Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
302 PM CDT Sat Aug 29 2015
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 20z...
* isolated showers through 22z.
* Light south winds becoming variable after 00z to northeast less than
* Ceilings lowering to IFR and possibly LIFR. In addition fog is
expected to develop and lower visibilities to around 1sm.
Visible satellite imagery shows considerable cloud cover across
the region...with surface observations indicating very light
winds. A weak frontal boundary was oriented across northeast Illinois
stretching southwest...with wind directions becoming variable on
either side of this boundary this afternoon. A few showers
continue to linger with some embedded thunder near Ord...however
it appears thunder will be ending by 20z and showers will slowly
dissipate in coverage through the remainder of the afternoon.
Guidance indicates ceilings will quickly lower around 00z or shortly
after...nearing IFR and possibly trending further towards LIFR.
Winds will remain light overnight...which will allow fog and low
ceilings to linger through daybreak. Areas of dense fog are expected to
develop as well...but should remain west of Ord/mdw. At this time
have opted to include mention of 1/4sm fog at dpa/rfd in current
//Ord and mdw confidence...updated 20z...
* high confidence in wind dir/speed.
* High confidence in ceilings lowering towards IFR...medium confidence
in timing and LIFR ceilings developing.
* Medium/high confidence in precipitation ending at Ord/mdw by 20z.
* Medium/high confidnece in visibilities lowering to MVFR...low
confidence in visibilities lowering to IFR.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night through Friday...primarily VFR. Mostly light
southerly winds...though afternoon lake breezes could shift winds
to east or southeast during afternoons.
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
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