Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
432 am EDT Tuesday Mar 11 2014
the aviation section has been updated below.
issued at 423 am EDT Tuesday Mar 11 2014
A warm springlike day will give way to a strong system
that will bring rain and then snow to the area tonight and
Wednesday. Strong winds with the system will be in place on
Wednesday as well. Quiet weather with the exception of slight rain
chances Friday night until Sunday when another storm system arrives
bringing chances for mixed precipitation through Monday.
Near term /today/...
Issued at 423 am EDT Tuesday Mar 11 2014
Quiet day before the storm arrives under warm advection. Should see
some clouds from advection but these will do little to limit warming
as 850 mb temperatures climb to around 10c. Look for highs in the
60s to around 70.
Short term /tonight through Thursday night/...
Issued at 423 am EDT Tuesday Mar 11 2014
Forecast focus is on storm system tonight through Wednesday. NAM is
a northern outlier with the low track so generally disregarded it.
Fairly similar features seen in GFS and 12z European model (ecmwf) so generally
averaged them...but discussion will focus on specifics seen in the
At 6z GFS shows strong frontogenesis across the central and southern
counties with temperature profiles well above freezing and a
southwesterly lljet of around 50 kts. Decent isentropic lift from 0
to 6z with this feature. With elevated instability available
continued with mention of slight chance for thunder.
850-700 layer frontogenesis is phenomenal across the northern half
or so of the area at 12z according to the GFS /over 50 and over 100
over the far northern counties/. Q vector convergence aloft over 30
for the entire area and over 50 across the northern half or so. Of
note in the cross section though is that the area of negative epv
/enhanced instability/ is showing up just below a decent intrusion
of dry air aloft which could be falsely enhancing the potential
instability. Dry air rushing in aloft could also cut down on
dendritic growth thereby cutting down on snow amounts.
Shortwave showing up as pressure advection on a pv surface moves
through between 12 and 18z Wednesday. At this point looks like system
will be zipping through the area and majority of precipitation could be
coming to an end across much of the area by around 18z.
As far as precipitation type is concerned...forecast soundings show
thermal profiles of all rain across the entire area through 6z.
Between 6z and 9z colder air starts to move in from the
northwest...but looks like a short-lived warm layer remains aloft
which could allow for some brief sleet before changing over to snow.
By 15z soundings show all snow even south of the area so have the
entire area snow starting then. In between 9 and 15z should see snow
move in from the northwest and see a similar transition from
northwest to southeast of rain to a mix of rain/brief sleet/snow to
As far as snow amounts are concerned...looks like the northern
counties could see 3-5 inches...with around 2 or so central and
trace to 1 in the south. With the strong forcing in the north
couldn/T rule out 6 inches somewhere...but with the warm ground to
start out and the dry air moving in quickly aloft to shut things
Don/T expect to see that widespread even in the northern counties.
Went with a Winter Weather Advisory for the northern counties with
the anticipated snow amounts.
Another big concern will be the wind with this system. A tight
pressure gradient develops across the area as the low moves through.
Could see wind gusts around 40 miles per hour. Combining these with any snow
could lead to some pretty serious visibility issues...especially in
the northern counties which could see a period of moderate to heavy
snow. For now will keep the Winter Weather Advisory confined to the
northern counties with the heaviest snow but could see the potential
for expanding it southward if changeover to all snow occurs faster
than currently forecast.
High pressure builds in Wednesday night and dry then through
Thursday night with temperatures remaining below normal.
Long term /Friday through Monday night/...
Issued at 427 am EDT Tuesday Mar 11 2014
Dry conditions can be expected on Friday with zonal flow aloft.
However...a weak cold front will traverse central Indiana on Friday
night. Moisture appears to still be limited with this front...and
latest initialization only pulls in slight chance probability of precipitation for rain.
Further out...a large ridge of high pressure establishes itself over
the western U.S. With a Broad Cut-off low over eastern
Ontario/western Quebec. This pattern will be conducive to precipitation
chances as weak waves rotate around that cut-off low. The best
chances will be on Sunday and Monday with a plethora of precipitation types
ranging from rain...snow...to a mix of rain/snow. Slightly colder
air will accompany this pattern with highs falling back into the 30s
and 40s for the second half of the extended period.
Aviation /discussion for the 11/0900z taf update/...
Issued at 430 am EDT Tuesday Mar 11 2014
mostly VFR through 03z Wednesday followed by lowering flight
conditions after that.
A few outlying tafs may experience a period MVFR fog near
sunrise...but with a big dewpoint spread at most taf sites...will
downplay this a little from the earlier forecast.
Weather depiction and model soundings indicate mainly high clouds
with some middle clouds at time through the day Tuesday. Low pressure
over the Central Plains will move to the central Mississippi Valley
Tuesday evening. This system will spread rain into klaf...khuf and
kind by late Tuesday evening with conditions quickly lowering to IFR
after 06z Wednesday.
Winds will be light southwest overnight and southwest up to 10 knots
Tuesday. Winds Tuesday night southeast 5 to 8 knots except northeast
near 10 knots at klaf.
Winter Weather Advisory from 4 am to 4 PM EDT Wednesday for
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