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FXUS63 KLOT 181933
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
233 PM CDT Mon Mar 18 2019

.SHORT TERM...
1210 PM CDT

Through Tuesday...

A weak disturbance in northwest flow will lead to an expansion of
cumulus/stratocumulus this afternoon. Low level lapse rates are
steep and this will promote isolated to widely scattered shower
development. Visible satellite suggests that most of the clouds
are shallow initially, there is some limited vertical growth
suggesting some development is expected, and we are already seeing
some echoes on KLOT radar this afternoon. Given low wet bulb
temperatures, showers could mix with snow or even graupel (small
ice pellets). These showers will taper off fairly quickly after
sunset.

High pressure will pass through central Illinois and shift to the
eastern Great Lakes on Tuesday. Winds will shift to southwesterly
and allow for some additional warming, and many areas will make a
run at 50 degrees or so. Meanwhile, high clouds will spread
overhead as an upper low will sink into the northern and central
Plains.

KMD

&&

.LONG TERM...

Tuesday night through Monday...

233 PM...Primary forecast concern is a period of rain late
Tuesday night through Wednesday morning. Additional concerns are
high temps through the weekend and possible lake breezes.

Low pressure will move across Ontario Tuesday night with a trough
moving across the western lakes region as another low moves from
the central Plains to central IL Wednesday morning. This second
low will bring a period of rain to the region early Wednesday
morning and the trend is for this to extend a bit further north
than previous forecasts...perhaps as far north as the I-88/I-90
corridor. Have bumped pops up quite a bit across the area with
categorical pops across the southern cwa. Storm total qpf could
reach a quarter/third of an inch...but most of this should fall
south of the Fox/Rock basins. Precip type appears to be mainly
rain. There could be some mixed wet snow flakes as the precip
develops. May also see some brief sleet as the low levels
saturate. Confidence is somewhat low for high temps on Wednesday
and will likely be dependent on the extent of any sunshine by
afternoon. Still have high temps near 50 which may be a tad too
warm to the east and perhaps a few degrees too cool in the west.
As a cold front moves across the area in the afternoon...there is
at least a chance for a few showers but have maintained a dry
forecast for now.

A large ridge of high pressure will move across the region Friday
and gradually shift southeast on Saturday and Sunday. With this
high nearby...lake breezes will become a concern and with the air
over the lake as cold as it is in mid March...there could be some
significant temperature gradients if the winds turn onshore. From
this distance...it appears an afternoon lake breeze would move
inland Friday...with prevailing southeasterly winds Saturday and
then a south/southeast wind Sunday affecting mainly the immediate
IL shore. But confidence is too low to include this in the
forecast from this distance. Opted to leave the forecast as is
with high temperatures generally in the 50s with some modest
cooling along the lakeshore. With full sunshine...highs inland may
be warmer than currently indicated. Next chance of rain appears
to be next Sunday afternoon/night. cms

&&

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

-Isolated showers this afternoon, some with snow or graupel
-Lake breeze late this afternoon

Expect an expansion of cumulus/stratocumulus this afternoon.
Ceilings may initially be MVFR but could transition to a lower
VFR. Low level lapse rates are steep and this will promote
isolated shower development. Visible satellite suggests that most
of the clouds are shallow initially, there is some limited
vertical growth suggesting some development is possible, and we
are seeing a few showers already popping up on radar. Given low
wet bulb temperatures, showers could mix with snow or even graupel
(small ice pellets).

Meanwhile, the lake breeze is currently at the shoreline. Model
guidance brings it through GYY and to the east side of MDW before
it appears to again wash out. Winds elsewhere will maintain a W
direction, but general winds will trend toward a lighter southwest
tonight and maintain southwest on Tuesday. High clouds will
spread in tonight into Tuesday, but it should be a dry daytime
period.

KMD

&&

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

$$

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