Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
224 PM EDT Friday may 29 2015
the long term section has been updated below.
issued at 245 am EDT Friday may 29 2015
Changeable weather is in store this weekend. Warm and humid
southerly flow ahead of an area of low pressure will bring chances
for showers and thunderstorms to central Indiana this afternoon
and tonight. A cold front is expected to sink out of the upper
Midwest late tonight and arrive in central Indiana on Saturday.
This will bring continued chances for showers and thunderstorms.
On Saturday night and Sunday...low pressure is expected to pass
along the front and bring more showers and thunderstorms to
central Indiana...along with cooler temperatures on northeast
Through the next work week...small chances for showers and storms
will linger as warmer and more humid southerly flow slowly returns
as the lingering frontal boundary lifts northward.
Near term /today/...
Issued at 245 am EDT Friday may 29 2015
Surface analysis early this morning continues to show a broad
trough of low pressure stretching from the upper Midwest...south
across the Central Plains to central Texas. The Bermuda high was
in place providing and easterly flow to southeastern United States and a
warm and humid southerly flow to the Ohio Valley and central
Indiana. Water vapor shows a broad trough in place across the
Central Plains along with a couple short wave ejecting
northeastward amid the flow aloft. Radar was quiet and dew point
temperatures were in the moist lower 60s.
GFS and NAM continue to suggest weak...poorly organized short
wave pushing northeast to the Ohio Valley today ahead of the main
upper trough over the Central Plains. Time height sections show
saturated lower levels. Forecast soundings also show favorable
conditions for convection with convective temperatures near 80
along with steep lapse rates and cape over 1000 j/kg. Although
confidence is low in the models handling of the upper
support...moisture and instability available leads to favorable
conditions for thunderstorms and rain development. Should any upper support
arrive...that will just be gravy in the convection/precipitation chances.
Thus will trend for probability of precipitation at or above mavmos. As for temperatures...little
to no advection appears in place as 850mb temperatures remain pretty
steady state. Thus will trend toward persistence for highs today.
Short term /tonight through Sunday night/...
Issued at 245 am EDT Friday may 29 2015
Main forecast challenge during this period will be probability of precipitation. GFS and
NAM both suggest the upper trough slowly begins to edge east
tonight...pushing dynamics across the Ohio Valley. Forecast
soundings continue to show deep saturation through the night.
Lingering diurnal convection may also be a continued factor.
Forecast soundings continue to show deep saturation also. Thus
with plenty of moisture available...will continue to trend toward
the categorical probability of precipitation of the mavmos tonight. Given the expected rain
and warm...southerly flow in place...will trend lows warmer than
Rains for rain on Saturday appear quiet favorable also. GFS and
NAM suggest the upper trough sags south across Indiana on
Saturday providing good lift. Forecast soundings once again show
deep saturation...with cape available loft. Meanwhile at the
surface the frontal boundary is expected to be pushing across
central Indiana...providing good low level convergence.
Finally...surface low pressure is expected to push northeast along
the front on Saturday afternoon into Saturday night...providing more
forcing. Forecast soundings show precipitable water near 1.70-1.80
inches. Thus will once again trend probability of precipitation at or above mavmos on
Saturday and again on Saturday night. As for temperatures will trend
Saturday highs cooler than mavmos and stick close to mavmos for
Saturday night lows as a new air mass in introduced in the wake of
Forecast soundings on Sunday show some lingering lower level
moisture during the morning hours...however the middle and upper
level dry out quickly as cold front sag farther southeast along
with the upper support. Furthermore...into Sunday night the
troughing aloft appears to flatten somewhat. Meanwhile at the
surface cooler and drier...Continental northeast flow takes
place...providing further drying and subsidence. Thus other than a
few lingering morning showers/drizzle/sprinkles lingering on
Sunday morning...will trend toward a dry forecast for Sunday
afternoon and into Sunday night. Given the expected clouds and
cooler NE flow during this time will also trend lows at or below
Long term /Monday night through Friday/...
Issued at 223 PM EDT Friday may 29 2015
Ensembles are showing good agreement on an upper ridge building
across the area through the long term. Ensembles are also agreeing
on a western U.S. Upper low digging south and getting cut off over
the Gulf as the aforementioned ridge builds. In the operational runs
there is a suggestion that moisture/upper forcing from this upper
low could swing north into the southern counties and bring some
thunderstorm chances despite the presence of the upper ridge
Thursday and Friday. Chances would be more likely during the day
with daytime heating available to help the forcing overcome the
inhibition of the upper ridge. Temperatures will warm from near
normal to start the period to about 10 degrees above normal going
into the weekend.
Aviation /discussion for 291800z taf issuance/...
Issued at 1252 PM EDT Friday may 29 2015
Widely scattered showers are popping up around central Indiana
just before 1 PM. Thus far no cloud to ground lightning is being
reported with these...but with instability available expect that
lightning will develop. With spotty coverage at best at this point
and no impetus for more widespread development may continue with
thunderstorms in the vicinity for the sites through the afternoon into the evening hours.
Confidence is low regarding thunder impacting any particular
site...but with aforementioned showers already across central
Indiana think dropping any thunder mention is premature. Could
introduce deteriorating conditions overnight and Saturday with
more development expected ahead of the cold frontal passage. Wind
shift from the cold front will likely occur Saturday evening and
thus will not be included in this issuance.
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