Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service northern Indiana
354 PM EDT Sat Aug 23 2014

Synopsis...
issued at 344 PM EDT Sat Aug 23 2014

An upper level disturbance tracking across the western Great Lakes
will bring showers and thunderstorms to portions of southwest lower
Michigan and North Central Indiana through early this evening.
The coverage of showers and thunderstorms will diminish this
evening as an upper level ridge continues to build across the
region. Warm conditions will continue into Sunday with most areas
remaining dry. There is a slight chance of an afternoon
thunderstorm across northwest and west central Indiana. Low
temperatures tonight will range from the lower 60s across
southeast lower Michigan...to around 70 across northeast Illinois.
High temperatures on Sunday will reach back into the middle 80s.

&&

Short term...(this evening through sunday)
issued at 344 PM EDT Sat Aug 23 2014

Main forecast items to address for the short term will be convective
potential through Sunday.

Mesoscale convective system across Illinois has become more diffuse over past few hours with
an overall weak background shear environment. Outflow has allowed
for regeneration of showers and thunderstorms across northeast
Illinois in moderately unstable environment. Weakening cold pool
with this mesoscale convective system and eastward progression into a more weakly forced
environment across local area lends some questions as to how far
east to bring higher probability of precipitation this evening. Will confine categorical
probability of precipitation to far western portions of forecast area with expectation
convection may be more scattered in nature across Southern Lower
Michigan and North Central Indiana. Severe threat appears to be low
given meager downstream shear...but slow movement of storms and
regeneration along composite outflow could yield some localized
heavier rainfall amounts across the far west. Outside of heavy
rainfall potential...secondary concern could be some pockets of
slightly stronger wind with greater precipitation loading associated with
stronger storms...but generally of sub-severe nature. Isolated
cells should continue to develop in advance of main area of
showers and thunderstorms with moderate instability and several
pockets of weak synoptic convergence/lake breeze convergence.

Will maintain some slight chance probability of precipitation through the evening across
mainly southwest half of the area as residual convectively enhanced
vorticity slowly drops southeast across the area. Will keep northeast
locations dry with weaker elevated instability expected and a less
favorably proximity to middle/upper forcing with aforementioned vorticity
maximum.

For Sunday...drier low level air will continue to advect westward in
response to southward shifting anticyclone across the eastern Great
Lakes. This should contract axis of appreciable low level moisture
across the middle MS River Valley/western Great Lakes. Difficult to
latch onto any substantial forcing mechanism for tomorrow...and
given best instability across the far southwest have included slight
chance probability of precipitation during peak heating. Have made just some slight downward
adjustments to maximum temperatures for Sunday with persistent low level
easterly component keeping brunt of low level thermal ridge just
west of the area.

&&

Long term...(sunday night through saturday)
issued at 344 PM EDT Sat Aug 23 2014

Hot/humid/mainly dry weather will persist into Monday and Tuesday as the
local area remains on the northern periphery of a high amplitude
subtropical ridge...with afternoon heat indices likely reaching the
middle-upper 90s each day. A frontal boundary will gradually shift
southeast on the southern fringe of shortwave jet energy/weak height
falls working northeast into the upper Midwest/northern lakes by the
middle of the week. This feature working into a very moist/unstable
airmass will bring increasing chances for showers/storms and heavy
rainfall by Wednesday-Thursday...with middle level cap, weak
forcing/flow, and Reliance on convective outflow or upstream
convection to sink into the area suggests much lower chances for
convective initiation Monday night-Tuesday. Only adjustment beyond
Thursday was to add low chance probability of precipitation...mainly for collaboration
purposes...with large model differences regarding timing/speed of a
longer wavelength trough modeled to propagate east through the
Midwest/Great Lakes.

&&

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 152 PM EDT Sat Aug 23 2014

Convective complex across northern Illinois has exhibited some
trend to southeast propagation over the last hour and expecting
this overall trend to persist through the remainder of the
afternoon. Will have to monitor for outflow accelerating ahead of
this line to possibly be focus for additional thunderstorm
development at ksbn late this afternoon. In the meantime...will
also need to monitor for potential isolated development along
weak lake breeze which has formed...with cumulus being more vertically
developed recently toward kmgc-kvpz. Some potential of thunder
will exist at ksbn into the evening hours as remnant vorticity maximum from
Illinois convection drops southeast through western Indiana. With
best near term potential of storms south and west of
terminals...will hold off on inclusion with the 18z tafs. Setup does
appear favorable for fog and stratus to redevelop overnight...and
have continued trend of previous issuance to IFR/lower end MVFR
conditions overnight. Slightly drier air to advect eastward on
Sunday and middle level ridge building into the area should tend to
discourage thunder chances for Sunday...with east winds of 10
knots or less continuing.

&&

Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&

$$

Synopsis...marsili
short term...marsili
long term...steinwedel
aviation...marsili



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

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/nwsnorthernindiana
www.Twitter.Com/nwsiwx
www.Youtube.Com/nwsnorthernindiana

National Weather Service Glossary of Abbreviations