Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241539

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
939 AM CST Sat Feb 24 2018

939 AM CST

Precipitation this morning and early afternoon will be fairly
light. South of I-80 the profiles support rain or drizzle. Farther
north low level east wind have dried out the lower atmosphere.
Therefore any precipitation would be minimal and short lived, but
for a few hours could briefly be flurries or maybe some brief
sleet. Warm advection does pick up a little bit this afternoon but
moisture from this morning`s wave will be easing, thus low chances
of precip which by this time would be light rain wherever it
occurs. Better chances for organized rain will be tonight. The
disturbance tonight is pretty dynamic out ahead of a cold front.
Precipitable water values will get above an inch in some spots, thus
expect a good batch moderate to briefly heavy showers. High based
elevated lapse rates do also warrant a low thunder mention mainly
during the mid to late evening hours. Instability progs mainly
graze our SE areas that were hardest hit with rain earlier this
week, but cannot rule out an isolated strike elsewhere.



330 AM CST

Through tonight...

Low pressure developing over the Southern Plains will deepen and
track overhead or just to our northwest over the next 24 hours,
bringing a prolonged period of precipitation followed by the
passage of a cold front late tonight.

The first part of the forecast period this morning could be
somewhat interesting as light rain moves into an area with
temperatures at or just above freezing. Farther upstream over
portions of Iowa and Missouri there are a handful of stations
reporting mixed precipitation but closer to home the readings are
mostly rain. In general temperatures seem to be nudging above the
freezing rain thresholds as the rain becomes more established in a
location, but as the boundary layer is still in the process of
saturating during the onset of precipitation there could be a
brief period of light freezing rain or even ice pellets. Did not
feel this was going to be widespread or persistent enough to
warrant an inclusion in the forecast, but if trends go a different
direction this would certainly prompt an early morning update.

The first round of precipitation looks to stall somewhere around
the I-80 corridor due to the influence of drier air to the north.
Much of the forecast area except for southern portions could end
up dry this afternoon before the next round approaches this
evening. Along with the rainfall comes a small chance of
thunderstorms. The initial SPC outlook for today included much of
our area in the general thunder category and our southeast third
in a marginal risk for tornadoes and damaging winds. Based on
internal coordination between local forecast offices and SPC this
likely will be pushed back farther south with the next update, so
do not plan to emphasize the severe threat at this time. Local
thinking is that shear is plentiful and instability is present but
limited and likely not surface based this far north.

Beyond precip trends, the other story for tonight will be the
increasing winds after midnight, with gusts by morning likely
exceeding 30 mph especially behind the passage of the cold front
expected during the predawn hours.



330 AM CST

Sunday through Friday...

After an active period today and tonight, ridging becomes more
established for the late weekend into early next week. In
addition to helping us see some sunshine again, the position of
the ridge to our south will allow for 2-3 days of warmer
southwest flow. Even Sunday might see a bit of a warmup after the
cold postfrontal start to the day, but Monday thru Wednesday look
much more promising for widespread 50s and perhaps a few locations
in the 60s farther south.

The next weather system arrives midweek and at least at this
time range appears to be potent. Deterministic models are not in
agreement regarding the strength and track of the low but early
indications are that we stay in the warmer sectors and see
primarily rain, perhaps with some thunder, and another round of
strong winds.

Friday again looks dry with seasonable temperatures, but timing
changes for the midweek system could bring corresponding changes
to the late week trends.



For the 12Z TAFs...

Having a difficult time getting a good handle on the areal extent
of MVFR CIGS this morning with higher clouds obscuring the lower
cloudiness on satellite imagery this morning. Overall, there has
been a trend toward increasing coverage of MVFR CIGS, so opted to
keep conditions MVFR through the day, though worth noting some
guidance suggests there could be some temporary improvement at
times today.

Precipitation over central IL is lifting fairly quickly
northeast, but should encounter drier air mass, suggesting that
the coverage and intensity of the precip should be waning as it
continues moving toward the terminals. High-res guidance is
suggesting some of this precip could make it into the Chicago area
terminals later this morning into the early afternoon, so have
added a tempo for some light rain, but probably wouldn`t be enough
to wet runways if it occurs. Cannot totally rule out a touch of
sleet at the onset of any precip, but again, minimal impacts
expected at most.

Conditions will deteriorate more quickly this evening with CIGS
likely falling to IFR and potentially LIFR early this evening with
an increasing coverage of rain. Could see a brief period or two of
moderate to heavy rain ahead of a strong cold front which will
sweep across the terminals shortly after midnight.

Cold front should result in quick improvement in CIGS to MVFR and
a wind shift and a rapid increase in winds from the west-
southwest. Expect to see a several hour period with winds gusting
over 30kt overnight into the early morning hours Sunday. Skies
look likely to clear after sunrise Sunday morning, but gusty
west-southwest winds are expected to continue through the day.

- Izzi


232 AM CST

Low pressure over the TX panhandle will rapidly deepen as it
quickly tracks northeast across the western Great Lakes tonight
and continue to rapidly deepen as it moves into Canada Sunday.
Expecting a period of easterly gales to develop ahead of the low
this evening, except over far southern portions of the lake where
milder air over the colder lake will result in somewhat more
stable conditions. For that reason, opted to run with a small
craft advisory this evening for the easterly winds for the
nearshore water. Behind the front, look for a period of high end
southwesterly gales behind the fast moving and quickly deepening
low. Quite concerned that there will be a window, perhaps up to 6
hours long or so, of southwest storm force winds over the far
northern portions of the lake, especially toward the Straits.
Strongly considered a Storm Watch for that area, but opted to just
introduce storm force gusts in the north for now and will let the
day shift get a look at latest guidance later this morning and
consider upgrading to a storm warning at that time. Gales will
quickly subside later Sunday afternoon and evening from south to
north with fairly quiet conditions on the lake the first part of
next week before the next storm approached later in the week.

- Izzi


LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...6 PM Saturday to 3 AM Sunday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 AM
     Sunday to 3 PM Sunday.

     Gale Warning...LMZ777-LMZ779...7 PM Saturday to 3 PM Sunday.




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