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Area forecast discussion 
National Weather Service Lincoln Illinois
1249 PM CDT Tuesday may 5 2015

issued at 1045 am CDT Tuesday may 5 2015

15z/10am surface analysis shows frontal boundary extending from
north-central Illinois into central Iowa...generally along a Des Moines line. Widespread rain
continues to fall north of the front: however, mostly sunny and
dry conditions prevail to the south across central Illinois.
Temperatures have already risen well into the 70s across all but
the far northern kilx County Warning Area. Based on the 12z upper air sounding and
assuming mixing up to about 750mb similar to yesterday
afternoon...highs today should reach the lower to middle 80s. Have
adjusted temperatures up by a few degrees accordingly.


Short term...(today)
issued at 332 am CDT Tuesday may 5 2015

A frontal boundary is oriented west-east across central Illinois this
morning with precipitation in isentropic ascent region largely to
the north of the boundary. Models indicate the boundary and
precipitation will shift slowly northward today. Weak disturbances
moving along the boundary will produce some enhancements in
convective shower coverage in the vicinity...early this morning from
around the Illinois River northwestward...and late in the afternoon
north of the I-74 corridor. For the most part...convection should be
weak at this point with only weak middle-level instability remaining
through the night...and 0-6km bulk shear up to 25-30 kts this
morning diminishing substantially for the afternoon. 1000-1500
joules of surface-based cape should be able to develop with
afternoon heating along/north of I-72 and can't rule out isolated
thunderstorms southward to I-72 as a result.

Long term...(tonight through monday)
issued at 332 am CDT Tuesday may 5 2015

Ongoing convection along the boundary over northern Illinois this
morning not only driving the weather throughout the day, but into
the overnight as well. GFS and NAM continue to keep a bit of precipitation
pretty far south, as far south as a line from Havana to Bloomington.
However, the rap/RUC and the hrrr are pushing the area of convection
well to the north through the morning with the boundary. When the
secondary wave ripples through the region after 00z, it would be
displaced north of the forecast area. The 4km WRF has just that solution,
starting more in line with the rap/RUC and hrrr...and keeping the forecast area
dry after 00z, but a little more generous with some isolated
instability showers. Have kept isolated ts in the north, mainly along
and north of I-74 for the early evening.

High pressure begins to build back into the region later tonight,
southerly winds pushing the front back to the north and out of the
forecast area through Wednesday night. Forecast remains roughly the same
beyond day 3... precipitation chances slowly edging back into west central
Illinois for Thursday as an upper level wave pushes out of the SW.
The energy is part of a general pattern shift to more southwesterly
flow with the upper level trough digging back in over the southwestern Continental U.S..
the surface low moves out into the upper Midwest and into Canada by the
end of the week, but in shifting to more southwesterly flow aloft,
opens the door to a series of shortwaves that keep the forecast wet
for the end of the work week and into the weekend. Temperatures rather
warm through the forecast with overcast and rainy skies keeping the
overnight lows up a bit, but likely not nearly as warm as the trend
in the guidance. Have knocked them down a couple degrees. Southerly
flow and a relatively warm airmass keeping the highs a bit above
norms, even with the passage of the first low skirting to the
northwest...the middle levels remain warm until the main trough over the SW
lifts into and out of the Great Lakes. Cooler air in the midlevels
is only around for 36 hours day 7/8 before the warm airmass
reestablishes itself.


Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1249 PM CDT Tuesday may 5 2015

Latest surface analysis continues to show nearly stationary
frontal boundary draped across north-central Illinois from just
north of kgbg to south of kpnt. Winds in the vicinity of the
boundary are light/variable at kpia...but are southwesterly at 10
to 15kt further south at the remaining terminals. High-res models
suggest the front will sink slightly southward later this
afternoon and evening...resulting in a wind shift to the east/NE at
both kpia and kbmi. Models differ on the exact timing...with the
hrrr showing the shift at about 22z. After that...the front will
remain in place along/just south of I-74 throughout the night.
Have continued with a light east/NE wind at kpia and kbmi
accordingly...with southerly winds at kspi and kdec. The front
will finally get a solid push northward by Wednesday
morning...allowing winds to become southerly at all taf sites.


Ilx watches/warnings/advisories...


short term...Onton
long term...hjs

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