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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 231545

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1045 AM CDT Tue Jul 23 2019

1035 AM CDT

No major changes needed to the inherited forecast this morning.
Temperatures continue to slowly warm through the 70s and will top
out either side of 80 this afternoon inland, while a nascent lake
breeze appearing on TDWRs will push westward through the early-
afternoon hours and will keep the immediate lakeside areas in the
mid 70s. The simple water vapor satellite RGB shows one shortwave
pinwheeling into northern Indiana, with another quickly dropping
southward across the U.P. of Michigan. 

The combination of fairly dry near-surface air (surface dewpoints
around 60), and notable mid-level subsidence inversions on ILX 
and DVN's morning soundings should limit our diurnal instability 
gains today. However, a few hundred J/kg of MLCAPE, coupled with 
the approach of the aforementioned shortwave and enhanced 
convergence near the lake breeze could be sufficient to spark a 
few showers and storms. The current PoP configuration looks good 
and captures this scenario well. May have to nudge PoPs up 
slightly across our northeast as the cumulus field develops. If
instability/moisture were greater, we'd be a bit more concerned
with a threat for strong storms given fairly fast unidirectional
flow (35 kts at 500 mb). However, the overall dearth of more
appreciable thermodynamics looks to keep any of the stronger
storms under control, with a threat for perhaps some gusty winds
in any stronger cores due to the presence of steep low-level lapse



225 AM CDT

Through Tonight...

To have back-to-back no chance of precipitation days with 
pleasant temperatures remains elusive, as today there will be a 
chance for isolated showers and storms this afternoon through mid 
evening. Overall confidence is fairly high that any coverage will 
be minimal. The severe threat with any storms that do develop is 
low but could get a couple gustier cells in the Wisconsin to 
northern Illinois region. Otherwise highs will near 80 today, just
a few degrees shy of normal.

Early this morning finds the area east of a large 1025 mb high
over the central Plains, a pattern supportive of cool and dry
advection that has resulted in mid 50s temperatures in numerous 
outlying locations. As for the mid to upper troposphere, it is 
not as quiet, at least upstream, with two pronounced circulations 
dropping south-southeast into the Great Lakes. The first of these 
will pass over Michigan this morning, while the second one will 
track slightly further west and graze the area later this 
afternoon into early to mid evening. The 700 mb temperatures will 
be reinforced cooler to around 0C with this second wave. Both the 
forcing for ascent and this mid-level cooling will be there to 
support updraft development where there is any low-level 
convergence. Synoptically there is little, but the afternoon lake 
breeze in far northeast Illinois will be one today. Another will 
be a subtle surface trough, almost more on the mesoscale, dropping
southward with the second wave into far northern Illinois early 
this evening or so. 

The lake breeze is anticipated to develop early this afternoon and
not race inland in Illinois given ambient north-northwest winds 
of 10-15 mph gusting 20 mph. With highs closing in on 80 and dew
points in the upper 50s, RAP and NAM forecast soundings just reach
their convective temperature with very narrow, elongated CAPE
profiles thanks to fairly dry low-levels. This area and downstream
drifting to the south-southeast near the IL/IN state line may see
a few afternoon cells pop. A majority of CAMs try to show at least
a couple cells, although some are overdone on moisture. By later 
afternoon, the second short wave and its low-level trough focus 
will approach. In Wisconsin, there is likely to be isolated to 
scattered convection with this second wave, and some of this will 
at least drift close to the state line by 6-7 p.m. and maybe into 
northern Illinois. With inverted-V type soundings and 
unidirectional cloud-layer flow, there will be enough momentum 
for some cells to have some modest downdrafts and outflows, again 
a little more favored in Wisconsin than in our CWA.

Any activity should dwindle quickly after sunset. Tonight will 
again see lows in the 50s in outlying locations and lower to mid 
60s in the heart of the Chicago metro.



213 AM CDT

Wednesday through Monday...

Generally, a quiet period of weather is expected first several days 
of the long term forecast period as the surface high pressure 
spreads across the region.  However, a weak shortwave dropping 
through the nwly flow aloft on Wednesday afternoon may bring a 
slight chance for some showers and thunderstorms near the IL/WI 
border.  Much of the guidance is at least hinting at the chance for 
some pcpn, though the GFS is likely generating too much QPF, the 
upper forcing from the shortwave may at least generate some 
scattered showers or thunderstorms, mainly during the afternoon 
hours.  Otherwise, conditions should remain dry, with a general 
warming trend through the weekend.  High should start out the period 
in the lower 80s on Wednesday, though a lake breeze developing in 
the afternoon should limit lake front locations to the middle 70s. 
By Thursday and through the weekend, swly low level flow should be 
strong enough to allow warmer air to reach the lake front.  Highs 
should be back in the middle 80s by Thursday and the upper 80s to 
low 90s for Friday and through the weekend.  The next significant 
chance for any pcpn will likely not be until Sunday night or Monday 
as a surface trough/cold front approaches the region.


For the 12Z TAFs...

For the Chicago area aviation forecast today, concerns are:

- Potential for scattered showers and isolated thunderstorms 
  mid-afternoon through mid-evening

- Wind shift to northeast associated with a lake breeze this 

North-northwest wind speeds will increase through this morning
with gusts in the mid teens by late morning and sporadically 
upper teens possible into early afternoon. The near north 
direction of the wind by early afternoon will allow a lake breeze
to develop and confidence in this reaching ORD and MDW is medium-
high. The specific timing is low-medium, although 20Z-22Z seems 
to be a consistent overlap in current guidance. Winds will turn
back to the northwest but light at some point this evening.

This lake breeze boundary may serve as a focus for isolated
showers and possibly storms by 19Z. Better potential for isolated
to scattered showers and storms will be over Wisconsin associated
with an upper disturbance during the afternoon. That disturbance 
will move southward and at least graze the area late afternoon and
early evening. While a thunderstorm chance at area airports, it is
low confidence and there's a little more confidence that there
will be spotty showers around, so felt a VCSH was the better
forecast message.