZARDS AND IMPACTS
Regarding snowfall potential, widespread light to moderate snowfall
appears likely across all of the area Thursday night into Friday.
Heaviest snow should be concentrated along and just to the north of
the parent vort max, which looks to take a track across central
Illinois through central Indiana. As such, our northern counties
should see the highest snowfall amounts with decreasing totals to
the south. There has been a noticeable trend in latest guidance
towards the north with this feature...with reduced amounts from
Indianapolis southward. Should this trend continue, then even less
snow is possible than currently forecast. Overall, from 1 to 3
inches, with a south to north gradient, is the current most likely
scenario Thursday night into Friday. A reasonable worst-case
scenario would be a stripe of 4 to 5 inches, mainly across our
north.
Regarding temps, strong cold air advection is shown in all guidance
regardless of snowfall totals. Additionally strong winds of near 30
knots are depicted by various CAMs. Temperatures falling back into
the teens and single digits, combined with the wind, may lead to
prolonged sub-zero wind chills yet again Friday into Saturday.
Sunday morning has the potential to be the coldest start, as surface
high pressure slides in leading to great radiational cooling
potential with fresh snow cover in place. Below zero ambient air
temperatures are not out of the question.
FORECAST CONFIDENCE
Models are in unanimous agreement with the vort max diving southward
tomorrow, bringing snow to the region. Confidence for snow is high.
As for amounts, that is a different story. As mentioned before,
there has been a noticeable trend northward today within various
guidance. Given that guidance still hasn`t settled on a track just
yet...confidence in snowfall totals is only moderate. Still, even
with the more northerly track at least an inch should fall across
most of the Indy metro area northward. Confidence with cold and wind
behind the system are high, and with the light fluffy high ratio
nature of the snow it may get blown around quite easily.
Next week, NAEFS and ECMWF situational awareness tables continue to
show 99th percentile, or higher, 500mb heights and PWATs. Given the
range we are looking at, this is quiet impressive and implies
ensemble agreement in an anomalous set up. Teleconnections trending
positive add weight to this, and confidence is unusually high
regarding a pattern shift to warmer and potentially wetter
conditions next week. Confidence is low regarding the fine details
such as the track of individual waves of low pressure and
precipitation timing/totals.
&&
.Aviation...(00Z TAF Issuance)
Issued at 556 PM EST Wed Jan 17 2024
Impacts:
- Low level wind shear overnight for most sites
- Areas of light snow Thursday, with KLAF most likely to see ceiling
and visibility impacts
- More widespread snow and lower ceilings/visibility after 00Z Friday
Discussion:
Much of the first half of the TAF period will see some mid and high
clouds. A low level jet will aid in producing marginal low level
wind shear at most sites after 03Z (ending by 15Z or so).
Light snow will develop late tonight and impact mainly KLAF in the
12-15Z time period Thursday. This will bring MVFR conditions to
that site. Other areas of light snow will be across central Indiana
as well from 12-18Z Thursday. Confidence in coverage and location
of these areas is low, leading to just a VCSH mention at the other
sites. Any snow could bring MVFR conditions.
Additional more widespread and impactful snow will develop Thursday
night.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
Update...50
Short Term...CM
Long Term...Eckhoff
Aviation...50