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Area forecast discussion 
National Weather Service Indianapolis in
1003 EST Sat Feb 28 2015

Update...
near term discussion for the rest of today has been updated below.

&&

Synopsis...
issued at 304 am EST Sat Feb 28 2015

A frontal system is expected to move through the area tonight and
Sunday. In the wake of this front...high pressure will build into
the area by Monday. A stronger low pressure system may affect the
area towards the middle of next week.

&&

Near term /rest of today/...
issued at 1003 am EST Sat Feb 28 2015

Satellite indicate lots of middle level clouds over all but the far
northeast parts of our region. Models soundings indicate some
breaks may occur across far northeast sections this
afternoon...otherwise mostly cloudy skies will be the rule. Overall
current forecast is on track. Will keep low chance probability of precipitation far west
late today. Highs late this afternoon will range from the lower to
middle 20s north to near 30 south.

&&

Short term /tonight through Monday night/...
issued at 304 am EST Sat Feb 28 2015

Main item of interest during this period is a short wave trough that
is expected to drop southeast across the Great Lakes on Sunday. The
surface reflection associated with this feature doesn/T look too
impressive...but models suggest a sustained 35-40 knots low level jet
across the area from tonight into Sunday evening...indicating a
rather long duration precipitation event. Will go with high probability of precipitation all
areas tonight and Sunday...with probability of precipitation tapering off Sunday night as
low level jet moves off to the east.

Models are coming into better agreement on the thermal profiles
during this event...but some of the ensembles are still warmer than
the operational models. As a result...there is still a possibility
that the warmer air may get farther north than forecast. At this
time...the majority of the data suggest mixed precipitation may
start working its way into the far southern zones during the pre
dawn hours of Sunday...probably reaching as far north as the central
zones during the day Sunday. Due to the expected long duration of
the event...looks like a high end advisory type situation. The lack
of a good surface feature lends to lower confidence of reaching
warning criteria over a widespread area at this time. Planning on a
Winter Weather Advisory for all areas starting this evening and
lasting through the day Sunday. Model quantitative precipitation forecast and precipitable waters
flowing into the system suggest 4-6 inches possible over the
northern half of the forecast area...with progressively lower
amounts farther south as precipitation becomes more mixed with time.
Brief period of dry weather expected Monday as surface high pressure
move through...before lift/precipitation threat moves back into the
area by late Monday night ahead of the next system. Will continue
with chance probability of precipitation for mixed precipitation late Monday night.

Low level thickness forecasts suggest the GFS MOS temperature guidance
for the rest of the short term doesn/T look too bad...so only minor
adjustment planned.

&&

Long term /Tuesday through Friday/...
issued at 335 am EST Sat Feb 28 2015

Active pattern continues through midweek as a broad moist southwest
flow persists downstream of an amplified upper trough over the
intermountain west. Confidence remains high in significant weather
impacts through Wednesday as low pressure tracks through the region
along a slow moving frontal boundary.

Model consensus capturing the southwest flow aloft and surface wave
track into the lower Great Lakes by Tuesday night well. Differences
however exist with respect to strength of the surface low with the
op GFS and bulk of the GFS ensemble members offering a much stronger
feature than the global models. Appears a stronger interaction with
the northern stream energy may be partially responsible for the
deeper surface wave. These differences not likely to be ironed out
for another 24-48 hours but the big picture remains the same as a
sloppy wintry mix early Tuesday gradually transitions to rain for
Tuesday afternoon and night before colder air changes precipitation back
towards snow during the course of the day Wednesday.

Thermal profiles are strongly suggestive that freezing rain will be
the main precipitation type Tuesday morning as raw surface temperatures on bulk of
the 00z models remain subfreezing while warm advection intensifies
through the boundary layer. The expected snowpack will be a factor
in near surface warming and as has been discussed...may have to
wait until snow tonight/Sunday gets down before the models
accurately Sample impacts. The icing potential looks to make an
absolute mess for Tuesday morning with surface temperatures warming above
freezing by late morning/early afternoon.

Still feel the heavy rain over frozen ground and several inches of
snow on the ground present a growing and increasingly significant
flood potential for central Indiana. Precipitation water values remain
exceptionally high and near the climatological maximum for early March
between 1.25 and 1.50 inches by Tuesday evening as a strong 65-75kt
850mb jet moves through the Ohio Valley. Still appears 1-2 inches of
rain will fall Tuesday afternoon through early Wednesday before the
front gradually sags south of the Ohio River. In addition...feel
comfortable in introducing an isolated thunder mention into southern
portions of central Indiana. Scattered convection will bring
periodic higher rainfall rates and only exacerbate the flooding
concerns. Stay tuned.

Some light snow is possible on the back side of the precipitation shield
Wednesday and Wednesday evening as colder air spreads south. Much
colder and drier air then returns for the end of the week.

&&

Aviation /discussion for 281500z taf update/...
issued at 919 am EST Sat Feb 28 2015

Only needed minor tweaks to kind taf. Waiting on latest data/trends
before making any changes to snow start time.

Previous discussion follows...
VFR conditions throughout the day. Rapidly deteriorating to IFR and
lower this evening into the overnight in snow.

Expansive middle level deck continues to overspread central Indiana
early this morning. May briefly see the sun this morning with the
deck expanding and lowering through the course of the day. East/southeast
winds at 5-10kts can be expected through late afternoon.

Deeper moisture and isentropic lift will arrive over the region by
early evening...with snow quickly developing at all terminals.
Conditions will worsen quickly with the onset of snow...with
heavier snowfall and widespread 1sm and lower visibilities impacting
the terminals overnight. Ceilings will drop below 1000ft this
evening and remain there all night. Winds will become light and
variable late tonight.

&&

Ind watches/warnings/advisories...
Winter Weather Advisory from 7 PM this evening to 7 PM EST
Sunday for inz021-028>031-035>049-051>057-060>065-067>072.

&&

$$

Synopsis...jas
near term...jh
short term...jas
long term....Ryan
aviation...Ryan/50

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