Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
659 am CDT sun Jul 5 2015
issued at 317 am CDT sun Jul 5 2015
High pressure over the eastern Great Lakes will continue to control
the weather across central Illinois today...providing warm and dry
conditions. After patchy fog dissipates by mid-morning...mostly
sunny skies will be the rule for the remainder of the day. Forecast
soundings suggest mixing up to about 4000kt...resulting in afternoon
high temperatures in the middle 80s. Humidity levels will begin to
increase as well...as light southerly return flow brings middle to
upper 60s dewpoints into the area.
Long term...(tonight through saturday)
issued at 317 am CDT sun Jul 5 2015
Short-wave trough skirting along the Canadian border will push a
cold front into central Illinois late Monday into Tuesday. Models
continue to be in good agreement concerning timing of system...with
front remaining west of the Mississippi River until Monday night.
Airmass ahead of the boundary will become highly unstable as
dewpoints rise into the lower to middle 70s. NAM indicates even
higher dewpoints reaching the upper 70s...but think this may be a
bit too aggressive. Following the more conservative GFS still
produces SBCAPE in excess of 3000j/kg Monday afternoon/evening.
Despite the impressive instability...wind fields will be quite light
with 0-6km shear values of only 15-20kt. As a result...am expecting
a low severe weather risk as the front arrives with perhaps a few
marginally severe wind gusts with some of the storms Monday evening.
Main threat may be localized flash flooding due to precipitable
water values approaching 2 inches and overall slow storm movement.
Before the front arrives...breezy and very warm conditions will be
on tap Monday with highs climbing into the upper 80s to around 90
degrees. Have included a low chance pop along/west of the
Illinois River during the afternoon...but think best rain chances
will hold off until evening. Model consensus takes front to the
I-55 corridor by 12z Tuesday...so the far east/southeast kilx County Warning Area may remain dry
until Tuesday morning.
Front will sag southward on Tuesday...then will eventually become
parallel to the upper-level flow and stall across southern Illinois
Tuesday night. As seen in previous model runs...latest solutions
push boundary far enough south to end rain chances across the far
northern County Warning Area Tuesday afternoon and night...while precipitation continues
further south. Area of low pressure will ride along the boundary on
Wednesday...pulling it back northward slightly and spreading showers
and thunderstorms across the entire County Warning Area.
After that...model solutions diverge as upper-level ridge shifts
from the southeast Continental U.S. To the Desert Southwest late this week into
early next week. The GFS temporarily builds the ridge over the
Midwest...pushing the persistent frontal boundary back northward and
introducing a period of hot and dry weather for Friday and Saturday.
Meanwhile...the European model (ecmwf) is much weaker with the ridging over the
Midwest during that time frame and therefore keeps the boundary and
resulting rain chances in the vicinity. Both models come back into
good agreement early next week as the ridge establishes itself over
the southwest and a downstream trough digs across the Great
Lakes/Midwest. While the Gem looks very much like the hotter/drier
GFS...am reluctant to disregard the European model (ecmwf) because it is basically
advertising a continuation of the pattern we have been locked into
for the past several weeks. Despite likely continuation of
northwesterly flow or only weak ridging...do not want to fill the
forecast with rain chances each and every day through the extended
as dry periods will be likely. Will instead drop probability of precipitation to slight
chance Friday into the weekend until better model consistency is
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 658 am CDT sun Jul 5 2015
Patchy fog will linger through 13z, with only Dec down in the MVFR
category. Otherwise, VFR conditions will prevail the remainder of
this taf period. The forecast area will see high pressure depart
farther to the east today, allowing southerly winds to develop
behind the high. Wind speeds should remain less than 10kt. The
pressure gradient will begin to tighten tonight, keeping south
winds in the 7-8kt range overnight.