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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
104 am CDT Monday Jul 6 2015


No big changes to going forecast...but did tweak min T forecast
downward a bit in a few outlying areas and a few degrees upward
in the inland portions of the Chicago urban heat island based on
trends this evening. Overall...looking at Lower-Middle 60s outside of
Chicago and upper 60s to lower 70s in the city by early Monday.
Also added patchy fog mention to the grids late tonight into early
Monday morning outside of low temperatures will get close
to dewpoints and winds will remain light. Patchy fog has occurred
the past few mornings and the only airmass change tonight is
higher dewpoints and slightly warmer low temperatures. Do not expect
widespread dense fog however.

Regarding the persistent Canadian wildfire smoke that has been in
the sky the past week...the last visible satellite observation still showed
smoke over the evidenced by yet another red sunset here
at the office. But much clearer conditions were over the western
2/3 of Iowa and points hoping that we'll have a smoke
free day on Monday to support the forecast highs in the upper 80s
to lower 90s.



Short term...
315 PM CDT

Through tonight...

Quiet and seasonably warm weather will persist into the
evening...and the scattered to broken cumulus that was supported
by afternoon heating should begin to dissipate. Southerly winds
will relax a bit overnight but not enough to allow quite as much
radiational cooling as might be achieved under mostly clear skies.
Winds then begin to turn southwesterly and increase again toward
morning ahead of an approaching cold front.



Long term...
315 PM CDT

Monday through Sunday...

The weather of interest this week will be temperatures and muggy
conditions on Monday and Monday evening...thunderstorm chances and
intensity during mainly Monday night...and trying to time chances of
storms after that.

A progressive short wave trough presently over southern Alberta will
move along the northern Continental U.S. Border through the day Monday with a
more subtle shortwave or two...possibly convectively
the south. As low pressure continues to develop Monday across the
upper Mississippi Valley along an already present cold
front...pressure falls will envelop the area and lead to breezy
conditions. The low-level thermal ridge of 20-22c in the 850-925mb
temperatures should add at least a few degrees to the temperatures
today. The current thinking is that the thickest wildfire smoke
aloft which may have kept temperatures a few degrees cooler the
past couple days should be mainly east...and debris cloud cover is
not favored to arrive until middle-late afternoon. Upstream
observations confirm upper 80s are certainly doable...and think a
few 90 readings could be seen given climatology for these
850-925mb temperatures. Dew points will be creeping into the 70s
by afternoon given upstream observations and should maximum out in
the middle 70s during the evening. Heat index readings are forecast
to maximum out during the afternoon in the middle 90s.

Nwp guidance is handling this system with improving consistency. Due
to a thermal cap in advance of this system...forcing for showers and
storms look to be mainly confined to an axis just ahead of the front
with cooling middle-level temperatures and higher precipitable waters
at or above 1.75 inches. Have adjusted temporal resolution of the
forecast and can see the only exception of that being maybe north
central Illinois sneaking out a late afternoon storm. Otherwise
the 7 PM time and after is most favored for scattered/possibly
line of storms initiating from WI through eastern Iowa moving
eastward. The deep layer shear seen today in the Dakotas will be
less with this system...but enough /around 30kt/ to support some
areas of storm organization and propagation given the plume of
moderate to high instability and associated dcape forecast ahead
of the front. While the boundary layer will become more shallow
into the evening...continued increasing dew points and a 40-50 knots
low-level jet will allow for some severe storm potential into our
area...with north central Illinois favored more so due to the
earlier arrival and ability to tap more instability and just
closer to the deep layer shear. Will also have to watch for
temporary heavy rainfall rates which is of course always an issue
when precipitable waters get to 98th percentile...even if only in a narrow
plume. There is uncertainty on storm coverage going into
overnight since deep layer forcing appears to weaken even thought
convergence along the front is forecast to remain modest.

Beyond...the pattern midweek becomes more zonal with the cold
front becoming a west-east stationary front somewhere to the
south. This may try to inch northward given global
guidance...although the timing of specific short wave troughs from
Wednesday night through Friday will be key and those features are
challenging to do at this point. Temperatures will be below normal
on Tuesday and Wednesday...with onshore flow into downtown Chicago
likely to keep conditions in the 60s there on at least Tuesday.
Temperatures will moderate back late week into next weekend it




//Ord and mdw concerns...updated 06z...

* showers with possible thunder later this evening.


//discussion...updated 06z...

Quiet and dry conditions expected this morning with any precipitation
activity staying well west of the terminals at this time. Patchy
fog with increasing high level cloud cover is likely this
morning...with southerly winds observing a steady increase through
midday. Main forecast challenge for the terminals is precipitation trends
and timing. Am monitoring showers/thunderstorms across Minnesota
and far western Iowa...which could first impact rfd by late
morning. This development will likely drift eastward through
midday and possibily reach rfd...but should also observe a
weakening trend as it approaches. Best focus this forecast period
for all terminals will arrive later this afternoon and evening
along an approaching surface trough and frontal boundary.
However...guidance varies to the extent of coverage and intensity
of any precipitation as well as timing and duration. Based on latest
trends have included precipitation in the tafs with rfd having the better
chance to observe it earlier in the afternoon...and then the
remaining terminals not until later in the evening. Confidence is
lower at this some guidance is bringing this precipitation in
earlier. Other challlenges will arrive very late in the forecast
period...with low ceilings and wind speed/direction likely
becoming problematic.


//Ord and mdw confidence...updated 06z...

* high confidence with VFR conditions this morning/today.

* High confidence with wind speed/direction today.

* Low medium confidence with showers/thunder and timing/duration
later this evening.


//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday...chance morning thunderstorms and rain and possible MVFR ceilings...trending
towards VFR conds by afternoon.

Wednesday...slight chance of rain showers late.

Thursday through Saturday...chance of thunderstorms and rain.



220 PM CDT

With high pressure departing the lakes region and a low pressure
taking shape in the High Plains...southerly winds will continue to
strengthen across the lake. On these winds will come higher dew
point air. Webcam imagery from around the lake indicates haze that
may be a little more thicker already over the water. Concerns with
dense fog will increase during the day and evening on Monday when
dew points of 10-20 degrees higher than the lake water temperatures
move over the lake.

The offshore southwesterly winds on Monday afternoon and evening
look to reach Small Craft Advisory criteria along the Illinois shore
and it is possible along the Indiana shore. Will go ahead and issue
a Small Craft Advisory on this shift for the Illinois shore. As for
the open waters...the stability will keep gusts far below what they
could be Monday winds just off the deck /within
700 feet/ across the northern part of the lake are forecast to be
40-50 knots. Will mention a few gale force gusts possible across the
north and that will be particularly true from the area of the
north buoy to the u.P. Shore.

The system cold front will move southward down the lake overnight
Monday night. The onshore flow from this on Tuesday...if a little
stronger than forecast...could bring waves to near Small Craft
Advisory criteria along mainly the Indiana shore.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...noon Monday to 10
PM Monday.



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