Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Thu Mar 23 2017

.UPDATE...934 AM CDT

Concerns today include chances of showers late this afternoon/this
evening, and even hints at some thunder potential.

Latest RAP/HRRR and 12z NAM guidance depict that current dry air
mass over the area will lead to shower activity struggling to
really get going, other than some sprinkles. Once the lower
moisture gets in place during the warm frontal passage, we lose
the deeper moisture over Iowa which appears to move through in
the mid-late afternoon. There are some pretty steep lapse rates
upstream that advect eastward with the elevated warm layer
depicted well on KABR and KOAX 12z upper air soundings with
700-500 mb lapse rates in excess of 7-7.5 deg/km. Have not
introduced thunder chances here given the concerns mentioned above
about how much we will see anyways given the narrow corridor of
forcing ahead of the warm front. But the time period of interest
for any activity would be in the 5pm- 9pm time frame. Storms
remain possible over the lake briefly this evening.

KMD

&&

.SHORT TERM...
330 AM CDT

Through Friday...

Lots to talk about this morning, in the short term, precip
chances, temps, and cloud cover are the biggest concerns.

Have continued the trend of lowering pops for today as low level
dry air and weakening of approaching shortwave both look to pose
impediments against more robust precip chances. Still concerned
that there could be a shower or sprinkle, but chances of anything
measurable this afternoon and evening appear low.

Extensive cloud cover upstream is moving into the area and
expected fairly cloudy skies today. Significant low level warm
air advection doesn`t arrive until tonight, so with cloud cover
today have trended highs toward the lower side of guidance.

Developing warm front will lift north this evening, which should
shift the focus of nocturnal convective/precip chances well north
of our cwa. In addition, strengthening southerly winds should
begin to draw warmer and more moist air north into the area.
Evening lows could dip into the upper 30s/low 40s, but expect a
fairly significant rise in temps through the night with temps in
the 50s by sunrise Friday morning.

Temperature forecast is tricky Friday and bust potential is quite
high. There are essentially 2 different lines of reasoning, the
first being the domestic models (NAM & GFS) which suggest low
stratus will blanket the area and the NAM keeps highs in the 50s
and GFS only in the 60s. Conversely, there is the foreign model
camp (ECMWF & GEM) which keeps the area largely stratus free
Friday and sends temps soaring well into the 70s, even upper 70s
for Chicago in the GEM. The ECMWF has very strong support from its
50 member ensemble with very minimal spread.

Using backward trajectories in the models, the low levels (<1000m)
Friday afternoon originates from the Carolinas, while slight
higher up the 1000-2000m parcels Fri PM are forecast to originate
over the NC/NE Gulf Coast. Both of these points of origin are dry
and void of stratus now. There is some patchy stratus beginning to
form over lower TX early this morning, but evening soundings
suggest this moisture is fairly shallow and would tend to advect
more to our west and southwest Friday afternoon. Given this, have
trended our sky grids and temperature grids toward the warmer
foreign model camp and bumped highs into the 70s. Given the strong
low level winds, higher late March sun angle, the forecast
afternoon soundings from the GFS and NAM look suspicious under
mixed and cloudy. Obviously if the more stratus solutions pan out
then highs Friday will be much cooler.

Izzi

&&

.LONG TERM...
330 AM CDT

Friday night through Wednesday...

Big upper low will work its way out into the central/southern
plains Friday afternoon with showers and thunderstorms likely to
break out along the associated surface cold front. This activity,
in a weakening phase, will begin to spread northeast into our CWA
Friday night. In addition, a strong Canadian high will move east
into Ontario Friday night and likely push the warm front to our
north back south as a back door cold front. Again, there are model
differences with foreign models more aggressive with the southward
movement of this backdoor front and the domestic models holding it
farther north. Climo this time of year and the very cold
lake/marine air mass make me think that the ECMWF/GEM models
probably have the right idea with the farther south placement.

The impacts of this frontal position on temperatures is huge. The
blended model guidance with highs in the upper 50s to mid 60s is
likely substantially too warm north of the front, especially near
the lake. After collaboration with DVN/MKX, nudged temps downward
some north of I-80, especially near the lake, however if the
ECMWF/GEM frontal positions are correct, then it will be MUCH
colder Saturday north of the front. The operational 00z run of the
ECMWF has highs in the lower 40s at ORD, mid 40 at RFD, and in the
upper 30s near the lake, with a sharp temp gradient and 60s south.
There is more spread in the ECMWF ensembles for Saturday, so
confidence is lower supporting the small adjustment downward we
made in temps for now. Like Friday, temp forecast for Saturday has
a fairly high bust potential.

Behind the lake enhanced front, fog, possibly marine enhanced, is
a pretty good possibility and have introduced some fog into the
grids. The large, slow moving cyclone will become increasingly
vertically stacked/quasi-barotropic, so instability will be
decreasing with time, but some threat of thunder could linger into
Saturday, particularly if there are any breaks in the clouds
allowing for sfc heating. Otherwise, looks for showers to develop
and pinwheel around the low through the weekend. There will likely
be dry hours in between the rain bands, but overall the weekend
looks cloudy with several showery periods.

This upper low is expected to move east of the area by Sunday
night, but medium range models have another dampening southern
stream short wave moving into the confluent flow in its wake
Monday. This system could bring some showers, mainly to our
southern CWA and points south Monday. Should be a break in the
precip after that system heading into mid-week, but with winds off
the lake and probably some cloud cover sticking around not
expecting temps to get too terribly warm, even though the polar
jet will remain to our north holding back any real push of cold
air. Does look like there could be another big upper low moving
toward the area late next week.

Izzi

&&

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

Concerns center around

1) A brief window of showers this afternoon/early evening
2) low level wind shear later tonight
3) increasing south-southwest winds on Friday
4) a smaller chance of lower cloud development late tonight into
early Friday.

The warm front will make more headway northeastward and bring
gradually lowering cloud bases through the afternoon. The weaker
energy will likely bring some isolated shower activity through the
terminals mid to late afternoon based on spottier areas of bumpy
cloudiness on visible satellite and some elevated IR cloud tops.
The earliest concern would be south of the main terminals, and
then northeast of them tonight. Based on the drier low level
southeast winds and concerns about coverage have held onto the
VCSH mention in the TAF but it is likely many areas are dry. This
activity may increase over the lake this evening where some
t-storms are possible.

Concern will shift to increasing south winds and especially low
level wind shear tonight. With the warm front moving in, a strong
inversion will develop. Low level winds around 1500-2000 ft will
increase to around 50 kt late this evening and overnight, so it
appears to be a decent set up for LLWS. More of the winds will mix
in on Friday as the overnight low level jet eases somewhat and
the surface pressure gradient increases. Winds will be SSW but at
this point winds look to remain below cross-runway thresholds for
ORD given a bit more shallow mixing, though gusts to the mid 20s
are reasonable, and possibly upper 20s if mixing is stronger.

The wild card on Friday is the stratus. At this point the
moisture over the plains is what some guidance is suggesting will
come in late tonight as either some patchy fog or some very low
cloud. NAM/GFS time heights hint that this will get here and it is
concerning given the pattern, but short term guidance and lack of
precip keeps confidence low on the morning cloud occurrence. Some
lower cloud may make its way up here later in the day.

KMD

&&

.MARINE...
229 AM CDT

Surface high pressure over the eastern Great Lakes will shift
eastward to the central Atlantic Coast later today. As this occurs
winds over the lake will become south and be on the increase
today. Winds up to 30 KT are likely at times this afternoon and
tonight, especially over the northern half of the lake.

A cold front will then shift down the lake Friday before likely
becoming stalled near the southern end of the lake Friday night
into Saturday as surface high pressure builds eastward across
southern Canada. An area of low pressure will then gradually shift
east-northeastward from the Plains to southern Lake Michigan by
Sunday. Northeasterly winds up to 30 KT will set up over northern
Lake Michigan by Friday night following the cold frontal passage.
With time the winds will gradually veer easterly later Saturday
into Sunday. Over far southern Lake Michigan, however, expect
lighter and more variable wind directions, depending on where the
surface boundary stalls and if the surface low tracks overhead on
Sunday.

With the possibility of the frontal boundary stalling out over
southern Lake Michigan Friday night into Saturday, dense fog may
develop in this region. This fog may persist through late Sunday
as the surface low shifts over the southern end of the lake.

KJB

&&

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

$$

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