Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1150 am EST Wednesday Jan 28 2015
the aviation section has been updated below.
issued at 300 am EST Wednesday Jan 28 2015
A couple of low pressure systems will bring chances of precipitation
to central Indiana Thursday and again for the second half of the
weekend into early next week. Temperatures will go from near to
above normal early in the period to well below average by early next
Near term /this afternoon/...
Issued at 936 am EST Wednesday Jan 28 2015
Surface analysis late this morning shows high pressure centered
over Ohio and eastern Indiana. Infrared pictures shows high Colorado streaming
in the upper Midwest into Illinois and Indiana. Radar was quiet.
The surface high is expected to slowly drift east this afternoon.
Plenty of sunshine is expected as forecast soundings suggest
convective temperatures are unreachable. Thus a mostly sunny sky
is expected. As for temperatures...given temperatures in the lowest 850mb...the
ongoing highs are on the mark. Little to no changes needed at this
Short term.../tonight through Friday night/
issued at 300 am EST Wednesday Jan 28 2015
Focus is on chances for precipitation tonight into Thursday night.
Models are close enough that a blend was used.
Isentropic lift will ramp up this evening with 850mb winds
increasing to around 50kt. However this lift will have little
moisture to work with. Will keep the evening dry but increase the
Overnight...additional forcing will move in as an upper
trough approaches. However...moisture is still pretty limited in the
lowest levels. Thus only went slight chance to low chance category
probability of precipitation starting about 08z. Temperatures will bottom out early in the
evening then rise overnight with good warm advection. By the time
any precipitation would start...the atmosphere will be warm enough
for just rain.
A couple of upper waves will move through the area Thursday.
Best combination of forcing and moisture occurs between 12z and 18z
Thursday. However...even during that period the best moisture
arrives after forcing has peaked. Thus just went chance probability of precipitation most
areas...with a thin area of likely probability of precipitation in the far northeast.
Chances for rain will diminish through Thursday afternoon as forcing
weakens. Temperatures cool quickly aloft...but surface temperatures
stay mild for a while. Thus only introduced a mix of snow and rain
late in the afternoon.
Kept slight chance probability of precipitation across the eastern half of the area Thursday
evening as system exits. Continued with slight chance probability of precipitation northeast
overnight Thursday night to account for any lake effect snow that
makes it into the area.
High pressure will then move in and provide dry but cool conditions
Friday and Friday night.
Long term /Saturday through Tuesday/...
issued at 249 am EST Wednesday Jan 28 2015
Primary focus for the extended period remains on the growing
potential for a high impact winter weather system across the Ohio
Valley for the second half of the weekend. 00z model guidance has
come in generally with a more phased look to the upper jet structures
consequently producing a stronger surface wave tracking through the
After a dry day Saturday as high pressure moves across the
area...focus will turn to developing low pressure over Texas early
Sunday. This system will lift northeast into the Ohio and
Tennessee valleys as an upper trough amplifies over the Central
Plains. Variations are present between the different models which
is to be expected. The ggem and op GFS are particularly aggressive
in phasing the upper energy over the region by late Sunday...
consequently producing a deepening low going sub-1000mb Sunday
night as it passes through the area. European model (ecmwf) is flatter in the upper
levels initially lending to a faster moving wave with the phasing
energy aloft not taking place until after the system passes. The
individual GFS ensemble members have yet to organize into a
consensus surface low track at this point...showcasing the
differences with respect to the degree and timing of the polar and
subtropical jets phasing.
At this point...the forecast track is in a fairly ideal location
for snow to be a predominant precipitation type for most of the region.
As mentioned on Tuesday morning however...the overall handling of
low level warm intrusions this winter by models have been
problematic and track of the 850mb low across central Indiana on
the bulk of the extended guidance does nothing to quell that
concern at this time. With this storm still 4-5 days out...it remains
premature to speculate on details until a greater model consensus
can be achieved.
The main takeaway at this early stage is that confidence is
growing in a winter storm to impact the region late Saturday night
through early Monday. Will carry mainly snow over the northern
half of the forecast area with a rain/snow mix on Sunday across
southern counties. Should snow end up as the predominant precipitation
type...potential exists for several inches of accumulation. Stay
Much colder air will be drawn south into the region for early next
week behind this system with potential for coldest temperatures in about
3 weeks with highs struggling to get out of the teens in some
locations Monday and Tuesday. Could see lows near or just below
zero in spots Tuesday morning.
Aviation /discussion for 281800z taf issuance/...
issued at 1150 am EST Wednesday Jan 28 2015
VFR conditions with high and middle clouds expected through this
evening. However...rain and near MVFR stratocu is possible after 08z
and throughout the day on Thursday as a cold front moves through the
area from the west. The front should reach laf and huf around 14z
Thursday and ind and bmg around 16z.
Winds this afternoon will be from the southeast and south to around
15 knots. Then...winds will be pick up late evening from the south
with gusts to 20 knots or more. Finally...with the frontal
passage...winds will shift to the west and northwest to 15 knots and
gusts to 25 knots.
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