Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
223 PM CDT Friday Jul 31 2015
222 PM CDT
A boundary is finally exiting the southeast County Warning Area with a few isolate
showers firing along it over Northwest Indiana. High pressure is
spreading over the upper Midwest while low pressure remains over
Hudson Bay. Between ample mixing and a tightened pressure gradient
between the high and low...west to northwest winds are gusting up to
30 miles per hour this afternoon. Gusts will continue until mixing eases this
evening. Expecting dry conditions through the night with lows
around 60 except for middle to upper 60s downtown.
The nam12 features a weak vorticity streamer moving through southern WI
and northern Illinois early Saturday morning. The NAM also indicates
isolated showers and maybe a thunderstorm will form in association
with the vorticity streamer. Have very low confidence in any precipitation
occurring and coverage...but also did not have enough confidence to
go dry either. So a few isolated light rain showers with perhaps a
couple of strikes of lightning are possible along the WI/Illinois state
line early Saturday morning...then east of a Rockford to Midway
Airport line middle to late Saturday morning. Any lingering showers
will then move southeast over Northwest Indiana early Saturday afternoon.
My confidence decreases rapidly after the sun comes up as we warm
and dry due to mixing. Therefore...keep an eye out for a stray
shower or storm Saturday morning over far northeast Illinois and Northwest
Indiana...but the rest of the day time hours Saturday look dry.
Raised high temperatures over west central and central Illinois by a degree or
two as the warmest locations will be under the warmest upper level
air. The whole warning area will see highs in the middle to upper 80s
with heat indices about the same as the high temperatures. West winds are
once again expected but with the weaker gradient...gusts in the
afternoon will only be up to 20 miles per hour.
251 am CDT
Saturday night through Thursday...
500mb trough axis will be pivoting east across the eastern Great
Lakes Sat...while ridging persist across the western Continental U.S.. oper
solutions continue to feature a middle-level wave developing Sat afternoon
across the plains...and quickly lifting east and reaching the
forecast area late Sat night. Instability will be on the increase as
the wave slides overhead...however timing is less favorable for
well organized/developed thunderstorms. Some of the guidance
members suggest arrival of the wave will be after 6z sun.
The 500mb trough axis continues to send a few weak lobes of
vorticity south across the upper Midwest sun/Mon. Marginal
instability will linger sun/Mon...so will maintain the mention of
showers/thunderstorms. Although at this time expect the best chance
for thunderstorms would occur in more of a diurnal component...in
the afternoon/evening timeframe both days.
Ensembles then trend back towards the western ridge
developing...helping to bring a slightly stronger trough back across
the Great Lakes region towards mid-week. Minimal spread continues to
be advertised amongst most ensemble members...suggesting a
relatively high confidence. A frontal boundary will push across the
region Tuesday...with unseasonably cooler air steadily advecting south
with a thermal trough setting up over the Great Lakes region. This
will transition temperatures from the middle/upper 80s sun/Mon...to around 80
Tuesday...and then middle/upper 70s for the remainder of the extended
periods. Chances for showers and isolated thunder looks minimal
beyond Tuesday...and could end up remaining dry as some guidance
suggests weak anti-cyclonic flow lingering overhead.
//Ord and mdw concerns...updated 20z...
* west/northwest winds with gusts around 25kt this afternoon.
* Isolated showers/thunderstorms possible Saturday morning.
Surface trough moving through the area early this afternoon with
VFR conditions in place...and with west/northwest winds increasing
at this time. VFR conditions will remain today into tonight while
any shower/thunderstorm development this afternoon remains well
south of the terminals. These stronger westerly winds will likely
persist through early evening...with expected gusts around 25kt.
There does appear to be small window Saturday morning for isolated
precipitation to move across the terminals. They will likely be only
showers...but with thunder definitely not out of the question.
//Ord and mdw confidence...updated 20z...
* high confidence with wind speed/direction.
* Medium confidence with showers/thunderstorms Saturday morning.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...mainly VFR. Chance of showers/thunderstorms late.
Sunday...chance of thunderstorms and rain. Southwest winds.
Monday...chance of thunderstorms and rain. Northwest winds.
Tuesday...dry/VFR. Northeast winds.
Wednesday...slight chance of thunderstorms and rain. Easterly winds.
Thursday...slight chance of rain.
255 am CDT
The gradient tightens up again today...with west/northwest winds
creating some gusty conds mainly for the southern half. This will
create hazardous conditions for small craft through this
afternoon with 20-25kt gusts...then after sunset the gradient
diminishes allowing winds to become less gusty. With high pressure
drifting east across the Ohio Valley Saturday...winds will be
slowly turning southwesterly. The gradient tightens late in the
weekend as another area of low pressure approaches Lake Michigan
from the west. Winds may become gusty Sun afternoon. A stronger
low and frontal boundary will arrive Monday night into
Tuesday...with winds turning northwest to north.
Lm...Small Craft Advisory...nearshore Winthrop Harbor to Gary in until 7 PM
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