Area Forecast Discussion
Issued by NWS Chicago, IL

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964
FXUS63 KLOT 270533
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1233 AM CDT Thu Apr 27 2017

.UPDATE...
851 PM CDT

Made a few tweaks to going forecast for tonight, primarily to slow
timing of onset of precip, especially NW CWA and also to back off
on thunderstorms chances.

Warm conveyor belt, with associated plume of showers and
thunderstorms stretches from the lower MS Valley north into NW
Indiana early this evening. The more robust thunderstorm activity
is confined to the eastern flank of this swath of precip and has
been and appears likely to continue to mostly miss our CWA just to
the east. Area of trailing stratiform rain with some ocnl
lightning does extend west and this shield of rain has begun to
spread into our southern and southeastern CWA.

GOES 16 Water vapor imagery nicely depicts the sharp trough
beginning to take on a more negative tilt over the MO Ozarks
region this evening. As this trough lifts north tonight, look axis
of precip over our eastern CWA to either begin to expand westward
as flow backs in advance of the amplifying trough. Instability
likely to be minimal, so any thunderstorm threat appears small
as we head into the overnight hours. While threat of convection
appears low, feed of fairly moist air with PWATS of 150-200% of
normal should set the stage for periods of moderate, to perhaps
briefly heavy rain overnight into early Thursday morning.

Precip rates unlikely to be heavy enough to produce any flooding
threat, however, portions of SE CWA could see totals over an inch.
Another axis of heavier totals is possible over NW CWA where
TROWAL feature should linger steady rain into Thursday morning.
Its possible totals in this area could approach or perhaps even
exceed an inch as well.

- Izzi

&&

.SHORT TERM...
309 PM CDT

Through Thursday...

Main concern for the short term forecast period will focus on heavy
rainfall associated with showers and thunderstorms moving into the
region by late this afternoon or early evening and continuing
through the night.

Latest guidance and observed trends indicate that much of the
remainder of the afternoon should be dry.  While vis satellite
imagery indicates an area of towering cu developing in the warm
sector air not covered by layered mid and high cloud, there is no
indication of pcpn development on radar and SPC meso analysis still
indicates a small amount of CIN where MU CAPE levels are 500 j/kg or
higher.  Sfc dewpoints are slowly creeping up into the upper 50s and
expect dewpoints to climb further to around 60 F, especially for
locations east of the I-55 corridor.  Latest pwat analysis are the
region is around 1.2 inches and model guidance suggests that pwats
should increase to 1.5 inches or slightly higher overnight. Ongoing
convection is still well south of the CWA, in the St. Louis area and
spreading east into central IL.  Latest guidance suggests that the
convective activity will gradually build northward, possibly
reaching the Pontiac area by 6:00 - 7:00 PM and then continuing to
spread north and east overnight.  While the models had been
advertising that the main long wave trough aloft would be fairly
progressive, the latest guidance suggests that the progression of
the upper long wave and associated sfc front should be relatively
slow.  While there is still some severe thunderstorm threat tonight,
currently feel that heavy rainfall and the potential for flooding
with be the greater threat.  Expecting widespread rainfall amounts
of 1.5 to 2.0 inches storm total QPF from late this afternoon
through early tomorrow afternoon.  Locally higher amounts will be
possible invof stronger storms.  With upper level winds remaining
sly through the night, with a 50 kt low level jet right on top of
there area, some echo training will also be possible.

The models remain relatively consistent in lifting the system newd
away from the region, with some wrap-around rain possibly lingering
through late morning or early afternoon tomorrow.  Following the
passage of the associated cold front, temperatures will be much
lower tomorrow, with highs only in the lower to middle 50s or even
some upper 40s close to the WI border.

&&

.LONG TERM...
317 PM CDT

Thursday night through Wednesday...

Very wet and at times stormy conditions expected over the weekend
for northern Illinois and northwest Indiana resulting in renewed
flooding concerns.

A broad upper trough that covers the western 2/3 of the country is
expected to become increasingly amplified late in the week through
the weekend a strong shortwave digs across the Intermountain West
and into the Desert Southwest over the weekend. A wide corridor
of deep southerly flow will develop from the Gulf of Mexico into
the Midwest transporting high theta-e air into the region. PWats
start to increase to around an inch during the day Friday as a
lead low amplitude shortwave trough moves across the region. This
will support at least a chance of showers though instability at
this point isn`t impressive with only an off chances for a
thunderstorm or two. Meanwhile, as the strong shortwave rounds the
base of the longwave trough Saturday, cyclogenesis will occur
with a surface low lifting from the Southern Plains into the mid
Mississippi Valley Saturday night. A 50kt low level jet out ahead
of the low will send another surge of moisture into the local area
with PWats further increasing to around 1.5-1.7 inches Saturday
night while a corridor of strong moisture convergence on the nose
of the low level jet slowly lifts across northern Illinois and
northwest Indiana and serves as the focus for potentially strong
thunderstorms and heavy rainfall. Both the GFS and ECMWF paint a
corridor of 2-3 inches 24 hour QPF through midday Sunday which
raises concerns for renewed rises on areas streams and rivers,
especially given a potential round of heavy rainfall tonight and
additional rain Friday. Guidance still needs to work out some of
the fine details on the axis of heaviest rainfall, but those with
hydro interests or in flood prone areas should stay in tune with
forecast updates over the next few days.

Low pressure begins to occlude over the Upper Midwest Sunday
evening and night while the dry slot wraps across northern
Illinois and northwest Indiana. Still have some potential to see
some additional light rain or drizzle within the low level
cyclonic flow which persists into Monday. High pressure is
expected to build across portions of the Great Lakes by midweek
and should provide a couple days of respite from the
precipitation.

Deubelbeiss

&&

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

1233 am...Forecast concerns include rain this morning...cigs and
winds through this afternoon.

A broad area of low pressure extends across Lake Michigan to
central IL and will consolidate into a low over southern Lake
Michigan later this morning. Until that time...low confidence for
wind directions with an easterly component likely at the Chicago
terminals. Once the low lifts north of the terminals...winds will
shift to the northwest and then more west/southwest after sunrise.
The low is expected to deepen and the gradient is expected to
tighten quickly. As a result...westerly winds should rapidly
increase with forecast soundings suggesting gusts into the
mid/upper 20kt range...possibly higher. These strong/gusty winds
may diminish some this afternoon and then will diminish under
10kts this evening.

Light to moderate rain will continue into the mid/late morning
hours with cigs expected to lower to ifr across much of the area
though only medium confidence on cig trends this morning.
Visibilities will also lower into the 2sm-4sm but expect these to
improve as winds shift westerly toward daybreak. Cigs will slowly
lift through mvfr later this morning and should become vfr this
afternoon...scattering out tonight. cms

&&

.MARINE...
251 PM CDT

A very active period of weather is expected across the Great Lakes
region through the weekend and into next week as a series of lows
lift across the region. An elongated trough that extends along
much of the Mississippi Valley this afternoon will shift east and
consolidate into a low that lifts north across Lake Michigan
tonight and eventually to James Bay early Friday. An area of
weaker flow (col area between the departing low and new low over
the Southern Plains) will lift north across Lake Michigan Friday
into Friday evening. North to northeast winds increase again
Friday night into Saturday as another low lifts from the Southern
Plains into the eastern Great Lakes. Yet another and deeper low is
expected to develop over the lower Mississippi Valley early
Sunday and lift north to Lake Michigan early Monday. Gales appear
possible out ahead and behind this low early Sunday though
Monday.

BMD

&&

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

$$

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