Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 270546
Area Forecast Discussion
National Weather Service Lincoln IL
1146 PM CST Sat Nov 26 2016
Issued at 858 PM CST Sat Nov 26 2016
Main concern overnight is the potential for an expanding area of
stratus/fog advancing northward through central Illinois after
midnight tonight. The HRRR and RAP guidance continue to agree on
advancing the low clouds and fog currently in southeastern
Missouri, northeast across central Illinois into northeastern IL.
The GFS is less pessimistic with clouds and fog, but have
significantly increased clouds and added fog across our entire
forecast area for tonight into tomorrow morning. The clouds could
work to keep lows slightly warmer, and we already were slightly
above guidance numbers so will hold the lows as previously
forecast, ranging from 33F in the east to 37F in the west. The
remainder of the forecast looks on track, with rain chances
increasing Sunday afternoon in our far western counties. Updated
forecast info is already available.
.SHORT TERM...(This evening through Sunday)
ISSUED AT 211 PM CST Sat Nov 26 2016
Some welcome sunshine is occurring across much of the forecast area
this afternoon. The exception is over the northeast corner, where
diurnal cumulus started forming around midday from Decatur eastward.
While the western fringes have quickly faded, the clouds have been
tougher to exit areas around Champaign and Danville. Temperatures
over most areas had reached the 50 degree vicinity by 1 pm.
Surface ridge will continue to slowly shift eastward across the Ohio
Valley tonight, and southerly winds will keep temperatures up as a
result, mainly in the mid 30s over the CWA. Main question will be
for any fog development. Both the NAM and RAP soundings are showing
very moist profiles below 970 mb with dry air above it, with its
visibilities (and the associated HRRR) tanking overnight. Meanwhile,
the the GFS soundings are indicative of more mixing of the boundary
layer during the day, and only reduce visibility to around 3-4
miles. Leaning more toward the GFS scenario at the moment, but will
include mention of patchy fog after midnight over the area.
The storm system over the Plains will start cranking up quickly on
Sunday as the deep upper trough currently coming onto the California
coast ejects northeast overnight. While clouds will steadily
increase, it still looks like most of the daylight hours will be dry
in central Illinois. Have limited most of the PoP`s to the far west,
with slight chances as far east as Springfield and Peoria.
Temperatures expected to reach the lower 50s again despite the
clouds, as the southerly winds ramp up.
.LONG TERM...(Sunday night through Saturday)
ISSUED AT 211 PM CST Sat Nov 26 2016
Precip will be spreading across most of the CWA Sunday night as the
low pressure area in the northern plains continues to deepen and
move further north. This period of precip Sunday night will be the
first of two waves of precip to move across the area. Probabilities
of moderate showers with this first wave will be quite high and qpf
during the 12hr period will be near one half inch, with the highest
amounts being north of I-72. There will be a lull after this wave as
the main cold front will still be lagging back to the west during
the day Monday morning. The highest pops during the morning will be
in the south and southeast as this will be where any lingering
precip associated with the first wave should be. Pops will increase
again during the afternoon as the second wave moves northeast into
the eastern half of the CWA, mainly east of I-55. There should be a
good amount of instability in the area, given the amount of warm and
moist air that should be advecting into the area ahead of the
system. So, this wave will bring showers and isolated thunderstorms
to the area Monday afternoon through Monday evening, with the best
chance of thunder being associated with the best instability, Monday
evening. QPF will be higher with the second wave and all models now
agree that the axis of heaviest qpf should be across the
southeastern portions of the CWA, where qpf values should exceed one
inch. The main front of this system will move through the CWA Monday
evening, but showers will still be possible during the overnight
hours as a mid level trough rotates through the area. Winds will be
southeast ahead of the system and will be breezy/gusty due to the
tight surface pressure gradient and strong low level winds...even
during the Sunday night time frame. Speeds will begin to taper off
once the precip moves through the area later Monday night. Beginning
Wednesday, and through the remainder of the forecast period, surface
winds will be westerly with winds not very strong.
By Tuesday morning, the front and associated weather will be well
northeast and east of the area, so dry weather can be expected into
the middle of the week. The mid level low pressure area associated
with the surface system will remain in the western Great Lakes
region, keeping the CWA under some cyclonic flow aloft. As the upper
level system finally moves east during the middle of the week,
partly to mostly cloudy skies will be possible, but no precip is
expected at this time. Surface high pressure will finally build into
the region for the end of the week and into next weekend, while the
mid levels transition into a two stream setup, with one upper level
low in the southwestern US and the other over the Great Lakes.
Temperatures will be quite warm through Monday night, due to the
strong warm air advection forecasted ahead of the weather system.
The cold air looks to lag quite a bit behind the precip, so Tuesday
should be above normal as well. The cooler, below normal, air looks
to hold off until the latter part of the week.
.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1146 PM CST Sat Nov 26 2016
Main concern tonight will be the potential for IFR/LIFR fog
and/or stratus advancing north through central IL after 09z. The
HRRR, RAP and NAMNest have been slowing down the arrival of the
clouds/fog with each run this evening. Satellite images are even
slower than the high resolution models, so there may be even more
delay of those conditions. Have enough confidence that we should
see some fog and low clouds, have continued with TEMPOs from 11z-
14z for LIFR vis/IFR cigs. Will be monitoring satellite trends, high
res model updates and forecast soundings to see if prevailing LIFR
conditions need to be adjusted later tonight. Models indicate fog
may linger until 16z-17z for the northern terminals, before
conditions improve to VFR.
Rain chances increase later Sunday afternoon for PIA/SPI, as the
next system approaches from the Plains. Forcing for precip
maximizes west of I-55 Sunday evening, so have prevailing rain and
MVFR conditions developing from west to east Sunday eve.
South-southwest winds look to remain steady at 9-13kt the rest of
the night. Winds will increase more dramatically Sunday evening
with the onset of rains, with S-SE winds at 13-15G26kt.