Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
345 am CDT Sat Jul 26 2014
322 am CDT
Synopsis...unsettled weather is expected today with severe storms
possible along and south of I-80. Cooler conditions move in
tomorrow and persist through the rest of the week.
Today and tonight...
showers have sprouted over northwest and north central Illinois early
this morning at the nose of the low level jet. Expecting showers
and a few thunderstorms to fester south of I-88 before pushing
east of the warning area by middle morning. Not expecting any severe
weather with these showers and storms as a significant cap is in
place along with limited instability.
Upper level high pressure remains over the southern U.S. And it is
still expected to retrograde over the SW U.S. By Monday night. The
upper level low over southern Saskatchewan will move over WI Sunday
morning. At the surface...the low over the plains is slowly
trekking east and will move over central Illinois tonight. The second low
over southern Saskatchewan will slide southeast and move over Lake
Michigan Sunday morning.
A warm front is over central Missouri and it will move north into
the southern forecast area this morning. Dew points will rise into
the 70s. Precipitable water values will also increase to around 2 inches this
evening...resulting in torrential downpours with any storms. While
the nams 4000+ j/kg of cape seemed a little excessive...instability
will still be plentiful with most guidance suggesting values of
3000-4000 j/kg south of I-88. The 0-6 km shear in excess of 50 kts
and steep lapse rates aloft look very favorable for organized
storms capable of severe weather...namely winds and hail...in
central Illinois including the southern forecast area. The 0-1 km
shear is maximized over the far southern portion of the
County Warning Area...south of a Pontiac to Rensselaer line. Storm Prediction Center mentions a chance
for tornadoes in the same area...but thinking tornadoes will not
be the main threat. Once again heavy rain will be a primary threat
along with damaging winds and large hail.
Have medium confidence in timing and convective Mode. Have probability of precipitation
increasing from the early afternoon through this evening primarily
along and south of I-80. The best chance for storms will be this
evening into tonight as forcing is maximized ahead of the low and
its cold front. Could see some isolated storms at first in this
area...maybe even supercells...but the main show should be a line
or storm complex south of I-80 this evening.
Finally...lowered high temperatures today as expecting thick overcast skies
to remain in place. The clouds will limit heating and instability.
Based on 850 mb temperatures alone...however...still thinking we will
easily make it into the middle to upper 80s today. If clouds thin...we
could see spots reach 90 degrees.
the upper level low weakens and becomes an open wave over Lake Michigan
Sunday and we remain on the west side of the upper level trough
through the rest of the week. The surface plains low also weakens
and merges with the Canadian low...and then shift east to over New
England Monday morning. A second cold front moves through the region
Sunday afternoon/evening ushering in another round of well below
Expecting showers and storms to form over the lake Sunday morning as
the upper level low moves over WI. Bands of vorticity associated
with the low will provide lift for scattered storm development
across most of the forecast area Sunday afternoon. Severe storms
are not expected as instability will be weak.
High temperatures will get into the low to middle 80s...but will feel temperatures
fall quickly behind the cold front late Sunday afternoon/evening.
Overnight lows will be around 60.
Monday through Friday...
we remain on the west side of the upper level trough next week and a
surface high moves over the region Tuesday. In general looking at
below normal temperatures that slowly warm to around 80 by the end of next
week. Have a few periods of showers and storms in the
extended...but thinking storms will be very diurnal. Also not
expecting a ton of activity...as the forcing source will be weak
vorticity streamers that travel around the upper level trough.
//Ord and mdw concerns...updated 09z...
* MVFR ceilings possible through mid-morning.
* Small threat of rain showers/thunderstorms and rain today and this evening...with greater
storm coverage across southern tracon area namely this evening.
* Possible wind shift to east with a lake breeze late this
A diffuse low-level trough from Minnesota down to Kansas continues to inch
eastward. Moisture continues to slowly increase across the area
ahead of this. With that increase...2000-3500 feet ceilings are
widespread across northern Iowa/WI/northern Illinois and expecting that
over the taf sites through at least daybreak and likely a few
hours longer. There is some indication Rockford could drop down to
near 1000 feet around daybreak. Warm air advection ascent may be
enough prior to daybreak to spark a few rain showers/thunderstorms and rain but mainly south
of the taf sites. Convection expected prior to daybreak across
parts of South Dakota into northwest/NC Iowa is expected to weaken this morning as it
shifts east. So while cannot rule out and rain showers/thunderstorms and rain today...the
forcing is pretty nebulous despite high moisture around. So have
kept the tafs primarily dry. The better focus tonight will be
along a warm front boundary draped across central Illinois and in.
Winds are a challenge today due partly to the light gradient and
morning cloud cover likely to inhibit mixing to some degree. It
would appear a lake breeze will develop but how far inland this
propagates is the main uncertainty. Have opted to include in this
set of tafs but not mention a wind shift at Ord until late in the
day/early evening...as any lake breeze arrival would likely be later.
//Ord and mdw confidence...updated 09z...
* low in whether ceilings will drop below 3000 feet this morning...however
if they do confidence is high they will remain above 1500 feet.
* Low in any rain showers/thunderstorms and rain through the day today but any are likely to
* Low in wind direction through much of the taf including whether
an afternoon easterly shift will occur at both airports.
//Outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...chance of thunderstorms and rain.
Monday...VFR. Gusty north-northeast winds.
Tuesday through Thursday...generally VFR.
217 am CDT
An active period for summertime is upcoming over the lake but first
today and tonight will see southerly winds ahead of an approaching
surface low. Winds actually look to be light enough in Indiana and
Illinois nearshore zones to allow for a lake breeze to develop
for temporary afternoon onshore flow. The surface low will evolve
eastward over the middle of the lake on Sunday morning with cool
air advection enveloping the lake by late in the day and
especially in the evening. Northerly winds within unstable
thermal profiles over the water will support 20-25 knots sustained
winds with potentially some gusts knocking on the door to gale
strength Sunday night and early Monday. Waves are likely to build
where they were with the last cool advection episode earlier this
week /Wednesday/...both over the open waters and nearshore areas
of Illinois and Indiana. The flow will diminish into Tuesday but
continue from the north before high pressure settles in midweek.
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