Area Forecast Discussion
Issued by NWS Chicago, IL

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697
FXUS63 KLOT 301758
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 PM CDT Sun Apr 30 2017

.UPDATE...
1115 AM CDT

Continue to monitor atmospheric recovery upstream that will work
close and likely into the southern CWA late today and this evening,
with a potential window for some quick-moving severe storms,
generally between 6 pm and 11 pm and along southeast of a line
from Peru to Evanston. In addition, while showers will likely
show some undulation in coverage through the afternoon, any will
remain efficient rain producers, with an increased flooding
potential given the now very wet ambient conditions.

Short wave and an apparent mesoscale convective vortex (MCV) has
now moved east of the area. While there is some suppression in
mid-level forcing behind this, generally broad moist advection in
the lower levels as seen on the 12Z ILX sounding is continuing
areas of light rain/drizzle. This should be the case into the
afternoon along with an uptick in arcs of northeastward moving
showers as upper diffluence ahead of the well-developed
closed low increases. The ILX radar indicates a possible MCV in
south central Illinois that would cross into the forecast area
during the early to mid afternoon also favoring an increase in
coverage and temporary rain rates. Thunder potential will have a
gradual ramp up but elevated lapse rates are somewhat marginal and
lightning has struggled in central Illinois this morning. Will
have a lower trigger for flood warnings over the areas with 3+" of
rain yesterday given the saturated soils.

The primary surface low of 997 mb will continue to deepen across
eastern Kansas, with an eastward extension/triple point showing
signs of taking shape into central Missouri. Further east, the
initial warm front is responding with northward progress late
this morning, not far south of I-74. The true more effective warm
front is still well south, modulated by ongoing widespread
rain in southern Illinois. This front will begin to move
northward this afternoon, likely quickly late in the day owing to
warm advection response. Per the suite of high-resolution
guidance, would presently expect this to be near I-74 around 6 pm
with dew points into the mid to upper 60s up to this feature,
though confidence still is somewhat limited on that placement.
Accordingly, models still show a fair spread on instability, but
generally indicate some narrow mixed-layer CAPE of 300-1000 J/kg
making it into the southeast, and a few up as far north as I-88.
Feel the upstream 12Z SGF sounding had a good sample on what the
environmental wind fields would look like near or in the southeast
CWA early this evening. As the mid-level speed max associated
with the dry slot nose moves overhead this evening, and given the
oblique orientation of the shear vectors to the incoming
boundary, it seems that a few, quick north-northeastward moving
discrete storms are probable. Whether they are able to persist
long enough in the effective warm sector, or near warm sector air
for a realized tornado or strong wind potential into our CWA will
continue to be assessed. Again the most favorable area within the
defined Slight Risk would be along/east of I-55 and south of I-80.

MTF

&&

.SHORT TERM...
325 AM CDT

Through tonight...

The main forecast concerns continue to focus on the impacts of a
very dynamic and slow moving storm system expected to impact the
sensible weather across the region through Monday. However, of
most concern is the threat for more moderate to briefly heavy
rainfall today, and also the possibility for severe storms late
this afternoon into the evening.

The upper level low is centered over the Texas Panhandle early
this morning with surface low pressure taking shape over
northeastern OK into eastern KS. While round one of the heavier
rain ended across the area several hours ago, expect rain to shift
northward back over the region early this morning as another
convectively enhanced disturbance shifts northward over northern
Illinois. This will result in another period of light to
moderate rainfall through late morning. Some embedded
thunderstorms will also be possible with this activity.

With this morning convective activity in the region, it will
likely aid in keeping the effective warm frontal boundary south of
much of my CWA until at least mid day. Thereafter, the warm front
should begin to gradually shift over my southern counties this
afternoon as the parent surface low shifts northeastward towards
northeastward into Iowa while becoming occluded. As this occurs,
a tipple point is likely to set up over western Illinois later
this afternoon. Farther east, it appears the warm front may
struggle to get north of the I-80 corridor through the afternoon.
As a result, another chilly day in the 40s is likely over much of
northern Illinois with east-northeasterly flow off of the lake
continuing. To the south of the front, however, temperatures are
likely to warm into the lower 70s.

The main concerns for later today into this evening with be the
threat of severe thunderstorms as a cold front approaches from the
west. There remains questions as to the extent of surface based
convection over the area during this period due to extensive cloud
cover and periods of showers at times into the afternoon.
Nevertheless, there will be a conditional threat for severe
thunderstorms this afternoon, and this looks to be especially the
case along and really south of I-80. While buoyancy is not
expected to become overly impressive tied to the relatively
unimpressive mid-level lapse rates and little to no insolation
this afternoon, the wind field over the area, associated with the
larger scale storm system, is expected to be very impressive. In
fact, given that the near surface flow is likely to remain backed
south- southeastward near the surface warm front beneath south-
southwesterly 850 MB flow around 50 KT, low level shear/helicity
will also be very impressive and could support some tornados.
While this is a conditional threat on there being enough
instability to generate convection, it does appear at this time
the main threats would be south of I80. This situation will need
to be monitored closely through the day.

Very heavy rainfall and possible flash flooding could also become
an issue later today into this evening given the rain that has
already fallen, and the likely high rain rates expected with any
thunderstorms. The good news is that these afternoon and evening
storms should be moving quite quickly, which would limit the heavy
rain rates over any given location, unless any training convection
develops. Otherwise, the thunderstorm and heavy rain threat
should wane from west to east through the evening as the cold
front moves across the area.

KJB

&&

.LONG TERM...
336 AM CDT

Monday through Saturday...

The large upper low and the attendant surface low will continue
gradually shift towards the Upper Great Lakes on Monday into
Monday night. Rap around moisture will continue to result in some
additional light showers over the area on Monday and possibly into
Monday night before ending. It appears that much of the extended
period will be mainly dry. However, another upper trough is
expected to dig across the central portion of the country around
midweek and this may drive another period of rain over portions of
Illinois late Wednesday into Wednesday night. At this time it
appears the best chances may remain south of, or just clip my
southern counties. Otherwise expect temperatures to remain on the
cool side, mainly in the 50s to lower 60s.

KJB

&&

.AVIATION...
For the 18Z TAFs...

The primary concerns with the ORD and MDW TAFs are LIFR/IFR cigs
this afternoon and a very gradual lifting expected through the
afternoon and evening, then a couple hour window of thunderstorm
potential this evening, followed by gusty southwest winds on
Monday.

Northeast winds across the TAF sites have locked in IFR cigs,
though a very gradual lift is favored into the afternoon as
slightly warmer air creeps in. Pockets of drizzle and light rain
are likely early, and then more showery activity mid-late
afternoon. Some of the showers are likely to cause variability in
ceilings, but in general the northeast flow at ORD and MDW should
support IFR cigs. The confidence in cigs remaining below 800 ft
through early evening is low...and there is certainly a chance
they could ease a little higher, or bounce a little bit right
around that threshold.

Arcs of showers along with scattered storms are likely to develop
and move northeast across the area this evening. Confidence in
TSRA at the airfields is somewhat low, but in timing if it occurs
is medium-high. Ceilings should show a sharper rise late this
evening/early overnight, possibly to VFR.

Monday will see wraparound cold advection with MVFR clouds and
scattered showers. The wind gusts could reach 35 kt out of 210-220
degrees.

MTF

&&

.MARINE...
237 PM CDT

Headlines...Will go with a gale warning for the south half and
nearshore waters this afternoon through most of tonight. While gales
will subside, small craft conditions will likely continue through
Sunday. Gales are then expected over the northern half of the lake
Sunday afternoon and night, and upgraded the previous gale watch to
a warning.

A weak high is over western Ontario and the western Great Lakes
while a low is forming over Texas.  The tightened pressure gradient
will keep northeast winds at 30 kt and they will increase to 35 kt
gales over the south half late this afternoon.  Both the low over
Texas and the high over Ontario will lift north through Monday and
gales will form over the northern half of the lake Sunday afternoon.

The low will pass over the western Great Lakes Monday night and
reach Quebec Tuesday.  The low`s cold front swings through late
Monday night/early Tuesday morning. Southwest to west winds may
increase to gales over the southern half Monday night into Tuesday.
High pressure moves over the lake Wednesday and then shifts
southeast of the lake late in the week.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 7 AM Monday.

IN...Flash Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until 7
     AM Monday.

LM...Small Craft Advisory...nearshore waters
     until 4 PM Tuesday.

&&

$$

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