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Area forecast discussion 
National Weather Service Lincoln Illinois
849 PM CDT Thursday Sep 3 2015

Update...
issued at 849 PM CDT Thursday Sep 3 2015

Quiet conditions across central Illinois this evening a couple of upper
level lows remain to the north from Wisconsin through the Great
Lakes region. A cold frontal boundary and associated surface
convergence near the Illinois/WI border resulted in a line of
thunderstorms this evening...but current radar trends and model
forecast trends indicate this will weaken and remain to the north
through the night. As a result...mostly clear skies...lows
reaching to around 70 and dewpoints in the upper 60s can be
expected. Some areas of light fog and haze are expected by
morning...but shallow moisture in evening sounding indicates any
fog should be patchy and/or thin. No significant updates
anticipated this evening.

&&

Short term...(tonight)
issued at 244 PM CDT Thursday Sep 3 2015

Weak frontal boundary/wind shift line as of 200 PM was located
between Milwaukee and Chicago with middle and upper 70 dew points
pooling along the boundary over parts of southwest Wisconsin into
northeast Iowa. Across our area...dew points once again have mixed
down into the middle and upper 60s while early afternoon temperatures
were in the lower 90s producing heat index values mostly in the
middle 90s. Main concern in the short term will be chances for rain
tonight as the weak front approaches our far northeast/northern
counties before becoming stationary. Except for the NCEP WRF-nmm,
most of the high res models suggest the bulk of the convection will
stay north of the area this evening before dissipating towards
midnight, so will not add any mention across our north and northeast
at this time. Otherwise, another warm night as early morning lows
struggle to drop into the upper 60s to lower 70s by dawn accompanied
by a light southerly breeze. May see some light fog and haze around
again by tomorrow morning, but it appears coverage across our area
will be patchy at best, so will not include in the grids.

Long term...(friday through thursday)
issued at 244 PM CDT Thursday Sep 3 2015

The forecast through Sunday will be dominated with heat and
humidity, as an upper ridge in the plains slowly drifts eastward.
The advancement of the ridge will be slowed by a couple of upper
shortwaves stalled over the western Great Lakes. Those two waves
will be responsible for shower and thunderstorm chances in our
northeast counties tomorrow and Saturday, mainly northeast of a line
from Lacon to Paris. We kept the slight chance probability of precipitation in that area for
both days. Otherwise, expect highs in the 89 to 94 range each day,
with afternoon and early evening heat index readings of 95 to 102.

The next concern is the timing of the pattern change for next week.
The system that is expected to break down the ridge is currently in
British Columbia. The models are in general agreement on dropping
that low south of British Columbia into Oregon on Saturday, then
lifting it northeast in Manitoba by Monday and stalling it
there. During that time, a surface low is expected to develop
ahead of the upper low in western South Dakota on Saturday, then
move into northwest Ontario on Sunday night-Monday, dragging a
dissipating cold front into Illinois on Monday. That front is
projected to stall out across Illinois as it becomes parallel to the
steering winds, providing scattered storm chances from later
Sunday night through Tuesday. There are varying solutions as to
short wave progression along the front, but with only marginal
convergence along the front, will keep probability of precipitation in the low chance
range through Tuesday.

Additional questions remain on the timing of the subsequent push of
cold air to give US our airmass change. The European model (ecmwf) and Canadian are
both showing a muddier picture of that next cold frontal passage,
while the GFS is a bit more progressive and gets the front east of
our counties by late Wednesday afternoon. The ec/Canadian bring the
second front into Illinois on Tuesday night into Wed, but indicate yet
another shortwave riding along the front starting late Wednesday/Wednesday
night, keeping precipitation chances into Thursday. Will keep some chance
probability of precipitation southeast of I-55 Wednesday night, and only slights in the far south
counties on Thursday for now.

Relief from the heat should develop starting on Tuesday, but more
significantly on Wednesday and thurs, when highs only climb into the upper
70s to lower 80s.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 604 PM CDT Thursday Sep 3 2015

VFR conditions are forecast this taf cycle. Diurnal cumulus will
quickly dissipate this evening...and with mostly clear skies and
light winds overnight some patchy light fog is possible. Used
persistence from last night to bring some lower visibilities (near 5sm)
to kbmi and kcmi after midnight. After sunrise any light fog will
dissipate. Light southwest winds should stay under 8 kts...and
scattered diurnal cumulus with bases around 4k feet will be common
through Friday afternoon.

&&

Ilx watches/warnings/advisories...
none.

&&

$$

Update...Onton
short term...Smith
long term...shimon
aviation...25

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