Area Forecast Discussion
Issued by NWS Chicago, IL

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418
FXUS63 KLOT 300535
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 AM CDT Thu Mar 30 2017

.SHORT TERM...

A closed off upper low continues to spin northeastward across
south-central Kansas this afternoon with an associated surface low
following suit. An initial wave of forcing is moving northeastward
across the forecast area with bringing areas of light rain with
it. A persistent feed of lower level dry air has been helping to
erode this band of light rain, but some coverage has been
maintained. Expect this to continue across the central and eastern
part of the forecast area over the next couple hours with some
areas seeing a little light rain or even just sprinkles where
something does fall.

Radar mosaic also shows another area of ascent, oriented more
west to east, in conjunction with a corridor of mid level warm
advection. Scattered to numerous showers have developed with this
with the best coverage across southeastern Iowa and northeast
Missouri at 240 pm CDT. This forcing will cross the area a little
later this afternoon and very early this evening bringing another
window for scattered shower activity. It appears coverage will
wane for a short time after this activity lifts north but the
primary area of forcing will arrive in the western forecast area
later this evening and spread east tonight. Expect that this will
bring a much more widespread area of showers/rain with it.
Elevated instability spreads northward overnight and thunder is
possible just about anywhere, though areas along and south of
Interstate 80 have the best chance. East to northeast winds will
increase this evening and gusts may push 30 mph through the
overnight hours.

MDB

&&

.LONG TERM...
222 PM CDT

Thursday through Wednesday...

Main concerns will for the long term forecast period will center
around pcpn/TS chances associated with 2 srn stream upper closed
lows lifting out of the swrn CONUS.

The models are coming into better alignment with the timing and
strength of the first in the series of srn stream closed lows,  By
Thursday morning, the upper low center should lift into the srn
plains with the associated sfc low in ncntrl OK and a warm front
extending east into the Ohio Valley.  Strong high pressure over ern
Canada will keep generally east to northeast flow over the region.
The sfc pressure gradient should strengthen through the day
Thursday, with wind gusts arnd 30 mph developing.  As the system
continues to lift to the northeast, pcpn will continue through the
day.  Latest guidance suggests that the instability axis should be
suppressed south in the morning, with lingering TS chances generally
south of I-80 in the morning and gradually spreading north to the
Wisconsin border through the day.  With deep layer moisture also
increasing through the day, with pwats in excess of 1 inch over the
area, periods of moderate to heavy rainfall will be possible.  Some
area rivers already running high from previous rainfall, additional
rises will be likely, and some river flooding will be possible.  The
temperature forecast for Thursday also presents some challenges as
it will be highly dependent on how far north the warm front will
lift.  At this time, expect that the warm front will get hung up
near the srn tip of Lake Michigan.  The nrn portions of the CWA
should be in the cool ely flow, north of the warm front and temps
should top out in the lower to middle 40s.  South of the front,
temps should be able to reach into the lower 60s.  Latest guidance
would suggest that the front will lay out near the I-80 corridor,
but there could be some bust potential to the temperature forecast
if the front can surge a little farther north or is suppressed more
to the south.  Expect rain to continue through Thursday night and
possibly linger into Friday morning as the main sfc low lifts newd
through nrn IL/IN.  Any lingering rain should end by Friday
afternoon.  As the sfc low passes northeast of the CWA, winds will
shift from generally ely to more northerly, allowing for cooler
conditions to overspread the entire CWA, with highs Friday only in
the lower to middle 40s.  The far sern portions of the CWA should
see temps peg 50 F.  Should the low take a slightly slower track,
the warmer air could linger over the ern portions of the CWA a
little longer.

There should be a lull in the pcpn Friday night through Saturday
night as shortwave upper ridging builds across the midwest as the
first upper low exits to the northeast and the next srn stream
system begins to lift out of the swrn CONUS.  While confidence is
relatively high in another round of pcpn associated with this next
system, timing of onset is still questionable as the longer range
models diverge somewhat on the speed with which the upper low lifts
newd.  Generally favor the slower ECMWF solution with this system as
the GFS has a bias of lifting these types of systems too fast.  So,
will carry chance PoPs spreading into nern IL by Sunday morning,
there is a strong chance that the onset of this pcpn could be
delayed into Sunday afternoon or evening.  The general trend with
this system should be very similar to the first system, though there
are indications that is should take a more southerly track.  The GFS
is not only trending faster, but is also trending farther north,
through central IL/IN, while the ECMWF is trending farther south,
through the Ohio Valley.  Again, favor the more southerly track of
the sfc low as well, which would limit pcpn potential to lighter
rain and any associated TS potential to the far srn portions of the
CWA, if any TS can even spread that far north.

&&

.AVIATION...
For the 06Z TAFs...

An upper low rotating over northeastern KS late this evening will
continue to pump moisture rich and weakly unstable air into
northern Illinois and northwest Indiana over the next 36 hrs or
so providing off and on showers along with a few thunderstorms
through much of the TAF valid period. Have identified two favored
time frames for thunderstorms at the Chicago terminals. The first
will be overnight into the predawn hours and is currently ongoing
over portions of central Illinois. The second time frame is less
certain, with timing differences in the models, but would either
be late morning or early to mid afternoon Thursday. Leaning on the
latter period for now as it is supported by both NAM and RAP
guidance. Have tempos in for TSRA to cover these two time frames
where the forcing is stronger and more focused. For RFD, guidance
indicates that instability will not overspread that area
overnight, so only have one tempo for TSRA during a better period
of forcing late Thursday morning. Outside of the two
aforementioned periods of better forcing, there will be continuous
more nebulous forcing throughout the period which will support
off and on showers with no strong signal for any breaks in the
precip at this time. Overall forcing weakens some after 00Z
tomorrow evening and we begin to lose some of the deeper moisture,
so would expect precipitation to taper to drizzle.

ENE winds will stay moderately strong and gusty overnight with
gusts into the low to mid 20 kt range. Strong winds in excess of
40 kt off the deck may be able to occasionally mix down,
especially with showers resulting in a few sporadic gusts of
around 30 kt. Winds should taper some through the morning and
afternoon tomorrow, and slowly back towards the north and
eventually north-northwest tomorrow evening/overnight as the
surface low lifts across central and downstate Illinois.

BMD

&&

.MARINE...
253 PM CDT

Northeast to east flow will increase through the remainder of the
afternoon and this evening as low pressure lifts from the southern
to central Plains. The low is expected to track east from Missouri
tonight to southern lower Michigan by Friday morning. East winds
are expected to increase to 30kt tonight into Thursday with
occasional gale force gusts to 35 kt possible, mainly across the
south half of the lake. The low will continue east to the eastern
Great Lakes Friday night and then merge with another low off the
mid Atlantic coast Saturday. Winds over Lake Michigan will back to
the northerly, and remaining moderately strong. There is an
increasing chance for more widespread gale force winds Friday into
Friday night in the northerly flow. This pattern will keep Small
Craft Advisory conditions in place for the Illinois nearshore
waters into Saturday, with the likelihood that a Small Craft
Advisory will be needed for the indiana waters as well when winds
turn northerly on Friday. High pressure will begin to build over
the region on Saturday allowing winds to relax some over the
weekend before another low tracks through the Ohio Valley early
next week.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Burns Harbor until
     4 AM Saturday.

     Small Craft Advisory...Burns Harbor to Michigan City IN until 3 PM Thursday.

&&

$$

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