Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
943 PM CDT Tuesday Jun 30 2015
943 PM CDT
For evening update...
Minor tweaks to going forecast this evening...mainly with respect to
hourly grids for wind and temperatures...as well as sky cover with earlier
lake breeze/secondary cold frontal boundary.
Weak surface low pressure was pulling away from the forecast area
across the central Great Lakes this evening. Primary trailing cold
front extended southwest across southern in/IL...while a combined
lake breeze/secondary cold front had surged inland across NE Illinois/northwest
ind and east central Illinois since late afternoon. While cooler and drier
air was spreading into the area on north-northeast winds...an area
of fog had initially developed behind the boundary across the lake
and briefly impacted areas near the Lake Shore. Web cameras indicate
that fog has largely dissipated...lifting into a low ceiling and
scattering as lower dew point air arrives. 11-3.9 micron infrared imagery
indicates only scattered low clouds around the south end of the lake
at this hour...with more extensive cloudiness limited to the
northern half or so. Farther west...cirrus was spilling into the
Mississippi Valley in association with the upper jet streak entering
the western side of the upper trough over the eastern Continental U.S....which
will likely keep skies partly cloudy in nature through the night.
Surface high pressure spreading into the northern Great Lakes and
upper Midwest will build south into the Midwest Wednesday...setting
the stage for a few days of dry weather.
210 PM CDT
Decided to reduce the precipitation outlook significantly for this
afternoon. The main upper level wave is well to our east over
eastern Michigan and Ohio and thick cloud cover is north of I-80. Was
originally thinking a few showers would develop along the lake
breeze...but the cloud cover will likely limit any vertical growth
especially since most places have not warmed as quickly as
originally thought. Kept a slight chance of thunderstorms across
southern Ford...Iroquois...and Benton counties this afternoon
although without any clear forcing...have low confidence in any
showers or storms forming. The latest rap analysis does feature
around 1500 j/kg of cape south of I-80 so if any forcing does
develop...storms are possible.
Expecting dry conditions through Wednesday as the next convective
system will pass to our south over central Illinois. To play it
safe...put a slight chance of storms south of a Pontiac Illinois to Fowler
Indiana line late Wednesday afternoon...but have medium-low
confidence in storms getting that far north tomorrow.
Did not make any major changes to temperatures for tonight and tomorrow.
With increasing cloud cover overnight and dewpoints in the middle 50s
to low 60s...low temperatures will get into the middle to upper 50s. Flow will
be off of the lake tomorrow resulting in unseasonably cool temperatures
with highs in the 70s. Locations along the lake will likely stay in
the low to middle 60s.
313 PM CDT
Wednesday night through Tuesday...
Most of the remaining forecast period will likely see below
normal temperatures and with drier conditions. This will be the case
Wednesday night as expected showers/thunderstorms across the
Central Plains likely drift east southeast towards a more
moist/unstable airmass...while drier air and little to no
instability are present across the County Warning Area. Did leave slight chance
probability of precipitation for showers across the far southern County Warning Area for some variability
in guidance trying to inch this precipitation closer to the County Warning Area.
However...a more likely scenario is for this development to remain
to the south. Thursday and Thursday night will likely be dry as
high pressure dominates the region...but there still is some
variability among model guidance handling of an approaching upper
level trough and associated surface reflection...and associated
precipitation on Friday. Although the best precipitation axis should remain just
south of the County Warning Area...did leave slight chance of showers for the
southern half of the County Warning Area due to this uncertainty. The remainder of
the weekend is looking dry for most areas but towards Sunday and
especially by early next week...a return to southerly flow will
allow a warmer and more moist environment to spread back north
across the region. Isolated to scattered shower/thunderstorm
development is appearing more probable on Sunday...but with
continued moist advection and increasing flow aloft early next
week...more widespread development will become more likely. Timing
and placement issues are present during this time....but more
organized development does appear possible in the Monday to
Tuesday time frame.
//Ord and mdw concerns...updated 00z...
* northeast winds around 10kt early this evening.
* Possible MVFR ceilings this evening...overnight and Wednesday
* Northeast winds around 10kt Wednesday afternoon. Cms
Area of low clouds/fog over the lake has been hugging the shore
line and will soon move into Northwest Indiana. Confidence
regarding how low ceilings/visible may drop at gyy is low...but there is
a potential for a short period of LIFR/vlifr conditions. Lower
ceilings have remained generally east of Ord/mdw but as sunset
approaches...its possible some of this may move inland. Otherwise
MVFR ceilings over northeast WI and Northern Lake Michigan may move
south later this evening and overnight. Some guidance suggests
low clouds developing across the area overnight. Still some
uncertainty regarding trends and how widespread any lower ceilings
become...so backed off to just scattered mention with this
forecast and trends will need to be monitored.
Northeast winds around 10 knots will slowly diminish this evening
and may turn back more north/northwest by Wednesday morning. Wind
directions will likely turn back northeasterly Wednesday afternoon
with a potential lake breeze pushing wind speeds back to around 10
//Ord and mdw confidence...updated 00z...
* high for winds through the period.
* Low for MVFR ceilings this evening through Wednesday morning. Cms
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday through Saturday...dry/VFR. Northeast winds.
* Sunday...slight chance of thunderstorms and rain. Easterly winds.
* Monday...chance of thunderstorms and rain. Southwest winds.
404 PM CDT
As low pressure departs the lake this afternoon...winds are
quickly shifting to the north. As this occurs...speeds are also
quickly increasing to the 15 to 25 knots range over much of the lake.
These speeds will likely spread through the remaining southern
portions of the lake through early this evening. Although the
strongest speeds should remain away from the nearshore...do think
10 to 20 knots winds will be possible in these areas this
afternoon/evening. With high pressure spanning the Great Lakes
region through the end of the work week...northerly winds will
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