Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
628 PM CDT Sat Aug 29 2015
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 00z tafs through 00z Sunday evening)
issued at 628 PM CDT Sat Aug 29 2015
A decaying cold front is passing across the forecast area early
this evening, and will cause the winds to become light and
variable for several hours this evening and possibly most of the
night. That will lead to the potential for areas of dense fog to
form, especially farther northwest around pia. Fog for the other terminal
sites should remain MVFR, but hrrr and rap output keep expanding
the area of dense fog farther south, so will monitor fog closely
for possible 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, with MVFR clouds at the
other taf sites for a few hours after sunrise. Then low clouds
should clear out as diurnal mixing increases and dry air washes
out the low level moisture.