Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service northern Indiana
409 am EDT Sat Jul 12 2014

issued at 408 am EDT Sat Jul 12 2014

High pressure will give way to an area of low pressure advancing
eastward out of the Central Plains. A few showers and perhaps an
isolated thunderstorm are possible this morning with generally
cloudy skies expected this afternoon with highs around 80.

A deepening upper level disturbance over Ontario will propel a
strong cold front into the southern Great Lakes late tonight with
a good chance for showers and thunderstorms. Lows tonight will
range in the 70s.


Short term...(today and tonight)
issued at 408 am EDT Sat Jul 12 2014

Western bound of lakes ridge slw to erode per 00z radiosonde observations and no doubt
affected by generally weak rtn follow inadv of lead SW trough
progressing through southern Ontario this morning. Much btr follow noted
further upstream across NE/western Iowa west/active convn developing there in
earnest this morning.

Near term trends would continue to suggest addnl bubbling in middle level
echoes seen across western County Warning Area westward into central Illinois as ll Theta-E ridge advts
northeastward however based on RUC/hrrr yet is likely to decay after daybreak
based on weakening/veering ll follow. Will grid accordingly.

Dvlpmnts this afternoon/evening likely tied to upstream small mesoscale convective system taking
shape across Iowa west/evolving outflw boundary dictating placement of
upstream redvlpmnt this afternoon. However downstream cloud contamination
locally will most likely hamper sufficient destabilization this
afternoon into this evening inline west/most 00z model guidance and see little
reason not to back off considerably on prior probability of precipitation until lt tonight
timed to more substantially height amplification across Ontario and low level jet
response through the western Ohio Valley. Regardless severe chances appear


Long term...(sunday through friday)
issued at 408 am EDT Sat Jul 12 2014

Main focus of long term period is on precipitation/severe weather chances
Sunday with unseasonably cool weather the only other issue for next
week. Deep pv anomaly will gradually drift south into the Great
Lakes on Sunday...pushing a cold front into the region. Nocturnal
low level jet ahead of this front will advect a very high Theta-E airmass into
the region by Sunday morning. Forecast soundings show precipitable water values
around 2 inches with 850mb dewpoints holding in the middle teens.
Expect convection will be ongoing here or immediately upstream to
start the period given strong low level convergence in a very moist
and modestly unstable environment. This activity will be associated
with a prefrontal trough/moisture gradient ahead of the main front
and will gradually push southeast as the day progresses. Diabatic
heating and decent midlevel lapse rates expected to support
1500-2000 j/kg of MLCAPE ahead of this boundary by afternoon. Exact
timing will be key to precipitation coverage and severe weather chances for
our County Warning Area. Latest higher resolution nwp shows this boundary pushing
through our area a bit too early for diurnal destabilization to
fully for our southeast third. Always dangerous
to get too specific with convection forecast over 24 hours away but
if current projections hold...could be enough instability in our
southeast counties (along and south of US-24) to capitalize on
healthy deep layer shear profiles and support a low-end severe
weather risk Sunday afternoon. Multicell clusters with damaging
straight line winds will be the main threat. Heavy rain will also be
possible given very moist environment. Not confident enough to go
categorical but likely probability of precipitation easily warranted within this corridor.
Will hold with just chance probability of precipitation in our northwest counties given
unfavorable timing.

Main cold front and negative Theta-E advection will arrive Sunday
night and support a brief dry period through Monday morning.
Significant differences noted between GFS/NAM and European model (ecmwf)/Gem for
Monday afternoon and Monday night forecast. Non-NCEP models
projecting surface wave development ahead of robust vorticity lobe
swinging through the southwest Great Lakes that leads to another
round of light-moderate rainfall for our County Warning Area. NCEP models keep upper
low further northeast with dry conditions for our area. Suspect
there will be at least some scattered showers late Monday as vorticity maximum
swings through the region but coverage/amounts remain uncertain.
Will maintain just low chance probability of precipitation for now.

Unseasonably cool weather still expected for the middle of the week
as Canadian airmass is drawn south by impressive longwave trough.
850mb temperatures drop to around 5c by Tuesday morning. Slow moderation
expected late Tuesday into Wednesday but still expect highs at or
below 70f with lows touching 50f. This is roughly 15 degrees below
normal for middle-July and will be flirting with record territory.
Gradual warming trend continues through the end of the week with dry
conditions persisting.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 408 am EDT Sat Jul 12 2014

VFR conds expected to hold this the period as surface anticyclone gives way to
deepening warm sector ahead of amplifying SW across Ontario. Isolated thunderstorms and rain
across central Illinois early this morning may reach toward the terminals around
daybreak but will likely remain anchored south along nose of stronger ll
follow/Theta-E rtn.


Iwx watches/warnings/advisories...


short term...T
long term...agd

Visit US at www.Weather.Gov/iwx (all lower case)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations