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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

                            
000
FXUS63 KLOT 241801
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
101 PM CDT Mon Jun 24 2019

.UPDATE...
1045 AM CDT

Continue to monitor thunder potential for the remainder of the
morning hours and then the potential for stronger storms with a
severe risk for the late afternoon/early evening hours.

Late this morning, a couple embedded vort maxs within a broader
area of isentropic lift have focused a couple pockets of embedded
thunderstorms within a greater area of rain and showers. Expect
that any thunderstorms through the remainder of the morning hours
will be sub-severe, though an occasional wind gust of 30-40 mph 
is not out of the question. The greater severe threat is still 
expected for the late afternoon/early evening hours associated 
with the main long-wave trough and sfc cold front cross the 
region. Modest mid-level lapse rates and adequate low-level 
moisture will be in place along with sfc forcing from the cold 
front convergence. However, have some concern over the whether 
there will enough clearing and impacts on sfc instability. Latest 
satellite imagery shows some clearing over ern Iowa/nwrn Illinois,
just ahead of one of the vorts over central Iowa. So, the 
potential remains for severe thunderstorms this afternoon/evening 
with damaging wind and large hail being the primary concerns.

&&

.SHORT TERM...
330 AM CDT

Through Tuesday...

Today through Early this Evening:
Main concern is the risk for strong to severe thunderstorms
capable of large hail and damaging winds during the mid to late 
afternoon and possibly into the early evening.

Early this morning, the area resides in the moist warm sector of
low pressure over northern Wisconsin, with temperatures and dew
points in the mid 60s to lower 70s. The primary feature of
interest that will drive the threat for thunderstorms this
afternoon is a stout mid-upper trough over the central and
northern Plains that will pivot eastward through this afternoon. 
This upper trough had unusually cold h5 temps in the minus mid to
upper teens Celsius analyzed on the 00z RAOBS, with tropopause
height possibly just below 500 mb.

Large scale ascent will increase as the short-wave trough
approaches this morning along with a anomalous for late June 
90-100 kt 300 mb jet streak nosing eastward. Anticipating that
there will be an uptick in showers this morning due to the
increasing large scale ascent interacting with moist air mass in
place. Through mid day, thunderstorm coverage should be low due 
to plentiful cloud cover and forecast soundings indicating minimal
instability.

Turning to this afternoon, there may be a small gap between the 
morning/mid day showers and the next round organizing. A dry slot
associated with the trough will wrap into the region, which could
allow at least partial sunshine to break out, along with southwest
winds gusting up to 30-35 mph. The core of the trough/upper low 
will be to our north, but cold pool of it will bring h5 temps down
to the minus low-mid teens Celsius over the area. This will bring
freezing levels down close to 10kft AGL. Upper 70s temps and mid 
60s Td with the unseasonably cool temp profile aloft would enable 
moderate instability up to 1500-2000 j/kg to be realized if there 
are sufficient breaks in the clouds. Conceptually, on the leading 
edge of the strongest ascent with the upper trough (with strong 
mid- level PVA), a broken arc of low- topped thunderstorms should 
blossom in the uncapped environment just to the west of the CWA in
the vicinity of the Mississippi River.

While there are uncertainties in this setup regarding how much
instability can be realized and coverage of thunderstorms during
the prime period of concern, there are things going for it in
support of the Slight Risk issued by SPC for a large portion of
the CWA. With seasonably fast flow aloft, effective southwesterly 
deep layer shear of around 35 kt (locally 40 kt) will support
storm organization. Given corresponding cold temps aloft/low
freezing levels, most intense updrafts, including isolated
supercells, could produce severe criteria hail, despite
equilibrium levels of less than 30kft. In addition, steep low
level lapse rates could support damaging downburst winds. With
limited directional shear, unclear how prevalent supercellular
structures are or if there's more mixed mode multi-cell,
supercell, and broken line segments. These storms will be moving 
at a fast clip off to the northeast at 40-45 mph, so duration of 
strong/severe thunderstorm risk in any given location should be 
fairly brief on the order of 2-3 hours. Approximate timing of 
severe risk from west to east is 2pm-7pm or so. The latest NAM 3km
Nest simulated reflectivity has a reasonable depiction of the 
expectations for today.

Mid Evening Tonight through Tuesday: 

Cold front passage will quickly follow the exit of anticipated
broken line of convection, which should shut most of the precip,
though can't rule out a stray shower while the trough is still
overhead. A cooler and temporarily drier air mass will move into
the area, along with clearing skies, with early Tuesday AM lows
close to seasonable upper 50s-lower 60s. 

Tuesday will start out pleasant and dry under close to full sun. 
It appears that return flow warm advection should ensue fairly 
quickly by mid day, though it's uncertain how quickly surface dew 
points recover and how high they get during peak heating. 
Southwest winds gusting up to 20-25 mph and 925 mb temps warming 
to the lower 20s support highs for most locations reaching the mid
80s, with isolated upper 80s possible. There is some concern for 
thunderstorms in the mid to late afternoon, as models are 
indicating fairly strong instability in 2000-3000+ j/kg range 
(though with aforementioned dew point uncertainty in how high dew 
points get which could limit these values) and little/no capping.
Big question is exactly what will trigger convection, with
possibility of a subtle wave in brisk west-northwest flow aloft.

Considering that there is a large variance in the models and
possibility of insufficient forcing to get convection to develop,
added only slight chance PoPs for now for Tuesday afternoon. 
There's a conditional risk for strong to severe thunderstorms with
more than sufficient northwesterly deep layer shear of 40+ kt and
moderate to strong instability. See the Long Term discussion for 
additional details on Tuesday evening/night. 

Castro

&&

.LONG TERM...

Tuesday night through Monday...

252 AM...Forecast concerns this period are convective chances 
and high temperatures.

A weak trough/cold front will move into the area Tuesday night
with a chance of showers/thunderstorms. Convective trends are
still quite uncertain Tuesday and how activity may trend into
Tuesday night. Current guidance trends would support activity
developing well to our southwest and then moving southeast. Thus
best potential may end up in the southwestern cwa. Any precip 
that does materialize in the cwa will likely be weakening/exiting 
early Wednesday morning.

Broad weak high pressure will settle across the area Wednesday
which is looking mainly dry but there could be some isolated
afternoon showers or thunderstorms in the far southeast cwa.
Westerly winds will be diminishing in the afternoon and with the
high overhead...a lake breeze is possible. Otherwise with plenty
of sunshine high temps should be well in the 80s.

The upper ridge will slowly build northeast across the region
Thursday into Friday with high temperatures warming in the upper 
80s and perhaps into the lower 90s as forecast by mex guidance. 
These temps look on track provided the area remains dry with sun. 
There does appear to be some potential for convection to develop
across MN and move southeast both Wednesday night and again
Thursday night before the upper ridge more firmly builds into the
western Great Lakes region this weekend. Have maintained low
chance pops for this uncertainty. Overall flow at the surface
looks weak during the end of the week and there should be some
lake breeze potential...perhaps remaining close to the lake if
they form. cms

&&

.AVIATION...
For the 18Z TAFs...

Forecast concerns include...

Thunderstorm potential for the afternoon/early evening. 
Strong/gusty winds afternoon/evening.

Patchy mvfr cigs in rain and isold/embedded thunderstorms continue
over far nern IL/nwrn IN in an area of broad isentropic lift in
advance of a long-wave trough aloft and associated cold front.
latest radar/satellite imagery shows the dense cloud cover
thinning and breaking up across the region, allowing some isold
shra/tsra development across the region. Expect that this activity
should remain relatively low coverage through much of the
afternoon. The greater chance for more organized and stronger
thunderstorms will be associated with a cold front sweeping across
the region. At issuance time, the cold front was moving through
swrn Wisconsin and ern Iowa with broken lines of thunderstorms
developing.  

Latest guidance continues to indicate that the progression of the
cold front will be relatively fast. Still anticipate that the
duration of storms directly impacting the individual terminals
should be relatively short, so will maintain a 2 hour tempo for
thunder.

Swly winds with gusts into the 20kt range will persist in advance
of the cold front. Winds will shift more wswly-wly behind the 
front with gusts increasing into the 25-30kt range. Winds should
slowly diminish through the evening.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters 
     until 1 AM Tuesday.

&&

$$

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