Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1104 am CDT Thursday Oct 2 2014

Discussion...
317 am CDT

Rainfall and the potential for severe thunderstorms will be the main
focus of this complex forecast period...with unseasonably cold air
spreading in behind a strong cold front for the weekend. Water vapor
imagery shows a massive upper trough over most of central and
western North America with numerous waves embedded within the flow
triggering areas of thunderstorms. Several of these waves are lined
up from Texas/OK to eastern Wisconsin. At the surface...loosely
organized low pressure is over Oklahoma with a warm front extending
eastward from it across central Missouri and south-central Illinois.
Warm/moist ascent is driving clusters of showers and thunderstorms
across much of northern Missouri and central Illinois in conjunction
with at least one of the above mentioned upper waves.

Today and tonight...the upper wave aiding the shower/thunder
development over the local area will lift off to the northeast this
morning with current bands of showers and embedded thunder will exit to
the northeast by middle morning or so. Some spotty showers/thunder may
linger through early afternoon but there is not much support for
large scale precipitation development through midday or even early
afternoon...with current convection also helping to limit the
incoming moisture flux. The surface low will begin to lift northeast
and become marginally better organized through the afternoon as the
main upper trough axis currently pushing toward western Kansas/OK/TX
advances eastward. It does appear that one or several middle/upper
waves will lift northeastward ahead of the main trough with at least
one currently producing new convection across Oklahoma. This may
result in an increase in convection locally from middle to late
afternoon...especially across the western County Warning Area. Another potential
source for convection will be the northward advancing warm frontal
boundary which is expected to pass this afternoon and be north of the
area by this evening. Confidence in timing/coverage details from
about middle morning through middle afternoon is limited but will continue
with the idea that there will be a minimum of activity during this
time...though there may be some activity around. The surface low
remains elongated and tracks northward along an inverted trough axis
from northern Missouri to central Wisconsin by evening. The main
upper trough axis will continue east as well helping to focus new
convective development along the cold front trailing the low. This
will lead to steadily increasing coverage of showers/storms across
the western County Warning Area late in the afternoon with the bulk of the activity
crossing the area this evening and tonight as the low deepens over
Northern Lake Michigan and the upper trough axis approaches. An
organized line of storms is expected to develop but the main
question is how far north in the area it may extend.

Heavy rainfall is a concern with precipitable water values of
1.6-1.7 inches. Much of the area should see around an inch of rain
with higher values near 2 inches possible if a line of particularly
well organized convection does cross the area ahead of a more
widespread area of less intense showers and storms. Severe potential
will be a challenge with it conditional on the extent of the
activity across the area today. Better instability would likely
need to be advected in with the passage of the warm front. Deep
layer shear will be strong as will the overall large scale forcing
so even weak instability could produce severe weather...with
guidance showing around 1000 j/kg of cape developing by late day
which would be plenty to increase the severe threat. Damaging
straight line winds will be the primary threat but a well developed
squall line could spawn a few short lived tornadoes provided strong
low level instability develops and there is sufficient low level
turning. Unfortunately there are still too many uncertain variables
to have good confidence in the threat for severe weather.

Friday and Saturday...the surface low will be moving into central
Ontario Friday morning with the cold front exiting the eastern County Warning Area.
The exit of rain to the east may lag the front thanks to the upper
trough axis still crossing the area. Another push of energy will be
racing southeast across the plains Friday night spawning
amplification of another upper trough axis which will cross the area
Friday afternoon. This will likely bring another round of showers
with it. Yet another waves races down the back side of the now very
amplified longwave trough helping it to close off over Wisconsin
Friday night which then leads to another embedded trough axis
pivoting across the area Saturday...leading to additional low
chances for showers during the morning/midday. The bigger story
however will be the push of cold air brought in behind the cold
front Friday. 850 mb temperatures will crash to about -2c by Friday evening.
Highs Friday will likely occur early with temperatures falling into the
upper 40s/lower 50s by evening with gusty west-northwest winds. Lows
will fall into the upper 30s with frost development unlikely thanks
to winds and clouds as well as some showers. Highs Saturday will
only reach the lower 50s and some areas of the northwest may be held
into the upper 40s. Lows Saturday night will should fall into the
middle to upper 30s as a weak ridge axis arrives by Sunday morning.
Winds may stay strong enough and cloud cover extensive enough to
preclude widespread frost development. Middle level temperatures warm later
Sunday but clouds and precipitation may temper warming so am expecting
highs in the middle to upper 50s with 60 possible far south.

Sunday through Wednesday...the expansive upper trough and associated
cyclonic flow will remain in place through Tuesday before it starts
to transition northeast and much lower amplitude flow builds in.
This will set the stage for the passage of embedded shortwaves and
therefore periodic shower chances with Monday looking to be a period
of higher chances for precipitation. Middle levels will gradually warm though
some cooling and warming will be modulated by the passage of the
waves but overall surface temperatures will be on a slow upward trend into
middle week...with upper 60s/lower 70s possible again by Wednesday.

Mdb

&&

Aviation...

//Ord and mdw concerns...updated 16z...

* increasing chances for showers/thunderstorms late this afternoon
into the evening...with showers likely persisting overnight.

* MVFR and IFR likely tonight into Friday morning...LIFR possible
late tonight.

* Wind shift to the west northwest with frontal passage early Friday
morning...with increasing speeds through the end of the forecast
period.

Rodriguez/kb

//discussion...updated 12z...

Bulk of most activity remaining outside of the terminal areas this
morning...but with some isolated light showers moving overhead.
Expect current showers/thunderstorms to the south to remain to the
south as this precipitation drifts to the east this morning. With the
departure of the showers this morning expect relatively dry
conditions for most of today...outside of a stray shower or two.
An isolated area of MVFR ceilings have developed across northeast
Illinois this morning...while a more solid area of low clouds is
situated over the eastern Iowa and far western Illinois area. The
isolated area should dissipate over the next couple of
hours...while the low clouds to the west remaining in this
location for most of today. The only terminal which will likely
observe an extended period of MVFR ceilings today will be rfd.
Activity will likely increase very late this afternoon into the
evening with the arrival of better large scale support for showers
and thunderstorms. Still some uncertainty with exact timing and
duration but at this time...a later start time does appear likely
and have reflected this in the tafs. Thunderstorms will likely
remain overhead for a portion of the evening but with instability
diminishing overnight...feel that a trend towards more showers
will occur overnight and then continue into Friday morning. The
arrival of the better large scale support will be accompanied with
the arrival of a surface trough from the west. This will mark the
arrival of already present IFR ceilings...with these ceiling
likely continuing through Friday morning. There is the potential
for LIFR as well...but confidence is low with this possibility at
this time. Frontal passage should quickly move across the terminals Friday
morning...with winds quickly shifting to the west and observing a
continued increasing trend.

Rodriguez

//Ord and mdw confidence...updated 16z...

* medium high confidence with wind speed/duration.

* Medium high confidence of showers/thunderstorms...low medium
confidence of timing.

* Medium high confidence of MVFR/IFR tonight...low medium
confidence of LIFR.

* High confidence of wind shift and speeds with frontal passage Friday
morning...medium confidence of exact timing.

Rodriguez/kb

//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...

Friday night...chance of rain showers.

Saturday...chance of rain showers.

Sunday...slight chance of rain showers.

Monday...chance of rain showers.

Tuesday...VFR.

Wednesday...VFR.

Rodriguez

&&

Marine...
441 am CDT

Surface ridge of high pressure spans the Atlantic coast and low
pressure is over the Southern Plains this morning...with south to
southeast winds currently being observed across the lake. The
surface pattern has tightened this morning and has resulted in
some slight increase in speeds...mainly across the south half of
the lake where 15 to 25 knots winds are being observed. These current
speeds will likely diminish this morning...with south winds of 10
to 20 knots being observed for the rest of the day. The low pressure
over the Southern Plains will lift northeast to the upper
Mississippi Valley today...and then continue northeast to the
central Great Lakes tonight into early Friday morning while also
strengthening. This will result in a varying wind direction as
well as an increase in speeds through Friday. Although winds will
be increasing through this period...it appears that speeds will
really ramp up late Friday afternoon into the early evening. West
gales do appear to be a definite possibility Friday night into
early Saturday morning over the southern half of the lake...and so
issued a gale watch. The nearshore waters will be on the fringe of
the strongest winds but with the Indiana nearshore waters possibly
observing these gales as well...did go ahead and include a portion
of the Indiana nearshore zones. As the low continues north through
Ontario to Hudson Bay Saturday and Saturday night...west winds to
30 knots will likely continue before diminishing on Sunday.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...gale watch...lmz080-lmz567-lmz669-lmz671-lmz673-lmz675-lmz744-
lmz745-lmz777-lmz779-lmz868-lmz870-lmz872-lmz874-lmz876-
lmz878...4 PM Friday to 9 am Saturday.

&&

$$

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

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/US.Nationalweatherservice.Chicago.Gov
www.Twitter.Com/nwschicago
www.Youtube.Com/nwschicago

National Weather Service Glossary of Abbreviations