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Area forecast discussion 
National Weather Service northern Indiana
530 PM EDT Thursday Aug 21 2014

issued at 310 PM EDT Thursday Aug 21 2014

A series of upper level disturbances will move across the Great
Lakes and Ohio Valley regions tonight. These disturbances will
interact with a stalled frontal boundary across the region and
abundant low level moisture to produce a good chance of showers
and thunderstorms tonight into early Friday. An upper level ridge
will build into the region Friday night and Saturday with hot and
humid conditions...along with the possibility of isolated
thunderstorms. Low temperatures tonight will drop into the upper
60s and lower 70s. High temperatures on Friday will range from the
lower to middle 80s across central lower around 90
across northeast Illinois and central Indiana.


issued at 530 PM EDT Thursday Aug 21 2014

Growing increasingly concerned west/conv cluster over NE Illinois late this
afternoon. 21z surface observation/visible Sat imagery show outflw induced baroclinic
zone along a kvpz...krcr...khhg line and large/very unstable
Reservoir S/SW across central IL/in. Suspect west/gradual increasing
moist southwesterly inflow into this ftr through this evening will yield
both a low end severe wind/isolated hail risk. More importantly a
risk for flooding as cells tend to parallel both ll boundary and middle
level shear vector suggesting echo training and heavy rain likely
in vicinity of of the Highway 30 corridor.

Will make some sig adjustments to going probability of precipitation/weather through this
evening to reflect above thinking west/zone forecast product amendments forthcoming.


Short term...(this evening through friday)
issued at 310 PM EDT Thursday Aug 21 2014

The next in a series of small scale short waves across southeast
Wisconsin will drop east southeast over the next few hours
resulting in increasing coverage of showers and thunderstorms.
Convection earlier this afternoon has stabilized conditions
somewhat for surface based parcels...but well-established axis of
pooled low level moisture will allow for moderate elevated
instability and scattered thunderstorms. Showers also appear to be
focusing late this afternoon on axis of differential heating from
vpz-rcr-aoh. Shear profiles will be steadily on the increase over
the next several hours belt of stronger upper level
westerlies overspreads the southern Great Lakes region. Limiting
factors to instability should tend to mitigate severe potential
over the next several hours. Next upper level short wave
progressing through mean ridge across eastern Iowa/northwest
Illinois should allow for continued broad isentropic ascent over
low level boundary across the region...with continued thunder
chances late evening into the early portion of the overnight. Some
low end severe potential may exist if upstream convection can
congeal later this evening...which cannot be ruled out given
strengthening background shear profiles. Other concern for tonight
will be possible Hydro impacts. Showers/storms earlier today laid
out an axis of 1-2 inch rainfall amounts from western Starke
County into Grant County. This corridor should represent best
overall elevated instability tonight...and with little
displacement of nearly 2 inch precipitable water axis and warm cloud depths
greater than 12k feet...cannot rule some localized heavy rainfall.
With still some uncertainties on how upstream convection will
evolve tonight...and a fairly sizable temporal gap from this
mornings heavy rainfall...have opted to hold off on any Hydro
headlines at this time.

Aforementioned Iowa short wave will become more sheared in nature
across the Ohio Valley/southern Great Lakes tonight into early
Friday morning and provide best forcing to eastern portions of the
area by afternoon. Middle level height rises will slowly build in
from the west during the day also...which should tend to
limit convective coverage across far western areas by late
afternoon. Have maintained high chance probability of precipitation into Friday
morning...with gradually diminishing probability of precipitation from west to east
during the afternoon. If good deal of insolation can be
realized...temperatures could reach into the lower 90s across
southwest portions of the area late afternoon...with heat indices
pushing near the 100 degree mark. Greater cloud cover/precipitation
should allow for heat indices remaining in the lower 90s across
the eastern half.


Long term...(friday night through thursday)
issued at 310 PM EDT Thursday Aug 21 2014

Upper level ridge will build northeast into the Great Lakes region
this weekend into early next week which will support a trend toward
hot/mainly dry weather. Retained low chances for showers/storms Friday
night into Saturday...mainly southwest where low level Theta-E
gradient remains anchored as cooler easterly drainage flow into
our northeastern zones limits northeast mix of diffuse instability
gradient/boundary. Precipitation chances remain nonzero into Sunday and
Monday given an increasingly moist/unstable boundary
layer...although warming/drying aloft under upper ridge axis suggest
holding with a dry forecast (10 pop) still the way to go.

Gradual eastward drift of a western Continental U.S. Upper trough into the
north-central US will eventually flatten upper ridge in place...
with moisture channel/Theta-E fold into the lower lakes supporting
increasing chances for showers/storms. Models continue to differ
with timing/amplitude of this feature resulting in low confidence
pop/temperature forecast Tuesday and beyond.


Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 129 PM EDT Thursday Aug 21 2014

First wave of showers and thunderstorms will continue to exit
Northeast Indiana over the next few hours...with attention for
remainder of afternoon and evening turning upstream with a couple
of additional short wave progressing through upper ridge. Some
lingering IFR conditions will persist over the next hour or two
before improving after 20z. The next short wave across southern
Wisconsin should allow showers to move back into North Central
Indiana late this afternoon. A relative stable bubble persists
across North Central Indiana in regards to surface based convective
potential...but suspecting enough elevated instability will be
present for some thunder potential as the southern Wisconsin short
wave shifts eastward this afternoon. Better chance of thunder
still appears to be in store for late evening into the overnight
as next upstream Iowa short wave approaches the area with
persistent isentropic upglide associated with low level boundary
across the region. Have kept in vcsh mention after 12z with a more
muddled picture at this forecast distance in terms of latching
onto any significant additional short waves. Gradually middle level
height rises toward the end of the period should diminish
convective potential later Friday afternoon.


Iwx watches/warnings/advisories...


short term...marsili
long term...steinwedel

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