Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
344 PM EDT Friday Jul 25 2014
the synopsis...near term and short term sections have been updated
issued at 344 PM EDT Friday Jul 25 2014
Scattered thunderstorms are possible tonight as a warm front lifts
through the area. A warmer and much humid air will build into the
Ohio Valley for Saturday. As a cold front approaches from the north
by the evening...widespread thunderstorms will develop across
central Indiana and have the potential to produce severe weather.
Additional chances for thunderstorms will come Sunday as a strong
upper wave tracks into the Great Lakes. Cooler air will return for
much of the upcoming week as a deep upper trough once again develops
over the eastern half of the country.
Near term /tonight/...
issued at 344 PM EDT Friday Jul 25 2014
Weakening area of convection moving through the middle Mississippi
Valley east into the Wabash valley this afternoon. Precipitation lightening
up considerably as it moves into a much drier airmass over central
Indiana. The extensive middle and high level clouds keeping temperatures
down in the 70s over most of the region at 19z.
Focus will be on precipitation chances for the region tonight as a warm
front moves into the area. Dewpoints remain in the 50s this
afternoon with drier boundary air slowly being eroded away. Deeper
moisture beginning to advect into the forecast area...with
mesoanalysis indicating a steady rise in precipitable waters above 1 inch ongoing.
This trend will continue overnight as the warm front lifts through
the region. Short range model guidance focusing better convective
chances will exist over northern counties as the front lifts to the
northeast...with increasing elevated instability through the night.
Will maintain highest chance probability of precipitation north of I-70 with low probability of precipitation
Temperatures...warmer overnight with the passage of the warm front. Expect
most areas to bottom out in the lower to middle 60s before beginning
to rise towards daybreak. Models were close overall and took a
general MOS blend.
Short term /Saturday through Monday/...
issued at 344 PM EDT Friday Jul 25 2014
Forecast challenges focus on threats for thunderstorms and severe
weather over the weekend...with a growing potential towards a
significant severe weather episode focused on Saturday evening into
The passage of the warm front early Saturday will usher in a warmer
and much humid airmass across the Ohio Valley as the flat upper
ridge pokes into the region from the southwest. Arrival of a strong
upper level wave from the Canadian prairies Saturday morning into
the Great Lakes on Sunday will force a retrograde of the upper ridge
and an amplified downstream trough over the eastern U S by early
next week. Northwest flow aloft will align across the Midwest and
Ohio Valley as this transition takes place...with a series of waves
traversing a slowly sagging cold front. This will result in multiple
opportunities for thunderstorms along with a persistent severe
weather threat existing late Saturday possibly into Sunday.
While cloud debris from overnight convection likely to still be in
the region Saturday morning...expect little to no precipitation through
early afternoon with enhanced forcing near the front back to the
northwest of the forecast area and capping developing with warmer
air aloft. While the general theme as to how the convective setup
evolves later Saturday has broad model support...NAM and sref both
generating more widespread convection faster than other models with
greater storm impacts as early as middle/late afternoon. Believe this
is premature considering the presence of the cap and that 700mb
temperatures should be in the 12-13c range through much of the afternoon.
While scattered convection is possible as the afternoon progresses
due to the increasing amounts of low level moisture and high levels
of instability...the more widespread storms are likely to hold out
until evening at earliest as the cap weakens and low level jet...bl
shear and deeper forcing increases.
Highest confidence for significant impacts from convection and
severe weather are beginning Saturday evening into the overnight as
the cold front drops into the region and a surface wave ripples
along it. A lot of favorable factors support rapid convective
expansion during the evening...with explosive upscale development
into an mesoscale convective system. A developing 35kt low level jet and high instability
possibly in excess of 3000j/kg will serve as the fuel for the
complex...pumping progressively deeper moisture into the Ohio
Valley. Precipitable waters are impressive at near 2.25 inches...about 2 Standard
deviations above normal. Forcing and diffluence aloft along with the
arrival of bl shear values approaching 40kts will further enhance
convective intensity and coverage Saturday night. Of further
interest from a climatological standpoint and serving to strengthen
confidence in significant convection...many cips analogs off both
00z and 12z data highlight past high end mesoscale convective system events producing a
potpourri of severe weather and flooding impacts for central Indiana
and the Ohio Valley.
The presence of such high precipitable waters and surface dewpoints likely into
the low/middle 70s combined with high instability supports the
potential for widespread damaging winds as the primary impact from
an mesoscale convective system Saturday evening/night. Considering available shear/helicity
and potential interaction of mesoscale boundaries...all modes of
severe weather would be on The Table. In addition...remain concerned
about torrential rainfall and flooding considering how high precipitable waters
are and that freezing levels near 15kft would support warm rain
processes within convection. Backbuilding convection is another
concern due to the orientation of the low level jet from west to
east. Finally...lightning will also be a factor...essentially
non-stop at times within the complex.
Sunday forecast remains highly conditional on what occurs Saturday
night. Deeper moisture is focused to our southeast...but the
approach of the stronger upper low into the Great Lakes Sunday
afternoon providing another boost in bl shear and forcing aloft.
Still feel mesoscale convective system Saturday night grunges out the airmass in its wake
for much of Sunday with limited instability. With that being
said...the presence of forcing aloft with the approach of the
anomalously strong upper wave warrants a continuation of the
potential for thunderstorms and possibly another shot for severe
weather. Should any sunshine take place Sunday...favorable
thermodynamic profiles with steep lapse rates would increase the
severe storm risk. For now...will carry high chance probability of precipitation.
The upper wave will become absorbed in the mean upper trough...
amplifying it across the eastern part of the country by Monday.
Passage of a short wave to our east Sunday night into Monday could
produce a few showers...especially over the northeast half of the
Temperatures...trended towards mavmos guidance for temperatures throughout the
period. Mav guidance has a better handle on Saturday temperatures today
with highs likely in the 85 to 90 degree range. Expect any near
90/low 90s to be confined to the lower Wabash valley...meaning indy
will remain without a 90 degree high in 2014. Heat indices could
rise into the middle and upper 90s over southwest counties Saturday
afternoon. Much cooler air builds in Sunday night and Monday...
expect highs 10-15 degrees below normal Monday.
Long term /Monday night through Friday/...
issued at 216 PM EDT Friday Jul 25 2014
The long term will start out dry and cool with high pressure
building over the area. With an upper trough over the eastern U.S.
Cooler than normal conditions will continue through the long term
with temperatures slowly warming but still not getting back up to
normal. Weak shortwaves moving through the main northwest flow
pattern could bring in small thunderstorm chances Wednesday through
Friday especially across the northern counties. With a wide spread
in ensemble solutions for that time will leave the allblend slight
to low chance probability of precipitation alone.
Aviation /discussion for 251800z taf issuance/...
issued at 115 PM EDT Friday Jul 25 2014
Currently looking VFR through the period. Large area of showers
and thunderstorms over central Illinois through eastern Missouri
has been weakening and continuing to head to the southeast. Based
on current motions and lack of moisture over Indiana think this
trend will continue...but could see a brief period of showers at
khuf sometime around 20z or so. Don/T think this will impact
flight category. Elsewhere expect increasing cloud cover but with
VFR ceilings. At klaf models are showing potential for some
thunderstorms during the overnight and kept these as a thunderstorms in the vicinity from
8-14z with still a decent amount of uncertainty on exact location
and timing. Expect to see more widespread storms over the sites
late Saturday night but at this point that/S outside of the taf
period. Winds should remain 5-10 kts during the overnight and
think this should preclude any fog development.
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