Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1020 PM EDT Thursday Jul 24 2014
The aviation section has been updated below.
issued at 345 PM EDT Thursday Jul 24 2014
Dry and pleasant weather is expected through Friday as high pressure
drifts east of the region. A frontal boundary will set up across the
lower Great Lakes on Saturday with warm and humid air south of the
boundary over central Indiana. As multiple waves track along the
front...chances for thunderstorms and potentially severe weather
will increase over the weekend. Cooler air will return for much of
next week as a deep upper trough once again develops over the
eastern half of the country.
Near term /rest of tonight/...
issued at 958 PM EDT Thursday Jul 24 2014
Update...minor tweaks made to current gridds mainly to add in a
little more high clouds overnight over western and central sections
based on current satellite. Temperatures could be a tad warmer in a
few places...but models generally favor current forecast.
Portion of previous near term discussion follows...
Despite progressively drier air and subsidence aloft overspreading
the region...presence of low level thermal trough contributing to
the cumulus field. Cumulus coverage will gradually diminish into early evening
as stronger heating is lost and the subsidence aloft eventually wins
out. Rest of the night will be quiet and refreshingly cool once
again. Generally clear skies expected overnight...with potential for
thin high cirrus from convection over the upper Mississippi Valley
to invade the region from the northwest towards daybreak.
Temperatures...with the center of the high nearby and clear skies for much
of the night...good setup for radiational cooling in place. Expect a
decent split between rural and urban areas on temperatures. Normal cool
spots are likely going to make a run at 50 degrees late tonight.
Light easterly flow off the city will keep kind warmer.
Short term /Friday through Sunday/...
issued at 345 PM EDT Thursday Jul 24 2014
Forecast challenges are plentiful...as the current stretch of dry
pleasant weather gives way to a much more active pattern with
potentially multiple opportunities for severe weather over the
The short term period will be highlighted by a transition to a
familiar theme this Summer...as northwest flow aloft develops
between the flattened yet strong ridge over the southern High Plains
and an anomalously strong upper low tracking across the Canadian
prairies. The upper low will move southeast over the weekend...
eventually arriving over the Great Lakes by Sunday. In doing
so...the upper wave will assist in forcing the ridge west into The
Four Corners region while amplifying our old friend the eastern
By far the pick day of the short term will be Friday with the region
still firmly under the control of slowly departing high pressure.
Expect a good deal of sunshine with a slow but steady increase in
middle and high level clouds as warm frontal convection takes place to
the northwest of the forecast area. Not out of the question that
some of this convection may make it into the northern Wabash valley
prior to sunset Friday...albeit in a weakened condition as it moves
into a drier and more stable airmass over central Indiana. Storm
coverage is likely to increase overnight Friday however as a low
level jet noses into the middle Mississippi Valley...with moisture and
elevated instability increasing across the northern half of the
Hoosier state. Will carry chance probability of precipitation Friday night mainly over the
northern counties with lower probability of precipitation further south.
Other than some initial convective debris Saturday morning...surge
of warm and increasingly humid air will quickly advect into the Ohio
Valley...making the pleasant conditions experienced today and
tomorrow a long distant memory. Convective redevelopment Saturday
afternoon is there...but some questions do exist that make
confidence lower than desired at this point on severe weather
potential prior to Saturday evening. Strong heating over the region
south of a frontal boundary will support a highly unstable
environment with progressively deeper moisture arriving during the
day. However...models continue to advertise a rather Stout cap with
700mb temperatures at 12-14c well into the afternoon which is likely to
limit coverage and intensity of storms over the region. Expect
scattered storms...with a potential uptick in coverage towards
evening as the cap weakens.
Highest confidence in potential for storms and severe weather comes
Saturday night as the cap completely erodes for much of the area
while a strong surface wave tracks along the boundary. With abundant
instability...exceptionally deep moisture with precipitable waters in excess of 2
inches being fueled by the nocturnal low level jet and nice middle and
upper level forcing available...signs point towards upward
convective development impacting the forecast area overnight...most
likely into an mesoscale convective system. The presence of deeper shear also lends itself
to more organized robust convection with all modes of severe weather
on The Table. Torrential rainfall and localized flooding will also
be possible with the tropical airmass in place. Feel comfortable in
introducing likely probability of precipitation Saturday night over the entire forecast
Sunday forecast is highly conditional on what occurs Saturday night.
While deeper moisture will be shifted to the southeast...stronger
upper low will be arriving into the Great Lakes Sunday afternoon
with renewed surface wave development. Available instability could
be problematic...especially if expected convection Saturday night
completely grunges out the airmass in its wake for much of Sunday.
With that being said...the presence of renewed forcing aloft with
the approach of the upper wave supports at the very least a
continuation of the potential for thunderstorms and possibly another
chance for severe weather. For now...will carry high chance probability of precipitation.
Temperatures...generally trended towards mavmos guidance for temperatures
throughout the period with the lone exception of Saturday. Mav seems
overly aggressive in attempting to bring 90+ air into the southwest
half of the forecast area. While 850mb temperatures of 21-22c noted on
model consensus by Saturday afternoon...the arrival of an
increasingly moist and humid airmass is not supportive of widespread
90 degree weather... particularly if any scattered convection can
fire as well. Undercut mav temperatures by 2-4 degrees across the board.
Long term /Sunday night through Thursday/...
issued at 156 PM EDT Thursday Jul 24 2014
At the beginning of the period there are some low chances for
departing showers and thunderstorms due to a departing cold front
Sunday night. And then surface high pressure looks to build in and
provide dry and cooler than normal conditions across the area yet
again. Temperatures will be well below average for most of the long
term. Highs will be in the 70s through at least middle week. A fast
moving short wave may approach and cross the area on Thursday and
have included low chances at this time. Allblend guidance appeared
to handle this forecast well...so no need to deviate too far from
Aviation /discussion for the 25/03z taf update/...
issued at 1020 PM EDT Thursday Jul 24 2014
No significant changes necessary. Previous discussion follows.
Issued at 659 PM EDT Thursday Jul 24 2014
Largely VFR through the period. Some potential for MVFR visibilities
in light fog tonight at the outlying sites. Will handle with tempo
groups as fog may be transient in nature.
Afternoon cumulus beginning to dissipate and skies should be mostly
clear within the next few hours and remain so much of the night.
This combined with light winds may promote some fog...but not
expecting worse than MVFR at this time.
Any fog will quickly burn off Friday morning. Some scattered cumulus
and increasing middle cloud expected ahead of next system.
Winds will be less than 10kt throughout the period.
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