Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231842

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

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.



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.



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.



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

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.






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