Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
634 PM CDT Tuesday Mar 31 2015
328 PM CDT
Tonight through Thursday night...
Main concerns during this period are the potential for patchy fog
development late tonight...warmer temperatures on Wednesday...and shower
and thunderstorms late Wednesday night through Thursday night with
cold front slowly sagging across forecast area.
Weak high pressure has spread into the area this afternoon in the
wake of the low that passed across the region last night. The
combination of the weak surface ridge and the lake breeze and its
associated mesoscale-high have led somewhat variable wind directions
across the County Warning Area and little low level drying as indicated by surface
dew points still in the middle-upper 30s. The high will continue to
drift east across the area tonight...with light variable winds and
generally clear skies expected to result in patchy fog development
after midnight away from the city. Forecast soundings from the rap
and NAM both indicate shallow moist hydrolapse layer of a couple
hundred feet as overnight temperatures lower to the middle-upper 30s by 12z
With the high off to the east on Wednesday...low level winds
become east-southeast during the morning...then strengthen and
become more southerly with gusts 25-30 miles per hour especially across the
western County Warning Area by afternoon as surface pressure falls tighten the
pressure gradient across the plains and upper Midwest in response
to deepening low pressure north of the Canadian border. These
breezy/gusty south winds will pump warmer air into the area with
only a little patchy high cloud cover anticipated...with which
model low level thermal forecasts support afternoon highs from the
upper 60s far east to the lower 70s across the west. Early east-
southeast winds will keep the Illinois Lake shore a little
cooler...though even those areas should see temperatures warm as
wind becomes south in the afternoon and primarily off-shore. Have
maintained forecast highs a few degrees above warmest guidance.
Breezy south winds continue Wednesday night...keeping overnight
temperatures in the 50s as low level moisture begins to increase.
Attention then turns to the potential for showers and
thunderstorms later Wednesday night...as the Canadian low trails a
cold front into the Mississippi Valley and a short wave approaches
in strong southwest flow aloft. Guidance is in pretty good
agreement that the axis of low level moisture and cape/instability
remains along and just ahead of the cold front...which moves
across Minnesota/WI/IA Wednesday afternoon and evening...and approaches
northwest Illinois after midnight or toward Thursday morning.
Strong/severe storms will likely develop well west of the County Warning Area
across the plains...with activity becoming weaker as it moves east
into our area early Thursday while stronger storms settle into
Kansas/MO into stronger low level instability axis. Still...scattered
thunder still appears to be likely as elevated cape in the 400-500
j/kg range spreads across northern Illinois overnight. Given the pre-
frontal warm sector moisture plume...approaching short wave and
strong upper jet (with some right rear entrance region divergence
potentially enhancing large scale lift) have boosted probability of precipitation to
likely across the western County Warning Area late Wednesday night...and to
categorical across the area Thursday morning as the front begins
its slow trek across the forecast area. The large scale upper
trough opens up...becomes positive-tilt and elongates across the
region Thursday...with the strongest middle-upper level forcing
lifting off to the northeast. However...favorable jet divergence
pattern aloft and low level forcing associated with the slow
moving cold front is expected to maintain potential for showers
and thunderstorms across the area as the front becomes parallel to
upper flow and slows its southeastward movement. Evolution of
early morning convection and cloud cover will be as important as
the location of the front itself with respect to redevelopment of
stronger convection across the area Thursday afternoon...and
though confidence in the is somewhat low have generally followed
guidance for frontal location by early afternoon along an
approximately Chicago to Peoria to Columbia Missouri line.
Afternoon destabilization is expected along and south of the
front...with guidance indicating MLCAPES from 700-1400 j/kg by
peak heating. Coupled with deep layer 0-6 km bulk shear increasing
to 50-60 kts...the potential would exist for severe or at least
organized (linear) storms...especially with 40-50 knots southwest
flow noted at 850 mb. Storm Prediction Center has included the far southeast tier of
counties in a "marginal" severe outlook for Thursday (day 3) and
this is prudent...though much will depend upon location of front
or pre-existing outflow from morning rain/thunder and threat could
easily end up being farther south of the County Warning Area. Rain potential and
at least slight chance of thunder looks to linger into Thursday
night over the southeast 1/2 of the forecast area...as main upper
trough axis approaches with continued support deep layer ascent
from left exit of next upper jet streak and low level baroclinic
zone north of the surface cold frontal position.
With front moving slowly to the southeast...cooler air does not
make much of an initial significant intrusion to the County Warning Area...with
temperatures Thursday expected to range from the low-middle 60s behind the
front in the north...to near 70 in the south but cloud and precipitation
dependent. Also...winds may shift more north/northeast along the
Illinois Lake Michigan shore which could produce cooler temperatures especially
later in the afternoon along the North Shore. Cooler air does
gradually spread into the region later Thursday night into Friday
308 PM CDT
Friday through Tuesday...
Main concerns during the long term period will center around
precipitation chances and temperatures as we look to be headed into
a fairly active period into early next week.
At the start of the period...model guidance is in decent agreement
in the progression of a northern Pacific system shifting over
Central Plains states and up over the lower Great Lakes and Ohio
Valley region by Friday evening. This looks to result in another
period precipitation across the area Friday and into Friday evening.
Model guidance is also in good agreement that the surface low track
will be to the southwest of the area...roughly from southern
Illinois northeastward across central Indiana. This track could put
parts of the area in line for a chance over to heavy wet snow Friday
The potential for a strong band of fgen forced rain/snow within an
evolving deformation zone could result in enough dynamic cooling to
result in this change over...especially considering a colder air
mass is forecast to filter in over the area during the day.
However...this system is too for out to get a good idea on where
exactly the strong mesoscale forcing will occur. So...overall...a
lot can change with this system as it is still yet to even dig
onshore over the West Coast. It also is not a favorable diurnal time
of day for accumulating snow. However...with the potential for
strong mesoscale forcing...a band of heavy wet snow can not be ruled
out...with some accumulations possible late Friday into Friday
evening. Confidence on any amounts at this time remains low due to
the inherent uncertainties in the actual thermal profile and areas
of best dynamic forcing.
Things look to warm up and dry out for a period on Saturday under
southwesterly flow. However...a cold front is expected to shift Down
Lake Michigan late Saturday and Saturday night...and this front
could end up stalling out for a period late Sunday through Tuesday
of next week over northern Illinois as the flow aloft begins to
become parallel to the surface front. If this does occur...there
will likely be a sharp temperature gradient across the area...with
the potential for low to middle 40s far north and northeast Illinois
and into 50s and 60s south. With the front possibly in the area...it
also is possible for extensive low cloud cover and periods of
An area of low pressure...driven by what could end up being a decent
middle/upper level disturbance...could move along this frontal boundary
at the very end of the period Tuesday night...and this could result
in more active showers and thunderstorms over the area...with even a
threat for strong or even severe storms south of the frontal
//Ord and mdw concerns...updated 00z...
* strong/gusty south/southeast winds Wednesday. Cms
Lake breeze boundary remains near the east side of Ord having not
moved west of the Airport. Still expecting it to move west...as it
dissipates this evening with winds becoming southeasterly under 10
kts at Ord. Winds will eventually become light and variable at 5
knots or less this evening. The gradient will begin to tighten
Wednesday morning and continue to tighten during the day. East or
southeast winds under 10kts will turn more southeasterly through the
morning and then southerly by afternoon as speeds and gusts slowly
increase. There remains some uncertainty regarding how strong
speeds and gusts will become by afternoon. Maintained speeds in
the middle teens with gusts into the 20kt range. But its possible
this could be low depending on how much mixing occurs and how
strong the gradient becomes. Further northwest at rfd...gusts well
in the middle/upper 20kt range are possible.
Mostly sunny skies and dry airmass today should limit fog
potential tonight into Wednesday morning. But some shallow ground
fog is possible as well as potential fog near the lake. Confidence
too low to include any mention in the tafs with this forecast.
Some middle/high clouds are possible Wednesday. Cms
//Ord and mdw confidence...updated 00z...
* low for wind direction this evening.
* Medium for wind direction Wednesday...low for speeds/gusts. Cms
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday...shra/tsra. MVFR/IFR likely. Gusty southwest winds
* Friday...rain or snow. IFR likely. Gusty north winds.
* Saturday...dry. Gusty westerly winds.
* Sunday...chance of rain. IFR possible. South winds shifting
* Monday...chance of thunderstorms and rain. East winds.
316 PM CDT
Winds will increase out of the south over the lake late Wednesday
and Wednesday night in response to strong pressure falls out area of
a Stout area of low pressure expected to shift eastward over Ontario
into Thursday morning. Winds of 25 to 30 knots are expected during this
period. Following the systems cold frontal passage...winds are
expected to increase out of the north for a period on Friday and
Friday evening as another area of low pressure develops over the
Southern Plains and shifts northeastward over the Ohio Valley. Wind
speeds with this could also reach up around 30 knots...with some low
end gale force winds not out of the question over Southern Lake
Michigan late Friday and Friday evening.
Later in the period a prolonged period of east-northeasterly flow is
expected to set up over Lake Michigan. Although these winds will not
be all that strong in magnitude...the prolonged nature of them could
result in some 4 + foot waves along the Western Shores of the lake
into early next week.
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