Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
213 PM EDT Thursday Sep 18 2014
the long term section has been updated below.
issued at 1255 PM EDT Thursday Sep 18 2014
High pressure over Ontario will keep quiet weather in place across
Indiana tonight. A ridge aloft of high pressure over the Rocky
Mountains is expected to build east into the Ohio Valley on Friday
through Saturday...providing dry weather. Temperatures will warm
up slightly through the first part of the weekend as the surface
high shifts eastward and warmer air arrives on southerly winds.
A weak cold front is expected to push across the region on late
Saturday night and early Sunday...bringing chances for rain then.
Another high pressure is then expected to arrive in the region for
Sunday night and Monday keeping dry weather across central Indiana
through the middle of the next work week along with near normal temperatures.
Near term /tonight/...
Issued at 1255 PM EDT Thursday Sep 18 2014
Surface analysis early this afternoon shows a broad area high
pressure centered over Ontario dominating weather across Indiana
and the Ohio Valley. Cool and dry northeast surface flow was in
place across Indiana with surface dew point temperatures in the upper 40s.
Main forecast challenge tonight will be temperatures.
The high pressure system over Ontario is expected to dive southeast
toward New England tonight...and this will result in our light
northeasterly flow across the area to become more southeast and
southerly for Friday and Friday night. Forecast soundings continue
to indicate a dry column and time heights continue to show subsidence
..thus a mostly clear sky will be expected along with the light
Given our expected dew points in the middle 40s will trend lows at
or below persistence given our ideal radiational cooling set-up.
Short term /Friday through Sunday/...
Issued at 1255 PM EDT Thursday Sep 18 2014
Main weather challenge during this period will be probability of precipitation. Models are
in pretty good agreement during this period and will use a blend.
Water vapor imagery shows ridging in place over the Rocky
Mountains this afternoon. GFS/NAM suggest this ridging is expected
to push eastward and flatten through Saturday...keeping any upper
level dynamics well north of central Indiana over the Great Lakes.
Meanwhile at the surface the large high pressure system is
expected to push to New England by Friday afternoon and out to the
Atlantic on Saturday. However the high continues to push a large
surface ridge west...across the virginias and into the Tennessee
River valley. Forecast soundings and time heights remain dry
through Saturday afternoon...with unattainable convective temperatures.
Thus will use a dry forecast until then with partly cloudy days
and mostly clear nights. As the high pressure system pushes
east...warm air will arrive in central Indiana on southerly winds.
Thicknesses and 850mb temperatures climb during this time. Thus will
trend highs at or above mav each day.
On Saturday night the GFS and NAM suggest a cold front pushing out
of the upper Midwest and into central Indiana. Forecast soundings
fail to show deep saturation...but do not show a completely dry
column either. 310k isentropic surface shows some moisture
arriving along the surface...over 6 g/kg...however isentropic lift
is marginal to poor. Surface convergence appears very good however
as a good wind shift and sharp trough is expected with the front.
Thus expect at least some King of light amounts of precipitation to break
out. Will trend probability of precipitation to the high chance or low likely categories
for Saturday night and early Sunday. Some clearing is expected on
Saturday afternoon as the the front should be clearing the state
and high pressure over the northern plains begins to build across
Given the expected clouds and rain...will trend lows on Saturday
night warmer than mavmos and highs on Sunday cooler than mexmos.
Long term /Sunday night through next Thursday/...
issued at 212 PM EDT Thursday Sep 18 2014
Ensembles indicate a deep upper trough will dig into the eastern
parts of the country early next week. Appears this trough will be
progressive in nature...as the ensembles suggest heights will rise
quite a bit towards the end of period as trough lifts off to the
northeast and upper ridging moves into the Midwest.
Frontal system that is expected to affect the area over the weekend
should be off to the southeast at the start of the extended.
Ensembles indicate very little threat for precipitation as upper
heights rise. Will keep the extended dry.
Aviation /discussion for the 181800z tafs/...
issued at 1251 PM EDT Thursday Sep 18 2014
Scattered diurnal cloud cover based around 040 expected to dissipate
Based on expected crossover temperatures...patchy MVFR visibility
restrictions may develop in the rural areas towards 190600z.
Surface winds 050-070 at 6-8 kts this afternoon will diminish to 6
kts or less after sunset.
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