Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
639 am CDT Friday Aug 28 2015
344 am CDT
Through Saturday night...
Main forecast concerns/challenges are with precipitation trends later
today into tonight...and into the weekend.
Latest satellite imagery depicting upper level with several vorticity
lobes rounding the base...progressing east through the northern
plains early this morning. Although associated precipitation remains well
west of the County Warning Area at this time...fairly dense middle/high level cloud
cover already stretching east into northern Illinois. Radar
imagery showing some weak returns at this time...however...this is
picking up these clouds with any precipitation likely not making it to
the surface. Have maintained a dry forecast early this morning
through midday for the entire County Warning Area as strongest forcing with the
first wave lifting east/northeast staying to the west/northwest.
Some guidance is trying to hint at some stray light showers over
portions of northern Illinois this morning but with rather dry low
levels in place this morning...per 00z observed soundings and
latest forecast soundings...think any forcing owing to weak warm air advection
will not be able to overcome this drier air. Do bring slight
chance to chance probability of precipitation over far northern Illinois by middle to late
afternoon today as this middle/upper level trough and associated
energy move closer to the County Warning Area...but once again feel that the best
forcing will still be to the north and northwest. Have maintained
showers and did not include thunder...with instability staying to
the west of the County Warning Area. High temperatures will be limited today due
to the middle/high clouds in the 70s...and did lower them slightly to
the low/middle 70s across northern Illinois where more dense clouds
and precipitation will likely be located.
Higher chances for precipitation will arrive later this evening and
especially overnight tonight for most of northern Illinois...as
shortwave energy on the backside of this trough and attendant
surface reflection will push into northern Illinois. Did maintain
slight chance for thunder with the likely/Cat probability of precipitation over northern
Illinois tonight...despite lacking instability. Strengthening low level jet
and stronger forcing could offset this lacking instability and
allow thunder...similar to what is occurring now over Iowa. East
central Illinois and Northwest Indiana wont observe the higher
chances for showers/thunder until Saturday morning and Saturday as
this trough pushes through. Confidence with overall precipitation trends
on Saturday and Saturday night does lower with guidance varying to
the extent and location of best forcing. Did maintain high chance
probability of precipitation for showers across much of the County Warning Area Saturday and then took
these chance probability of precipitation eastward through time during Saturday night.
Instability will try to increase on Saturday...but still think it
wont be high enough to warrant any wording other than slight
344 am CDT
Sunday through Thursday...
End of the weekend into the start of next week will see this
middle/upper trough slowly exiting to the east...with clouds and
precipitation chances likewise exiting during the day Sunday. Guidance
varies to the evolution of this exiting system but likely scenario
for our area will be for upper level ridging with weaker flow
aloft. Conditions will likely be on the dry side through early
next week...outside of some possible afternoon thunderstorms on
Tuesday due to increasing moisture/instability...while a warming
//Ord and mdw concerns...updated 12z...
* wind shift to easterly likely late this afternoon
* chance of rain showers/slight chance of ts tonight into Sat morning
A slow moving storm system over Iowa will weaken as it moves
closer to the region tonight into Saturday. Area of showers and
isolated thunderstorms and rain over Iowa will continue to inch eastward but progress
should be slow and leading edge could erode some in drier air mass
over our region with focus initially likely more into Wisconsin.
With time the showers and perhaps a ts or two will sag south into
north Illinois tonight (mainly late) into Sat morning. Some guidance
suggests precipitation will be decreasing in coverage/intensity by this
time...lending to some uncertainties in how to best convey threat
in tafs. Other than precipitation...which could possibly result in some
MVFR ceilings by Sat morning...only other concern will be winds this
afternoon and evening. Initially light southerly winds will likely
shift to easterly late this afternoon...possibly enhanced by lake
in chgo terminals. Could see a brief period of 10-12kt winds but
suspect winds will largely end up just below 10kt from easterly
direction through tonight.
//Ord and mdw confidence...updated 12z...
* high confidence in wind shift to east later this
afternoon...low- medium confidence in precise speeds and timing
* medium confidence in precipitation chances/timing
* high confidence in remaining forecast elements
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
Saturday night...slight chance of thunderstorms and rain...otherwise VFR.
Sunday...VFR. Northeast winds.
Monday through Thursday...VFR. Southerly winds...possible
afternoon wind shifts to easterly with lake breezes each
157 am CDT
Fairly typical Summer pattern likely to persist through the
upcoming week with generally no significant winds/waves
anticipated. Weak surface low over the corn belt will track slowly
east into northern Illinois/southern Wisconsin through Saturday
before dissipating. The main result will be a backing of winds to
easterly over southern portions of the lake later today into this
evening before veering southerly again late tonight and freshening
up just a bit...but likely less than 20kt.
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