Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1240 PM CDT Wednesday Apr 1 2015

Update...1047 am CDT

Minor changes to going forecast today...mainly increasing high
temperatures a couple of degrees in some locations.

A warm day with well above normal temperatures is still on track today.
Sunny conditions in place this morning will remain...with any middle
level clouds to the north expected to drift to the east this
afternoon. Warm air advection with increasing southerly flow along with strong
mixing today will allow temperatures to rise into the 70s today for most
locations...with middle 70s likely across the western County Warning Area. Put a
little more detail in the maximum temperature and hourly temperatures for locations
in far northeast Illinois along the initial east
southeast winds this morning will become more southerly. So
despite temperatures still holding into the 40s at this feel
that high temperatures in the middle to upper 60s could still be realized
later this afternoon as the strong southerly flow limits any
onshore component during that time.



Short term...
400 am CDT

Through Thursday...

April will get off to a warm and breezy start today...but the
quiet weather will quickly change tonight. West/v imagery this
morning shows midlevel riding building into the Midwest...with
strong upper disturbance riding over the top of the ridge in the
northern High Plains. This has spurred cyclogenesis and a ~995 mb
low over western North Dakota. Height rises will be accompanied by
strong 850/925 mb warm advection...with values of +10 to +15
celsius at 925 mb supportive of highs in the lower to even middle
70s in a few spots...aided by close to full sun. Exception will be
far northeast Illinois shore being kept cooler by southeast winds.
Slight onshore component is expected earlier in the day in
downtown Chicago...resulting in a slightly slower warming trend
than inland areas...but 70 is still a good bet in the late

Regarding the winds...our area will be sandwiched between
departing high pressure ridge to the east and frontal trough of
strong low pressure over the northern plains. As frontal trough
advances...pressure gradient will tighten and fairly impressive
pressure falls will result in increasingly gusty south
winds...especially in north central Illinois. The strong southerly
winds will continue tonight...though with some uncertainty on top
end magnitudes. The gusty winds and continued warm air advection will result in
quite a mild night tonight...with lows in the lower to middle
50s...which is on par with normal highs for early April.

Attention will turn to strong ~50 knots 850 mb low level jet late
tonight and upper disturbance punching into frontal zone. Guidance
indicates enough elevated instability supportive of maintaining
thunder in grids...but question is timing of possibly weakening
area of showers and storms into northwest County Warning Area. Some convective
allowing guidance keeps convective activity outside of the County Warning Area
until after 9z and overall have slowed trends a bit from previous

Thursday morning into the early afternoon appears to be prime
time for widespread showers and thunderstorms immediately ahead of
the front. Have continued to lean toward a slower frontal
passage...keeping especially the southeast County Warning Area in the warm sector
into the early-middle afternoon. Only a few hundred joules of cape
prognosticated and of a tall/narrow profile...but strong low level shear
and decent low level cape supports the marginal risk for severe
weather per Storm Prediction Center outlook. Main threat would be gusty winds given
marginal lapse rates...though cant rule out an isolated tornado
threat in our southeast...especially if a secondary low can form
along the cold front. Bigger concern could be a period of heavier
rain given anomalously high 1.25 to 1.5 inch precipitable waters advected into
the area. After the warm start to the day...expecting highs in the
middle to upper 60s...with a few spots in the southeast County Warning Area possibly
tagging 70. This is despite the clouds and precipitation.



Long term...
352 am CDT

Thursday night...

Passage of the cold front and drying of low levels Thursday night
will push precipitation to the far southern portion of the forecast
area...and maybe even into central Illinois and out of the local
area altogether.

Friday and Friday night...

This break from precipitation likely will be short lived as the
front stalls to the south and a center of low pressure moves along
this stationary boundary into the Ohio Valley on Friday.
Precipitation wraps around the north side of the low...and for a
while it appeared that accumulating snow would be a good possibility
Friday afternoon as colder air filtered into the area. The latest
guidance suggests that temperatures will remain just a bit too warm
to support snow until very late in the day or into Friday evening
when probability of precipitation drop considerably with the exit of the low. Both the GFS
and ecm show an inch or so of snow late Friday even with marginal
surface temperatures. Areas south of the Lake May have the best chance
since with north winds they will not get quite as warm during the
early part of the day and will cool faster toward evening.

Saturday and Easter Sunday...

High pressure sliding to our south and increasing heights aloft will
support a quiet and seasonably mild Saturday. As the high moves
southeast away from the area Saturday night...the next frontal zone
drops south across Wisconsin. Return flow on the west side of the
departed high will support milder temperatures Sunday but only
marginally increased am not including more than slight
chances of light rain Saturday night and Sunday despite the presence
of this boundary.

Monday and Tuesday...

Early next week the GFS and European model (ecmwf) show similar agreement in
developing a large low pressure system across the plains and lifting
it into the western Great Lakes. Shower and thunderstorm chances
increase Monday and especially into Tuesday as this system
approaches but at this range will just mention a chance of thunderstorms and rain for
both days.



Fire weather...

Southerly flow today and tonight will gradually bring increased
moisture to the area along with mild temperatures and eventually
thunderstorms on Thursday. However...peak wind gusts around 25
miles per hour this afternoon will approach minimum relative humidity values
of around 30 percent...especially in windier and drier areas west
of I-39. When winds are higher than the relative humidity...there
is an increased risk that fires will get out of control. At this
point conditions do not appear to be windy and dry enough to
prompt red flag warnings today...but those conducting prescribed
Burns should use extra caution.




//Ord and mdw concerns...updated 18z...

* gusty south winds developing this afternoon.

* Small chance for thunderstorms and rain late tonight into Thursday morning.

* Low level wind shear potential tonight.


//discussion...updated 18z...

An area of low pressure moving eastward across southern Canada will
continue to deepen as it moves over southwestern Ontario this
evening. Overall this will continue to lead to an increase in
southerly winds across the area through this evening.
spite of this surface wind appears that a low level
jet featuring 50+ knots south-southwesterly winds at around 2,000 feet
above ground level will develop this evening...likely setting the stage for low level wind shear
overnight. A cold front...associated with the southern Canada low...
will shift across the area by early Thursday afternoon...with west-
northwesterly winds likely in its wake.

The other concern during the period is the possibility for a period
or two of showers with some embedded thunderstorms...particularly
very late tonight and again by late Thursday morning. Uncertainty
still exists on the extent of the thunder with this activity...but
it does appear that an upper level disturbance...currently moving
northeastward over southwestern Missouri...will shift over portions
of northern Illinois by daybreak Thursday. This could result in some
scattered shower and thunderstorm development over the terminals
after 09 UTC. Finally...another area of showers and perhaps some
additional storms look to move over the area by late morning into
the early afternoon ahead of the approaching cold front.


//Ord and mdw confidence...updated 18z...

* medium-high in wind trends.

* Low-medium in extent of thunderstorms and rain late tonight into Thursday morning.

* High in remaining elements.


//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...

Thursday night...VFR likely.

Friday...chance light rain and MVFR. Gusty north to northeast

Saturday...VFR. Chance -ra at night.

Sunday...chance -ra. South winds shifting to east.

Monday...chance -shra and IFR/MVFR. Slight chance thunderstorms and rain at night.

Tuesday...chance thunderstorms and rain and MVFR.



458 am CDT

The lake will be in between a frontal trough over the plains and
high pressure departing to the east today. A tightening pressure
gradient will result in strengthening southerly winds...especially
this evening through tonight. Have issued a Small Craft Advisory
for the nearshore starting at 00z this evening...though given
magnitude of pressure falls spreading over the is
possible that it may need to be started early. Expecting southerly
speeds to 30 knots ahead of the front tonight...with a very mild air
mass limiting mixing. Cannot rule out a few gale force gusts
despite the stable conditions...especially on the central and
northern portion of the lake. The mild and moist airmass could
also result in fog developing...though confidence is low in how
widespread it will have gone with patchy wording in
the Gulf late tonight through middle day Thursday.

Front trailing from low pressure moving to near James Bay by
Thursday evening will cross the lake during Thursday
afternoon...ending the small craft winds in the nearshore...latest
for the Indiana shore. West winds relax Thursday night then turn
north for Friday as a low pressure area rides up the Ohio Valley.
30 knots winds are possible for the south half depending on the track
of the low...along with small craft winds and waves for the
nearshore. Winds will back west and then southwest on Saturday.
Uncertainty increases on some guidance indicates a
fast moving low crossing the lake...while other guidance has high
pressure over the region.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz743-lmz744-lmz745...7 PM Wednesday to
1 PM Thursday.

Small Craft Advisory...lmz740-lmz741-lmz742...7 PM Wednesday to
10 am Thursday.



Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations