Scientific Forecaster Discussion
fxus63 klot 271614
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1114 am CDT Thu Jul 27 2017
1106 am CDT
Expect cloud cover to be a bit slower to erode, but do gradually
expect some clearing as a lower dewpoint airmass to the north
filters in. Some instability south generally along and south of
I-80, with a nice non-linear depiction on GOES-r visible
satellite of enhanced cumulus growth, will allow a few showers
and maybe a storm this through the afternoon. This axis will
continue to drift south through the day.
319 am CDT
The day will begin with light and variable winds at most locations,
but during the morning a prolonged period of northeasterly flow will
become established and stay in place through the weekend. Showers
related to the late evening and overnight convection have mostly
exited the area as of the predawn hours, but there are multiple
signs that rain chances may linger into the morning and early
afternoon. First is the presence of a low pressure circulation
along a frontal zone straddling the area. Next is the very moist
airmass still in place. Third is the general agreement among
models of some weak shortwave energy aloft this morning. Overall
am not expecting much if any activity, but will leave a mention of
at least slight chances for most of the area through the morning,
and of chance pops in our southeast County Warning Area through mid afternoon.
With the northeast flow in place, temperatures today look to be 5
to 10 degrees cooler than yesterday.
The strengthening north-northeast winds in the wake of the low
will align well with the fetch of Lake Michigan, and as a result
build waves along the Illinois and Indiana shores. For Illinois,
the tightening pressure gradient and gusty winds move in early to
mid afternoon and for Indiana mid to late afternoon. So have
issued a beach hazards statement for a dangerous rip current
threat beginning at those times. With onshore flow anticipated
through at least early Saturday morning, have run the beach
hazards statement through then, and it is possible it will need to
be extended through midday Saturday.
319 am CDT
Friday through Wednesday...
The cool northeasterly flow that sets up this morning will
continue through the weekend, aided by a dome of high pressure
moving into the area. Subsidence will dominate through the
middle of next week as the high pressure center very slowly makes
its way across the area. It is not until later on Wednesday that
models agree in pushing the high off to the east and allowing for
better moisture and increasing precip chances to start returning
to the area. Temperatures through the week look to be very
seasonable for late July into early August, with cooler conditions
along the lake through the weekend during the period of onshore
for the 12z tafs...
The concerns for the Chicago area tafs today are:
- patchy IFR/MVFR clouds through mid-morning with a small chance
- north-northeast winds steadily increasing in speed, reaching 10
kt around 16z
Winds have turned northeast at most airports early this morning,
albeit still light. These speeds will slowly increase this
morning and then more quickly this afternoon, with gusts to become
common. Some gusts are probable even after dark, and then again
becoming common during the day Friday. Confidence in wind
direction being north-northeast for most of the taf period is
high, and is medium-high in forecast wind speed.
The back edge of the IFR cloud threat should move south with rfd
the most favored taf Airport to have IFR through mid-morning.
Cloud bases should lift/scatter some through the day, trending
VFR. Some clouds may develop in the northeast flow off the lake,
although those are likely to be no lower than 2500 ft.
An upper wave will move south over Michigan late tonight into
early Friday morning. While any showers are likely to remain to
the northeast, there is a chance for some MVFR clouds associated
with this near daybreak Saturday.
205 am CDT
An active couple days to end the workweek are in store for Lake
Michigan. A somewhat broad surface low south of the southern part
of lake by about 150 miles this morning, will move east-southeast
today. Behind this, the pressure gradient will strengthen between
this low and an incoming high to the northwest. This will drive
increasing north-northeast winds that will spread southward this
afternoon, with gusty winds reaching the Illinois nearshore early-
mid afternoon and the Indiana nearshore shortly after. A few
gusts exceeding 25 kt are possible late day into the evening
across the south. In response to the wind increase, waves will
build quickly. So have a Small Craft Advisory beginning near the
wind increase forecast time for both the Illinois and Indiana
The high gradually spreads south on Friday, while the low deepens
in the mid-Atlantic. In response, gusty north-northeast winds
will continue, with 25+ kt gusts across the south Friday afternoon
and evening. Waves will accordingly remain high and should remain
4+ feet into early Saturday morning. There is potential for these
waves to linger even through midday Saturday, so the Small Craft
Advisory may need to be extended.
Illinois...beach hazards statement...ilz006-ilz014...3 PM Thursday to 10 am
In...beach hazards statement...inz001-inz002...5 PM Thursday to 10 am
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...3 PM Thursday to
10 am Saturday.
Small Craft Advisory...lmz743-lmz744-lmz745...5 PM Thursday to
10 am Saturday.
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