Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 181142 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
642 am CDT Tue Sep 18 2018

Short term...
358 am CDT

Today and tonight...

A weak cold front was sagging into far northern Illinois early this
morning, with a typical surge noted along the Illinois Lake Michigan
shore. GOES infrared satellite imagery and surface obs indicates some
low cloud cover spreading into far northeast IL, along with a
modestly breezy wind shift to the northeast. Front doesn't have
much push to it otherwise, and thus is expected to continue to
only slowly sag across the northern Illinois and northwest in today.
With a lack of strong cool advection behind the weak front, temps
will remain well above average in most areas today, with highs
expected to range from the low to mid 80s north, to around 90
again south of the I-80 corridor. Lake Shore areas will likely be
limited to the 70s, due to the onshore winds.

With the front laying out across the area, some potential for
isolated to scattered showers and thunderstorms will exist.
Forcing is not particularly strong however, and therefore trying
to detail timing and location is somewhat difficult. Regional
radar mosaic depicts an mesoscale convective system currently across parts of southern
Minnesota/western WI, and much of the high-res cam guidance continues to
depict this activity eventually reaching northern Illinois later this
morning/midday, albeit in a decaying form. A low-amplitude short
wave evident over South Dakota and northern Nebraska will approach
the area this afternoon, which combined with weak convergence
along the diffuse frontal boundary and the diurnal peak in
heating/instability, will continue to present a scattered
thunderstorm threat this afternoon.

Focus for additional thunderstorm development then shifts back to
our northwest tonight, on the nose of the low level jet axis
across northern Iowa and Minnesota. Some of this activity may slip into
northwest/northern Illinois toward morning, thus have pops primarily
north after midnight tonight.



Long term...
343 am CDT

Wednesday through Monday...

Stalled frontal boundary will continue to impact local weather
through mid-week, with periods of scattered thunderstorm potential
and a large north to south temperature gradient making for an
active forecast period.

Focus for thunderstorm development again looks to be from
northern Iowa into Minnesota early Wednesday, where the nose of a broad
southwesterly low level jet will impinge on the elevated
baroclinic zone north of the surface frontal boundary. High-res
guidance at 12z Wednesday depicts an ongoing mesoscale convective system in this region,
upstream of the forecast area, which propagates southeast along
the baroclinic zone into northern Illinois during the morning.
Convection is expected to weaken and decay as it slides to the
east-southeast, though persistent southwest flow and moist
advection atop the frontal zone will likely continue to support at
least scattered shower/thunderstorm activity across especially
northern parts of the forecast area during the day. Farther to the
west, an upper level trough across the western Continental U.S. Begins to
develop eastward across the intermountain west and Rocky Mountain
regions through Wednesday night. Associated mid-level height falls
spread across the plains, inducing surface low pressure to
consolidate and lift toward mid-Missouri Valley Wednesday night.
This lifts the stalled front north as a warm front across the cwa
overnight, and develops stronger moisture transport into the
region as the low level jet spreads into northern Illinois/southern WI.
This maintains the threat of thunderstorms across the forecast
area, especially north, overnight into Thursday morning, as the
warm front continues to lift north. Some potential exists for
locally heavy rainfall Wednesday night into Thursday morning from
northeast Iowa into southern WI, on the nose of the low level jet
and associated moisture advection/transport, and this will have to
be monitored for close proximity to far northern Illinois.

Upper level trough continues to lift across the northern plains
and into the upper Midwest on Thursday, with surface low pressure
tracking north of the area across Minnesota and WI. This will allow the
warm front to lift north of the Illinois/WI border, with a very warm,
moist and breezy warm sector spreading across the forecast area.
Shower and thunderstorm coverage should be fairly sparse, if any,
once the front lifts north of the area in the morning. While
temperatures on Wednesday will likely be a range from low-mid 70s
across far northern Illinois to the upper 80s closer to the front across
the southern cwa, the entire area should warm up Thursday. Low
level model thermal fields would support temperatures reaching the
lower 90s in most areas. This will come close to or potentially
reach or surpass record high temperatures Thursday afternoon.

Low pressure continues to deepen as it lifts northeast across the
northern Great Lakes Thursday night. The trailing cold front
pushes southeast overnight, with an increase in shower and
thunderstorm chances along and ahead of the front. The combination
of 1500-2000 j/kg MLCAPE values in the warm sector late Thursday,
and strong southwesterly flow aloft, suggests the potential for a
few strong/severe storms late Thursday, with the greatest threat
across the northwest parts of the forecast area where storms will
arrive earlier with less diurnal cooling. Storm Prediction Center has included the lot
cwa in their day 3 outlook, with a slight risk for far
north/northwest Illinois. The cold front will continue to push southeast
across the forecast area into Friday, with a few storms
potentially lingering in the southeast through midday or early

Surface high pressure is then progged to build across the upper
Midwest and western Great Lakes region Friday night, with large
height rises across the region in the wake of the departing upper
trough. Guidance has come into better agreement in pushing the
cold front well south of the forecast area, south of the Ohio
River, overnight. Thus, cooler and drier weather arrives for the
weekend, for the most part, with highs ranging from the 60s near
Lake Michigan to the lower and middle 70s farther inland and south
of the I-80 corridor. Overnight lows may dip into the 40s in a
few of the typically cooler spots across far northern Illinois. The
exception to the dry part of the weekend is the ECMWF, which while
farther south than its previous runs, still brings the remnants
of a tropical disturbance across downstate parts of IL/in. Have
maintained a low chance pop mainly Saturday night for areas south
of the I-80 corridor for this, though the GFS and Canadian models
maintain dry conditions. Surface high pressure over the Great
Lakes eventually drifts off to the east on Monday, allowing the
development of some weak return flow into the Mississippi Valley
and a slight moderation in temps to start the new week.




Record high temps could be jeopardy Thursday, here
are the current records:

Chicago 92 (2017)
Rockford 92 (1920)


for the 12z tafs...

The main concerns continue to focus on timing the best chances for
thunderstorms at the terminals.

A deck of IFR cigs has moved over portions of northern Illinois early
this morning. This appears that it will be short lived, however,
as outflow associated with thunderstorms in southern WI is now
pushing south into the area. This outflow will likely help to
scatter out this deck of low cigs. Winds will also likely become
briefly gusty up around 20 kt for a short period at the terminals
following the passage of the boundary through 14z. I attempted to
cover this threat with a tempo for gusty north winds.

Thunderstorms look likely to impact rfd shortly after 12z through
about 14z this morning as activity shifts over this area from the
west. Farther east towards the Chicago area terminals, there
remains a good amount of uncertainty on if any of these storms
will hold together long enough to have an impact on them. If the
current storms moving over northwestern Illinois do hold together they
could reach Ord by around 14z this morning. We will continue to
monitor and amend the taf it needed.

Another round of showers and thunderstorms remains possible this
afternoon after 20z at the Chicago area terminals, and slightly
earlier at krfd. For this period, we have the mention of a thunderstorms in the vicinity.
Another upper level disturbance to our northwest over western
Iowa and southwest Michigan is expected to shift to the east-southeast
through the day and thunderstorm activity associated with this
disturbance could impact the area later this afternoon. Confidence
on this afternoon activity is low to medium at this time.

Following this afternoon activity it appears that the threat of
additional showers and storms going into tonight will remain
rather low across the area, with the better focus likely to our
north-northwest. However, some of the storms to our northwest
tonight could try to work there way southeast into our area by
early Wednesday morning. So the morning period will need to be
monitored for the possibly of another round of storms.



302 am CDT

A rather active period of weather is expected on the lake for most
of the week.

A cold front is currently shifting down the southern third of the
lake early this morning. Winds north of this front are northerly
in the 10 to 20 kt range, though a brief period of stronger winds
up around 25 kt is likely with the frontal passage. This front
will stall out south of the lake today, so northerly winds of 10
to 20 kt are likely to continue over the lake through the day.

The winds are then expected to veer from a northeasterly
direction tonight into Wednesday to the south-southeast by late
Wednesday night. This is expected to occur in response to an area
of low pressure taking shape over the plains. The low is expected
to track northward over the upper Great Lakes by late Thursday. As
it does so, the frontal boundary settling just south of the lake
today will move back northward over the lake as a warm front into
Thursday. In the wake of this warm front, southerly winds are
likely to increase significantly later Thursday into Thursday
night as the surface gradient tightens in advance of the
approaching surface low. Currently in appears that south-
southwester gales to 35 kt will be possible, especially over the
northern end of the lake Thursday night. However, there is some
uncertainty in how high the winds may get on the lake due to the
fact that a rather warm airmass will be moving over the lake.
Winds only a couple hundred feet off the water are forecast to
increase up to 40-45 kt, so this will have to be monitored

Expect the winds to shift back to the northwest late Friday as
another cold front shifts over the lake. Wind speeds will likely
remain elevated with this northerly wind shift Friday night, but
surface high pressure looks to set up over the lake this weekend,
so wind speeds are likely to abate for a period over the weekend.



Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations