Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 101939
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
239 PM CDT Sat Apr 10 2021

.UPDATE...
115 PM CDT

Forecast remains mostly on track. Upper low is presently moving
into Missouri while the associated 995 mb surface low is centered
just north of St. Louis. This position is well south and west of
yesterday`s model run forecasts for this time frame. Rain and
showers cover most of Missouri, eastern Iowa, Illinois and
Indiana, with precipitation mostly moving towards the north-
northwest. There is presently a lighter intensity area of
precipitation pushing across the I-55 corridor, with this expected
to uptick in intensity into mid afternoon. In addition, there has
been an area of isolated thunderstorms that has moved from south
central Indiana almost due north, and is now located in north
central Indiana, reflecting an area of increase instability. With
the low still to our southwest, cyclonic winds are from the east-
northeast at 10 to 15 mph. Temperatures in most areas have
fluctuated very little this morning, although counties to the
southeast have warmed up into the 60s due to low level warm
advection and earlier brief clearing in that area.

Rain is expected to continue this afternoon into evening, with the
focus for the steadiest and occasionally heavy rain continuing to
be primarily along and west of I-39 as has been the case through
mid day. To note, Rockford has received just under a half inch of
rain in the past 6-hours, while Sterling impressively at 1.01".
As mentioned, the eastern half of my forecast region is weakly
unstable. A 700 mb vorticity maximum is moving through, combined
with warmer temperatures and 200 J/kg of CAPE, vs. less than 100
J/kg further west. So there is a chance of isolated afternoon
thunderstorms I-80 and south through about sunset.

Guidance continues to indicate up to a couple hundred J/kg of
MLCAPE advancing northward through mid afternoon and then getting
pinched off as cool and dry advection wraps in with the well
defined system dry slot. With solid vertical shear, a lower end
and low confidence strong to severe risk exists primarily in the
form of winds capable of causing minor tree damage (with a very
low brief weak tornado risk). The Storm Prediction Center
maintained the level 1 (marginal) risk for severe storms, but
confidence in occurrence is low, and should be limited in coverage
and duration if any strong storms do occur. We`re watching a fine
line of strongly forced showers sans lightning advancing
northward across central Illinois right on the dry slot, which may
be capable of producing gusty winds if it remains sufficiently
organized into our southern CWA.

Details for evening and overnight precipitation, and the
potential for areas of fog, some dense, will be addressed with
the full forecast issuance later this afternoon.

BKL/Castro

&&

.SHORT TERM...
256 AM CDT

Through Sunday...

Main Points / Forecast Changes:

- The storm system is expected to track slightly slower and to the
southwest compared to what was incorporated into the previous
forecast package

- Many hours may now end up dry through early afternoon, especially
east of I-39.

- A low-end threat for gusty showers remains generally south of I-88
between 3 and 7 PM

- A soggy night is in store with mist, drizzle, fog, and light rain
showers

Discussion:

As expected, a surface low pressure system is in the process of
consolidating in southeastern Missouri immediately upstream of an
incoming negatively-tilted upper-level trough. The consensus of
deterministic and ensemble model guidance is for the low to deepen as
it lifts northward into Illinois this morning, with a path modestly
slower as well as southwest of the track incorporated into the
previous forecast package. Curiously, there appears to be a
consistent signal that the low will "wobble" southwestward this
afternoon near the western bulge of Illinois, before finally lifting
northeast directly over northeastern Illinois tonight.

Based on the new track and speed of the low pressure system, rain
showers will arrive later than the previous forecast. Now, rain is
expected to start falling across central Illinois toward daybreak
and north-central Illinois toward the noon hour. Additionally, the
steadiest rain is now expected generally along and west of I-39 from
late morning onward, with scattered showers toward the east. This is
a somewhat sizable change compared to the previous forecast, as many
areas including the Chicago metropolitan area into northwestern
Indiana may be dry through early afternoon! For such areas, the best
chance of rain will likely end up along an arcing band of showers
forced by a northward-moving warm front mid to late afternoon. As
the low then passes overhead this evening, pockets of fog, mist,
drizzle, and light rain showers will spread over all of northern
Illinois and northwestern Indiana, making for a soggy night. Some
fog may become dense just north of the warm front and directly
beneath the low, as well as along the lakeshore where the relatively
moist airmass collides with the cool Lake Michigan waters. Showers
are then likely to continue into Sunday, with the greatest coverage
across northwestern and central Illinois.

In terms of the severe weather threat... the now forecast wobble in
the low pressure system track casts doubt on whether the strongest
kinematics will overlap with the narrow, northward-moving warm
sector, which itself will likely stall just south of the Chicago
metropolitan area thanks to northeast winds off Lake Michigan. Even
so, 0-1 km shear of 20-25 kt within a narrow zone of 250-500 J/kg 0-
3 km CAPE will likely support elongated and low-topped supercell
structures along the warm front. With in-cloud temperatures
potentially remaining warmer than -20 C due to relatively low
forecast equilibrium levels, there may not even be lightning within
the arcing band. However, localized gusty winds strong enough to
down tree limbs cannot be ruled out consistent with the Storm
Prediction Center threat level of 1. The greatest threat will be
from 3 to 7 pm and generally south of I-88.

Finally, concern for strong non-thunderstorm winds has decreased due
to the expected track of the low. While breezy northeast winds are
likely this morning generally along/north of I-88, and gusty
southerly winds are expected generally south of I-80 this
afternoon, winds will slacken this evening as the surface low
passes overhead. And as has been alluded to in previous forecast
discussions, the threat for flooding is quite low due to
antecedent dry conditions and relatively low river water levels.

Borchardt

&&

.LONG TERM...
239 PM CDT

Monday through Saturday...

Extended range continues to feature a blocky upper pattern, with
an elongated area of low pressure from the upper Mississippi
Valley to the lower Great Lakes region at least through mid-week.
Upper level ridging develops around Friday, before another upper
level trough develops into the region next weekend. Overall, the
week looks to feature temperatures near our seasonal averages for
mid-April, though with some lake-cooling for the metro area during
the latter half. A few chances will exist for some diurnal
showers, though no strong signal for widespread precip is evident
until the end of the period.

Surface low pressure associated with our weekend storm system
will be off to our northeast on Monday, and is expected to
merge/phase with another area of low pressure across the upper
Midwest associated with another mid-level short wave trough
tracking east from the northern Plains. Some re-deepening of the
surface low will help to maintain a fairly tight surface pressure
gradient across IL and IN, producing gusty westerly winds in the
35 mph range during the diurnally better mixed daytime hours. Cold
advection with the area of elongating mid/upper level low
pressure will also steepen up low level lapse rates, with some
forecast soundings (particularly the ECMWF/GFS) depicting some
potential for scattered showers across the northern half or so of
the cwa late Monday morning and afternoon. Temps look to reach
60/low 60s across our far east and southern counties, while areas
farther north and west are limited to the upper 50s where cold
advection is more effective.

The upper and lower level patterns remain similar Tuesday, before
the elongated upper low to our north eventually moves off to the
east Wednesday. Breezy west-northwest surface winds and some
limited potential for daytime sprinkles/showers continue, though
model soundings become more divergence with the depth of the steep
low level lapse rates. With somewhat shallower mixing, the shower
potential appears fairly minimal at this point and will maintain
dry forecast conditions during the mid-week period. Temperatures
continue to hang in the 50s/near 60 which is right about our
average for this part of the season. Winds do look to eventually
turn more north-northeasterly Wednesday into Friday, in response
to surface high pressure building north/northeast of the region in
the wake of the departing low. This will result in lake cooling
across northwest IN and northeastern IL, with daytime temps along
the shore likely limited to the upper 40s.

Medium-range guidance depicts some spread in the development of
upper troughing across the northern Rockies/northern Plains and
upper Midwest into this weekend. There is general agreement
however in height falls developing into the area Saturday, with
veering low level winds bringing some return of moisture to the
Mississippi Valley. Saturday thus appears to be the best potential
for more widespread light light rain.

Ratzer

&&

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

Aviation Forecast Concerns:

* Deteriorating cig/vis into IFR conditions this afternoon as
  rain spreads back across the terminals.

* Potential for period of lower IFR vis and LIFR cigs in light
  rain/drizzle and fog this evening and again later tonight.

* Slow improvement Sunday morning to IFR cigs/VFR vis with
  scattered showers lingering.

* Breezy northeast winds this afternoon become lighter and more
  variable in direction tonight, then northwest Sunday.

Midday surface analysis shows deepening low pressure over far
western IL. Broad warm sector with south-southeast low level flow
was spreading moisture northward across IN, while steadier rain
was occurring over western and northwestern IL. After a break in
lighter precipitation across the Chicago metro terminals this
morning, expectation is that more widespread rain will develop
across the area in the next few hours with ceilings and visibility
deteriorating into IFR range as cool northeast flow off Lake
Michigan is moistened by this precip. A period of heavier
convective showers is likely later this afternoon into early this
evening, as weak instability spreads into the area from the south.
Forecast soundings depict fairly weak "skinny" CAPE profiles, with
the instability generally shallow enough to preclude much in the
way of thunder especially farther north of the I-80 corridor. So
while an isolated thunderstorm may approach the Chicago terminals
later this afternoon, the coverage and certainty are low enough to
keep thunder mention out of the TAFs.

Inverted trough (occluding warm/cold frontal boundary) lifts into
the area by early this evening, with precipitation likely becoming
more showery. Decreasing winds, moist boundary layer and
convergence along this boundary appear to support the potential
for lower IFR vis and potentially LIFR ceilings for a time this
evening, with light rain/drizzle lingering. May see some
improvement for a time late evening as winds turn southeasterly
across the metro terminals, though confidence on exactly where
this boundary ends up (it will be in the vicinity with the center
of the surface low tracking northeast along it) is somewhat low.
Although much of the model guidance suggests the lowest vis/cig
conditions this evening, the weak gradient associated with the low
moving across the area would support LIFR conditions (especially
ceilings) lingering into early Sunday morning before winds turn
northwest and begins a slow improvement into low MVFR cigs/VFR
vis during the day. Scattered light showers/sprinkles will linger
as well.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Northerly Is. until 10 PM Saturday.

&&

$$

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