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Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
1029 am CDT Thu Jun 21 2018

Update...
1023 am CDT

Moderate to heavy rain continues in a corridor of northeast
Illinois, as part of a larger area rotating around the broad
closed low centered across the Iowa/MO border region. The rain in
northeast Illinois has been around 0.75" per hour with isolated
over an inch rates. Those are rates consistent with the tropical-
like atmosphere analyzed this morning, and with recently wet
conditions these are very close to flash flood criteria rates. Am
concerned with a heavy rain area moving north-northeast out of
far southern Cook County as of 1020 am, as that could push a few
basins over. It is moving fairly quickly though which is why have
not issued at this point. We have received a handful reports of
minor / flood advisory flooding, and will be watching for a
potential issuance of a Flash Flood Warning for Cook County,
especially if that aforementioned heavy rain area slows.

While rain looks to continue in this axis beyond this into the
afternoon, there continues to be signs that it will ease in
intensity. While leaning that way, any rain still may be enough
to continue at least minor flooding issues through the afternoon.
Attention later today may temporarily shift to the southwest County Warning Area
as the surface low dumbbells its way into central Illinois.
Already an axis of modest cape, including over 100 j/kg of 0-3km
cape, that has developed across western and central Illinois in a
clearing swath. Scattered convection has developed in the
Iowa/Missouri/Illinois border region, and could see some low-
topped supercells approach the southwest forecast area. Convective
winds and a tornado or two would be the threats mid-afternoon
into early evening.

This convective area may set the stage for the heavy rain
potential tonight as it moves northward/expands into the County Warning Area.
Still some variance on guidance on where exactly that may set up.
No changes to the Flash Flood Watch at this time.

Mtf

&&

Short term...
330 am CDT

Through Friday...

Slow moving upper level low pressure in a relatively high
precipitable water environment will make for messy couple of days
for some areas across our region. Expect cool and wet conditions,
with areas under and closer to the low getting breaks/showery
weather, and areas farther north potentially getting stuck in
extended periods of rain with embedded thunder. Heavy rain and
localized flooding appear the main concern, and with low flash
flood guidance due to wet grounds, flash flooding may become an
issue for some areas as well.

The 0z ilx sounding depicts the moist and relatively warm airmass,
with a 1.89" precipitable water and 13,500 ft freezing level. This is a good
environment for efficient warm rain processes. Instability does
look to remain a bit more tempered the next few days with all of
the cloud cover. But with even lower instability, embedded thunder
remains possible which will enhance the warm rain processes.
Certainly with more instability this becomes even more concerning.

The first wave will come through the area this morning into early
afternoon. These will certainly have heavy rain and maybe some
localized flooding potential, but this axis may be a bit more
progressive than what may be coming later tonight. Areas south of
I-80 may get in on some breaks today, and as such will
destabilize a bit more. As the upper low continues to shift east,
additional shortwaves will pinwheel ahead of the low. Pnt forecast
soundings depict up to 1000 j/kg of cape and 25 kt of shear with
decent low level lapse rates supporting some localized stronger
wind gusts in our southern counties. Mid level lapse rates are
weaker, thus the severe coverage does not appear to be that large,
with instability dropping off farther north, but it appears that
some stronger storms will bear watching across west central Illinois and
close to our area later this afternoon.

Tonight the upper low will build into south central Illinois. This will
setup and easterly feed into the area, and therefore storms will
likely train over similar areas wherever the higher Theta-E axis
on the back side of the system sets up. Model run accum quantitative precipitation forecast
certainly suggests many areas when all is said and done will see
in excess of an inch, with 3-4" totals not out of the question in
this setup. The challenge will be where this higher axis will be.
At this point areas along and north of Chicago are favored. Gefs
plumes for Ord have 1.2 to 4.5 inches of quantitative precipitation forecast indicating the
potential this system has for some areas, with mean currently
holding at 2.75".

With all that in mind, some type of flood message is warranted
given that rivers across northern Illinois are running high, storm total
quantitative precipitation forecast ranging from 1-5" both from raw guidance and gefs ensembles
(showing potential is there) will go out with a Flash Flood
Watch through tonight, that may need to be honed in location.
Rivers will likely rise after getting a good bump already from
recent rains. While rates may not be at The Levels for flash
flooding or all areas (and the potential for some initial
breaks), the potential is there, especially if these get over more
urban areas.

Kmd

&&

Long term...
329 am CDT

Friday through Wednesday...

Not too many changes to the blended model guidance given the
active short term weather. The upper low will pull away Friday
night, with the heavier rains easing Friday afternoon and even
more so in the evening. Zonal flow over the weekend and northeast
low level flow with no significant waves suggests somewhat cooler
but generally dry weather. Cannot rule out some rain showers at
times as there is some weak energy in the zonal/northwest flow aloft.

Ridging aloft and southeast flow suggests Monday will be dry with some
warming. Another upper low will cross the plains and arrive across
our region mid week, suggesting more inclement weather is
possible, depending on if the low moves through our area or if it
stays north. Beyond that time frame, there are hints of a heat
dome building in our near the region, but it is not a clean one
that may be muddled with clouds and occasional precip. Did nudge
temps up a tad closer to 925 mb climatology.

Kmd

&&

Aviation...
for the 12z tafs...

645 am...forecast concerns remain periods of moderate/heavy rain
and associated cigs/vis along with low potential for thunder.

Forecast trends appear on track as moderate rain is now developing
across eastern Illinois. This activity will lift north across the
Chicago area terminals this morning with periods of heavy rain
possible. Only medium confidence for how far northwest this
activity will extend into the rfd area. Not much thunder with this
activity and not expecting much but an isolated thunderstorm is
still possible so maintain vicinity mention for now but chances at
the terminals are low. Better chances for thunder will remain
across central Illinois and mainly this afternoon/evening. Timing any
lulls in the precip will remain challenging but there is still a
consistent signal for a lull later this afternoon into early this
evening with another area of moderate/possibly heavy rain late
this evening into the overnight hours. Confidence for timing and
location steadily decrease further out into the forecast. Showers
will likely persist into Friday morning.

Visibilities have lowered into the 1-3sm range even in areas of
little rain currently. Its possible vis remains 1-2sm for much of
the day but confidence on prevailing vis is low. Cigs are mainly
IFR/LIFR and should slowly lift through IFR and possibly low MVFR
later this morning. There will likely be some variability with
cigs as precip intensity changes. Cigs will likely settle into
low MVFR this afternoon if the lull in precip develops and remain
low MVFR for the rest of the period...with IFR possible.

Easterly winds will steadily increase this morning into the
12-16kt range with gusts possibly as high as lower 20kts from
late this morning into this evening. Winds will slowly turn more
northeasterly overnight into Friday morning with speeds expected
to remain 10-15kts. Cms

&&

Marine...

416 am...low pressure over Iowa this morning will move to central
Illinois Friday morning and then to near Lake Erie on Saturday. This
will allow an easterly gradient to steadily increase today into
tonight with a period of 20-25kt easterly winds expected on the
southern portion of Lake Michigan. Some higher gusts to 30kts are
also possible...likely on the far southern part of the lake. These
winds will turn more northeasterly on Friday and then diminish
Friday night as they shift northerly. Northerly winds will then
persist into Sunday when a cold front will move south across the
region. This front will maintain northerly winds...which will
likely increase in speed behind the front. High pressure is then
expected to move across the Great Lakes Monday and then steadily
shift east Tuesday. Cms

&&

Lot watches/warnings/advisories...
Illinois...Flash Flood Watch...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012-ilz013-ilz014-ilz019-ilz020-ilz021-ilz022
until 7 am Friday.

Beach hazards statement...ilz006-ilz014...7 PM Thursday to 4 am
Saturday.

In...beach hazards statement...inz001...7 PM Thursday to 4 am
Saturday.

Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744...7 PM
Thursday to 4 am Saturday.

&&

$$

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