Area Forecast Discussion
Issued by NWS Chicago, IL

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484
FXUS63 KLOT 231949
AFDLOT

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

.SHORT TERM...
139 PM CDT

Through Friday...

Initial concern centers around shower chances this afternoon and
evening, possibly even a brief t-storm, then on winds and temps
for Friday.

Cloud tops continue to cool a bit upstream of more organized
convective development across the northern plains and upper
midwest this afternoon closer to the upper level shortwave.
Meanwhile in our area upper level heights continue to increase
this afternoon as was evidenced by our early scattering of clouds
this morning. In spite of the advection of modest lapse rates
aloft, with the dry southeast lower level flow, lack of a
strengthening organized wave, and poorly collocated moisture, the
warm front will struggle to do much over our area initially, but
there hints that that forcing could be good enough to get some
showers, and quite feasibly a storm, with the better chances
closer to Chicago and even more so once it gets out over the lake
and adjacent areas to our north and east. This window will be
narrow for most locations. The better lapse rates may not arrive
with the best precip timing and thus have confined thunder chances
along the WI border and just offshore of Chicago over the lake,
as there is some inhibition even with the most unstable parcels
and most of the echoes are not very deep.

The roller coaster ride of spring will take an uphill course, as
are in for quite the airmass change late tonight and Friday. The
warm front will come cruising on through on impressive south-
southwest low level flow.  While earlier this morning most
locations were sitting with temperatures in the mid to upper 30s
and dewpoints in the upper teens, after warming temperatures
overnight with the warm frontal passage, expect to kick off the
day in the 50s with dewpoints in the mid to even upper 40s.

The wild card to start the day will be any potential stratus
development and the impact on the temperature forecast. 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 of a stout inversion
and increasing moisture, but the propensity of short term guidance
(RAP/HRRR/SREF) and lack of precip (unless it occurs) would
suggest the low cloud would be slow to materialize and that some
of this low moisture is overdone. It is possible the lower cloud
over the southeast will work its way up here through the day,
which would also have implications on highs on Friday. Either way,
it will be well above normal, just possibly not as high as 925
climo would suggest, but that 70 degree mark is certainly within
reach. Breezy southwest winds will be in place as well.

The warm front appears to remain north of the Wisconsin border
through the day before high pressure across southern Canada begins
to shove it back south as a cold front, possibly reaching the WI
border by early evening. The upper low that will plague our region
for the weekend will be slow moving on Friday, and with some
shortwave energy kicking out ahead of it, but not appearing to be
enough of a trigger with the front north to do much until possibly
late afternoon.

KMD

&&

.LONG TERM...
249 PM CDT

Friday night through Thursday...

Very active weather period the coming week as upper level ridging
in the short term breaks down as a parade of upper level
troughs/upper lows then move onshore the Pacific coast and across
the mountains and plains and into our area through the period.
With such... expect some decent 3 day rainfall totals between 1
and 2 inches Friday night through Sunday (the majority coming on
Saturday/Saturday night) with widely varying temperature spreads
from north to south across the forecast area.

First upper low ejects into the plains Friday night and into the mid
Mississippi Valley Saturday. Expect showers and a few thundershowers
to break out in the isentropic lift ahead of the low/vicinity of the
warm front which stalls near the IL-WI state line Friday night and
early Saturday. Meanwhile a strong surface high pressure area over
northern Ontario/James Bay will drive a cold front south across the
Great Lakes early in the weekend.  As this cold front collides with
stalled warm front associated with plains low... chilly lake water
in the lower 40s will help continue to push the front south. 12z
NAM and GFS runs coming more in line with this thinking and
earlier ECMWF and Canadian GEM solutions.

A brisk east cyclonic flow over far northern Illinois in March
rarely makes for nice picnic weather... and with such a pattern
setting up for Saturday... with periods of rain... expect a
breezy raw day especially near Lake Michigan. Meanwhile south of
I-80 expect max temps to still climb well into the 60s in the
warm sector as the low begins to occlude late in the day Saturday
through Saturday night. Advection of mild moist air over
undercutting shallow cold front and marine layer also sets the
stage for areas of fog... possibly dense in spots... over Lake
Michigan and nearby adjacent land areas in northeast IL given
easterly boundary layer flow.

Expect rain periods to become more intermittent Sunday with an
overall decreasing POP trend as the system becomes more occluded
and gradually fills as it only slowly slides off to the east...
held up by east coast ridging.

The next in a series of upper troughs ejects quickly out of the
mountains and into the central plains Sunday night/early Monday.
Weak surface and upper level ridging brings a short lived break in
the rain Sunday night before the next round of a period or two of
rain moves into the area from the southwest as the system`s
surface low moves into Missouri by daybreak Monday. Seeing the
typical timing and location differences in model solutions by this
time with blend yielding a responsible solution to run with. With
this low following a slightly more southward track then the
previous... a continued NE flow will yield a wide max temperature
spread across the area again Monday with 40s far north and
immediate lakefront areas ranging to mild readings into the 60s
far south sections of the forecast area.

A ridge of high pressure both surface and aloft builds into the area
Tuesday bringing a period of tranquil weather lasting into the day
Wednesday with temperatures near to slightly above seasonal
averages. Upper ridge then amplifies briefly ahead of next in the
series of upper level troughs which at this team appear to bring
rain changes back into the area on Thursday.

Ed F

&&

.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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