Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271827
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
127 PM CDT FRI MAY 27 2016

.UPDATE...1043 AM CDT

We are in between waves of precipitation after some scattered
shower and isolated thunderstorm activity this morning. Remnant
shower activity/possible mcv in central Illinois continues to
rotate northward. Any initial activity may just be showers in
this decaying precipitation. But there are several embedded
shortwave disturbances that will also move northward this
afternoon/early evening in a relative warm period and in area of
better instability than in central Illinois which after the
initial activity clears additional scattered storms are likely.
Cloud cover will limit heating today. Therefore the combination of
weak to modest instability and generally weak shear...the severe
threat today is low and reflected in the marginal risk for severe
storms focused farther west. The concern for scattered storms this
afternoon/evening will be localized heavy downpours in the
moisture laden and weak storm motion environment...along with some
gusty winds to 35-40 mph.

KMD

&&

.SHORT TERM...
401 AM CDT

Today through Sunday...

Forecast challenges focus primarily on attempting to identify
timing of shower and thunderstorm clusters through the Memorial
Day holiday weekend. High perceptible water air mass and slow
storm movement will present the potential for locally heavy
rainfall with these storm clusters.

Upper air analysis and GOES water vapor imagery depict a broad
upper trough across much of the central and western CONUS this
morning...with a closed upper low over eastern Colorado.
Downstream to the east...upper ridging was noted over the
Atlantic and across the mid-Atlantic and southeastern states.
Model guidance is in good agreement in depicting amplification of
the upper ridge across the northeast through the weekend...while
the Plains upper trough takes on a negative-tilt orientation and
slowly lifts northeast toward the upper Mississippi Valley
Saturday and eventually across western Ontario and the western
Great Lakes through Sunday. An elongated area of surface low
pressure from the Texas Panhandle into south-central Kansas is
progged to move slowly northeast across the upper Mississippi
Valley and northern Lakes region in association with the upper
low. East of the track of the upper trough/surface low a very
warm and moist will remain in place across the region...with
embedded smaller-scale short wave troughs and MCV features
modulating areas of convection. Identifying and timing of these
more transitory features...while difficult...will be key to
attempting to add some degree of detail to timing the most likely
periods of convection to affect the forecast area through the
period.

In the near-term...one such feature is noted over northeastern
Missouri and western Illinois early this morning. SPC RAP-
based mesoanalysis data depicts this activity occurring within an
elevated warm frontal zone...featuring 25-30 kt south-southwest
flow and strong moisture advection and frontogenesis within a
region of 500-1000 J/kg of MUCAPE. High-res guidance including the
HRRR and 4km NAM indicate this activity will continue to lift to
the north-northeast early this morning...though with a weakening
trend noted in forecast reflectivity fields. Current radar trends
suggest greatest potential for convection early this morning will
likely be for western portions of the CWA...where weakening but
persistent moisture transport is depicted. With persistent
warm/moist advection it is hard to rule out additional scattered
convection at just about any time today...though it appears that
this afternoon and evening will be the next period with a greater
potential for convection given diurnal instability max and guidance
indications of energy from another mid-level wave lifting out of
Arkansas. After this evening...have generally maintained low
chance pops into Saturday before more organized forcing develops
late Saturday and Saturday evening...as more significant height
falls develop across the upper Midwest with the approach of the
negatively tilted upper trough...and a cold front which trails
the surface low and pushes across the forecast area overnight.
For this period...have carried high chance/likely pops
particularly across the western portions of the CWA where forcing
and frontal passage will be juxtaposed more closely with the
diurnal instability maximum during the afternoon and evening
hours. Increasing mid/upper level wind speeds will yield stronger
deep-layer bulk shear at this time as well...supporting organized
storms and the potential for strong to severe storms. SPC day 2
outlook does bring the marginal severe risk into the western
counties of the forecast area. Precip threat then looks to
diminish substantially Sunday and Sunday night behind the cold
front.

Temperatures through the weekend are expected to continue to
be above average for late May...with low level thermal fields
supporting low-mid 80s for highs today and Saturday. Considerable
cloud cover and the prospects for areas of precip will tend to
moderate the warmth just a bit however...and have carried lower
80s over much of the area per slightly warmer GFS MAV guidance.
Despite slightly cooler low level temps Sunday...more sun with
drier westerly flow behind the front should still yield lower 80s
in most areas.

Ratzer

&&

.LONG TERM...
407 AM CDT

Monday through Thursday...

A brief period of short-wave ridging develops behind the departing
upper trough Sunday night and Monday...with dry weather expected
across most of the forecast area. ECMWF and GFS do hint at the
potential for some light precip west and northwest of the area and
have maintained a slight chance of isolated thunderstorms for
parts of north central Illinois...though with little large scale
support especially during the daytime hours Monday feel any
activity should be fairly isolated. Low-mid 80s and somewhat less
humid conditions should make for fairly nice weather for Memorial
Day.

Medium range model solutions then depict another deep trough
developing across the Northern Plains as we head into mid-week.
This will support a return of more humid southerly flow and an
increasing chance for shower and thunderstorms again in the
Tuesday-Wednesday period as the trough moves east across the upper
Midwest. 00z run of the ECMWF brings a cold front through late
Wednesday which would likely dry things out by Thursday...though
the GFS remains slower with the front and lingers the precip
threat into Thursday. Temps continue to run above normal through
the period as we move into June.

RATZER

&&

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

A disturbance aided by earlier morning convection over central and
southern Illinois will continue northeast through the area this
afternoon. Showers and scattered thunderstorms have already
increased in coverage and should continue to do so through the
afternoon. We do not expect everywhere to see steady precipitation
all afternoon, but confidence is high enough with the vort max
moving through into the moist and weakly unstable environment to
include a tempo TS in the taf with medium confidence. Lower
visibility is possible than advertised if a storm trains over one
TAF site given the moist airmass favorable for heavy downpours.

Winds will vary between south-southwest to slightly southeast with
gusts approaching 25 kt at times. We do not anticipate the winds
going farther southeast than a 170 direction for any more than a
short period of time, but it cannot be ruled out.

Additional waves of precipitation are possible this evening and
again overnight, but confidence on coverage is too low at this
point to include more than a vcsh this evening as instability
weakens, and have held dry beyond this period. Again, precip
chances remain possible beyond this time but will wait for clearer
guidance on this timing. MVFR conditions are possible late
tonight, especially north and west, and have continued the mention
at KRFD.

KMD

&&

.MARINE...
159 AM CDT

The main weather concerns over Lake Michigan will be the continued
dense fog today...especially over the northern half of the
lake...and also the potential for more thunderstorms.

Light winds and high a dew point air mass over Lake Michigan is
resulting in dense fog...especially over the northern half of the
lake. It appears that this fog will persist through the day
today...and possibly well into tonight as well. As a result...we
have extended the dense fog advisory through late tonight for the
north end of the lake. It appears that although some fog could
persist over the south end of the lake...that the fog advisory there
may be allowed to expire later this morning.

An area of low pressure over the plains...is expected to shift
northeastward over the upper Midwest late Saturday. This storm
system will result in the possibility of a few waves of showers and
storms over the next couple of days. Additionally...it appears that
south winds will be on the increase into the 15 to 20 KT range for
Saturday and Sunday as the low lifts into Ontario.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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