Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 191750
Area Forecast Discussion
National Weather Service Lincoln IL
1250 PM CDT FRI AUG 19 2016
Issued at 1046 AM CDT Fri Aug 19 2016
Relatively featureless WSW upper level flow is over central IL
this morning with shortwaves propagating eastward just south and
just north of the area. Following thunderstorms over north-
central IL overnight an outflow pushed southward to somewhere
between I-72 and I-70 before stalling out around sunrise. Can`t
find much of a discontinuity in wind or dewpoint fields at this
point with SSW winds around 5 mph and dewpoints low 70s. There is
a large stratus field from close to I-72 southward promoting lower
temperatures in the mid 70s compared with around 80 to the north.
Stratus should take a few more hours to dissipate with daytime
heating. By afternoon a slight chance for isolated thunderstorms
will develop with moderate instability over the area. Some areas
that could become a better focus for convection might be the far
northern regions of the forecast area near a convective boundary
currently near I-80, and somewhere along the late morning edge of
the stratus bank. Expect highs today reaching the mid 80s I-72
southward where daytime heating will be diminished somewhat due to
morning cloud cover...to the upper 80s northward. Updates to
morning PoPs due to above mentioned morning trends.
.SHORT TERM...(Today and tonight)
ISSUED AT 322 AM CDT Fri Aug 19 2016
Scattered showers will continue this morning across portions of
central and eastern IL. As the outflow boundary continues to move
south, the showers should continue to dissipate early this morning.
So not expecting them to get very far. Outflow boundary has reaches
down to I-72 and thinking is it will not get much further. Then in
the morning as the sun breaks through some of the clouds and winds
become more southerly again, the boundary will begin to lift back
north across the area. Isolated showers will still be possible along
this boundary with isolated thunderstorms possible during the late
morning and through the afternoon, over the whole CWA.
Then as the cold front approaches from the west this evening, pops
will increase from the west and southwest. Pops will become likely
in the west later tonight...several hours after midnight. Though the
front does not move through the CWA til Saturday, showers and storms
will begin across the CWA well out ahead of the front late tonight.
Any storms that move into the area later tonight, associated with
the front, are not expected to be severe as the stronger dynamics
and instability will remain back with the front today through
Should be plenty of sunshine today, so temps will be warm again,
reaching into the mid to upper 80s by this afternoon. Clouds and
pcpn tonight will temps remaining around 70.
.LONG TERM...(Saturday through Thursday)
ISSUED AT 322 AM CDT Fri Aug 19 2016
Northern stream and subtropical jets are expected to interact
across the Midwest Saturday providing increased quasi-geostrophic
lift with the right entrance region of the northern stream segment
over Ontario enhanced by the left exit region of a subtropical jet
streak over the Middle Mississippi Valley. Best forcing appears to
be over the Great Lakes, but some enhancement is likely as far
south as Central Illinois as the surface system deepens to our
North. Models are in good agreement in timing of the upper wave
and associated surface cold front.
With significant upper support, thunderstorms are expected to be
ongoing Saturday Morning along and ahead of the front which should
be near the Mississippi River at 12z. The front is expected to be
relatively progressive with the front reaching the Wabash River
around 00z Saturday Evening and rain chances ending soon after
frontal passage. Amount of instability available for modulating the
intensity of the convection Saturday afternoon east of I-55 will be
dependent on overall cloud cover and whether enough breaks develop
to push temps into the 80s. Models currently suggest abundant clouds
in place should limit strength of storms but time heights do
indicate an increase in synoptic forcing during the afternoon across
the eastern half of the forecast area and would not rule out some
strong gusty winds with the more intense storms.
In the wake of the front, 850mb temps are expected to plummet with
both ECMWF and GFS suggesting temps at 850mb dropping around 10C
between 18z Saturday and Sunday. Would not be surprised to see
diurnal cloudiness develop early Sunday as coldest mid-level temps
move across state with the main trough axis enhancing lapse rates.
This cloudiness should inhibit warming Sunday and lead to temps 10-
15 degrees below normal for mid to late August.
The trough axis moves east of Illinois by Sunday Night and with
surface high pressure over the mid-Mississippi Valley and rapidly
dissipating cloudiness early in the evening, temps Sunday Night may
be some of the coolest we have seen since May. NAM from 00z drops
temps well into the 40s north of I-72/Danville by 12z Monday
Morning. Lows near these values would be near record levels for
August 22. Will not go quite that cold yet but will start trending
lows downward as the last three runs of the GFS have also pushed
temps lower each time. Fog potential may also become issue depending
how much rain falls Saturday.
Building shortwave ridging develops over Illinois as the week
progresses and 500mb heights climb 60-80 meters between Monday
morning and Tuesday afternoon. This should lead to a gradual
increase in temps ahead of the next large-scale trough expected to
begin to impact the region late Wednesday into Thursday. Some
differences in timing evident in the GEFS plumes and Will go with a
blend of determinalistic models for temps and rain chances mid-week.
.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1230 PM CDT Fri Aug 19 2016
Generally MVFR ceilings this afternoon as stratus from near I-72
corridor southward slowly dissipates and additional cumulus
develops to the north. As significant breaks look like they will
be less common than MVFR ceilings...have at least TEMPO MVFR cigs
at all sites. Another feature is an outflow boundary propagating
southward from northern IL morning convection. This looks to reach
KBMI around 20Z and have included VCTS from 20Z-24Z. Doubtful this
feature will reach any additional sites so chances of
thunderstorms this afternoon/evening look too slight for mention
in TAFs at this time. Overnight...a cold front approaching from
the west will bring increasing chances for thunderstorms after
07Z-10Z...at which point VCTS and MVFR cigs introduced into TAFs.
Precipitation and lower cigs look to become more prevalent after
10Z-14Z. Winds will remain southerly through the period and should
remain less than 10kts.