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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1115 am CDT Friday may 29 2015

Update...

1058 am CDT

Showers have already kicked off this morning with soundings
showing weak but uncapped instability in place. Middle level lapse
rates are poor across the area...around 5 c/km which has resulted
in slow growth of convective activity and very little if any
lightning thus far this morning. Models show little improvement in
these lapse rates through the day...and with temperatures already
in the upper 70s to low 80s and dew points in the middle to upper
60s...dont expect too much in the way of diurnal increase in the
sb/MLCAPE unless we see significant clearing in the cloud cover.
Refined probability of precipitation through the morning to show current line of showers
over The Heart of the County Warning Area slowly lifting northeast across the
Chicago metropolitan. This afternoon...lack of any focus and only
broad/modest ascent makes it hard to pin down any details on
timing or area so blanketed the County Warning Area with 35-45 percent probability of precipitation.
Updated zones to include scattered showers with isolated
thunderstorms through the afternoon.

Deubelbeiss

&&

Short term...
339 am CDT

Through Saturday...

A warm front extending from NE Iowa into the southern suburbs of
Chicago and into Indiana continues to make northward progress during
these early morning hours. The relatively comfortable middle 50s
dewpoints in northern Illinois will soon be replaced by muggy 60s
dewpoints. Aloft...a closed middle level circulation is noted in water
vapor imagery over Wyoming...with multiple shortwave troughs/vorticity
maxes embedded in broad southwesterly flow across the plains and
extending into the local area. Some shower activity is ongoing
across eastern Iowa and extending back into Missouri in
conditionally unstable and moist (pws of 1.5 or so). Some
isolated showers/earlier T-storms near kgyy are occurring over our
area associated with a fast moving upper jet segment...along with
an increase in low level flow per overnight amdar soundings and
local vwp analysis. A few hundred joules of elevated instability
do exist across the area this morning.

The challenge today will be in timing and coverage of showers and
embedded thunderstorms given the passing of subtle shortwaves in
the SW flow aloft. The entire area...with the exception of over
the lake...will be firmly entrenches in the warm sector by
daybreak. We will be uncapped much of the day...but instability is
expected to be tempered. Weak upglide on 305k isentropic surfaces
are noted over NE Illinois...slightly better off to our west.
Therefore expect coverage will remain limited over the area
initially. This lift is expected to increase over the remainder of
the area through the morning/early afternoon as these shortwaves
progress through the area. Attempting to latch onto these waves in
Iowa/Missouri and at least moderate confidence from decent
agreement in hi-resolution guidance in progging showers to
increase late morning to around midday. Recent guidance is backing
off a bit on this solution...but the pattern still supports
scattered activity later in the morning. There could be some
activity not long after daybreak...but it should be isolated at
best. This is initial morning/early afternoon activity does not
pose a significant risk for severe weather...though locally heavy
downpours would be a concern in the increasing high precipitable water airmass.

Concern then increases later in the day/evening as a cold front
slides southeast that a marginal severe weather threat exists.
Additional scattered thunderstorms are expected ahead of this
front. We will still be unstable...though to what degree will
depend on any earlier convection. The front will also intact with
an increasing middle level speed maximum as a positively tilted upper
trough approaches. With this increased wind shear and modest
instability... locally heavy rainfall and some stronger wind gusts
look to be the main hazards. Marginally severe hail is also
possible in deeper updrafts with modest 500-300 mb lapse rates.
Still coverage across our local area may be tempered as the front
lingers to the west and may be arriving a bit past peak
timing...especially closer to the urban corridor.

Broad western upper trough will be crossing the area tonight along
with the surface cold front which is being driven by a strong upper
wave in southern Canada. Some differences exists with how quickly
this wave moves through...specifically when comparing the WRF-NAM to
the global models...and any impact of surface waves potentially
slowing the frontal progression over land. Impressive fgen field
with the front per the NAM may lead to some focused moderate to
heavy rainfall. The global models push the core of the precipitation
through tonight into Saturday morning. The impacts of the timing
of this front and its southward progress will directly correlate
to whether any additional thunderstorms will develop over the
southeastern portions of the County Warning Area Saturday afternoon...and the the
temperature progression away from lake areas during the day.

Kmd

&&

Long term...
356 am CDT

Saturday night through Thursday...

The extended period is shaping up to be a bit quieter...and
certainly initially much cooler. Sunday will feature plenty of
sunshine...and will also feature a significant temperature gradient
from near lake areas only in the 50s to near 70 in the far
southwest. Cooler conditions make little headway out of the area as
an upper trough slides by to our north. Surface high pressure
remains off the northeast maintaing an onshore flow as well. Upper
level ridging will build Tuesday through Thursday. Surface high
pressure will be slowly pushing eastward which will allow onshore
winds to turn more southeasterly and a warming air mass each Day.
Lake breezes can be expected each day as well...keeping lake areas
on the cool side for early June.

Kmd

&&

Aviation...

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

* scattered rain showers/isolated thunderstorms and rain middle-late morning through middle
afternoon.
* Chance of rain showers/thunderstorms and rain this evening.
* Scattered rain showers/thunderstorms and rain late tonight/Saturday morning.
* Gusty southerly winds middle morning through late afternoon.
* Wind shift to northeast Saturday morning.
* IFR/LIFR Saturday morning.

Cms/rc

//discussion...updated 16z...

Area of rain showers moving toward Chicago area terminals...with no
lightning activity thus far due to only weak instability. Rain showers
will pass through with visibility impacts but pulled thunderstorms and rain mention as it
appears unlikely any lightning strikes will develop before
arriving at the terminals. If some clearing in clouds can
occur...there may be a window for isolated thunderstorms and rain in the middle
afternoon period...but confidence is low.

Rc

From 12z...

Short term guidance still in good agreement with scattered
thunderstorms developing over west/central Illinois later this morning
as an upper level wave moves across the area. This activity will
then move across the terminals early this afternoon and made no
changes to timing...though some uncertainty regarding coverage
and duration remains. Convective trends into this evening may be
dependent on this expected activity. But there is some potential
for showers or a few thunderstorms this evening. Confidence is low
enough to keep the tafs dry for now...but will need to be
monitored.

Southerly winds will increase into the 10-12kt range this morning
with higher gusts developing. Winds will turn a bit more to the
southwest this afternoon and if precipitation ends and clouds scatter...
could see some increase in the wind speeds/gusts until sunset when
winds will diminish under 10kts and then turn more westerly
overnight.

A strong cold front will move across the area Saturday morning.
There remains some timing issues but appears winds will shift from
west to north over a few hour period...then rapidly increase by
middle Saturday morning. Gusts into the middle/upper 20 knots range from
the northeast are expected Saturday morning.

As this front approaches late tonight/early Saturday morning...
showers and a few thunderstorms are expected to develop and then
transition into a period of moderate rainfall. This rain combined
with the winds shifting north and colder air overspreading the
area will push ceilings into IFR and possibly LIFR Saturday morning.
Cms

//Ord and mdw confidence...updated 14z...

* high for rain showers through middle day...medium in rain showers through middle
afternoon...low in direct thunderstorms and rain impacts at terminals.
* Low for rain showers/thunderstorms and rain this evening.
* Medium for rain showers/thunderstorms and rain late tonight/Saturday morning.
* Medium for winds through Saturday morning.
* High for wind shift Saturday morning...medium for timing.
* High for IFR Saturday morning...medium for LIFR.

Cms/rc

//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...

* Saturday night and Sunday...dry/VFR. Gusty northeast winds.
* Monday and Tuesday...dry/VFR. East winds.
* Wednesday...slight chance of thunderstorms and rain. Southeast winds.
* Thursday...chance of thunderstorms and rain. South winds.

&&

Marine...

335 am...a strong cold front will move across Northern Lake
Michigan early Saturday morning...then push south of the lake by
early Saturday afternoon. Strong high pressure will build across
the northern plains and upper Midwest Saturday into Saturday
night...which will strengthen the gradient behind the front. Winds
will shift northerly and increase to 30 knots with some gale gusts
possible. Could be a period of low end gales over the southern
portion of the lake from late Saturday morning through middle afternoon
but confidence too low to include prevailing gales. Winds will
turn more northeasterly Saturday night into Sunday...but the
gradient will be slow to relax across the southern portion of the
lake...so a prolonged period of 15-25kt is possible into Sunday
night. The high will shift east into Quebec Monday...leaving a
weaker ridge of high pressure across the western lakes into
Tuesday. Cms

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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