Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
1154 PM CDT Sat Aug 29 2015
issued at 900 PM CDT Sat Aug 29 2015
A lingering band of showers will continue across central and
eastern Illinois this evening, then begin to progress east after
midnight. The upper level trough axis extends across Illinois from NE to
SW, and will be the focus for the continued chances of precipitation
overnight. Clouds will primarily dominate the skies overnight, but
patches of clearing are showing up in eastern Iowa and far western
Illinois on the latest satellite images. Any clearing northwest of the Illinois River
will likely result in fog formation, with some areas seeing dense
fog. Have left a mention of fog for northwest of the Illinois River, but there
is some indications in the hrrr and rap that the fog could affect
spotty areas in the remainder of our forecast area. For now, will
not include fog everywhere, but will watch closely for any
clearing in the clouds east of the Illinois River, which would increase
fog potential there.
Low temperatures will be mainly held in the middle 60s due to cloud cover
and minimal airmass change overnight under light and variable
winds. Updated the weather/pop/sky grids to match expected trends.
The remainder of the forecast information appears on track.
issued at 228 PM CDT Sat Aug 29 2015
Latest surface map showing frontal boundary arcing from the
northeast tip of Illinois to east central Iowa...before extending
southwest into southeast Kansas. Precipitation has largely remained
light and scattered...and has been mostly east of I-55 the last
several hours. Decent slug of dry air aloft seen on water vapor
imagery south of a shortwave...which is currently dropping into
southern Iowa. Short-range model guidance is having a bit of
difficulty as to the amount of redevelopment...but the hrrr is
performing halfway decently and continues to suggest mainly scattered
showers. Thunder has been staying away from our area so far...and
while mesoanalysis shows respectable cape values of 1000-1500 j/kg
this afternoon...middle level lapse rates are fairly modest. Therefore
will continue mention with only isolated thunderstorms through
tonight...with the threat diminishing from northwest to southeast.
Some concerns later in the night with potential for some fog...
mainly across the northwest County Warning Area. Still quite a bit of lower
stratocumulus across most of Iowa and Wisconsin. Latest rap model
guidance suggests this contracting some more over the next several
hours...before significantly expanding again this evening with high
pressure expanding over the Midwest. Much of the area underneath
this high saw from 1 to several inches of rain in the last couple
days...and is most favored for dense fog potential. However...the
rap shows the fog field oozing southward after midnight...perhaps as
far as Peoria. That area already had patchy fog in the grids...but
am thinking it will be a bit more widespread and will go with areas
of fog instead.
Long term...(sunday through saturday)
issued at 228 PM CDT Sat Aug 29 2015
Showers and isolated thunderstorms could linger in the southeastern
part of the County Warning Area Sunday morning but then spread back west
some...covering the eastern half of the County Warning Area...mainly east of I-55.
This should be the end of the precipitation as the frontal system diminishes
over the area remainder of the weekend. Fog and lower clouds will
seen this morning along and behind the front...so thinking is that
the fog will continue into the first few hours of Sunday morning.
Have included a slight chance of showers for Sunday night...again
mainly east of I-55 until midnight...and then in the extreme southeast
Beyond this upper level ridging will build into the northern plains
and northern miss valley. On the surface...high pressure ridging will
build back toward the area from the southeast. The combination of
the two should help to keep the area dry through the beginning of
the week. Temperatures will be in the lower to middle 80s tomorrow...but
then warm into the middle to upper 80s for Monday into Tuesday.
The remainder of the week looks to be dry...though some of the
models continue to bring some spotty probability of precipitation in the area due to
southerly flow on the surface. For now...will only have slight chance probability of precipitation
for Wednesday across areas along and north of I-74 and east of I-57.
Remainder of the week with be dry...even into the beginning of the
Labor Day weekend. Temperatures will remain well above normal in the
extended period so heat could be an issue later...but dewpoints will
remain low so heat index values are not expected to get over 100
through the period.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1154 PM CDT Sat Aug 29 2015
Fog formation and low clouds are the primary concerns over the
first 6-9 hours of the taf period. The weakening cold front has
basically stalled out from NE to SW roughly along I-55 as a
stationary front. As a result of the weak pressure gradient in the
vicinity of the front, the winds will remain light and variable
overnight. That will lead to the potential for areas of dense fog
to form, especially north-northwest of the front around pia and bmi. Fog for
the other terminal sites could remain MVFR, but hrrr and rap
output continue to show areas of dense fog farther south from spi
to cmi, so will monitor dense fog closely for possible significant
visible reduction for the other terminals.
Tomorrow morning winds are forecast to return to the south-
southwest and remaining less than 10 knots through the day.
Clouds will possible dip to LIFR at pia and bmi (or any site that
experiences dense fog), with MVFR clouds at the other taf sites
for a few hours after sunrise. By middle morning, low clouds should
lift to VFR levels as diurnal mixing increases and raises the local.
Clouds are forecast to persist until later afternoon, as a lobe of
the upper trough retrogrades into Illinois from the E-se. Shower chances
will increase east of cmi and Dec, but we kept the taf dry for
Skies should begin to clear out by Sunday evening, with light
south winds through 31/06z.