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Area forecast discussion 
National Weather Service Lincoln Illinois
701 am CDT sun Jul 24 2016

Short term...(today and tonight)
issued at 325 am CDT sun Jul 24 2016

Abnormally hot conditions will continue for one more day, before a
cold front pushes through Illinois later this afternoon and tonight. There
is enough evidence that heat indices will reach advisory criteria
(105f) even across our northern counties, so we decided to add the
northern tier of counties back in the excessive heat warning (due to
cumulative effect of 4 days of heat advisory levels). High temps
should top out in the lower 90s, and dewpoints should reach the mid
to upper 70s, producing peak heat index readings of 102 to 107f. The
heat warnings will end at 7 PM for all of our counties.

Despite minor timing differences for frontal progression in the
short term models, the overall consensus has the cold front reaching
near the Illinois River around 00z/7pm this evening. The rap and hrrr
models seem to be acknowledging the capping inversion across central
IL, because they keep storms at Bay ahead of the front for much of
the afternoon. The forcing along the front should be enough to
trigger a line of convection that may contain strong winds and hail.
We remain in the Storm Prediction Center day 1 marginal risk for severe storms north of
I-70. The uncertainty in breaking the cap sufficiently is one
factor that has kept US below the slight risk to this point.

In light of the latest model trends, have kept the forecast dry
until this afternoon. We started with slight chances northwest of the Illinois
River at 20z/3pm, then ramped up to high chance pops by 00z/7pm,
while advancing the storm chances to the southeast as the front
progresses toward southeast Illinois. The most active period for storms in our
area appears to be this evening, as the low level jet intensifies
ahead of the front just after sunset. The main storm axis may be
along I-72 at that point, then progress into southeast Illinois after
midnight.

The airmass change behind the front should be delayed enough to keep
low temps in the low to mid 70s, with a few upper 60s possible
farther northwest toward Galesburg.

Long term...(monday through saturday)
issued at 325 am CDT sun Jul 24 2016

The slowing boundary moving through the Midwest sun/Mon will linger
the precip through Monday, slowly retreating to along and south of
the I-70 corridor by the end of the day. High pressure building just
to the west will keep a slight northerly component to the winds.
Temperatures retreat into the 80s for the first part of the work
week, and reduced relative humidity will keep heat indices in the 90s. However,
the 850mb temps only varying a few degrees throughout the week. Once
the ridge passes Wed night, more southerly direction to the winds
sets up and temps may start to edge up again. Concern with the
extended guidance at this point is that it is not handled well, and
the perpetual convective feedback at the end of the week is keeping
the guidance artificially low. That being said, a warm and muggy
summertime airmass will keep plenty of instability in place for
mesoscale influences, and pops returning to the forecast in an
increasingly northwesterly flow over the building western ridge.
Latter half of the week may see rounds of thunderstorms as
shortwaves top the ridge yet again. Whether or not the convection
materializes enough to warrant the lowered guidance in Max temps
remains a question.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 700 am CDT sun Jul 24 2016

The muggy airmass has produced MVFR vsbys in fog, which will
linger until 14z or so. Cmi is the only site that dipped into IFR
with 2 1/2sm. Once the fog clears, skies will be relatively cloud
free, with cirrus to start off the day, then some scattered cumulus as
the sun heats the boundary layer.

The majority of the high resolution models keep dry conditions
until around 20-22z, as the cold front reaches near pia. The pre-
frontal airmass should remain relatively capped until the frontal
trigger arrives. A line of storms should progress southeast from
late afternoon through the evening. The NAM and GFS show an
expansion of precip coverage after 06z along the front, but storm
severity should be relatively low at that point. Have just kept a
thunderstorms in the vicinity going from late afternoon through the later portions of the
taf period due to timing uncertainties.

Light south winds this morning will become southwest and increase
to 10-13kt this afternoon. After frontal passage tonight, winds will be
variable around 5 kt until west-northwest winds redevelop toward sunrise
tomorrow.

&&

Ilx watches/warnings/advisories...
excessive heat warning until 7 PM CDT this evening for
ilz040>057-061>063-066>068-071>073.

Excessive heat warning from 11 am this morning to 7 PM CDT this
evening for ilz027>031-036>038.

&&

$$

Short term...shimon
long term...hjs
aviation...shimon

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