Drought in Illinois, June 1 to 30, 2021

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 AM CDT Tue Jun 1 2021

.UPDATE...
700 PM CDT

Made some tweaks to the forecast into Tuesday. Out the gate,
decent mid-level wave paired with modest frontogenesis and
saturation just low enough to the surface has allowed for light
non impactful rain from the I-80 corridor in LaSalle County up to
the south side of Chicago. This banded light precip will gradually
sink south and east through the night and likely see coverage decrease.

The next change was to add in patchy fog overnight into early
Tuesday. Satellite trends suggest that the mid clouds draped
across the area should yield to a mostly clear/partly cloudy sky
near and northwest of I-55 by the late evening. Given very weak
pressure gradient and diminishing dew point spreads, there's a
signal on the guidance for fog development. Suspect the high-res
guidance, especially the HRRR, is overdone on footprint of dense
fog. However, do think that some patches of sub 1 mile visibility
are possible, especially over north central Illinois.

Finally, made some PoP tweaks into Tuesday. Looks like the morning
hours should be primarily dry, with isolated to perhaps widely
scattered showers possible south of I-80 in the afternoon.
Anticipating more sun today, supporting highs in mid-upper 70s
away from the lake, where early afternoon lake breeze cooling
will keep readings in the 60s. There may be isolated convection on
the lake breeze north of the IL/WI state line, but kept a dry
forecast in far northeast Illinois.

Castro

&&

.SHORT TERM...
226 PM CDT

Through Tuesday night...

An upper level trough and surface boundary is approaching from the
northwest, producing southwesterly winds with occasional gusts up to
20 mph, with weaker winds to the southeast. At the same time an area
of virga and showers associated with a plume of warm, moist air is
moving up from the southwest, creating mostly cloudy skies. The
cloud cover has reduced our heating a bit today, but with southerly
flow we still reached into the upper 60s, and expect to reach highs
of around 70 this afternoon.

We have dry air up to around 12,000 ft over our region. As the moist
air moves up from the Gulf, we will see the area of virga
precipitating into this dry air. This dry column will shrink as more
moisture is pumped into it, so I expect some rain showers this
afternoon into early evening, mainly along and south of I-80.
Dynamic conditions are not good for thunderstorm development, as the
better energy will remain south of our region. The showers will
reduce in coverage once the sun sets, and the area of precipitation
will shift off to our southeast overnight.

The southern half of the watch area will remain mostly cloudy
overnight, while northern counties will see partly cloudy skies.
With little change in the atmosphere (850 mb temperatures change by
less than 1 degree C over 24 hours), low temperatures will once
again be in the lower to mid 50s, with cooler temperatures to the
north.

The overnight break in showers will last until sunrise, but we have
another chance of rain Tuesday morning as a low pressure approaches
from the southwest. Once again the chance for showers will initially
be south of I-80 throughout the day. By late afternoon the chance
for showers will increase and spread further north, up to and along
I-90 Tuesday overnight. Winds will back to the southeast but remain
light. With continued southerly flow and extensive cloud cover, the
low temperatures Tuesday night will be a bit warmer, in the mid to
upper 50s.

BKL

&&

.LONG TERM...
226 PM CDT

Wednesday through Monday...

There may be a few episodes of rain this week, but it does not look
to be any big rainfall, with some areas remaining dry through the
extended period. The most likely periods of any rain come early
in the period, notably Wednesday. Expect a warming trend too.

Wednesday through Thursday: A strengthening mid-level trough digging
into the central Great Plains on Tuesday in conjunction with a
loosely coupled upper jet structure will interact with a modest
surge of low-level moisture to spread an area of showers and
possibly some embedded thunderstorms northeastward through the Ohio
River Valley on Wednesday. While EC/Canadian ensemble guidance has
really focused the bulk of the precip southeast of our area, the
GEFS and NAM still supports some of this precip shield getting into
the southeast half of the CWA during the day. Spottier coverage is
still possible for the remainder of the area with steepening
lapse rates (though still modest) in the afternoon. This will be
followed by a secondary area of showers and a few storms with the
passage of the mid-level trough late Wednesday night into Thursday
morning before subtle upper height rises commence. This would
again be focused along and southeast of I-57. Afternoon coverage
of any precip on Thursday should therefore be low outside of a
stray shower or storm at peak heating.

Friday through Sunday: A strong ridge building into the western
CONUS early this week will drift east across the remainder of the
CONUS through the upcoming weekend, suppressing the northern jet
stream and more organized precip well to the north. Meanwhile,
consensus guidance indicates at least a portion of Wednesday's
trough will become cutoff across the southern Great Plains and
meander in all cardinal directions across the lower half of the
Mississippi River Valley through the weekend. The result will be an
overall trend to unseasonably warm conditions across the western
Great Lakes late this week into early next week. While there may be
some isolated diurnal precip chances each afternoon, a primarily dry
period is expected as signs point to generally insufficient moisture
return into the area due to the location of the southern low.

KMD

&&

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

Concerns for this period are the development of shallow fog early
this morning away from the core of the Chicago metro and timing
of the lake breeze at ORD/MDW this afternoon.

An axis of -RA from southeast IA to the southern Chicago metro
Monday afternoon/evening provided enough moisture to foster MIFG
development where upper-level clouds have thinned. While the
initial axis of MIFG is south and west of RFD/DPA/GYY, the lower
visibilities are expected to expand/advect NE with the background
light SW flow. MVFR visibility will be common through daybreak,
with IFR visibility and potentially LIFR visibility becoming more
likely in the pre-dawn hours though sunrise. At ORD/MDW, the added
warmth from the urban heat island should be enough to limit
reductions in visibility to MVFR levels. However, upstream trends
will be closely monitored in the event LIFR ceilings are still
able to form and/or advect into Chicago prior to or around
sunrise.

Any MIFG or low ceilings will quickly mix out after sunrise,
leading to VFR conditions through the remainder of the period. A
lake breeze is expected to shift winds ESE at ORD/MDW early this
afternoon, though a gradual backing of the winds may result in
light S/SSE flow by a couple hours prior to passage of the lake
breeze.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

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Hazardous Weather Outlook...UPDATED
National Weather Service Chicago/Romeoville IL
724 PM CDT Mon May 31 2021

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-020030-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
724 PM CDT Mon May 31 2021 /824 PM EDT Mon May 31 2021/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Limited Fog Risk.

DISCUSSION...

Patchy fog development is forecast overnight west and northwest
of a Pontiac to Lake Forest line. There may be localized visibility
reductions under a half mile in any thicker patches of fog.

.DAYS TWO THROUGH SEVEN...Tuesday through Saturday.

Tuesday Morning...
  Limited Fog Risk early.
Wednesday through Sunday...
  Limited Thunderstorm Risk at times.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

LMZ740>745-020030-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
724 PM CDT Mon May 31 2021

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

  No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Tuesday through Saturday.

Wednesday through Sunday...
  Limited Thunderstorm Risk at times.

Castro

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