Scientific Forecaster Discussion
fxus63 kind 151454
Area forecast discussion
National Weather Service Indianapolis in
954 am EST sun Dec 15 2019
the near term and aviation sections have been updated below.
issued at 336 am EST sun Dec 15 2019
A frontal boundary will align just south of the Ohio River by late
today and remain nearly stationary through Monday night before
shifting away from the area. Deeper moisture will lift up and over
the front and across the area tonight and again late Monday and
Monday night...bringing two waves of wintry precipitation along with
accumulating snowfall for much of central Indiana. High pressure
will return with cold and dry weather Tuesday through the remainder
of the upcoming week.
Near term /rest of today/...
issued at 954 am EST sun Dec 15 2019
Forecast is in good shape. Some partial clearing has occurred this
morning, but this will change as mid and high clouds increase ahead
of the system today.
Tweaked pops based on latest hi-res model data, but overall trend
still looks good. Some of the latest models are shifting things
north a bit, so will have to keep an eye on this. Strong
frontogenesis will impact southern areas close to 00z, which could
lead to some brief intense snow bands.
Temperatures will be warm enough south for a mix of rain and snow to
start with, but the strong forcing will cool the column and allow
the precip to change to all snow.
Previous discussion follows...
Cloudy morning ongoing across the region as weak high pressure has
built in. Monitoring some scattering of the clouds over central
Illinois that may pass through the region later this morning. 08z
temperatures were generally in the 20s to lower 30s.
Much of today is setting The Table for the first higher impact
winter storm of the season which is likely to come in two parts...
the first of which will focus tonight.
The high pressure ridge extending from the upper Midwest into the
Ohio Valley will serve as the dominant feature for the region
through the first half of the day. Model soundings and relative humidity progs do
suggest some scattering of the clouds during the morning with a few
peaks of sunshine before clouds thicken back up midday into the
Focus will shift to an expanding area of precipitation currently
over the Central Plains early this morning and expected to surge
east across the Missouri and into the mid Mississippi Valley through
midday. This is associated with an area of enhanced lift north of a
developing surface wave and warm frontal boundary that will
eventually align from the Red River valley east/NE into the lower Ohio
Valley by late day.
Light precip will reach the lower Wabash valley by mid afternoon
then slowly but steadily expand east/NE through the early evening. Dry
air through the boundary layer will stunt precip initially before
evaporative cooling can promote a gradual top down saturation that
should enable increasing precip coverage by late afternoon and early
evening. While some rain will mix in initially...precip should
transition to predominantly snow with some light accumulations
developing over the lower Wabash valley prior to 00z as deeper
isentropic lift arrives. The approach of a strong low level jet will
only further enhance the lift into the evening and set the stage for
a period of moderate to locally heavier snowfall after 00z which
will be addressed in greater detail in the short term section below.
Temps...an abundance of clouds followed by the onset of evaporative
cooling later today should keep surface temps cooler than originally
thought. Guidance is capturing this well with highs in the low to
mid 30s likely by early to mid afternoon.
Short term /tonight through Tuesday night/...
issued at 336 am EST sun Dec 15 2019
Forecast challenges focus on details with a two part winter storm
for the region...with the highest impacts poised for this evening
and overnight followed by another round late Monday into Monday
Another 12-24 hours of model runs with the addition of the hi-res
data going out far enough has helped to clarify the details and
raise confidence on a number of parameters with the winter storm set
to impact central Indiana. Still some lingering questions which will
be discussed below...but confidence levels are high enough now to
address specific snowfall amounts...degree of precip mixing and
where this is most likely...and necessary adjustments to headlines
both in area and type.
Strong model agreement now in place with respect to location of the
frontal boundary tonight and Monday as it aligns from East Texas
northeast into Kentucky by Monday morning. Waves will track along
the boundary with a final wave Monday night helping to shift the
boundary south and east...and offer the secondary shot of snow to
the region which will pad amounts a bit from the first shot this
evening into the overnight.
The first wave will be ongoing at 00z tonight as discussed in the
near term section with precip overspreading the southern half of the
forecast area. With the boundary layer fully saturated by the
evening...growing signs at a 6 hour window or so where snowfall
amounts and rates may rip as the low level jet noses into the region
and interacts with the boundary. North of the front across the
southern half of the forecast area...a strong deformation band in
the 850-700mb layer will shift east/NE across the region into the early
overnight and further aid in a period with moderate to locally heavy
snowfall. Individual models still are fluctuating a bit on the
specific location of the deformation band and associated
frontogenesis which will impact where the heaviest snow falls...but
there is model consensus that the band will align across the
forecast area and likely in the vicinity of the I-70 corridor.
Will hit snow hardest from 00-06z with rates and coverage dropping
off during the predawn hours as the deeper forcing and the low level
jet shift off to the east. An overall colder solution noted via the
model soundings and boundary layer temps suggest any mixing with
freezing rain tonight will be confined to southern counties with any
icing generally under a tenth of an inch through daybreak Monday.
May see pockets of freezing drizzle lift north into the I-70
corridor closer to daybreak with the loss of more substantial
lift...but this should do little to impact expected snowfall totals
It now appears that there will be an extended period with little to
no precipitation from the predawn hours into Monday afternoon and
possibly all the way into the early evening hours as deeper forcing
refocuses closer to the boundary over Kentucky. Certainly cannot
rule out drizzle or a few areas of light snow. With surface temps
warming into the 30s...the snowpack laid down tonight will likely
slush up and compact a bit. The secondary wave that will force the
boundary south and east Monday night alluded to above will bring the
secondary round of predominant snow to the area as a deformation
band again aligns northwest of the surface low. Additional forcing
aloft ahead of an upper trough will aid in a period of snow Monday
night into Tuesday morning with additional accumulations that will
inflate the amounts that fall tonight. Snow will end Tuesday morning
with high pressure building in and clouds scattering through the
course of the day. Another front will swing into the area early
Wednesday with a reinforcing shot of cold Arctic air.
Considering all of the thoughts above...expect a broad swath of 2 to
4 inches of snow bisecting the forecast area tonight with the
highest axis near the I-70 corridor. Most of this will likely fall
in about a 6 hour period through the evening and into the early
overnight. The back side deformation snows Monday night could add an
additional 1 to possibly as much as 2 inches before snow ends
Tuesday morning. Any icing is expected to be largely confined to
southern counties and should remain under a tenth of an inch.
With the above thoughts...have decided to convert the current Winter
Storm Watch to a Winter Weather Advisory and expand it to encompass
the entire forecast area. While total snowfall amounts may near 6
inches within the heaviest axis of snow expected near the I-70
corridor...the fact of this coming in two separate parts tonight and
Monday night and minimal mixing with any freezing rain really does
not offer any justification to introduce a warning. Additionally...
amounts at or above 6 inches will likely be the exception and not the
norm. Will bump the start time for the advisory back to 22z this
evening and will continue to run it through Tuesday morning. Despite
the extended break with little precip on Monday...feel it is simpler
to just run with a long advisory to account for the potential for
additional snow Monday night. It should be stressed that going with
an advisory is not a downgrade to the watch but better fits the
expectations from this storm laid out above.
Temps...generally undercut guidance for high temperatures Monday and
Tuesday with an overall model blend for lows. Lows Tuesday night
will dip into the teens and could go colder with the addition of a
Long term /Wednesday through Saturday/...
issued at 240 am EST sun Dec 15 2019
Ensembles are in good agreement through most of the extended
period, as a transient upper ridge drifts through the area. By
next weekend, the ensembles indicate another upper trough will
move into the Midwest. Majority of the ensembles keep the more
significant energy closer to the Gulf Coast, although there are a
few that are farther north. If the farther north solutions come to
fruition, accumulating snow may be possible towards next weekend.
Will keep the extended dry for now, and monitor the trends with
respect to this next trough.
Aviation /discussion for the 151500z taf update/...
issued at 954 am EST sun Dec 15 2019
Lower cloud has exited kind so have removed these ceilings. Looks
like mainly mid and high clouds for much the time into afternoon
before lower clouds increase again.
Previous discussion follows...
MVFR ceilings around 020 beginning to scatter out over parts of
western Indiana. Short term models suggests these lower ceilings
should scatter out at the terminals by the mid to late morning
Otherwise, expecting increasing layered cloud above 050 later
today. IFR/LIFR visibility restrictions due to snow may begin
affecting the khuf/khuf terminals around sunset. These
restrictions are expected to spread over the remaining terminals
Surface winds 270-290 degrees at 7-10 kts this morning will
diminish around midday, and eventually become 090-120 degrees at
4-7 kts by sunset.
Winter Weather Advisory from 5 PM this afternoon to 7 am EST
Tuesday for inz021-028>031-035>049-051>057-060>065-067>072.