Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
518 PM CDT Tuesday Jun 18 2013
Discussion...
239 PM CDT
Stratus layer that was anchored over far northeast Illinois/northwest in
this morning has eroded with the warm air heating the surface. Visible
imagery indicates a few areas of cumulus clouds that have developed
this afternoon across northern Illinois. In addition some building cumulus
clouds were noted along a Pontiac to Kankakee line...in an area of a
stronger thermal differential. However the lift mechanism in this
area is minimal...and so a few isolated showers/thunderstorms may
develop but quickly dissipate. All precipitation over the County warning forecast area will
be diurnally driven...thus by sunset or immediately after sunset
precipitation will come to an end. Further south stronger instability was
positioned across central Illinois stretching west through northern
Missouri. Middle-level wave continues to pivot east across far northwest
Illinois this afternoon...and will maintain the northeast flow across much
of the County warning forecast area. As a result cool temperatures will persist this evening...with
minimal cloud cover from the diffluent flow aloft. Skies will become
mostly clear tonight...with the low level flow weakening. This should
provide decent radiational cooling with temperatures overnight expected to
be in the middle 50s south of I-80. Further north temperatures will be much
cooler to around 50...possibly the upper 40s for areas along and north
of a Rockford...Aurora...Waukegan line.
Confidence...high.
Wednesday through Thursday...
500mb trough axis slowly pivots to the eastern Great Lakes Wednesday...and
will continue to push east Thursday. The surface ridge will also begin to
push east...however an onshore or northeast to east flow will
persist into Wednesday for much of northern Illinois. The difference for Wednesday
will be that the 850mb thermal trough will linger overhead. With
abundant sunshine and surface heating...temperatures should warm into the middle/upper
70s or possibly the low 80s from LaSalle to Iroquois County.
For Thursday 500mb heights will begin to rise with flow beginning to
turn to the southeast in the llvls. A weak lake breeze may develop
and push into Lake County Illinois...which would most likely provide a
cooling trend in the afternoon hours. 850mb temperatures will steadily warm for
Thursday to around 14 to 16 degree c...with continued thin cloud cover. This
will allow temperatures to warm into the low/middle 80s. The caveat will be
areas of Lake County Illinois Post frontal passage that will allow temperatures
to fall into the 60s in the afternoon.
It does appear that enough dry air will linger through Thursday
aftn/eve...thus have held onto a dry forecast in this timeframe.
This pattern will be changing as we migrate into the extended
periods.
Confidence in temperature forecast...high.
Confidence in dry weather through Thursday...medium/high.
Friday through Monday...
minimal changes from previous forecast in regards to the extended
periods. Possibly the first stretch of 90 degree days is upon US for
sun/Mon/Tue.
Guidance has a warm frontal boundary lifting overhead Friday with
southerly flow ushering in addtl low level moisture. It does appear a
wave will also push over the County warning forecast area Friday...with potential for
showers/thunderstorms. Temperatures will be slightly warm for Friday into the
upper 80s...with a few 90 degree points across the County warning forecast area.
Ensembles for several days have continued to indicate that a 500mb
ridge across the central Continental U.S. Will advect east and become anchored
across the eastern half of the Continental U.S. By the weekend and possibly
persist through early next week. Additionally...guidance has prognosticated
850mb temperatures to rise to around 20-22 degree c for the weekend through early
next week. Consistency from previous runs suggests high confidence
in this warm stretch arriving. Then sun/Mon/Tue appear to be near
Carbon copies with temperatures easily warming to 90 degrees...possibly the
low 90s. Climatology based on the 850mb temperatures prognosticated and time of
year would suggest temperatures to be warmer as forecasted. A couple
limiting factors might be the potential for a series of waves
bringing instability and thunderstorm potential each day of the
extended...along with dew points prognosticated to be around 70 degree.
This could potentially keep temperatures in check in the middle/upper 80s.
Confidence in temperature forecast...high.
Confidence in precipitation/convective forecast...medium/high.
Beachler
&&
Aviation...
//Ord and mdw concerns...updated 22z...
* timing of diminishing gusts this evening.
Lenning
//discussion...updated 18z...
IFR/LIFR ceilings still persisting south of mdw from igq to gyy east
along the Indiana shoreline. Clearing from the east along with
erosion from the southwest seems to have slowed in the past hour or
so. While VFR conditions are now expected at Ord/mdw...will need
to monitor trends for gyy through the afternoon. Some cumulus along
with middle clouds possible through early evening...then sky clear expected
for the remainder of the period.
Northeast winds will remain in the 10-15kt range with gusts in the
16-20kt range into this evening with winds diminishing under 10kts
by middle/late evening then turning more easterly Wednesday morning.
As a weak wave moves south of the terminals this afternoon...there
could be some showers and possibly an isolated thunderstorms and rain but any
activity that develops is expected to remain south of all the
terminals. Cms
//Ord and mdw confidence...updated 22z...
* medium to high confidence on wind trends.
Lenning
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night through Thursday night...VFR.
Friday through Monday...periodic chances for thunderstorms...else VFR.
Bmd
&&
Marine...
137 PM CDT
A surface ridge of high pressure across the upper Great Lakes
region...will shift eastward towards New England by Wednesday
evening. Meanwhile...low pressure is expected to take shape across
the plains states. This pressure pattern will remain in place for a
few days into the weekend. Overall...this will result in a prolonged
period of southerly winds across the lake. Wind magnitudes generally
look to be in the 10 to 20 knots range...with a few periods of 15 to 25
knots southerly winds...especially later in the period.
There will also be increasing chances for a few periods of
thunderstorm activity Friday and into the weekend over the Lake. A
fairly active weather pattern looks to setup up across the great
the northern Continental U.S. And Great Lakes region.
In the shorter term...I have extended the Small Craft Advisory into
this evening for the Illinois shores and maintained the advisory
through late tonight for the Indiana shores. Local buoy observation indicate
waves around 4 feet...and with the onshore flow continuing to be in the
15 to 20 knots range ahead of the surface high...I do not see any
improvement in the wave conditions for several more hours.
Kjb
&&
Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz006-ilz014 until 4 am Wednesday.
In...beach hazards statement...inz001-inz002 until 4 am Wednesday.
Lm...Small Craft Advisory...nearshore Winthrop Harbor to northerly is until 10 PM Tuesday.
Small Craft Advisory...nearshore northerly is to Michigan City in until 4 am
Wednesday.
&&
$$
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