Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
955 PM CDT Tuesday Apr 15 2014
856 PM CDT
Very minor adjustments tonight. Surface ridge has become rather
diffuse...although observation would suggest the center is placed across
northern Indiana. The surface ridge influence stretches
north/northwest to northwest Ontario then south through the tenn
valley. However just west of this feature a surface low continues to
develop and slide east towards the western Great Lakes. The
gradient is much tighter just west of the County warning forecast area...so it appears the
light and variable winds will quickly turn southeasterly and
increase by daybreak.
Expect temperatures to still radiate efficiently over the next several
hours...however the rate of fall may decrease around midnight or
shortly after as southeast winds begin to increase and enough
friction aloft may inhibit the ability for the surface to continue to
cool. A few locations may even experience temperatures rising a degree or
two...mainly in our western County warning forecast area after midnight. Otherwise expect
temperatures to still fall into the middle/upper 20s to lower 30s closer to
340 PM CDT
Main forecast concerns/challenges are with cloud cover and temperatures
this evening...some up and down with temperatures over the next couple of
days...and then increasing chances for precipitation late in the work
In the near term...cloud cover remains over the County Warning Area and has
remained on the thicker side for most of the day. This has limited
daytime warming today...with temperatures struggling to get into the
upper 30s. Although cloud cover is on a diminishing trend...did
keep at least some partly cloudy mention into early this evening
with this stratocu deck continuing a slow diminishing trend. Any
remaining isolate flurries across the area will likewise
diminish...but more likely at a quicker rate than the cloud cover.
Then with middle/high level cloud cover to the west/northwest
remaining off to the west...a period of mostly clear skies will be
observed tonight. Despite warming aloft tonight...surface ridge
with relaxed gradient and clear skies should allow for another
cold night tonight with low temperatures in the 20s expected for most of
the County Warning Area. Confidence is somewhat lower for low temperatures across the
western County Warning Area tonight...as southerly picks back up with the surface
ridge quickly shifting to the east of the County Warning Area.
Nonetheless...another night with temperatures well below normal will
likely be observed.
Expect Wednesday to be dry...as warm air advection persists and helps start a
trend towards more normal temperatures with high temperatures expected to be in
the 50s. This warmer air will be riding along some strong
southerly winds...as pressure gradient really tightens up and
speeds increase into the afternoon. With middle/upper level trough
pushing east through the Central Plains during the day...high
level cloud cover will be on the increase. Forecast guidance in
agreement with this middle level trough to swing east just north of
the County Warning Area Wednesday night...with best forcing and precipitation to stay
north of the County Warning Area. Have kept the County Warning Area dry but will need to monitor
the potential for some warm air advection precipitation to possible develop across
southern Wisconsin...which could clip areas right along the border
in northern Illinois. Attendant surface low will lift through
southern Wisconsin Thursday morning while weakening...but with
most of the County Warning Area expected to be in the warm sector. In this warmer
air mass...temperatures will be in the 50s and low 60s across the
southern half of the County Warning Area on Thursday. It does look like some
slight cooling will occur late in the day/evening as a weak
front/trough moves through the County Warning Area. As this is
occurring...approaching longwave trough will help usher some weak
impulses which should aid in the development of rain Thursday
evening/night. Did increase probability of precipitation and coverage of probability of precipitation Thursday
night as precipitation is appearing to be more likely...but with some
uncertainty as to the exact placement of precipitation axis. Colder air
will be trying to work south across the area Thursday night into
Friday morning...with the rain/snow line trying to inch its way
close to the County Warning Area. Although...it does appear that this line will
remain north of the County Warning Area...as deeper moisture moisture/crystals
will be lost by the time thermal profiles would be supportive of
snow. Drying trend will occur during the day Friday with slightly
cooler temperatures in the 40s and 50s. Expect this return to cooler
weather to brief as warming trend is expected into the weekend.
//Ord and mdw concerns...updated 03z...
* light south-southeast winds overnight becoming strong south
Wednesday...with gusts 30-35 knots possible in the afternoon.
* Potential for low level wind shear late Wednesday evening as gusts
diminish...with winds 20045-50kt around 2000 feet above ground level.
Surface high pressure ridge along Illinois/in border and over Southern
Lake Michigan will continue to drift east this evening will allow
decreasing westerly winds to shift to the southeast over the next
1-3 hours. Lake breeze had developed and moved as far inland as mdw
before retreating a bit...but departure of ridge axis to east
should allow for a large scale shift to southeast winds across the
Focus then shifts to strong south winds expected Wednesday in
response to an area of low pressure developing into the upper
Midwest. Tightening pressure gradient appears to support sustained
winds around 20 knots by afternoon...with gusts into the 30-35 knots
range possible per forecast soundings depicting mixing to
4000-5000 feet above ground level. Surface low lifts into western Wisconsin
Wednesday evening allowing the gradient to ease slightly. Degree
of boundary layer decoupling somewhat uncertain after sunset with
strong winds continuing...though by late evening expect decreased
gusts. This sets up the potential for non-convective low level
wind shear however...with models depicting low level jet from
about 200 degrees at 45-50kt at 2000 feet above ground level.
Otherwise...current VFR stratocu deck expected to erode quickly
early this evening with strong subsidence developing and dry air
in place. Middle/high VFR clouds already racing southeast across the
northern plains should result in increasing middle-clouds later
tonight. Model time/height depictions indicate VFR middle deck will
remain across area in 10-15 kft range through Wednesday. Low
clouds will be absent...with southern portion of surface high
pressure ridge effectively blocking moisture return from the Gulf
//Ord and mdw confidence...updated 03z...
* medium confidence in gusts abating and true low level wind shear conditions
developing Wednesday evening.
* High confidence all other elements.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...chance rain late aftn/ngt. Mainly VFR.
Friday...chance rain/MVFR early...then VFR.
Monday...slight chance rain.
303 PM CDT
High pressure stretched from Manitoba to the western Gulf Coast will
work eastward tonight and consolidate into a better organized area
of high pressure over the Ohio Valley into Wednesday morning. Some
residual gusty winds across the far eastern section of Lake Michigan
this afternoon will diminish into the evening as the pressure
gradient weakens. Should see quite a bit of variability in wind
direction this evening before a steadier south wind develops over
the southern two thirds of the lake overnight...with an easterly
direction far north thanks to a developing warm frontal boundary.
South to southeast winds will increase markedly Wednesday morning
and should easily gusts to 30 knots across central and southern
portions of the lake...especially over the western half. Do see some
potential for gales though they may be short lived or
infrequent...or confined to the nearshore waters given the shore
parallel flow slight onshore component suggesting any lakeward
extent of mixing from land would be minimal. Illinois/in nearshore waters
will solidly fall into small craft criteria. Cannot rule out gales
there as well but the stronger winds just above the surface are
focused further west of the lake but will need to monitor.
Low pressure will track from Nebraska/South Dakota to Northern Lake
Michigan into Thursday bringing a cool front down the lake later
Thursday. This will shift winds to the north. The low will be
weakening as it passes and the pressure gradient between it and high
pressure to the north will be fairly baggy. This combined with a
fairly weak push of colder air will result in rather modest winds
for a frontal passage and speeds will likely be quite a bit weaker
with the frontal passage later Thursday into Friday. High pressure
ridging then works eastward across the lake Friday and Saturday
keeping winds on the light side. Another weak low may move into the
region later in the weekend.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Wednesday to 4 am Thursday.
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