Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
403 am CDT Friday Oct 24 2014
326 am CDT
Short term...through Friday night...
for the remainder of the night and into Friday morning...the initial
concern will be cloud cover...and impacts on visibility. Following
the passage of a weak surface trough/Pacific-sourced weak cold
front...weak high pressure has built across the region. A low
stratus deck has been in place following the frontal passage...but the stratus
deck is eroding in places...and with calm to light/variable winds in
place...the clearing locations are experiencing rapid fog
development. The fog is initially mostly impacting northwestern Illinois...around
the Rockford area...but as the stratus continues to
dissipate...additional fog may develop...but any dense fog should be
limited to north central and northwestern Illinois...away from the Chicago metropolitan
area. Except for locations that clear out...temperatures should
generally bottom out in the lower to middle 50s...except for some
isolated...sheltered low lying areas...which could radiate down into
the upper 40s.
For the remainder of Friday...the warming trend should continue as
upper ridging continues to build out of The Rockies and into the
Western Plains. The morning stratus and stratocu should dissipate
through the morning hours...but total sunshine will be limited as a
thick cirrus layer and some middle level cloudiness will overspread the
region through this afternoon and evening as a strong upper level
jet stream dives over the upper Mississippi Valley and into the
Midwest. The short range guidance is also indicating some shortwave
energy overtopping the ridge this evening...with some guidance
indicating a little quantitative precipitation forecast. Have a hard time introducing any
mentionable probability of precipitation with this system as moisture profile forecasts also
indicate little deep layer moisture. However...some of the high res
short term guidance is showing some increase relative humidity late Friday night
under increasingly southwesterly flow...but as flow off of the Gulf of Mexico
will be limited by high pressure ridging extending from the central
Appalachians through eastern Texas...feel that this increased relative humidity could be
more indicative of some fog potential rather than precipitation potential.
As for temperatures...under increasing low level warm advection and
building heights aloft...expect that temperatures for Friday will
likely be around 5 degrees higher than Thursday with highs in the
middle 60s. Friday night should remain mild with dewpoints
increasing into lower to middle 50s through Friday and into Friday
evening...so overnight lows Saturday morning should be unseasonably
mild...and only drop into the lower to middle 50s.
Saturday through Monday...
500mb trough axis will be pivoting east across the Great Lakes
region...with a trailing frontal boundary stretching south and
expected to push east across the forecast area Saturday morning.
Middle-level heights will steadily rise as the ridge parked across the
central Continental U.S. Pushes east...bringing milder air to the region Sat
with highs in the middle/upper 60s. The far southern County warning forecast area could touch 70
degree Sat. Temperatures sun are expected to be slightly cooler as the flow
begins to transition briefly into a quasi-zonal setup. However this
does not look to remain very long...as ridging returns for Monday and
temperatures should quickly warm into the 70s across much of the area. A warm
frontal boundary will lift across the area early Monday...as southerly
flow strengthens. Expect much of the day Monday to remain dry...however
late afternoon/evening a developing low across the Central Plains may help to
generate a few showers/thunderstorms late Monday.
towards the middle of next week ensembles continue to suggest the
ridging overhead will shift east...with flow trying to flatten.
Suggesting that a semi-zonal setup may develop by Wednesday...which will
likely result in periodic chances for showers. Temperatures should return to
more seasonal conds Tue/Wed.
//Ord and mdw concerns...updated 09z...
* MVFR ceilings during the early morning hours. Chance ceilings may scattered and
visible may drop to MVFR levels.
Following the passage of a weak surface trough...any residual precipitation
remains isolated and south of the terminals. However...there is sole
residual low level moisture and as high cloud pushes away...a band
of stratus can be seen in the latest satellite imagery. As a
nocturnal subsidence inversion lowers overnight...cloud bases
should gradually lower to lower end MVFR levels. There is a chance
for some patchy IFR ceilings during the early morning hours...especially
over northwestern Illinois where some rain fell earlier...which would most
likely affect rfd and possibly dpa. As winds becm light/variable
overnight...visible should drop to 3-5sm during the early morning
hours...and if the western edge of the stratus deck can erode over
northwestern Illinois before sunrise...there is a chance for rfd to drop into
IFR or even LIFR visible levels. However...that would be highly
dependent on the ceiling trends.
Through the day on Friday...another surface trough will track across
the Central Plains to the upper Mississippi Valley by early evening.
Winds will become south-southwesterly though the day...though with a weak
pressure gradient...wind speeds should be less than 10kt through
the day. For the early evening hours...some of the models are
suggesting that there could be some light precipitation with the passage of
the next surface trough. However...since forcing with this trough axis
looks to be rather weak...confidence is too low to add precipitation to the
tafs at this time.
//Ord and mdw confidence...updated 09z...
* medium confidence in ceiling/visible trends overnight.
* Low confidence in specific timing of improvement to VFR Friday.
* High confidence in all forecast elements through the day on
* Low confidence in the chances for precipitation Friday evening.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
* Friday night...mainly dry. MVFR/fog possible.
* Saturday...dry/VFR. Gusty west/northwest winds.
* Sunday...dry/VFR. Southeast winds.
* Monday...dry/VFR. Gusty south winds.
* Tuesday...chance of rain showers. Strong/gusty westerly winds.
* Wednesday...chance of rain showers. West-southwest winds.
215 am CDT
A weak ridge of high pressure extends from western Quebec
southwest through the central Great Lakes...into the Tennessee
Valley. This ridge will continue to drift east of the region this
afternoon...meanwhile a trough of low pressure over central Canada
continues to slide east towards Hudson Bay Saturday. The gradient
will remain light through this afternoon...then slowly turn
south/southwesterly. As the low nears Hudson Bay on Saturday...a
frontal boundary will trail and slowly slide south across Lake
Michigan. This will result in winds turning west/northwest
Saturday afternoon/evening. Cooler air will flow across the lake
along with a tightening gradient...resulting in increasing winds
nearing 30kt at times. Then another ridge of high pressure will
quickly slide southeast from the upper Midwest/northern plains
Sunday...steadily diminishing the gradient. Another trough of low
pressure is expected to develop early next week across the Central
Plains...then approach Lake Michigan closer to mid-week. This will
bring winds back to the southwest with occasionally winds gusting
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