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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 200601

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
101 AM CDT Thu Jun 20 2019

940 PM CDT

Mid level circulation will continue its eastward trek along and
south of the I-80 corridor this evening. Lighter precipitation
will continue along its northern periphery, while more convective
elements will form/redevelop ahead of it. Fortunately the MUCAPE
axis not only is diminishing but becoming even more focused
across central and southern Illinois, which will make lightning
coverage, which has been sparse even to this point, even less. 

While widespread flash flooding concerns are not high in the 
immediate term, with one hour rates generally under 1/2 inch, we
have had localized amounts to 1.8" in 3 hours (near Dwight, IL).
Therefore this initial batch of low echo centroid/efficient rain
producing showers can still bring some decent rainfall in a short
time. Therefore, we will hang onto the flash flood watch given the
PWAT axis in excess of 1.8" will spread into our southern
counties and into northwest Indiana this evening and overnight,
and lead to possible concerns with the additional development
expected overnight. 

Meanwhile, with the surface low now shifting farther east, winds
have become more northerly near the lakeshore. This coupled with
the more steady rain has allowed fog to lift, and thus expect 
patchy fog from this point forward given the rain will continue. 



247 PM CDT

Through Tonight...

Well-defined vort max on water vapor imagery early this afternoon
is cartwheeling across northern Missouri with an associated
expanding/developing warm conveyor to the east. Based on satellite
and radar loops, the center of this cyclonic circulation--arguably
convectively-reinforced--seems to be located over of just east of
Chillicothe, Missouri. To the east of this feature, a very moist 
airmass has spread north and eastward, with PWATs now analyzed in
the 1.6 to 1.9" range--nearing the 99th percentile for this time
of year. At the surface, a lake-enhanced cold front can be seen
dropping southwestward towards Rensselaer and Ottawa. To the north
of this boundary, conditions are cooler and stabilizing with a
rapid decrease in instability noted the farther north and east you
go towards the lake. 

As this aforementioned shortwave/developing surface low 
intercepts this southward-moving boundary, expect a proliferation 
of widespread light to moderate rainfall and embedded heavier, 
convectively-enhanced precipitation rates into the evening hours. 
Upstream observations have indicated rainfall rates inching 
upwards into the 1-2 in/hour range, with 30 minute rainfall rates 
nearing one inch in some instances. Radar imagery depicts classic 
low-echo centroid features in the embedded heavier cores currently
spreading east of Peoria and a lack of appreciable lightning 
activity confirms warm rain processes are dominating in this 
tropical airmass. Guidance has clearly been struggling with the 
evolution of the parent circulation, but has continued to latch on
to a farther north solution over the last several hours and this 
is confirmed based on satellite and radar loops. 

It appears as if a strung-out surface low will probably track
across our far southern tier of counties this evening and
overnight. As an associated 850 mb low deepens, guidance suggests
a pivoting deformation band may yield a longer residence time of
heavier precipitation into our CWA. Given all of this, we've opted
to hoist a Flash Flood Watch for our counties south of I-80 where
hi-res guidance is now keying in on a potential for widespread 
rainfall amounts of 1-3 inches with pockets of potentially 3-5 
inches of rainfall through Thursday morning. This threat could 
potentially spread a bit farther north and west of the current 
watch, but rapidly decreasing instability in the post-frontal 
airmass should keep overall rainfall rates in check. 

Continue to think the severe weather risk remains low, but can't
rule out some gustier winds across our far southern counties this
afternoon from precip-loaded downdrafts. 

Dense fog will continue into the early evening hours across the
nearshore waters. Conditions may improve for a period this evening
as rain begins, but a rapid deterioration in visibilities may re-
develop and attempt to push inland overnight and into Thursday
morning with a potential for pockets of dense fog across parts of
the region. 



328 PM CDT

Friday through Wednesday...

Thursday: The surface low and shortwave trough axis responsible for 
tonight's rain is progged to be over north-central Indiana around 
12Z Thursday. The back edge of showers should be shifting east of 
the I-39 corridor at this time, then gradually edge eastward through 
northwest Indiana by early afternoon. With the axis of heavier 
showers likely still over NW Indiana for a few hours after daybreak, 
will continue the Flash Flood Watch through 15Z. Clouds should 
briefly scatter west of the showers in the morning hours before a 
healthy cumulus field develops during the afternoon. Most high-res 
guidance attempts to initiate an isolated shower or two during the 
afternoon where enough heating behind the departing system builds 
convective cloud depths. As has been the case this month, guidance 
appears to be a bit too high on surface dewpoints, so may be a 
little aggressive on producing sufficient cloud depths to generate 
precip. Have therefore kept the forecast dry, but would not be 
surprised to see an isolated shower or two in the afternoon across 
primarily the west half of the CWA.

Thursday night and Friday: Dry weather is expected through much of 
this time as the transition to an active set-up for the weekend 
begins. However, the initial stages of WAA may be ongoing across the 
southern CWA Friday afternoon. This may be enough to generate a few 
showers and possibly an elevated storm or two, though chances do not 
look overly high.

Friday night through Monday: An active period of weather is expected 
during this time as several disturbances, some convectively 
enhanced/induced, round an amplifying mid-level ridge across the 
lower half of the Mississippi River Valley. The overall message of 
the potential for multiple rounds of thunderstorms, some severe, 
Friday night into Monday remains the same as an increasingly 
unstable air mass advects northward into the region. Additionally, 
high pwats up to 1.8" will support very heavy rain with any 
thunderstorms, increasing flash flood potential through the weekend.

Predictability in overall timing and intensity of convection remains 
low as convection on each day across the region and the central 
Great Plains will have significant impacts on convective evolution 
on the following day. However, the overall pattern combined with 
guidance suggests the first round of convection will lift NE toward 
the CWA Friday night into Saturday morning in response to 
substantial WAA and moisture transport on a 50kt low-level jet over 
northern MO and southern IA. Impressive steepening of mid-level 
lapse rates over this forcing will support robust nocturnal 
convection arcing from western IA into west-central IL in the 
evening. Modest 1-6 km effective shear around 30 kts along with 
decent MUCAPE within the steep lapse rates will support a risk for 
severe hail with any thunderstorm. It appears most of the 
significant convection should remain south and west of the CWA, but 
as the LLJ veers through the night, some severe convection may reach 
roughly the southwest half of the CWA through daybreak Saturday.

Uncertainty increases beyond Saturday morning as Friday night's 
convection may hinder the northeastward expansion of the warm sector 
through the CWA until late in the day and ultimately change where 
and when additional convection develops on Saturday. The forecast 
trend has been toward a slightly cooler Saturday with a decent 
amount of convective debris clouds around, but actual precipitation 
looks to be isolated to scattered at best given an overall lull in 
forcing. LLJ activity may then focus convection west of the CWA 
Saturday evening before veering and shifting storms into northern IL 
Saturday night. Additional rounds of storms will be possible through 
Sunday night until a trough axis clears the CWA during the day 



For the 06Z TAFs...

The Chicago area airports concerns for today are:

- Light to moderate rain ending in the couple hours after 

- IFR ceilings and MVFR visibility becoming prevalent this 
  morning, with IFR visibility in fog possible 10Z-14Z

- Winds 350-020 degrees with occasional gusts in the upper teens 
  through 14Z

A continued challenging aviation forecast. Low pressure moving
into and across central and northern Indiana early this morning
will eventually take the rain shield with it, but not until just
after daybreak at most TAF sites. Any IFR visibility reductions 
in rain will be temporary, however fog is possible as the rain
lightens and ends, primarily in the 10Z-14Z window. This will be
assisted by northeast winds off the lake, possibly steering in
developing fog from there. At the least, IFR ceilings are very
likely, most favored also after 10Z and prior to 16Z, though could
persist through early afternoon. The most likely solution on how
low clouds will drop is around 500 ft, but they could conceivably
come down to as low as 300 ft.

As the system pulls away, dry advection will be limited, and as 
such clouds will be slow to depart. IFR should lift to MVFR over 
Chicago this afternoon and eventual scattering and clearing of the
lower clouds. Confidence on an early afternoon time of lifting 
above the forecast 500 ft is low-medium. In addition, isolated 
light showers are possible again this afternoon, but mainly just 
for RFD.

As for winds at Chicago area airports, confidence in 350-020 
direction and around 12 kt in speed through the morning rush is 
high. Occasional gusts of 17-20 kt are likely. Confidence in 
specific wind direction diminishes some for the mid-late morning 
into early afternoon, but should generally stay within 350-020. 
Thereafter for mid-late afternoon, more of a 030 is favored.



IL...Flash Flood Watch...ILZ023-ILZ032-ILZ033-ILZ039 until 7 AM 

IN...Flash Flood Watch...INZ001-INZ002-INZ011 until 10 AM Thursday.

     Flash Flood Watch...INZ010-INZ019 until 7 AM Thursday.