Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
614 PM CDT Sat Aug 1 2015
issued at 314 PM CDT Sat Aug 1 2015
A short-wave trough evident on 19z/2pm water vapor imagery over the
western Dakotas will be the primary triggering mechanism for
thunderstorms this afternoon into tonight. As this feature tracks
eastward...storms will increase in coverage across the eastern
Dakotas into Minnesota this evening...then will spread into the
Great Lakes by Sunday morning. Further south...high-res models such
as the rapid refresh are suggesting another area of scattered
convection will develop along the remnants of an old frontal
boundary across southern Iowa/northern Missouri after midnight.
Some of this activity may spill into west-central Illinois toward
daybreak. Have adjusted probability of precipitation to go with dry conditions across the
board this evening...then have introduced low chance probability of precipitation along/west
of the Illinois River well after midnight. Overnight low
temperatures will range from the lower 60s south of I-70 where winds
will remain lightest beneath a slowly departing high pressure
system...to the upper 60s in the Illinois River valley.
Long term...(sunday through saturday)
issued at 314 PM CDT Sat Aug 1 2015
Forecast models are coming into a bit better agreement with the
evolving upper level pattern for next week as western ridge and
eastern trough flatten out into a semi-zonal flow by late in the
week. However...as this evolution takes place...a period of
unsettled weather will be in store for central and eastern Illinois in the
In the near term...an mesoscale convective system/cluster of thunderstorms is expected to
develop in southern Minnesota/northern Iowa overnight...farther north than
previously expected. The remnants of this may bring scattered
showers and T-storms to areas northwest of the upper Illinois River valley Sunday
morning. A weak cold front will also gradually move into northwest Illinois
toward evening. Plenty of warm air and moderate instability should
lead to scattered T-storms northwest of a Macomb-Bloomington line Sunday
afternoon...reaching a Quincy-Lincoln-Danville line during the
evening. As the front continues to make progress through central
Illinois Sunday night...the potential for scattered T-storms will
increase in most areas north of I-70.
On Monday the front will slow down in southern Illinois. The low level
wind flow...however...is expected to become parallel to the boundary
resulting in a rapid drop in low level convergence. This combined
with marginal instability should result in a few clouds and little
in the way of convection in southeast Illinois.
The GFS and European models are similar in showing a piece of a
shortwave trough cutting through the upper ridge position and
approaching the middle MS and lower Ohio Valley Tuesday...with
additional shortwaves expected to follow through early Thursday.
This upper level support will combine with the nearly stationary
front in southern Illinois to produce periods of showers and T-storms.
Will keep an eye on this for a heavy rain potential...but it appears
that there should not be enough deep level moisture for this to
occur. With much of the forecast area expected to be on the cool
side of the front...temperatures should be below seasonal normals
most of next week.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 615 PM CDT Sat Aug 1 2015
Predominantly VFR conditions expected through the 00z taf period.
Latest model guidance generally consistent with dry conditions for
central Illinois terminals until 17z...then bringing a chance for
thunderstorms mainly for northern terminals kpia-kbmi. Several
clusters of convection expected to develop to the west...then
either weaken or move past central Illinois to the north middle-morning
through afternoon Sunday...so chances for disruption of VFR
conditions are minimal and have only included thunderstorms in the vicinity at terminals
kpia-kbmi after 17z. Winds SW through the period...4-7 kts
overnight...increasing to 12-16 kts after 17z with gusts around 20
kts possible in the afternoon.