Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
839 PM CDT sun Aug 30 2015
839 PM CDT
Main concern is again with fog potential thanks to calm
winds and clear skies with exception of high-level haze from
wildfire smoke. One of the typical spots to fog up...karr...has
been down to 2-3 Michigan visibility as of this writing...but only a
few other locations with 5 Michigan visibility so far. Unlike last
night...GFS lamp guidance is not hitting the fog as hard. However
..hi-res guidance and NAM do show dense fog developing in predawn
hours. Will monitor trends for next few hours for possibly needing
to issue a dense fog advisory. Currently do not yet have enough
confidence to issue one. Updated zones will be sent shortly.
Tonight and Monday...
230 PM...primary forecast concern is dense fog potential tonight
into Monday morning.
Cloud cover has been slow to scatter this afternoon with some breaks
showing up across northwest Illinois and over portions of the lake. Cloud
cover is steadily eroding from the northwest across central WI. The
trend from guidance is for this scattering and possible clearing to
continue across the area this evening and have maintained this in
the grids but confidence regarding clearing is low. Cloud trends
will likely have a large impact on fog potential tonight. Winds will
become calm in many locations tonight and if clouds do scatter/clear
then as temperatures drop to the dewpoints...areas of fog will develop but
may become dense. Confidence regarding how widespread dense fog may
become is low and so no mention at this time...but trends will need
to be monitored into this evening. In addition...while fog did lift
over the nearshore waters...there still remains some patchy fog
which may thicken and then spread into far northeast Illinois this evening.
Chance of showers and perhaps an isolated thunderstorm this
afternoon into this evening has shifted a bit further south and may
just affect the far southern County Warning Area through early evening. Maintained low
chance probability of precipitation for this potential...but after this evening...dry
weather can be expected through Monday evening.
The cloud cover has held temperatures down this afternoon...but temperatures are
still warming through the 70s and some lower 80s may still be possible
where the sun breaks out. If the fog can burn off fairly quickly
Monday morning with at least partly cloudy skies...then middle 80s are
possible with cooler temperatures near the lake. These temperatures combined with
dewpoints in the upper 60s/lower 70s may push heat index values into
the lower 90s Monday afternoon. Cms
235 PM CDT
Monday night through Sunday...
Weak middle-level wave continues to be advertised by a few of the
operational guidance solutions...which could touch of a few
showers/thunderstorms across the region. Confidence is diminishing
with this scenario in the Monday night/Tuesday timeframe. Have trimmed back
probability of precipitation for Tuesday...but held onto a slight chance Tuesday afternoon for any
diurnally driven precipitation that may develop along the eastern half of
the County warning forecast area. Otherwise will maintain a dry forecast in the Monday night/Tuesday
Low level ridge centered over the middle-Atlantic region will keep a
southerly to slightly southwesterly flow over the forecast
area...with a plume of better moisture streaming north from the Gulf
of Mexico. This should maintain slightly elevated precipitable water values. Temperatures
will generally be in the middle/upper 60s at night and middle/upper 80s during
the afternoon hours.
Wednesday through Saturday... broad middle-level ridge remains centered
over the region...with the best forcing remaining shifted to the
north. This should keep the bulk of showers/thunderstorms in the
longer term north of the forecast area. Ensembles continue to
support a dry extended...although a diurnally driven shower/storm
could still develop but the organization or coverage should remain
Warm thermal ridge in the middle-levels continues to be prognosticated
underneath...which should allow temperatures to approach the upper 80s to
near 90. The limiting factor could be how much cloud cover develops
along with low level moisture. Later in the week heading into the Labor
Day weekend...ensembles are hinting at a pattern change that could
feature the strong middle-level heights lowering and bringing an active
pattern back to the region for next Saturday.
//Ord and mdw concerns...updated 00z...
* potential for fog development overnight.
High pressure is centered over the middle-Ohio River valley this
evening providing light winds and VFR conditions to much of the
region. A lake breeze has pushed across the Chicago terminals
turning winds easterly...which should veer and weaken overnight.
Light and at times calm winds overnight and clear skies should set
the stage for radiational cooling...though the relatively thick
smoke aloft that has settled overhead may hinder this process to
some degree. In addition...while guidance does hint at the
potential for fog development...it isnt as pronounced as
yesterday. Backed off on the timing several hours and tapered the
magnitude slightly at the Chicago terminals.
//Ord and mdw confidence...updated 00z...
* low confidence in visibility trends overnight.
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday through Thursday... VFR. South to southwest wind.
Friday through Saturday...VFR. South to southeast wind.
235 PM CDT
Rather benign pattern continues across the western Great
Lakes...which will likely allow areas of fog to once again develop
tonight and could linger into Monday afternoon/evening. A weak low
pressure will lift northeast of Lake Superior...which could allow
a slight increase in the winds for the northern portions of the
lake Monday/Monday night.
High pressure over the Central Plains will slide east towards the
Ohio Valley...then remain anchored through the end of the week. This
will maintain a southerly flow for much of the lake...with only a
brief bump in the gradient and winds Monday ngt/Tue.
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