Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
241 PM EDT Sat Apr 25 2015
The synopsis...near term and short term sections have been updated
issued at 241 PM EDT Sat Apr 25 2015
A low pressure system will drop southeast across southern Kentucky
and the southern Appalachians tonight. Widespread late afternoon and
showers and thunderstorms will be winding down from northwest to
southeast during the evening and overnight. Then...high pressure
will bring dry weather to central Indiana Sunday and most of next
week. The one exception is Wednesday...when a weak low pressure
system will bring a small chance of showers to the area.
Below normal temperatures will gradually warm to normal midweek and
slight above normal next weekend.
Near term /remainder of afternoon and tonight/...
Issued at 241 PM EDT Sat Apr 25 2015
The main focus to the near term will be first of all severe
potential across mainly southwestern counties and secondly probability of precipitation
and timing of the exit of the showers.
Models are in general agreement that upper low will weaken tonight
as it encounters confluent flow around the base of an upper low just
off the New England coast. The surface low will also weaken as it
quickly tracks southeast across southern Kentucky and the southern
Appalachians. This and high pressure moving in behind the low will
result in showers quickly ending from northwest to southeast this
evening and overnight. Will start off with likely probability of precipitation...drop down
to chance probability of precipitation south around 06z and no probability of precipitation after 09z.
An enhanced risk of severe weather continues this afternoon for
areas roughly near and south of a Bicknell to Loogootee line.
Meanwhile...there is a slight risk further north to near a
Farmersburg to Brownstown line and a marginal risk to around a North
Terre Haute to Columbus line. The enhanced risk area will be closer
to the western Kentucky warm front and subsequently to more low
level shear and forcing. Thus...isolated tornadoes per Storm Prediction Center day1
outlook through 6 PM or so there looks reasonable. A lot will depend
if the front can make it into parts of these counties thereby
realize the greater warm sector instability. At any rate...large
hail and damaging winds will also be a threat southwest. Only
general thunder is expected north of the marginal risk area I.E.
North of a North Terre Haute to Columbus line.
Instability forecasts suggest thunder threat should be over no later
than 03z across the southern counties and probably earlier than that
as system weakens substantially.
Cold air advection and clearing should allow temperatures to fall to
the middle 30s to the middle 40s per 12z MOS for overnight lows. The
coldest temperatures should be across northwestern parts of the
area...where clearing will arrive first.
Short term /Sunday through Tuesday/...
Issued at 240 PM EDT Sat Apr 25 2015
Main focus for the short term will be on temperatures and cloud
cover as broad area of surface high pressure from Hudson Bay to the
southern states should keep it dry. Model relative humidity time sections in good
agreement that very dry air will be in place aloft. However...ribbon
of moisture around 850 millibars and cyclonic flow suggests there
will be a fair amount of fair weather diurnal cumulus around each
afternoon with mostly clear skies at night. Could be some frost
around close to dawn each morning...but it looks marginal per
projected overnight lows and wind speed.
With model 850 millibar temperatures prognosticated to only a couple of
degrees above zero...prefer to go with temperatures at or slightly
below 12z MOS which translates to below normal temperatures with
afternoon highs from the middle 50s to the lower 60s and overnight lows
in the middle 30s to the lower 40s. Tuesday could be a few degrees
warmer though as 850 millibar temperatures rise a few degrees with
upper ridge over the area.
Long term /Tuesday night through Saturday/...
issued at 208 PM EDT Sat Apr 25 2015
Long term guidance continues to depict a poorly organized upper
level system passing to the south of the area and another system
passing to our northeast...leaving the forecast area largely
unaffected. Will go with a dry forecast throughout the period.
Northwest flow initially will keep temperatures seasonably
mild...with some warmup toward the end of the period as an upper
level ridge begins to impinge upon the area.
Blended initialization handled things well and few changes were
Aviation /discussion for the 25/18z tafs/...
issued at 102 PM EDT Sat Apr 25 2015
IFR initially will likely give way to MVFR this evening and then VFR
Area of rain and embedded thunder will impact most of the terminals
initially...and will carry a brief thunderstorms in the vicinity mention all but bmg. After
this area moves out showers may redevelop later this afternoon.
Cannot entirely rule out additional thunder but it seems unlikely.
Winds will gradually become more northerly as the low moves through
the region. Expect VFR conditions to return by daybreak Sunday.
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