Area Forecast Discussion
Issued by NWS Chicago, IL

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017
FXUS63 KLOT 260745
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 AM CDT Wed Apr 26 2017

.SHORT TERM...
245 AM CDT

Through Thursday...

A bit more active day is expected today across the region with a
series of upper level lows spinning across the plains with heavy
rain a considerable concern, along with some risk for severe
weather especially later today/this evening.

The presence of altocumulus castellanus clouds Tuesday afternoon
is well understood looking at the DVN/ILX 0z soundings. While
these are still very dry downstairs they depict the presence of
the elevated mixed layer with 700-500 mb lapse rates very steep,
8-9 deg/km but very high based above 700 mb. Instability therefore
was pretty small, but weak shortwave energy was able to tap into
this to trigger a few storms across northwest Indiana Tuesday
evening. The SGF sounding, indicative of the airmass to some of
the area headed our direction, depicts 1000 J/KG of cape and near
50 kt of deep layer shear. This coupled with -20C height of about
21K ft suggests that there are several thunderstorms concerns
later today.

Water vapor satellite depicts a fairly strong wave across
Oklahoma with some weaker waves ahead of it, and these will
continue northeast through north central Illinois this morning
where the best t-storm threat would be. Current showers to our
west are in a minimum of mid level lapse rates and thus no
thunder. RAP forecasts of instability creep up as this stronger
wave well southwest shifts in the coming hours, but does not get
too high with this wave. The airmass is still on the drier side
with this lead wave, so expect a decent cut off to the eastern
extent of thunderstorms this morning, but expect there could
certainly be some high based showers farther east as well. There
could be some hail with the stronger storms, but the main storms
should be through before instability increases to the levels of
severe concerns. Expect some weakening as the low level jet
wanes through the morning with a downtick likely.

More significant height falls will occur this afternoon as the
upper lows spread eastward, the upper level jet will also undergo
some strengthening indicating a system getting stronger, and
providing good forcing for ascent ahead of an advancing cold
front. This forcing will come atop and increasingly unstable
airmass mainly due to advection, but it will limited farther
north and especially west of I-55 due likely significant cloud
cover which could keep convection coverage less initially. Plus,
there are some weaker lapse rates aloft that will creep in from
the west across Illinois this afternoon and some stability with
the approaching cold front which will mitigate the severe threat
for areas northwest, but these do look to recover some later. Still will
need to watch any leading cells that remain discrete as they could
pose a localized severe threat.

The moisture transport kicks into a bit of higher gear tonight as
instability is waning, though instability will get nudged up some
at least before late evening, especially along the I-55 and
eastward corridor. This is the period where any line/bowing
segments lifting up from likely more active convection to the
south pose a more significant severe concern, as when this occurs
the low level shear really increases. The low level shear/helicity
values get fairly large as well. Instability will weaken later
this evening as we trend to a heavy rain threat as soundings
depict a very deep and saturated profile with PW values from
1.3-1.5" for several hours but little much less instability. There
will also be a very strong frontogenesis with the encroaching
cold front that it could really dump rain for a few hours. This
may end up for many areas being a more significant concern than
the severe threat. Overnight the upper low will still shift
through with some lingering fgen, so in spite of the cold front
stabilizing things overnight, modest rains could still fall late
into early Thursday in a band farther west, with some showers
underneath the low itself that could still have a little kick.
Fortunately the dynamic cooling does not look like enough to get
over to a snow profile (had to find a way to sneak snow into this
AFD) as hinted at by a NAMBufr sounding near RFD early Thursday.

The moisture axis and low will shift north in the afternoon ending
precip as much colder and drier air filters back in from the
west/northwest.

KMD

&&

.LONG TERM...
245 AM CDT

Thursday through Monday...

The concern for the weekend will be heavy rainfall and periods of
thunderstorms. The lowest GEFS ensemble member from 0z suite has
2.25 inches of QPF for ORD with Wednesdays and the weekend
precipitation, so this certainly indicates flooding concerns
exist, including a re- aggravation of rivers that have just
recently subsided.

The northern stream upper low will linger across the northern
Great Lakes Friday with a massive longwave trough enveloping much
of the country, but there is a small ridge axis that will setup
across the upper midwest ahead of developing low pressure out
west. Southwest flow ahead of the developing low will send waves
of precipitation through at times until this upper low really
gains some strength and eventually shifts northeastward through
our region. The first wave appears to be on Friday with a leading
warm advective wing. Precipitation could be initially light, but
NAM/GFS project a strong and strengthening upper jet with a
northward moving warm front to lead to some better rains, though
the locations are a bit hairy to project and could split us a bit
north and south. At this point thunder looks like a minimal
concern.

The northward moving front gets shoved south by the high pressure
area mentioned earlier. This will keep the area locked in a
cooler northeast wind pattern and could keep precipitation on the
lighter side on Saturday. Confidence is not high though as the EC
is still somewhat wet Saturday afternoon, but is possible the high
to our north keeps the better activity south.

The big issues appear to come Saturday night into Sunday. This
period holds the highest concern for very wet conditions and
the aforementioned flood concerns with significant warm advection
induced precip beginning Saturday night, which could the wettest
period, but alot to still iron out here. It will also be windy at
times, with thunderstorms re- entering the picture as well. More
to come on this once we get through Wednesday`s active weather.

KMD

&&

.AVIATION...
For the 06Z TAFs...

1235 am...Forecast concerns include thunderstorm potential later
this morning and again this afternoon into this evening...wind
directions this afternoon into this evening and cigs this
afternoon through tonight.

A fairly complex pattern is setting up across the region for the
next 36 hours or so. Low pressure over eastern IA will lift into
WI early this morning. A band of showers and possibly some thunder
is expected to move across the terminals after sunrise.
Instability is limited during this time period and confidence on
thunder coverage is low. A second low pressure over OK early
this morning will move to northeast IL this evening. Additional
showers and thunderstorms are expected to develop ahead of this
approaching low during the mid/late afternoon hours. While
thunderstorms are expected...only medium confidence on both timing
and location. Storms could be later than current 22z timing and
potentially be a bit further east...perhaps gyy/ikk/pnt. Trends
will need to be monitored later this morning. As the low
approaches a period of moderate to perhaps heavy rain is expected
during the mid/late evening...with lingering showers into early
Thursday morning.

South/Southeasterly winds around 10kts overnight are expected to
turn more to the south/southwest by mid morning. Confidence is low
regarding how much speeds/gusts will increase this morning with
cloudy skies potentially limiting mixing. Gusts into the mid to
perhaps upper 20kt range will be possible. Wind directions from
late this afternoon through mid evening will be problematic since
the low is going to move right over the Chicago terminals. Some
shift to southeast or east seems plausible but if the low were to
track just a little further east...winds would quickly turn
northerly and then northwesterly. Changes...perhaps large changes
to the wind forecast are possible with later forecasts. Eventually
winds will turn more to the northwest late this evening into early
Thursday morning.

Cigs will gradually lower through vfr today with a period of mvfr
possible with showers this morning. The mvfr cigs may continue
across northwest IL/rfd today. Cigs will then lower through mvfr
late this afternoon and into ifr this evening or overnight as the
low approaches. Only medium confidence on cig trends. cms

&&

.MARINE...

220 am...One area of low pressure over eastern IA will lift
northeast into WI this morning as it slowly weakens. A trailing
cold front extends to a second area of low pressure over the
southern plains. This low will move northeast over Lake Michigan
tonight. Southerly winds today will shift more southeasterly this
evening and then west/southwest Thursday as the low continues
moving into Ontario. Small changes to the track of the low will
have large impacts on wind directions. Thus...changes to wind
directions are possible with later forecasts.

A weak ridge of high pressure will move across the lakes region
Friday and remain nearly stationary across the northern lakes
Saturday...as high pressure then builds across Ontario. Low
pressure will develop over the southern plains and eventually
lift northeast to the western Great lakes Sunday night. The
gradient between the high to the north and this approaching low
will slowly tighten with northeasterly 30kts expected and the
potential for some low end gales...especially over northern Lake
Michigan by Sunday evening. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 11 PM Wednesday.

&&

$$

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