Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
859 PM CDT Sat Aug 29 2015
859 PM CDT
Main forecast concern overnight continues to be the potential for
areas of dense fog to develop. With weak low pressure that was
over middle MS valley dissipating to a trough...warm front extending
northwest from the low has become barely discernable. This front
served as a moisture convergence axis combined with forcing from
the upper trough axis pivoting gradually southeast across the
region to yield the persistent shower activity into this
evening...but the forcing is weakening as the trough shifts
east...so shower activity will finally taper off...latest in far
southeast. As of this writing...there was a low topped supercell
in Jasper County where better instability resided and about 20 knots
of 0-1 km shear according to Storm Prediction Center mesoanalysis. Thunder threat
should quickly wane in east central Illinois and south of Kankakee River
in Indiana over next few hours.
Turning to the fog potential...the weakening of the low and
extensive high pressure covering the eastern third of the Continental U.S.
Has set up a very weak pressure gradient. Despite plenty of
stratus...very moist low levels and low dewpoint depressions
should still support fog. Based off observation and webcams along
Wisconsin shore to the Illinois shore...visibility has been on a
downward trend this evening. Hrrr-x seems to have a good handle on
fog progression...which would mean areas of dense fog along the
shore would then spread inland overnight. Do not yet have enough
confidence to issue a dense fog advisory...but will issue an Special Weather Statement
for the Lakeshore counties and a graphicast to hit the fog
potential on our website and social media. Have higher confidence
in areas of dense fog focusing on counties along and north of
I-80...but wouldnt be surprised to see dense fog across the County Warning Area in
the predawn hours. Updated zones will be sent shortly.
Tonight and Sunday...
248 PM...forecast concerns are dense fog potential tonight...
lingering precipitation tonight and isolated thunderstorms Sunday
While much of the precipitation has ended this afternoon with just some
scattered showers across northeast Illinois and Northwest Indiana...there
remains the potential for showers into this evening mainly across
the eastern half or so of the County Warning Area. Maintained chance probability of precipitation slowly
dropping off into this evening though overall coverage of precipitation
trends is a bit uncertain. Any lingering showers overnight into
early Sunday morning should be confined to the far southeast County Warning Area.
As the low now over western Illinois continues to dissipate...the gradient
will become quite weak tonight with winds already shifting to light
east/northeast across southeast WI and far northeast Illinois where lower
clouds and some light fog has developed. Model fog/visible guidance all
show fog developing in these areas this evening then spreading to
the west/southwest overnight into Sunday morning. With MOS guidance
showing many locations tanking overnight. With the low levels
already saturated...this trend seems reasonable and added dense fog
over northwest Illinois...mainly northwest of an Ord to vys line. With
late August sunshine...expect this to burn off by middle morning
Sunday with clouds scattering out by afternoon. Assuming this
occurs...temperatures should be able to warm into the lower 80s. Though a
lake breeze will likely keep the Lakeshore areas cooler.
As temperatures warm...especially in the southeast where less fog is
expected...enough instability is present for isolated showers and
possibly a thunderstorm. This should be mainly diurnally driven
from early afternoon to early evening though with minimal
forcing...precipitation may end up quite sparse. Cms
301 PM CDT
Sunday night through Saturday...
For the longer term forecast period...a rather stagnant pattern is
setting up with high pressure sitting over the eastern Continental U.S. And a
trough of low pressure remaining nearly stationary over the Western
Plains. The upper level pattern will be a little more
progressive...but the progression will largely be confined to north
of the Canadian border...with little effect on the local area.
The general trend for the long term forecast period will be for a
warming trend...beginning on Sunday and persisting through the
remainder of the week. By Monday...a positive tilt upper level
ridge will extend from the Desert Southwest through the western
Great Lakes region...with general upper ridging persisting over the
region through the week. By Monday...temperatures should reach the
middle 80s across the region...but a southeast component to surface
winds is expected to develop through the day...which could limit
warming along the Illinois Lake Michigan shore to around 80f or
possible a little less. The remnants of weak tropical systems are
expected to undercut the upper ridge...tracking across the southern
Mississippi Valley...setting up southerly-southwesterly flow through the lower
levels...drawing some Gulf moisture into the region. This would aid
in the development of diurnal cumulus...with periodic chances for isolated-
scattered shra/tsra...generally during the daytime hours. The latest long
range model runs continue the trend begun yesterday in limiting
daytime warming to the middle to upper 80s through much of next week
with dewpoints in the upper 60s to lower 70s. So...with the absence
of any significant focusing mechanism...do not expect any
widespread organized precipitation through the period with generally warm and
//Ord and mdw concerns...updated 00z...
* a few lingering showers this evening.
* Lowering ceilings/visibility overnight.
A trough is draped from northeastern Illinois into west central
Illinois this evening serving the focus for scattered showers.
These showers are expected to continue to diminish with primarily
dry conditions expected overnight and the remainder of the taf
period. The trough axis will slowly sag south overnight with light
northerly winds overspreading the terminals. Widespread MVFR
stratus is noted upstream with an area of IFR/LIFR ceilings over far
southeastern Wisconsin. Ceilings are expected to lower at the
terminals overnight along with visibility dropping with the
development of fog. Conditions should be slow to improve tomorrow.
Light winds will continue through the day tomorrow.
//Ord and mdw confidence...updated 00z...
* high confidence in rain showers ending by around 01-02z at the
* Medium-high confidence in general ceiling/visibility trends
overnight...but low-medium in timing of IFR at 04z. Confidence
in IFR increases through and after 07z.
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...VFR. South winds.
Tuesday...VFR. Southwest winds.
Wednesday...slt chance thunderstorms and rain. Primarily VFR. South winds.
Thursday...slt chance thunderstorms and rain. Primarily VFR. Southeast winds.
Friday...slt chance thunderstorms and rain. Primarily VFR. Southeast winds.
301 PM CDT
A weak trough of low pressure extends from the middle Mississippi
Valley through Southern Lake Michigan as high pressure sits over
the eastern Continental U.S. And a slightly deeper trough of low pressure
develops over the Western Plains. The weak trough over Southern Lake
Michigan will persist long enough into this evening for Ely winds
over the southern portions of the lake while low pressure moving
through the Canadian prairies will keep winds over the northern
portions of the lake more southerly to southwesterly. Weak high pressure will be
over the south half of the lake Sunday...leading to lake breeze
development and onshore winds over the nearshore waters and light
and variable winds over the open waters of the south half of the
lake. By Monday...sly-swly winds will cover the whole lake by
Monday as a modest pressure gradient develops over Lake Michigan
as the high over the eastern Continental U.S. And the trough over the Western Plains
Lm...dense fog advisory...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745-
lmz777-lmz779 until 9 am Sunday.
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