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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
909 PM CDT Thu Jun 21 2018

908 PM CDT

No changes to going forecast this evening. Evening satellite and
upper air analysis depicts closed low drifting southeast across
northeastern Missouri, with deep moisture wrapping cyclonically
north-northwestward across Illinois/in to the east of the low. Water
vapor imagery suggests a short wave embedded in the flow over
central Illinois at this time, which is progged to rotate cyclonically
around the upper circulation and across the forecast area
overnight. High res model guidance continues to indicate that
showers will increase in coverage across the area over the next
few hours, as this wave begins to wrap into the area from the
south. Central and northern parts of the forecast area appear to
be in line for the greatest coverage overnight per various
NAM/hrrr/rap based guidance runs, with the best chance of
thunderstorms and deeper convective updrafts generally along/south
of the I-80 corridor where northern periphery of MUCAPE gradient
is located north of the low level stationary front occlusion. Even
persistent shallower convection farther north will be efficient
rain producers however, with dvn/ilx 00z raobs depicting pw's
around 1.60 inches. Earlier weak convective rains were able to
produce 1-2 inch per hour rainfall rates. With widespread 1-2 inch
rainfall amounts and localized 3-4+ inch amounts already having
fallen in the past 24 hours, will maintain Flash Flood Watch into
Friday morning with no changes.

Updated text/digital forecast products already available.



Short term...
252 PM CDT

Through Friday...

Active weather is expected to continue across much of the County Warning Area over
the next 18-24 hours as period of heavy rain are still expected
through tonight.

An upper-level low center indicated by impressive satellite imagery
just southeast of Omaha this afternoon will finally begin to edge
east-southeast late this afternoon, passing near stl late this evening before
pivoting back NE to near ind by late Friday afternoon.

A feed of moisture-laden air represented by precipitable water values around 1.75"
will interact with a low-level frontogenesis/deformation axis across
northern Illinois late this evening into Friday morning. Upper forcing
associated with a vorticity arm curring causing convection across
central will enhance deeper-layer forcing for the activity
overnight. The potential exists for an associated band of heavy
showers and some thunderstorms to remain nearly stationary across
northern Illinois as the upper-low pivots across southern Illinois. The exact
location of where a potential band of heavy showers aligns overnight
remains somewhat uncertain, but observational trends and short-term
model guidance suggests a narrow heavy rain band may set up between
the I-80 and I-90 corridors, including Chicago and possibly
Rockford. An additional 1-2" with locally over 3" of rainfall remain
possible late this afternoon into Friday morning for the
aforementioned area. Because of saturated antecedent ground
conditions and the potential for heavy rain to continue into Friday
morning, have extended the Flash Flood Watch to 15z Friday morning
and expanded the watch to include lake and Porter counties in Northwest

In addition to the heavy rain potential tonight, a marginal severe
weather risk exists for brief tornadoes generally SW of a line from
Dixon to Morris to Rensselaer into early evening. This case is a
hybrid cold-air funnel/non-supercell tornado set-up, with only the
potential for a weak tornado or two. Clearing has allowed MLCAPE
values to rise to near 1000j/kg across extreme SW portions of the
County Warning Area. Modest southeast flow at the surface has resulted in effective low-
level shear of 10-15kts. With local heights of 500-750m, conditions
are favorable for a brief touchdown with any stronger storm rooted
in the boundary layer. Upstream reports of funnel clouds in central
Illinois support this notion.

Given the slow progression of the upper-low to the east on Friday,
scattered showers should continue across much of the forecast area,
especially NE Illinois and Northwest Indiana, through much of the day.



Long term...
309 PM CDT

Friday night through Thursday...

Friday night and saturday: the closed low causing heavy rain today
and tonight will continue NE to near Detroit by late Saturday
morning. A few showers will likely linger into Friday night. Much of
the area should remain dry through the day on Saturday, but weakly
cyclonic flow combined with lingering low-level moisture will be
supportive of widely isolated light showers.

Sunday: long-wave troughing shifting southeast from northern Canada
will brush the County Warning Area to the NE as a convectively-induced shortwave
tracks eastward across Iowa. Interaction between these two troughs
appear less likely now than guidance indicated on Wednesday as the
longwave trough moves through earlier during the day. An isolated
shower or thunderstorm still cannot be ruled out depending on the
evolution of the shortwave tracking eastward toward the County Warning Area.

Sunday night through thursday: high pressure passing NE of the area
will bring a period of dry conditions under generally NE winds
Sunday night through at least Monday night. Deep troughing over The
Rockies will then bring increasing chances of precip Tuesday
afternoon into Wednesday.



for the 00z tafs...

Off-and-on showers will continue throughout much of the forecast
period as an upper low continues to slowly rotate across the
region. In the immediate near term, expect a relative lull to
continue at the terminals over the next couple hours with only
some very light rain and drizzle, but another band of moderate to
heavy showers is expected to pivot across northern Illinois late
this evening and overnight. Heavy rainfall rates are expected, at
times, and should be able to achieve IFR visibility within the
heaviest showers. Also cannot rule out a few thunderstorms in the
region, but thunder coverage should be low enough to preclude any
taf mention at this time. Threat for showers should begin to wane
at the terminals mid Friday afternoon as the upper low starts to
shift east into Indiana. Away from the influence of any heavier
showers or storms, winds should remain fairly steady state out of
the northeast through the period with gusts into the high teens to
around 20kt. Generally expect IFR ceilings through the period,
though may bounce between LIFR/MVFR at times as well.



309 PM CDT

An upper-level low pressure over the central Great Plains today will
drift from Missouri to the eastern Great Lakes tonight through
Saturday night. Expect a persistent period of 15-25 knot north to NE
winds through the weekend. However, a period of winds to 30 knots
are possible tonight into Friday morning across the south half of
Lake Michigan. High pressure will then build across the Great Lakes
region Sunday night into Tuesday, bringing another period of north to NE



Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz006-ilz014 until 4 am Saturday.

Flash Flood Watch...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
until 10 am Friday.

In...beach hazards statement...inz001 until 4 am Saturday.

Flash Flood Watch...inz001-inz002 until 10 am Friday.

Lm...Small Craft Advisory...nearshore waters
until 4 am Saturday.



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