Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1247 am EDT Sat Sep 5 2015
the aviation section has been updated below
issued at 127 PM EDT Friday Sep 4 2015
Weak but hot high pressure in place over the Ohio Valley will
provide warm and humid weather through the Holiday weekend.
Isolated showers and thunderstorms will be possible each afternoon
through the weekend...however most areas will remain dry...hot
A weak cool front is expected to pass across the area on Monday
night and this will bring some relief to the heat and humidity
during the next work week.
Near term /rest of tonight/...
Issued at 955 PM EDT Friday Sep 4 2015
evening convection has quickly tapered off over the last hour with
loss of heating. Current radar mosaic does still show some weak
isolated showers/thunderstorms across the central portions of the
forecast area. As a result...will keep probability of precipitation in the forecast
through Sat 06z over central and some southern counties to account
for any lingering activity. After that though...dry conditions
should prevail for the overnight hours. Overnight lows are still
expected to be in the upper 60s/low 70s. Current temperatures across the
area are generally in the low to middle 70s. Will keep mention of fog
in the forecast tonight...but development may be inhibited in some
areas due to lingering cloud debris. Updated grids have been
main forecast issue for tonight will be probability of precipitation. GFS and NAM continue
to suggest the cut off low aloft will retrograde and sag south
somewhat into northern Indiana overnight. Time heights only show
lower level moisture available...representing the very high
surface dew points. Forecast soundings continue to show a
relatively dry column with conditions only favorable for diurnal
convection during the early evening. Rapid refresh appears a bit
overdone with expected convection this afternoon...showing
widespread coverage. Given the recent trends and the lack of
ongoing precipitation will trend bearish on probability of precipitation. Will be unable to rule
out some low pop chances early this evening...particularly across
the north...but will attempt to trend toward a dry forecast
As for temperatures...there continues to be little change in the overall
pattern. Will stick close to persistence. Again with light
winds...mostly clear skies and very low dew point depressions
expected...favorable conditions will exist for fog. Thus will
continue with patchy fog mention...particularly away the urban
Short term /Saturday through Monday/...
Issued at 127 PM EDT Friday Sep 4 2015
Models remain in pretty good agreement during this period and will
use a blend. Upper flow remains highly amplified through Monday
with a strong ridge in place across the Great Lakes and the Ohio
Valley. Meanwhile at the surface the weak and poorly organized
high pressure system remains in place...providing weak southerly
flow and very moist dew points in the upper 60s and low 70s.
Little change in the overall pattern is expected during this
period and will continue with a persistence type forecast through
the period. Forecast soundings continue to suggest plentiful lower
level moisture with dry air aloft and no lift. Each day forecast
soundings show convective temperatures in the upper 80s to around
90...with cape diminishing each day...falling from around 2000
j/kg on Saturday to near 1200 j/kg on Monday. With little support
in the area...only chances for precipitation will be due to diurnal
heating. As the heat builds through the weekend...even that will
diminish some as cape lessens. Thus will probably need every low
probability of precipitation on at least Saturday to cover for the isolated pop up
shower...but may be able to trend toward a dry forecast on Sunday
and Monday depending upon collaboration.
As for temperatures...thicknesses and 850mb temperatures appear rather steady
state through the period. Thus will trend toward a persistence
type forecast fro highs and lows. Mavmos looks to have a pretty
good handle on that.
Long term /Monday night through Friday/...
issued at 215 PM EDT Friday Sep 4 2015
Models are close enough that the superblend initialization was
accepted for most items.
An upper ridge will flatten out and allow upper troughing to build
into the eastern USA. This will result in chances for rain and a
cooling trend during the long term.
A cold front will gradually work its way through the area during the
long term. The result will be chances for rain Monday night through
Thursday...with best chances Tuesday night into Wednesday.
High temperatures will start out in the upper 80s on Tuesday but end
up in the 70s Thursday and Friday.
Aviation /discussion for 050600z taf issuance/...
Issued at 1244 am EDT Sat Sep 5 2015
Could be some isolated convection during the overnight
particularly over northern parts of central Indiana...but at this
point looks like that will avoid the taf sites. Thus VFR sky
conditions with some lingering convective debris early and then
decreasing cloud cover toward daybreak. This should result in less
fog tonight then recent nights. Will just go with MVFR from 09z-
13z at the smaller airports.
Can not rule out a thunderstorm or two this afternoon at ind and laf
as an upper wave drifts southeast over central Indiana and interacts
with the warm...moist and unstable atmosphere. However...coverage
expected to be too low to include in the tafs. Should once again see
scattered to broken afternoon diurnal cumulus. Winds will be southwest 6
knots or less.