Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 302335
AFDLOT

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

.SHORT TERM...
229 PM CDT

Through Friday..

Main concerns are threat for strong to severe
thunderstorms this afternoon through early this evening south of
I-80 and unseasonably chilly and blustery conditions on Friday,
especially near the lake shore.

Continuing to monitor for the threat for strong to severe
thunderstorms south of I-80 later this afternoon, and particularly
south of the Kankakee River. The convection that has been lifting
north-northeast across the CWA is associated with a short-wave
disturbance ahead of the closed mid-upper low over Missouri. Given
strong moist transport flowing over warm front and PWAT values
around or over 1", main threat prior to this activity exiting per
latest radar trends is heavy rainfall and localized
ponding/flooding on some roads. This is especially the case with
saturated grounds due to what already fell overnight. Large areal
Flood Advisory that was issued remains in effect through 7pm.
Otherwise, can`t rule out some small hail to pea or dime size
(especially south of I-80) for stronger cores with cold
temperatures aloft/700 mb temps near or below 0 Celsius resulting
in fairly low freezing levels and wet bulb zero heights.

The main risk for strong to severe thunderstorms looks to evolve
over or just south of our southern CWA counties in the mid to late
afternoon, particularly south of the Kankakee River. What occurs
will likely be associated with arc of thunderstorms lifting
north-northeast from STL area. Good speed shear will be in place
with strong lower and mid- level flow, but directional shear from
850-500 limited given meridional nature of closed low. As low
pressure just north of STL lifts northeast this afternoon, surface
warm front will continue to progress north and likely reach areas
south of the Kankakee River, namely portions of Ford, Iroquois,
Benton, and Japser and Newton Counties.

Temperatures in the 60s and dew points approaching or just above
60 topped by relatively steep lapse rates could yield 500-1000
j/kg of SBCAPE near and south of warm front based off obs and
latest RAP forecast. In presence of supportive bulk shear,
embedded low topped supercell structures and small scale line are
favored. With earlier mentioned parameters in favor of hail,
threat for marginally severe hail could extend northward to near
I-80 with elevated MUCAPE north of surface warm front. Where warm
sector reaches, anticipating backed (southeast/east-southeast)
surface winds to result in decent veering in lowest 1km, which
could be enough low-level shear/SRH for an isolated tornado risk.
In addition, downdrafts from strongest warm sector storms would
pose a risk for damaging winds. Overall thinking hasn`t changed
much since previous update and will continue to pay close
attention to observational trends. The threat for any severe
weather should end during the early evening hours.

Approximately 1000 mb surface low will then track east-northeast
south of I-80 tonight into Friday morning, with system
deformation rain/showers diminishing and then shifting east on
Friday morning (with lowering PoPs in grids). Tight pressure
gradient between the exiting low pressure and high pressure around
1020 mb over the northern Great Lakes will result in strong and
gusty north to north-northeast winds, with gusts of 25-30 mph near
Lake Michigan. Expansive stratus will keep skies locked in
overcast, so temperatures will struggle to climb into the mid 40s
away from the lake. Strong northerly boundary layer flow down the
length of the still cold waters of Lake Michigan will keep
high temperatures even colder near the lake, in the upper 30s to
around 40. March will go out like a lion temperature wise with a
decided brisk/wintry feel, so be sure to dress accordingly.

Castro

&&

.LONG TERM...
229 PM CDT

Friday night through Thursday...

The upper level split flow pattern will continue through the weekend
and most of next week with some model signals toward a pattern shift
late next week/next weekend toward a more amplified pattern. Such
evolution would bring larger scale mean ridging established over
western North American and mean troughing in the east.

In the meanwhile, main forecast concerns continue to be on
periodic rain with scattered thunderstorms, first later Sunday
into early Monday, with a more potent system with the potential
for more soaking rains Wednesday into Thursday.

As the surface and upper low continue to exit to our east Friday
evening, surface ridging will continue to build into the area
from the west. This combined with rising heights aloft will help
yield a relatively pleasant early spring day for the area
Saturday, albeit chilly for areas near Lake Michigan with a
prevailing synoptic north flow. With north winds moving across the
40-44 degree surface water, expect max temps to hold in the 40s
with highs into the low- mid 50s as you further inland.

The next in the parade of upper level troughs will approach the area
Sunday. Despite a weakening trend as the trough ejects out of the
mountains and into the plains, expect clouds to thicken following
by rain overspreading the area west to east Sunday afternoon.
Better low level forcing remains well south with unphased upper
low moving into the mid Mississippi valley inhibiting better
moisture flow this far north. Still a period or two of light rain
likely with upper system and associated surface inverted trough
then exiting to the east across Indiana Monday.

With rising heights Tuesday expect a milder day but max temps will
again be held back near the lake with the prevailing NE flow.
Significant model differences in the next in the series of upper
lows... this one moving across the central plains Tuesday and
Wednesday. Forecast reflects blended solution with general agreement
toward a fairly dynamic system and potential for another round of
soaking rain and scattered embedded thunderstorms Wednesday into
Thursday.

Ed F

&&

.AVIATION...
For the 00Z TAFs...

635 pm...Main forecast concern this period are cigs along with
showers/drizzle and possible fog/low vis.

Low pressure over south central IL will move northeast to northern
OH Friday morning. This track will maintain northeasterly flow off
the lake into Friday morning. Most guidance keeps cigs in the 400-
600ft range through sunrise and then begins a slow climb through
ifr into low mvfr Friday afternoon. Followed this trend with
medium confidence but its possible cigs may dip lower tonight...
especially if any widespread drizzle develops. Cigs should start
to scatter from the north Friday afternoon and this clearing may
reach the terminals sooner than indicated in the 30 hour ord
taf...but confidence is low.

Current showers south of lot/gyy have been slow to lift north and
may begin to dissipate before reaching the rest of the terminals.
Opted to maintained shower mention this evening with uncertainty
as to how much additional shower activity may develop but much of
this evening could end up dry northwest of mdw/gyy. Additional
showers or drizzle are expected tonight into Friday morning.

With the expectation that low cigs will remain well into Friday...
not too confidence on fog producing visibilities too low and
maintained mvfr vis with this forecast but some guidance does
suggest vis may drop as low as 1sm. Currently expect any vis
reductions this low to be combined with drizzle but trends will
need to be monitored this evening.

Northeast winds 10-15kts with some higher gusts this evening will
slowly turn more northerly overnight and then remain northerly
for much of Friday. Speeds/gusts will increase Friday morning with
gusts into the mid 20kt range expected. Winds will begin to
diminish with sunset Friday evening and begin turning more to the
north/northwest. cms

&&

.MARINE...
156 PM CDT

Surface low pressure tracking across central Illinois this
afternoon will move east into Ohio overnight, with east-northeast
winds becoming north-northeast by Friday morning. Forecast
soundings support winds generally in the 25-30 kt range, though
there may be a brief period of marginal gale force gusts just
before sunrise on the northern portion of the western (WI) shore.
The gradient will begin to weaken Friday afternoon as the low
continues to pull away and redevelop off the Atlantic coast, and
weak high pressure spreads southeast across the upper Midwest and
western Lakes into Saturday. The surface high pressure ridge will
drift east across Lake Michigan by Sunday morning, allowing winds
to veer to the southeast during the day. A trough of surface low
pressure will approach from the west Monday morning, with low
centers passing north of Lake Superior and farther south across
the lower Mississippi/Tennessee river valleys. Medium range model
guidance depicts a stronger low may impact the Midwest Wednesday,
passing near or just south of Lake Michigan. This may result in a
period of stronger easterlies Wednesday which would shift to the
north-northwest Wednesday night into Thursday.

Ratzer

&&

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

&&

$$

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