Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
241 am CDT sun Oct 26 2014
236 am CDT
through Sunday night...for the remainder of the night
and into Sunday morning...and indeed through much of the day on
Sunday...clear skies and light winds will be the main feature as surface
high pressure slides across the area before settling into the Ohio
Valley Sunday afternoon. Overnight...the clear skies and light
winds should allow for ample radiative cooling...with temperatures dropping
into the middle to upper 30s by daybreak...except for the Chicago
area urban heat island...which should only drop into the lower 40s.
A few sheltered...low lying locations could even see temperatures
drop into the lower 30s as dewpoints continue to slowly drop into
the upper 20s by daybreak. After sunrise...temperatures should
climb rapidly under ample sunshine and a dry airmass. The one
concern will be temperatures along the lake front. The weak
pressure gradient and differential heating between the land and Lake
Michigan should allow for a lake breeze to develop. The main
question regarding the lake breeze will be timing since if it forms
late enough in the afternoon...Lakefront locations could see
temperatures reach into the middle 60s...before dropping off as the
boundary pushes inland. As for the remainder of the
County Warning Area...temperatures will still be above seasonal normal levels...
though not quite as high as Saturday...with highs expected to reach
into the upper 60s today.
Sunday evening into Sunday night...as the high continues to build
to the east...low pressure will develop over the Central Plains
while a deeper low develops over the southern Canadian rockies. The
developing surface low over the Central Plains will set a strong pattern
of warm advection...with a warm front developing from the low
center over the NE/Iowa border into the lower Ohio Valley by early
Sunday evening. Ahead of the warm front...strengthening southeasterly winds
will develop overnight as the warm front slowly lifts north. Warm
advection and isentropic lift north of the warm front will bring
increasing cloud cover through the evening as well as increasing
chances for precipitation. The strongest forcing will be in the path of the
surface low as it tracks northeastward through Iowa and into southwestern WI by early
Monday morning. While the County Warning Area will see increasing cloud
cover...the best chances for any precipitation will be north of the Illinois/WI
border in the zone of strongest isentropic lift...though the northern
tier counties could possibly see some light showers. With the
increasing warm advection and cloud cover overnight...temperatures
should remain relatively mild...with overnight lows in the lower
to middle 50s.
Monday through Tuesday...
the week kicks-off with very mild temperatures...as temperatures may tag
80 degrees across portions of the forecast area. However as can be
expected with very anomalous conditions...the potential exists for
thunderstorms in the late afternoon/evening timeframe Monday. Middle-
level ridging over the Great Lakes will begin to flatten early
Monday...as a robust 500mb trough digs into The Rockies. Surface flow
will be turning southwesterly and may back further to the south by
midday Monday. This will allow a warm frontal boundary to lift
north...allowing steady advection of much warmer/moist air into
the region. Dew points will push into the 50s...and possibly near
60 degree by middle- afternoon Monday. Guidance has been relatively consistent
with suggesting the llvls will become capped so not expecting much
weather prior to 21z Monday. Then the surface should warm enough to
dissolve the cap...with strong 0-3km shear developing by early
evening. Cape doesn't look to be too favorable within the environment
late afternoon/early evening...although MUCAPE could near 1000j/kg. Lapse
rates will steepen...and could support a few marginally severe
Precipitable water values will steadily increase in excess of 1"...nearing 1.5" by
late Monday aftn/eve. While it appears likely that storms will be
progressive given the strengthening low level flow...the moisture rich
environment and marginal 850mflux could warrant a few storms
producing moderate to brief heavy downpours. Overnight Monday with
continued chances of precip/storms...steadily shifting east by Tuesday
morning...the clouds will likely keep temperatures mild Monday night in the
middle/upper 50s to perhaps around 60 in the southeast County warning forecast area.
Trough axis will be pivoting overhead Tuesday...with a dry wedge working
in quickly by midday Tuesday and bringing an end to the precipitation. Surface low
will continue to deepen across northern lower Michigan Tuesday
afternoon...slowly lifting northeast into Ontario/Quebec. Much
cooler/drier air will be advecting back into the region Tuesday
aftn/eve...as temperatures may top out around 60/low 60s early in the day
before beginning to fall a few degrees Tuesday afternoon into the middle/upper
50s. Thermal trough continues to slide overhead Tuesday night...and with
p-cloudy skies...temperatures should cool into the upper 30s/low 40s.
Wednesday through next weekend...
Will mother nature provide a treat of dry weather Friday afternoon/evening or
will we be tricked. From middle-week into the upcoming weekend...the
pattern will be transitioning towards a ridge/trough setup across
the Continental U.S.. current ensemble forecasts paint a high amplitude ridge
developing across The Rockies stretching north into the Canadian
rockies Wed/thur...while troughing digs into the southwest Continental U.S.
And troughing digs into the southeast states. Then the first half
of next weekend the ridge will begin to flatten. A shortwave will
traverse the region Thursday bringing a chance for light precipitation Thursday/early
There has been some consistency in the later periods that temperatures will
cool Friday/Sat of next week into the 40s for highs with overnight lows
in the middle 20s/low 30s as a surface ridge arrives.
//Ord and mdw concerns...updated 06z...
* low level wind shear likely overnight with surface winds west-northwesterly at around 5 knots and northwesterly
around 30-35 knots at 1500 feet above ground level.
* Chance for an easterly surface wind shift with a lake breeze
West-northwesterly winds will continue to diminish to light/variable overnight as a
strong nocturnal inversion forms under clear skies. Winds aloft at
the inversion layer quickly becm northwesterly 30-35kt...setting up the
likelihood for low level wind shear...so low level wind shear has been added to the forecast.
Anticipate that with clear skies and a dry airmass in place...the
boundary layer will quickly become well mixed and the potential
for low level wind shear should end by 12-13z. As surface high pressure builds across
the region the remainder of the day will be clear and dry with
light winds. The weak pressure gradient should allow for the
development of a lake breeze. There is still some question as to
how far inland the boundary will push and how strong winds will be
behind the boundary. Given that the water-land temperature differential
is not that great...feel that the boundary will be slow to push
inland...likely not pushing through Ord/mdw until 22-23z. Also in
the absence of a strong push behind the boundary...winds should
remain less than 10kt while backing to Ely. Overnight Sunday
night...the high pressure will shift to the east as low pressure
moves across the plains. Winds will trend to southeasterly though the
evening and southerly overnight. Increasing warm advection through the
night should bring increasing cloud cover...but anticipate that
bases will remain at the lower end of VFR through the night.
//Ord and mdw confidence...updated 06z...
* high confidence in surface winds through early Sunday afternoon.
Medium confidence in timing of the lake breeze and wind speed
behind the lake breeze.
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...rain showers likely mainly during the evening with a chance of thunderstorms and rain.
Tuesday...chance of rain showers during the morning with a slight
chance of thunderstorms and rain. West winds.
Wednesday...VFR. West winds.
Thursday...chance of rain showers. West winds turning north.
Friday...slight chance of rain showers. NE winds.
217 am CDT
A few active periods setting up across the Great Lakes region over
the next few days...as weak ridging centered over the Ohio Valley
shifts east by this evening. This will allow flow to turn
southeasterly to southerly Monday. A developing trough of low
pressure stretching from the Central Plains north into the
northern plains will slowly consolidate into a low across lower
Michigan Tuesday morning. The gradient is poised to increase across
the northern tier and southern tier of Lake Michigan...with gusts
nearing 25kt and possibly a few gusts to 30kt Monday night. It is
possible small craft conds could develop Monday...but with much
warmer/moist air flowing across the slightly cooler lake the wind
gusts could be slightly diminished.
Then the low is expected to deepen as it lifts north across
Eastern Lake Superior Tuesday midday...with a tight gradient
developing once again across the entire lake and producing west to
northwest winds near between 25 to 30 knots Tuesday evening/Wednesday morning.
High pressure then slowly builds back into the region Wednesday/Thursday
with lighter west/northwest winds developing.
Lm...Small Craft Advisory...nearshore Gary to Michigan City in until 10 am Sunday.
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