Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
757 PM EDT Wednesday Aug 20 2014
issued at 605 PM EDT Wednesday Aug 20 2014
Periodic showers and thunderstorms are possible late tonight into
Friday as a warm front slowly moves north into the area. Heat and
humidiity will return by Friday as this front moves north. Lows
tonight will be in the 60s with highs on Thursday in the lower 80s.
Short term...(this evening through thursday)
issued at 251 PM EDT Wednesday Aug 20 2014
Tranquil afternoon so far in shallow Post frontal airmass. However ll drying
has not been as extensive as indicated in prior model solutions and
isolated rain showers northwest shld uptick some through this afternoon given band of
aggravated cumulus across extending eastward into Northwest Ohio.
Otrws primary difficulty is west/northward rtn of surface frontal zone inadv of next
SW disturbance rolling eastward through southwestern Canada this afternoon. While
model guidance is similar aloft...timing of strong ll moisture flux
differs and generally sided west/slwr solutions in light of current
complex working through southeast Illinois/SW in...thus slowed arrival of
likely probability of precipitation SW half until 12z. Active wmfntl zone expected to continue
on Thursday especially central/west under the Assumption that outflw/warm front
composite anchors surface frontal zone through northern in if not further
south. Bumped southwestern half higher yet cut far NE lower per consensus of
highres guidance. Prior temperatures likely too warm as noted in upstream
temperatures under extensive clouds/precipitation this afternoon and have cut maximum temperatures
some but likely not enough SW. Will let night shift take another
look at that.
Long term...(thursday night through wednesday)
issued at 251 PM EDT Wednesday Aug 20 2014
Warm front will continue to slowly lift northeast Thursday night
into Friday. Suspect an mesoscale convective system will develop somewhere over the upper
Midwest Thursday night with low level jet strengthening to around 30-40 kts
and strong Theta-E advection. This mesoscale convective system will eventually dive
southeast early Friday morning. Low confidence in exact track of
nocturnal convection but it could impact our County Warning Area. This combined with
potential for lingering convection from the afternoon supports at
least a high chance pop for Thursday night. There is a low chance of
severe weather into the overnight period with forecast soundings
indicating around 1000 j/kg of elevated instability. Heavy rain may
be the bigger threat with precipitable water values at or above 2 inches and
potential for training storms.
Uncertainty remains unusually high for precipitation chances Friday into
the weekend. Warm front will likely stall somewhere over the Great
Lakes and wash out a bit. 12z nwp suite continues to show widespread
convective quantitative precipitation forecast but forecast soundings also show a decent warm nose
around 850mb. Combined with a lack of forcing mechanism amid
building longwave ridge...suspect model quantitative precipitation forecast may be overdone.
Maintained just a low chance Friday into Saturday. Kept Sunday and
Monday dry though an isolated storm cant be completely ruled
out...especially in our northeast counties where the cap will be
weaker. Midlevel trough and surface cold front will eventually
arrive with a better chance of rain but latest models now suggest
that could be as late as Wednesday.
Somewhat higher confidence aspect of the forecast will be the heat
and humidity over the weekend. 850mb temperatures still look to be around
20c and most guidance agrees on high temperatures around 90f. Surface
dewpoints also expected to hover in the low to middle 70s...supporting
heat index values near 100f in our southern counties.
Aviation/update...(for the 00z tafs through 00z Thursday
evening) issued at 749 PM EDT Wednesday Aug 20 2014
Continued similar vein of prior taf issuance in latest iteration.
Meld public/near term grids toward tafs with respect to sig later
convective initiation for northern Indiana. Timing shifted to
later given well southwestward suppression of mixed layer cape lying out
from west central Iowa to mouth of Ohio River. Mlcin to north and
lack of cogent low level jet dynamics until closer to daybreak
supports lowered probability of precipitation this evening into early tonight. Suspect
some brief dips into IFR met conditions in heart of thunderstorm
complexes...though exact timing of fine mesoscale details beyond
scope at present.
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