Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
658 am CDT Wednesday Oct 1 2014
328 am CDT
A range of Fall weather is expected over the next week with fairly
mild conditions to start with a period of rain/thunder ushering in
temperatures much colder than normal for the weekend. Early morning
water vapor imagery shows a closed upper low slowly spinning over
Pennsylvania with a narrow ridge axis extending over the local area.
Of greater note is a broad longwave upper trough over much of
western North America. One closed upper low is spinning northward
across the northern plains into the Canadian prairies with a weaker
wave moving eastward into the Central High plains. Another wave is
digging into the Desert Southwest and will be a key player in our
weather going into Thursday/Friday. At the surface...high pressure
ridging is spread from Quebec southwest into the local area with a
strong low moving northward into southern Manitoba.
Today...the upper ridge and associated surface ridge will shift to
the east today resulting in a light southeast wind. Winds look to be
light enough to allow the lake to turn winds more easterly during
the afternoon. So while highs across the majority of the area will
warm into the 70s...warmest west and south...Lakeshore areas of NE
Illinois will likely be held into the low to middle 60s. Northwest Indiana Lakefront
areas may be spared the extra cooling unless winds are able to turn
northeast. Stratus may be lingering across the northern counties
through middle morning but should lift to the north by late
morning/midday. Middle and high clouds look to thicken across northwest
areas this afternoon.
Tonight...things start to become more active tonight as the
northernmost upper low lifts into central Canada and a series of
weaker waves currently moving across the central/Southern Plains
move northeast and act on the middle/upper level moisture plume on the
east side of the broad upper trough. This will push bands of showers
eastward into northern Illinois. The forcing is not terribly
coherent as it spreads in early this evening so will keep probability of precipitation
low...but forcing focuses overnight resulting in best coverage of
showers across the western half of the area...with less coverage
east. Elevated instability looks to be limited but still present
overnight so will include thunder mention mainly overnight. Overall
activity looks somewhat disorganized.
Thursday and Thursday night...this will be the main period for
rain/thunder as the upper wave now moving into the Desert Southwest
helps amplify the the longwave trough as it moves into the plains.
This will strengthen a surface low over the Southern Plains which
will track northeast and deepen as it reaches the u.P. Of Michigan
by Friday morning. This will bring several waves of showers and
storms as a warm frontal boundary develops and crosses the area
during the day with a cold front crossing Thursday night. Timing of
waves will need to be refined but right now the main period for
activity will be from afternoon through the overnight. Surface based
instability may spread into the County Warning Area Thursday afternoon but the warm
sector ahead of the low looks to be filled with clouds and scattered
precipitation. Focused forcing looks to be held back across
Missouri/Oklahoma/Kansas during the day with the upper trough axis
kicking the surface low northeast across northwest Missouri late in the
afternoon. This would result in a line of storms tracking
northeastward into the evening. Will need to keep an eye on how far
west of the area the low tracks with a further west track...as some
guidance is showing...leading to a better severe threat across much
of the County Warning Area thanks to a more robust warm sector and greater
instability in place to help maintain an incoming squall line
during the evening. Strong straight line wind gusts would be the main
severe threat. Heavy rainfall is a concern as precipitable water
values push 1.6 or 1.7 inches into Thursday night. Widespread
amounts of around an inch look likely with the extent of stronger
convection dictating potential for isolated 2+ inches. The limiting
factor would be that the waves of precipitation look to be moving steadily.
Highs Thursday will be tricky depending on the timing of precipitation
waves and cloud cover but with the area getting into the warm sector
and coverage of precipitation looking to be lowest during the day highs may
warm well into the 70s for most areas.
Friday and Saturday...the surface low will continue northeast with
guidance still in disagreement on where it will be by 12z Friday.
The front should be exiting the eastern County Warning Area if not having already
done so by Friday morning so some lingering convection may continue
east. A reinforcing upper trough will dive in from Canada and close
off before dropping right across the area late in the day/evening.
Strong cold advection will spread in as all of this occurs with 850 mb
temperatures falling to near 0c by evening with a gusty west wind. In
addition with the upper low crossing the area showers are a good
possibility...especially north of Interstate 80. Between the cold
advection and increasing shower potential high temperatures will probably
occur early then fall through the afternoon. Thus expect low to middle
50s if not cooler by evening for all areas...with lows falling to
around 40 away from the lake Friday night...with winds and cloud
keeping maximum cooling from occurring. Some showers may linger in the
east Saturday morning but expect a dry day beyond that with a sun
and cloud mix. Highs will likely only top out in the low to middle 50s
with breezy west to northwest winds. Weak ridging will move across
Saturday night but with broad cyclonic flow over the region thanks
to what will have evolved into a massive broad upper low over
Ontario...there will be a tendency for shortwaves to pass along with
bouts of cloud cover. As a result maximum cooling may be tempered once
again but still expect upper 30s for most areas Saturday night
bringing at least patchy frost potential.
Sunday through Wednesday...the big upper trough will be in place
into at least Tuesday as it will be reinforced a few times by waves
pushing into from central Canada. This will bring a few additional
chances for showers with some consistency in a wave passing later
Sunday/Sunday night. Each wave will bring a brief push of warm
advection but any notable warming does not look to occur until middle
week when upper ridging starts to move in from the west and the
eastern trough starts to de-amplify. Should see some temperature moderation
Sunday and Monday but am wondering if forecast highs are a little
warm...especially if we end up with cloud cover either day. Tough to
pinpoint best rain chances but Sunday afternoon through Sunday night
looks best right now.
//Ord and mdw concerns...updated 12z...
* east southeast winds through period...increasing to around 10 knots
* Increasing chances for precipitation this evening through overnight and
Thursday morning...with thunder possible.
Low stratus continuing to slowly push north of all the terminals
this morning...with latest satellite imagery showing the far
southern edge of this cloud shield just north of Ord. These clouds
will continue to push north this morning with VFR conditions
likely for the remainder of the day. Southeast winds are currently
staying under 10kt but do anticipate this to change by around the
14z time frame...when speeds should increase around the 10kt range
and remain this way through the afternoon. This southeast wind
will hold for the remainder of the forecast period...but likely
diminish this evening. Chances for showers and thunder will
increase later this evening as upstream forcing will allow for
continued development to the current area of precipitation over Iowa.
This precipitation will slowly move east northeast today reaching the rfd
area by early this evening...and then continue a slow eastward
push through the overnight hours. Although the instability axis
currently resides well west of the terminals...this will slowly
shift east as well. Confidence with regards to prevailing thunder
potential is lower due to questions to the extent of this eastward
push of the better instability...and higher with shower chances.
Potential is there for thunder and so have included in the taf
with the prob30 group.
//Ord and mdw confidence...updated 12z...
* high confidence with wind speed/direction.
* Medium confidence with precipitation trends.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night...periods of thunderstorms and rain. MVFR likely. IFR possible.
Friday...chance of rain showers.
Sunday...chance of rain showers late.
Monday...slight chance of rain showers.
Tuesday...slight chance of rain showers.
306 am CDT
Both winds and waves have continued to diminish/subside early this
morning...as the surface field has relaxed with high pressure
spanning from the upper Midwest northeast to Quebec. This high
pressure will shift east towards the eastern Great Lakes today
while low pressure over the northern plains lifts north into
Canada...and while a separate area of low pressure remains
situated over the Central Plains. Surface pattern over the lake
during this time will try and tighten up through the day but will
only result in an increase in speeds more towards 10 to 20 knots...as
southeast winds become more south later today into tonight. This
speed and direction will continue into Thursday as the low
pressure over the Central Plains lifts northeast reaching the
upper Mississippi Valley by Thursday night. Winds will likely ramp
up quickly Thursday night as this low moves to southern Ontario
while strengthening...with winds to 30 knots likely. A very tight
gradient on the southern end of this low pressure system as well
as much colder air moving over the lake will support winds to 30
knots Friday and Friday night as well as even a period of gales
possible. Then expect a slow downward trend in speeds through the
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