Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270744
AFDLOT

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

.SHORT TERM...
244 AM CDT

Through tonight...

Expect to wake up to a little different airmass then you may have
been used to this past week. Steady rains continue this early
this morning as a deep negatively tilted trough axis swing
northward. The flow is nearly due south to north with rain bands
following this trajectory. The system is still strengthening, so
in spite of a loss of instability, moderate rains will occur at
times as the cold front oozes eastward. There are several surface
lows that will consolidate. The system`s main surface low,
across central Illinois will pass northeast this morning,
reaching the southern tip of Lake Michigan in the coming hours,
then strengthen further across the eastern shores of the lake
this morning. Cold advection and quick pressure rises will lead to
a quick surge in west winds through mid to late morning, which
when coupled with falling rain for a little longer will make it
feel a bit raw outside.

Fortunately, in spite of the cool and continued breezy conditions
today, the low will quickly lift northeast late this morning, and
drier air will filter in from the back side of the system. Lower
clouds will lift some but not fully dissipate until later in the
day, and will be eroding from southwest to northeast. The clearing
will be brief as clouds will thicken again later this evening and
overnight. Chilly highs today will only reach the 50s in many
spots, but possibly only upper 40s in northern/north central
Illinois. Lows tonight fall into the upper 30s to mid 40s.

KMD

&&

.LONG TERM...
244 AM CDT

Friday through Wednesday...

Unfortunately for those with outdoor plans this weekend, we will
still need to forecast rain, and quite a bit of it. Expect very
damp conditions, and possible localized flooding as well.

By Friday morning our current weather maker will have lifted north
of the northern Great Lakes, while our next big upper low will be
taking shape out across the Western U.S.A. In between these two
systems across the upper midwest and close to the area a weak
ridge will at least initially keep the real deep moisture from
getting up this far north. Strong southwest flow between the deep
low and high pressure of the southeast US will set up favorable
pattern for bringing gulf moisture northward.

A lead wave on Friday will lift an elevated warm front through
Friday leading to a batch of somewhat lighter rain showers,
generally speaking, though they could be moderate briefly on the
nose of the low level jet. The surface warm front will likely get
into our far southern CWA but that appears to be it. The high to
our northwest will allow this boundary to sag back southward
through the night, thus a brief lull in the action.

The gulf will reload through the day Saturday and into Sunday. The
global models try to bring precip back north as early as Saturday
morning, whereas some higher res models keep cool northeast flow
in place to lock the moisture south a bit longer and keep the
morning drier, especially north of I-80. Eventually with a deep
low that is deepening, strong frontogenesis with cool northeast
flow to the north and very warm moist air to the south, and
finally the sheer strength of the southwest flow and the
magnitude of the moisture transport will allow the door to open a
bit more in our area. Expect the rain to increase through the
afternoon, with global guidance maximizing the moisture transport
Saturday night into early Sunday, with rain continuing off and on
Sunday. Thunderstorms enter the picture Saturday night into Sunday
as the surface warm front will lift north, and the upper low will
steepen lapse rates. If that weren`t enough, the upper low will
then swing through late Sunday with very large height falls
providing deep ascent for hopefully what will be the system`s last
hurrah, heavy precip wise.

Confidence in the general pattern, reaffirmed by decent model
agreement and high relative measure of predictability of the
actual upper low progression, is fairly decent, but the devil is
in the details as far as where each wave`s high QPF axis will
setup and how much of the high PWAT axis will lift this far north.
especially given how much rain will fall via convection to our
south that may limit how heavy the rain will get up here. Still,
current suite of guidance suggest we can generally expect QPF
numbers in the Friday through Sunday night period in the 2 to 4
inch range, and locally higher numbers are certainly possible,
with the main swath coming later Saturday into Sunday. PWAT values
in the whole area are at maximum moving average values for late
April, with the 1.6" PW values possible in some areas being at all
time highs recorded for the month of April. After modest rains
early Thursday, these numbers could at least lead to some minor
flooding, and will likely re-aggravate a number of area rivers.

The actual low center will still not be through until Monday
evening, so there will still be some showers around.
Behind the low expect much cooler and breezy conditions, with the
seasonally chilly airmass lingering into mid next week.

KMD

&&

.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...

226 am...Broad low pressure over Lake Michigan early this morning
will consolidate into a smaller/stronger low later this morning
as it lifts northeast to eastern Lake Superior this afternoon and
then into Ontario this evening. As this low lifts north...winds
are expected to shift west/northwest and increase...possibly to 30
kts. Depending on how strong the gradient becomes...there could
also be some brief gale gusts. Confidence is only medium and
trends will need to be monitored later this morning. Prior to the
westerly winds increasing...areas of fog will be possible which
may become dense but confidence is low.

A weak ridge will build across the lakes region tonight into
Friday morning and then slowly lift north Friday night and is
absorbed by a stronger high building across Ontario. Low pressure
will move from the southern plains Saturday to the western Great
lakes Monday. The gradient will tighten between these two features
with northeast winds increasing to 30 kts. A period of gales is
also possible but confidence is low. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...7 AM Thursday to 7 PM Thursday.

&&

$$

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