Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
244 am CDT Sat Sep 20 2014
243 am CDT
main focus in the near term will revolve around timing of
precip/thunderstorms...which guidance is still leaning towards a
midday/afternoon timeframe. The wildcard is the early morning convection
over central Iowa that stretches northeast into far southwest
Wisconsin. Dry air continues to linger across the forecast
area...although this will quickly change as the morning progresses.
Abundant moisture exists just west of the County warning forecast area with pwat's hovering
between 1.5-1.8" in much of Iowa/northwest MO. This moisture rich
environment is prognosticated to slide east by late morning...bringing dew
points into the middle/upper 60s and perhaps a few sites hitting 70 degree dew
points this afternoon...mainly south of I-80. Middle-level lapse rates remain
rather steep early this morning...so if the convection across Iowa
can hold together it is possible the elevated cores could strengthen
further and bring a low risk of severe storms to the area by late
morning. Hi-res guidance is pointing towards the early morning line
By late morning a middle-level vorticity...that continues to dive southeast
across the northern plains...should catch-up with the trough and
help to redevelop and strengthen thunderstorm activity shortly after
18z. The placement will continue to be a challenge...as some
guidance suggests that it could slip just past Chicago before
developing. Nonetheless have held onto the likely probability of precipitation from Pontiac
to Chicago early afternoon...then expect this channel of likely probability of precipitation to
slowly slide east. Middle-level vorticity maximum will be pivoting east across the
County warning forecast area by 20z...which should then bring a slow end to the stronger
storms from west to east shortly after this. The frontal boundary
appears to be lingering further west...but shortly after 00z this
boundary should arrive across the northwest forecast area.
Temperatures today will be a challenge. At first glance it appears the
highs forecast in the low 80s look to be in good shape. However with
the cloud debris from the early morning complex arriving this
morning...it is plausible that enough cloud cover lingers and holds
temperatures down in the 70s. The deep layered shear is prognosticated to steadily
increase by this afternoon as well...which could suggest some of the
strongest storms could produce a damaging wind threat and perhaps
become better organized.
middle-level vorticity will quickly Peel off to the east/northeast by
00z...and could see a quicker end to the convection just before 00z.
For now will show a steady decreasing trend in the precipitation...with
much drier/cooler air steadily advecting in from the northwest.
Winds will slowly turn northwest overnight. Wrap-around moisture
should hold off until just after daybreak sun...with much of the
overnight hours remaining dry. Overnight lows will likely hold in
the low 60s across the southeast County warning forecast area...while other areas fall into
the middle/upper 50s.
Extended discussion will be issued shortly...
//Ord and mdw concerns...updated 06z...
* S/SW winds with speeds around 10-12kt...will see gusts redevelop
by daybreak to near 20kt.
* Possible isolated rain showers/thunderstorms and rain west of Ord/mdw before 14z...then better
development to thunderstorms and rain after 17z through 21z.
* Patchy MVFR ceilings with rain showers/thunderstorms and rain through 02z. Winds turn
west/northwest by daybreak sun.
Infrared imagery early this morning shows minimal cloud cover over
northern Illinois/northwest in...however a complex of
showers/thunderstorms across central Iowa/southern Minnesota will
continue to slide southeast towards the area. Timing would suggest
the high clouds will arrive around daybreak. Guidance has shown that
some spotty rain showers possibly an isolated thunderstorms and rain could be going on at daybreak
in the west near rfd. Then there is some better agreement than
increased coverage of thunderstorms and rain will take place midday through the afternoon
and impact Ord/mdw. Main timing appears to be 18-21z. With the
stronger storms some patchy MVFR ceilings may occur. Winds will also
be a challenge as clouds will likely keep the gusts down around
18-22kt. But if clouds can thin or erode any midday...gusts could
be higher or near 25kt. Frontal boundary is slow to push through...so
some linger rain showers could occur after 21z before much drier air arrives
from the northwest. Southwest winds will then turn west/northwest
late tonight into early Sunday morning.
//Ord and mdw confidence...updated 06z...
* high confidence in all elements through daybreak.
* Medium confidence in timing/intensity of rain showers/thunderstorms and rain after 18z.
* High confidence in winds turning southwest to west/northwest
//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...VFR. Breezy northwest winds.
Monday...VFR. Light westerly winds.
Tuesday...VFR. Northeasterly winds.
Wednesday and Thursday...VFR. Southeasterly winds.
315 PM CDT
Primary concerns are south to southwest gale force winds on the
northern 2/3 of the lake late tonight into Saturday
morning...followed by strong north-northwest winds on Sunday into
Sunday night. Warm air mass is leading to stable conditions over
the lake but very tight pressure gradient between strong low over
Hudson Bay and expansive high centered over New England. This will
support high sustained speeds near or at times over 30 knots...and
frequent gale force gusts. Occasional gale force gusts are
possible on the southern 1/3 of the lake this evening. Another low
will form in the frontal trough of the cold front trailing from
the low over Canada and lift northeast on Saturday. Southwest
winds will gradually ease through the afternoon then yet another
low will form in the frontal trough Saturday night and move into
the eastern lakes on Sunday as high pressure spreads east from the
plains. Much cooler air mass and fairly tight pressure gradient
will result in northerly winds to 30 knots on Sunday...especially
over the south half open waters. There is some potential for gales
so will need to monitor this period. Fairly quiet conditions are
then expected early to middle work week as high pressure moves
gradually east across the Ohio Valley.
For the nearshore waters...southerly winds are still expected to
increase to Small Craft Advisory criteria for the Illinois zones late this
afternoon and then the Indiana zones this evening...with criteria
winds until middle Saturday afternoon. Small Craft Advisory conditions
are then possible Sunday into Monday morning...especially for the
Indiana zones where the largest waves are anticipated.
lmz669-lmz868-lmz870 until 10 am Saturday.
Small Craft Advisory...nearshore waters
until 4 PM Saturday.
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