Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1033 PM CDT Monday Jul 6 2015
813 PM CDT
The storm threat continues to diminish though periods of moderate
to briefly heavy rain with some embedded thunder are still
expected through early overnight...with the most
frequent/prolonged activity likely south of I-80. While plenty of
MUCAPE aloft of 1500-2500 j/kg...most storm clusters have
struggled to sustain likely due to very limited effective shear
in the absence of any pronounced forcing mechanism. In
addition...while the low-level jet is strengthening...moisture
transport is likely being robbed by significant storm coverage to
our southwest. Will still need to monitor for locally heavy
rainfall as a high precipitable water air mass /1.59 inches on 00z ilx radiosonde observation/
progresses over the area with some locations receiving successive
batches of rain.
Have adjusted temporal probability of precipitation to meet latest thinking...with still
likely probability of precipitation for most locations at some point during the
evening and early overnight...but took down the thunder
252 PM CDT
A line of showers is moving through northeast Illinois with additional
showers and storms over southern and eastern Iowa. The line of
showers with a few embedded thunderstorms will continue east and may
weaken further as the outflow boundary has started to separate
itself from the line of showers. Heavy rain...a few gusts of
wind...and a few funnel clouds have been reported with this line of
showers. The Funnel clouds are forming as boundaries intersect...but
the funnels are not located under any updrafts so tornadoes are not
Temperatures are falling into the 70s behind the line of showers and
with the thick cloud cover...recovery is very slow at best.
Therefore locations over northwest Illinois actually saw their high temperatures
early this morning as they will stay in the 70s. Have falling temperatures
also across Northwest Indiana as the storms roll through...but if the line
falls apart...temperatures will likely stay in the low to middle 80s.
The cold front stretches across northwest WI through southern Minnesota and
northwest Iowa and is prognosticated to swing through northern Illinois and Northwest Indiana
overnight. Another round of showers are expected ahead of the cold
front...but not expecting much if any severe weather. The latest
rap analysis shows 500-2000 j/kg of cape behind the line of storms
over north central Illinois...with the values decreasing as you get closer
to the Mississippi River. Shear values are also around zero...so
organized storms are not expected. Expecting the next round of
showers and storms to move into northwest Illinois after 5pm CDT and
continue across northern Illinois and Northwest Indiana through the night. This
round will also have lightning...heavy rain...and probably some
gusty winds...but widespread severe weather is not expected.
Given the lower than expected high temperatures today...lowered low temperatures
tonight as well. Dew points will remain in the middle 60s to around 70
so low temperatures should stick pretty close to the dew points. Expecting
lows to vary from the middle 60s around rfd to around 70 in Northwest Indiana.
325 PM CDT
Tuesday through Monday...
The front will continue shifting south through Tuesday morning
with northerly winds and cold advection enveloping the area. Such
as has been the trend with the system...did also speed up the end
of rain chances on Tuesday. Think shower coverage in the morning
will be scattered within Post frontal forcing...and any thunder
likely even more isolated. Stratus is likely to linger longer than
the rain chances as the cool air collides with the moist boundary
layer. It is certainly possible clouds stick around for southern
areas as well as locations near the lake through the afternoon.
High pressure should keep the area mainly dry into Wednesday
Temperatures both Tuesday and Wednesday look to be well below
normal with highs primarily in the lower 70s...with middle-upper 60s
near the Lake Shore. These readings across the forecast area are
10 to 15 degrees below normal for early July. In-between on
Tuesday night...any clouds should be confined to the southern County Warning Area
which will allow northern communications to dip into the
50s...even lower 50s for outlying locations.
Beyond...short wave impulses look to be key to rain chances with
one becoming more consistently targeted on guidance Wednesday
night. The window of instability return looks limited...so this
could be mainly just showers especially north of I-80. Each
successive impulse appears to bring the Theta-E gradient aloft
more so over the area which warrants storm chances almost daily
late in the week into the weekend. Temperatures also look to
rebound back to normal by the weekend.
//Ord and mdw concerns...updated 03z...
* rain may persist through early Tuesday morning...with a few embedded
* South winds around 10-14kt steadily turning west/northwest by
early Tuesday morning. Winds flip northeast and increase with gusts
to 22kt possible Tuesday midday.
* MVFR ceilings possible briefly with rain tonight...then more likely
Tuesday morning. Possible Tuesday morning ceilings could dip to IFR.
Channel of very warm/moist air continues to feed northeast across
northeast Illinois/northwest in. Rain and a few thunderstorms continues
to develop this evening...however expect the coverage of thunder
to slowly diminish. Rain may increase is coverage and intensity
later this evening...however some guidance is indicating that as
the frontal boundary slowly drops southeast overnight...this may
diminish and result in spotty showers overnight. Clouds will
remain thick overnight and bases may flirt with MVFR conds
overnight. As the boundary arrives overhead...MVFR ceilings will
spread in coverage and could perhaps dip to IFR conds around
daybreak Tuesday. Winds will be slowly turning northwest to
north...and guidance continues to indicate by middle-morning Tuesday that
wind speeds will increase and could become gusty to 20-25kt.
Clouds will slowly erode Tuesday afternoon...with VFR conds returning.
//Ord and mdw confidence...updated 03z...
* high confidence in rain over Ord/mdw...low confidence in when
rain will end.
* Low confidence in coverage/timing of thunder overnight.
* High confidence in wind dir/speed...high in gusts developing Tuesday
* Medium confidence in ceilings...high confidence in ceilings dipping to
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday...chance of rain showers slight chance of thunderstorms and rain.
Thursday through Sunday...periodic chances for showers and
252 PM CDT
Wind gusts were not as strong as expected across the nearshore
waters today as thick cloud cover limited mixing. A line of showers
that has a history of producing gusts up to 30 knots is moving across
the nearshore waters so will keep the small craft going through
00z/7pm CDT to account for shower related gusts. Winds will pick
back up tomorrow and a Small Craft Advisory has been issued for
points south of Chicago from 9am CDT Tuesday through 4am CDT
Wednesday. Flow will be on shore so wave heights along the Indiana
shore will grow to 4-7 feet and then come down Tuesday evening and
night. Winds diminish as high pressure spreads over the lake
Wednesday morning. Winds will be light and variable over the north
half of the lake into Wednesday evening while a weak low passes just
south of the lake Wednesday night. Winds will be northeast then
north Thursday afternoon. Winds remain less than 10 knots as the high
slowly shifts east through the end of the week with the next low
impacting the lake early next week. There will be additional
periodic chances of thunderstorms Wednesday night and then from
early Friday morning through early next week.
Illinois...beach hazards statement...ilz014...9 am Tuesday to 4 am
In...beach hazards statement...inz001-inz002...9 am Tuesday to 4 am
Lm...Small Craft Advisory...lmz741-lmz742-lmz743-lmz744-lmz745...9 am
Tuesday to 4 am Wednesday.
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