Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
640 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.
Long term Sunday through Friday...
A secondary trough of low pressure and cold front will shift
southward over the area Sunday morning and early afternoon.
Meanwhile...in the middle and upper levels...a significant
disturbance and speed maximum...now over southern Saskatchewan...will
drop southeastward across southern Wisconsin and northern
Illinois. This looks produce some scattered showers across the
area...especially during the middle morning and early afternoon
hours. The expected cloud cover and much cooler air mass will
likely only allow for temperatures in the lower to middle 60s
across northern Illinois Sunday. It also looks to be a breezy day
as high pressure builds over the Dakotas. West to northwesterly
winds early Sunday morning will shift northerly by the afternoon
in the wake of the front.
Surface high pressure will build eastward from the northern
plains...Sunday night and move overhead on Monday. This will
result in a couple of chilly nights across the area Sunday night
and Monday night...with temperatures potentially dropping into the
upper 30s in some areas.
The remainder of the week after Sunday looks very nice and dry.
Temperatures will moderate back into the 70s by middle week as the
surface high shifts east of the area.
//Ord and mdw concerns...updated 12z...
* southwest winds increasing...gusts nearing 20-24kt. Winds
veering west/northwest tonight.
* Storms developing between 17-18z and persist until 21-22z.
Possibly a few strong/severe storms.
* Patchy MVFR ceilings late this evening.
Infrared imagery shows considerable cloud cover steadily sliding
east/southeast across northern Illinois/northwest in. Some of the clouds
have eroded...but expect some degree of cirrus to persist through
this morning. In addition the regional radar mosaic indicates a
a broken line of showers/storms extending from central wisc
stretching southwest through southcentral Iowa. Guidance continues
to point towards this area of precipitation falling apart as it
slides east into northwest Illinois this morning. Winds will remain
southerly...and with some sunshine this morning mixing will
increase as will gusts. Expect by 14-15z southwest winds will near
10-14kt with gusts possibly reaching 20-24kt. Then the atmosphere
will become unstable by midday with an upper level system arriving
and helping to kick-off showers/thunderstorms by 18z. The best
location for development appears to be along a Pontiac to Chicago
line. Then thunderstorms will be sliding east/southeast through the
afternoon/early evening. The frontal boundary will be trailing which will
keep winds southwest/west then begin turning west/northwest late
this evening. Precipitation should come to an end around 00z for most taf
sites...then it appears possible some wrap-around moisture will
arrive around daybreak sun with MVFR ceilings returning.
//Ord and mdw confidence...updated 12z...
* high confidence in gusts developing. Medium confidence in timing
of winds turning west/northwest.
* Medium confidence in thunderstorms timing...high confidence in
storms ending by 22z.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...mainly VFR. Patchy MVFR ceilings early in the evening.
Breezy northwest winds.
Monday...VFR. Light west winds.
Tuesday...VFR. Northeast winds.
Wednesday through Friday...VFR. Generally southeast to south winds.
320 am CDT
Southwesterly gales will continue to ease this morning...then the
winds will shift west-northwesterly by this evening as a cold
front pushes over the Lake. A second and stronger cold front will
push down the southern half of the lake on Sunday morning as
another trough of surface low pressure develops and strengthens
over lower Michigan state. Once this secondary cold front moves
through...northerly winds across the southern half of the lake
could increase to gales for at least a few hours as the pressure
gradient strengthens. Given this potential...I have issued a gale
watch for Southern Lake Michigan on Sunday. Wind speeds will ease
Sunday evening as the gradient weakens. Surface high pressure will
move east just south of Lake Michigan early next week. This will
result in a period of westerly winds over the lake early next
lmz669-lmz868-lmz870 until 10 am Saturday.
lmz878...9 am Sunday to 7 PM Sunday.
Small Craft Advisory...nearshore waters
until 4 PM Saturday.
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