Frost and Freeze in Illinois, April 19 to 21, 2021

Area Forecast Discussion
National Weather Service Paducah KY
305 PM CDT Mon Apr 19 2021

.SHORT TERM...(Tonight through Wednesday night)
Issued at 305 PM CDT Mon Apr 19 2021

A dry cold front will move through our region tonight and settle
just southeast of the PAH forecast area. This will cause
temperatures to again drop to below normal readings from the upper
30s to lower 40s. On Tuesday, low pressure will move along the
front as an upper level trof moves over the middle Mississippi and
lower Ohio valleys. Model blends indicate scattered showers will
be possible by late Tuesday morning across our west and northwest
counties, with numerous showers spreading across our entire
region by late Tuesday afternoon and continuing much of the
evening. Showers will begin tapering off from the west by mid
evening, with showers ending in our eastern counties between 06z-
09z Wednesday. Our eastern counties could see some snow mix in
before ending, but it will not be anything significant.

As the showers end and skies begin clearing, colder air will be
filtering in from the west. At this point, the best chances of
freezing to just below freezing temperatures will be across
southern Illinois and most of southeast Missouri, and this is
where we have a freeze warning in effect from 06z-13z Wednesday.
Low temperatures may tie or break some records.

Chilly conditions can be expected Wednesday after our cold start
along with northwest winds, although at least we will see some
sunshine. High temperatures will be 12 to 16 degrees below normal,
with readings on reaching the lower to middle 50s.

For Wednesday night, models show a surface high moving overhead.
Forecast relative humidity time heights show some potential for
some low clouds, so despite calm winds, it does not look like
ideal cooling conditions at this point. However, we should see
temperatures drop into the lower to middle 30s. With dew points
within a couple of degrees of these values, frost remains a good
possibility. We will just have to wait and see where the future
guidance takes us.

.LONG TERM...(Thursday through Monday)
Issued at 305 PM CDT Mon Apr 19 2021

By Thursday morning, there will likely be quite a bit of frost
across the area, as a large area of surface high pressure will be
centered just to our west/northwest by 12Z. The center of this high
will slide southeast with time on Thursday and be southeast of the
area by Thursday evening. The low level flow will transition
Thursday night into Friday to become more southerly/southwesterly,
as this occurs. Meanwhile, an upper trough will steadily make its
way eastward across the central/southern Plains states on Friday.
Upper level moisture will be increasing across the area Friday with
the deeper moisture arriving late Friday night into Saturday.

While the NBM has generated some low PoPs coming into the area as
early as late Thursday night and into the day on Friday, believe the
most optimal rain chances will arrive Friday night into Saturday
when the lower levels moisten back up and recover from the unusually
dry air mass that will move in mid week. Due to many of the
GFS/ECMWF ensemble members hinting at incoming precipitation for
Thursday through Friday, seems prudent to leave the low PoPs
inherited by the NBM. Later on though, NBM PoPs pick up on this more
optimal time frame for rain as PWs peak, with PoPs in the likely
category, so minimal changes needed. This upcoming system is rather
progressive and models have a good beat on when it will exit the
area, with precipitation ending from west to east Saturday afternoon
or evening.

Sunday and Monday should be dry with warming temperatures, as an
upper level ridge builds in for early next week. In fact, after
Wednesday, high temperatures should gain several degrees each day,
ending on Monday with readings in the lower to middle 70s. This is
supported by the latest GEFS, which indicates a pretty high
probability of reaching over 70 degrees on Monday especially over
southeast MO, far west KY and far southern IL.

&&

.AVIATION...
Issued at 305 PM CDT Mon Apr 19 2021

Clear skies expected through much of the TAF period, with just
some high clouds approaching from the north. KMVN may have BKN
MVFR clouds by the end of the TAF period. Winds from the west to
southwest at 6-12 kts with gusts of 15-20 kts will decrease to
around 5 kts after 00z and become north with the passage of a dry
cold front.  No vsby restrictions.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Warning from 1 AM to 9 AM CDT Wednesday for ILZ075>078-
     080>094.

MO...Freeze Warning from 1 AM to 9 AM CDT Wednesday for MOZ076-086-
     087-100-107>111.

IN...None.
KY...None.
&&

SHORT TERM...RST
LONG TERM...CW
AVIATION...RST

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