Scientific Forecaster Discussion
fxus63 klot 280817
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
317 am CDT Mon may 28 2018
315 am CDT
The main weather story for this Memorial Day Holiday is the heat,
with daily record temperatures in jeopardy and a possibility of
chicago's all time may high temperature being reached (see below
climate discussion). There are mesoscale uncertainties with
regard to how early northeast winds establish themselves into
northern Illinois and that is a factor on just how hot today. The
multiple day duration of this early season heat and the
impressively high magnitude of today's temperatures again warrant
continued heightened messaging, even though the lack of moisture
prevents higher heat index readings and heat headlines.
Early this morning, widespread convection across northern and
central Wisconsin is of interest. Much of this remains elevated
and as of 300 am a consistent outflow signature is not
identifiable, though would expect to see this soon given the
amount of storms and high downdraft cape values analyzed above
the low-level inversion in the southern half of Wisconsin. How
far and quickly this northerly wind shift progresses southward
remains of low confidence with a potpourri of convective allowing
model (cam) solutions. Any outflow would likely arrive during the
mid-morning and slow in speed into north central Illinois, while
could be lake-enhanced into far northeast Illinois and Northwest
Indiana. If this surges more so in, it would obviously slow the
rate of temperature climb for a couple hours and enough to fall
short of forecast highs. Synoptically, a light easterly wind is
favored, so once onshore wind does setup in northeast Illinois
(chicago) it should continue through the afternoon.
As noted in the last evening's discussion, the 850mb and 925mb
temperatures sampled by last evening's dvn raob are at seasonal
records for the site and near or at it for ilx. This thermal ridge
should maintain its magnitude over the area today, again with
possibly some low-level mesoscale modulation in far northern and
northeast Illinois as described earlier. The 500mb high center,
also at the upper percentile for strength for late may, will drift
directly over the County Warning Area today. All of this synoptically points
toward temperatures a couple degrees warmer than yesterday if
uninterrupted by morning outflow. Have generally gone that way for
much of the County Warning Area except far northeast Illinois and far Northwest
Indiana due to anticipated earlier onshore flow. Highs in the mid
to upper 90s should be realized for much of the cwa, unless in
the diminishing scenario that the morning outflow progresses much
further into the County Warning Area. Cannot rule out the century mark tagged in
one or two spots.
Dew points again should mix out some this afternoon, even with
light easterly flow. Heat index readings are forecast to be near
the temperature readings.
The earlier mentioned outflow, possibly elevated by midday, will
likely lose any cooling influence by afternoon but could serve as
a focus for widely isolated convection. The mid-level high and
its suppression directly overhead favor less coverage than
yesterday. High resolution guidance generally depicts the
weakened, semi-elevated boundary/wind shift to be around or south
of I-80 during the afternoon. Similar to yesterday, if a storm or
two is able to Spring in the high cape/dcape atmosphere, it could
become strong, but coverage will be very low of any cells.
Lows tonight will again not drop very far, with minimums around
70 and slightly cooler Lakeside due to onshore winds.
315 am CDT
Tuesday through Sunday...
To be issued shortly.
315 am CDT
Today's heat will compromise daily records for may 28, as well as
Memorial Day record high temperatures. It is also possible, the
all time Chicago may temperature record is reached.
Daily records for Monday, may 28:
Chicago 95 (2012)
Rockford 93 (2006)
Daily records for Memorial Day (floating date):
Chicago 95 (2012)
Rockford 96 (1934)
All-time may record for chicago:
98 (may 31, 1934)
for the 06z tafs...
Closely monitoring convection over Minnesota and northern WI.
Guidance continues to suggest that outflow from said convective
complex will race south through the terminals this morning. I have
just enough confidence to turn winds NE a few hours earlier when
the boundary moves through. I have low confidence in how strong
the NE winds will be, and they could be more like 10g20kt for an
hour or so.
Winds veer to the east by the early afternoon with VFR
cumulus clouds. Winds diminish but remain easterly this evening
under mainly clear skies.
345 PM CDT
In the near term, there has been some fog reported over the lake.
Given this possibility, have included fog in the open waters
forecast for tonight. Winds over the lake don't appear to be
overly strong into the start of the week. Will see high pressure
build south out of Canada by late Monday night into Tuesday, with
a steadier northerly wind expected. By midweek, winds will likely
be variable as the next system approaches from the south and moves
across the lake.
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