Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1149 PM EST Wednesday Dec 4 2013
the aviation section has been updated below.
issued at 400 PM EST Wednesday Dec 4 2013
A boundary will drop through the area late tonight into
Thursday...before stalling across the Tennessee Valley. A series
of disturbances moving along this boundary late tomorrow into
Friday will set the stage for what could be the first significant
winter storm of the season across much of central Indiana. After a
brief break...another system will move east of the area late
Sunday into Monday...with the potential for more accumulating
snowfall then. Toward the end of the period...dry conditions will
return but temperatures will be very cold.
Near term /tonight/...
issued at 951 PM EST Wednesday Dec 4 2013
Warm front has moved north of central Indiana and with it the fog.
Do not expected areas in the warm sector or behind the cold front to
see any significant fog with winds staying up tonight and low Post
cold frontal dew points. Made changes to the hazardous weather
outlook and weather story to reflect this.
Previous discussion follows...
Boundary will sink through the area beginning late tonight...and
although atmosphere will be fairly dry at first...as moisture
moves north into the area on a 40kt low level jet may see some
showers along the boundary. With much of the area still dealing
with fog and low level inversion unlikely to weaken significantly
overnight...will carry areas of fog in the forecast as well.
On temperatures...consensus appears best with a boundary in the area and
significant gradient in min temperatures likely.
Short term /Thursday through Friday night/...
issued at 400 PM EST Wednesday Dec 4 2013
Short term focus is on potentially significant winter storm
early Thursday evening into Friday evening.
Models continue to set the stage for a potential winter storm. As
the boundary sinks south of the area and cold advection
continues...a series of disturbances moving along the boundary may
allow significant precipitation to occur at times. Area will be in
favorable position with respect to a 160-170kt upper level
jet as ample moisture streams northeast along and north of the
frontal zone. Model cross sections show increasingly potent
frontogenetic forcing Thursday evening into Friday in a saturated
airmass with strongly negative equivalent potential vorticity
values at times...suggesting that mesoscale forcing will pick up
any slack from moderate synoptic forcing and enhance the potential
precipitation...perhaps even resulting in some banding at
times...during the storm.
At this time...appears that precipitation through at least midday
Thursday will be in the form of rain. Model thermodynamic profiles
suggest a transition will begin and sweep across the area from the
northwest to the southeast late tomorrow afternoon through
tomorrow night. A potent warm layer centered around 800-850
millibars throws a complicating factor into the mix...as this
combined with the depth and strength of the low level cold air
suggest a significant amount of sleet could mix with any
snow...especially Thursday evening. This greatly complicates
resultant snowfall amounts...but at this time combined snow and
sleet amounts of three to six inches with some higher amounts
appear likely...with the highest amounts south of the Interstate
70 corridor. A brief period of freezing rain near transition time
may occur...but this would be transient at best and will leave out
of the grids at this time. Have gone with a Winter Storm Watch
across the southern half of the area as a result.
Consensus temperatures appear close to wetbulb during precipitation times and
this will be acceptable.
Long term /Saturday through Wednesday/...
issued at 209 PM EST Wednesday Dec 4 2013
European model (ecmwf) and GFS agree that a cold pattern is expected to persist for
this period. Cold...Arctic high pressure is expected to build into
the Ohio Valley on Saturday as the preceding storm system departs
farther south. With new snow pack on the ground along with cold air
advection amid cold north to northeast flow...will trend temperatures
lower than guidance with a dry forecast.
GFS and European model (ecmwf) suggest a continue very quick flow aloft on Sunday
into Monday as another trough aloft over the Central Plains sags
eastward and passes across the Ohio Valley on Monday. This will not
only give US another shot at snow...but in it/S wake provide yet
another reinforcing blast of cold air as by Tuesday a broad trough
of low pressure aloft will allow very cold air out of the Canadian
northwest to stream into the Ohio Valley. With the cold...dry air
mass in place...no precipitation will be expected on Tuesday and
Wednesday...however will continue to trend temperatures cooler than
allblend where possible.
Aviation /discussion for 050600z taf issuance/...
Issued at 1145 PM EST Wednesday Dec 4 2013
A cold front will be moving through the sites over the next few
hours shifting winds to the west and then northwest. Expect conditions to
deteriorate to MVFR and then IFR ceilings during the overnight. In
the afternoon should see rain begin at all sites but klaf and
expect this to transition to a mix of rain and sleet after 22-0z
and then a mix of sleet and snow 0-3z before transitioning to all
snow after 6z. Again expect precipitation to stay south of klaf which
should remain MVFR during this time. Elsewhere could see IFR
Winter Storm Watch from Thursday evening through Friday evening
Visit US at http://www.Weather.Gov/ind
Follow US on twitter and youtube at: