Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 191558

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1058 AM CDT Wed Jun 19 2019

1041 AM CDT

Going forecast is in pretty good shape in the immediate term 
here, although made some tweaks to temperature trends as a cold 
front is now oozing southward along the lake and inland across 
portions of northeastern Illinois. Low stratus and fog is 
occurring upstream across southeastern Wisconsin behind this 
frontal intrusion. At this time, think that any noticeably 
reduced visibilities will remain confined to the immediate 
lakeshore and nearshore waters this afternoon. Extended the 
mention of fog across the lake and immediately inland through the 
afternoon as a result. Think high temperatures have effectively 
been met near and north of I-90 and have trended temperatures 
either downwards or nearly steady here through the day. Farther to
the south, filtered sunshine should allow temperatures to rise 
towards 80 degrees south of I-80. 

A lead shortwave has zipped through the region, with another more
notable wave noted on water vapor loops immediately upstream in
northern Missouri. Modest warm advection and some 700-600 mb fgen
has resulted in spotty light rain showers in a northeast-southwest
arcing band along and north of I-88 this morning. Overall, it
looks like most of the region should remain precip-free through
the rest of the morning, however, as we await the main lobe of
forcing with this next incoming system. Guidance has been
struggling to handle these subtle and small-scale features, and it
looks like at least some convective enhancement to vorticity may
be playing a role in the model variability. Based on latest radar
and satellite trends, it does appear as if a farther north
solution may pan out here this evening, and morning guidance has
begun to latch on to this potential. As a result, have expanded
and upped PoPs northward this evening and tonight across the

With a fairly moist and destabilizing airmass in place and PWATs
continuing to ride upwards today, the flash flood threat may be
increasing for our southernmost counties as the aforementioned
shortwave/MCV combo intercepts this moist airmass. We may need to
consider a targeted Flash Flood Watch for counties south of I-80 
as this northward shift would bring the better dynamics into our
CWA this evening and overnight. The severe threat continues to
look pretty minimal, but some stronger wind gusts within precip-
loaded downdrafts will be possible across our south this afternoon
and into the early evening. 



305 AM CDT

Through Tonight...

Two dry days, albeit with morning fog, are now in the rear view
mirror as three dry days in a row continue very hard to come by in
2019. Much of the area looks to experience showers at some time 
today and tonight. The character of the rain will vary in
character some from north to south. More of scattered showers are
expected north of I-88 throughout today, while south is greeted 
with more periodic showers this afternoon and night with embedded 
scattered thunderstorms. The far southern CWA (along/south of 
Kankakee River Valley) remains the location where rainfall may see
the heavier rates and possesses a flood risk. That all said, 
confidence on nitty-gritty specifics is lower than typical in the
first 24 hours owing to both this system still maturing and a 
majority of model guidance struggling with ongoing trends.

Early morning satellite water vapor imagery indicates a
well-defined but compact mid-level wave across far northern 
Missouri. This is interacting with a lower to mid-level
baroclinic zone and associated cold front across southern 
Wisconsin into eastern Iowa and resulting in some weak ascent and 
frontogenesis. Showers in this area will continue to inch eastward
through the morning, likely becoming more scattered in nature 
with 40-60 percent coverage into north central and parts of 
northeast Illinois. Elevated instability characterized by MUCAPE 
of 300-500 J/kg provides forecast equilibrium level heights around
20k ft (near -20C) on RAP soundings, so cannot rule out a few of 
these showers having lightning.

The mid-level circulation will move over northern Illinois during
the early to mid afternoon. An area of differential positive 
vorticity advection in tandem with 850-700 mb warm and moist 
advection should support an increase in shower coverage. 
Convective allowing guidance generally supports this solution as
the most likely, but with a high amount of variance on how far 
north. Observational trends on satellite and regional mosaic in 
southern Iowa early this morning would support going a little 
further north than a guidance blend and have done so boosting
forecast shower coverage. The surface low reflection near 1003 mb
by afternoon is forecast to translate eastward across central 
Illinois, with still a buffer if further north in keeping low- 
level effective instability south of the CWA. However, the 
elevated instability with MUCAPE values around 1000 J/kg 
forecast will be induced northward. Expect there will be 
scattered storms, maybe even a cluster or two of semi-organized 
elevated storms in the southern CWA. Severe weather is mainly not
expected, but a strong more sustained storm or two is possible.

As a strong subtropical jet pushes eastward into the Lower Ohio
River Valley by this evening, the system will respond in slightly
amplifying and slowing. This has been consistently forecast over 
the past several days and its effect will likely mean showers 
continuing tonight to be fairly solid across the southeast CWA 
(east central IL/northwest IN). It's tough to say specifics on 
QPF, but the signal of deformation and deeper moisture convergence
and 2+ inches of QPF have diminished in our CWA. So feel 
confidence is enough in not going with a Flood Watch at this 
time, however some localized flooding certainly could materialize 
in favorable areas especially the southern CWA, and rivers in this
area will likely be on the rise into late week.

For temperatures today in any one place, if it rains and how early
will have an impact. More so though will be thickening clouds and
increasing northeast winds thanks to the synoptic gradient and 
possibly some lake enhanced push to the aforementioned upstream 
front. This should result in temperatures cooling some this 
afternoon especially in lake adjacent counties. Northeast winds 
in lakeshore areas will likely peak 20 mph or a little higher at 
times late day and evening. The onshore nature of this flow will 
result in at least a moderate risk for rip currents at exposed
beaches and will need to watch trends if a higher wind northeast
wind speeds. Also there will be potential for low-level 
saturation in this lake-land interface with the northeast flow 
into an already moist low-level, and a period of fog is possible 
by late afternoon. Confidence on this is very low for several
reasons, including that any showers would likely cause disruption
of the profile enough for any impacting fog.



341 AM CDT

Thursday through Tuesday...

By Thursday morning, much of the activity from Wednesday night will 
have exited to the east by around daybreak, though there may still 
be some lingering showers over nwrn Indiana.  As the main long-wave 
trough moves east and weak upper ridging builds across the Plains 
and Middle/Upper Mississippi Valley, the area will be under nwly 
flow aloft through Thursday night. Low-level moisture may remain 
sufficient to support deep enough diurnal cumulus to generate 
isolated showers during the afternoon, but there should be a greater 
chance for showers Thursday night as a shortwave tracks across the 
apex of the ridge.  The path of the shortwave looks to be across 
southern Wisconsin, so, while there should be enough forcing for 
some showers, the better chances for any thunder will be in closer 
proximity to the wave, north of the IL/WI border.

For Friday through early next week, the longer range guidance 
continues to indicate an active period across the region. Upper 
ridging will amplify Thursday through Friday night along the 
Mississippi River Valley, with the ridge axis crossing the CWA on 
Saturday. A return flow of warm/moist/unstable air from an 
increasingly open Gulf of Mexico will spread across the area, under 
this upper ridge. A series of shortwaves, convectively enhanced or 
induced convective activity over the Plains are expected to over-top 
the ridge bringing periods of showers and thunderstorms to the 
region.  However, there is considerable uncertainty to the exact 
timing of individual waves, particularly since the development of 
these waves will be highly dependent on the development of the 
upstream convection, which presents it's own difficulty. So, much of 
the long term forecast period will sound like a broken record, with 
daily chances for showers and thunderstorms. There will likely be 
dry periods intermixed with the periods of showers and 
thunderstorms, so the period will likely not be a total washout. 
With temperatures expected to reach into the lower to middle 80s and 
sfc dewpoints in the low 70s, and some models indicating a 
relatively strong shortwave crossing the area Friday afternoon/ 
evening, there will be some severe thunderstorm potential and SPC 
has highlighted the local area under a Marginal Risk for severe 
weather in the latest Day 3 Convective Outlook.


For the 12Z TAFs...

The concerns for the Chicago airport forecasts for today are:

- Scattered light showers mid-morning through early afternoon,
  likely increasing in coverage thereafter, with the higher
  thunder coverage favored to remain south

- MVFR clouds becoming more likely by afternoon, with a chance for
  IFR clouds off the lake today, with better potential later
  tonight into Thursday morning 

- Northeast winds turning near due north late tonight into
  Thursday morning

Scattered showers across eastern Iowa to southern Wisconsin will
slowly move eastward. The heart of these should not reach Chicago,
however some lighter showers in advance of these will likely move
over the metro. The primary upper disturbance of the weather 
system will start to influence the area this afternoon into 
tonight, resulting in shower coverage increasing across the 
region. RFD to ORD will be on the northern edge of this, but do 
think sustained showers will reach these airports. There is a 
small chance for thunder, but the better setup is south of Chicago
(does include the southern TRACON area).

Clouds will gradually lower today, likely reaching MVFR at some
point this afternoon. There are indications that there are patchy
IFR ceilings over the lake and that this may expand. As winds 
increase from the northeast into this afternoon, it's possible 
some of this is dragged near Chicago airports, but confidence is 
quite low on that. Later tonight, the signal for IFR increases, 
especially in the wake of heavy showers and before drier air moves
in, which looks to be late tonight into Thursday morning.

As for winds, a northeast direction will prevail with speeds
increasing into the afternoon. A near due north direction is
likely overnight and Thursday morning, though confidence is high
in sustained speeds of 12 kt or less.