Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 04/19/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 635 PM CDT Sat Apr 18 2020

.SHORT TERM...
245 PM CDT

Through Sunday night...

What a difference a day makes with full sunshine and upper 50s as
of 230 pm replacing the snowy ground and stratus of 24 hours ago.
As sort of a "gee whiz" fact for Chicago, today is the warmest
day immediately after a calendar day with 3+ inches of snow on
record for the city! That factoid even applies with 2+ inches.
There are actually only a few events that are even close, one of
which interestingly was last year (April 14 5.4 inches of snow,
April 15 52 degrees then April 16 73 degrees). So quite an
impressive and rare quick turnaround we have experienced these
past 24-36 hours!

Anyways, fairly quiet weather for this weekend apart from gusty
winds into this evening. We are firmly entrenched into the warm
sector of a quick-moving northern stream system at 988 mb in
Ontario. The 925 mb temperature on an early afternoon aircraft
sounding out of MDW was 6C and it is likely pushing 8C in the
west. This may be enough to reach 60 by late afternoon. Vertical
profiles are supporting mixing for the anticipated 40+ mph gusts
north of I-80, with Waukegan already having observed 43 mph. A
few gusts to 45 mph are certainly possible late this afternoon as
the low-level jet begins to both strengthen and move over the
area. While gusts may become slightly more sporadic after dark,
this type of gradient and overall setup often seem to support
modest winds continuing into the evening. Suspect that there will
be some 30-35 mph gusts even in the 7-10 p.m. time and maybe even
common depending on how temperatures and mixing hold up.

Satellite and especially radar imagery indicate this system is
moisture starved over the Upper Midwest. Gradually better upper
support will lend itself to more ascent over the northern and
central Great Lakes tonight, but the CWA will remain at the
southern periphery of this. Could see some sprinkles overnight
and early Sunday morning in northern and eastern parts of the CWA,
but see the chance for true showers being very low.

The system cold front will drop southward through the area Sunday
morning. This will turn winds off the lake by or near noon.
Temperatures should fall quickly into the mid possibly lower 40s
in downtown Chicago given the lake temperatures. Elsewhere inland
mid to upper 50s are favored.

A stronger southern stream system will be pushing into the Gulf
States Sunday and there is some influence on the baroclinic zone
into central Illinois and northern/central Indiana. This could
result in enough of a frontogenetic circulation for some light
rain to develop across the south, primarily Sunday afternoon.
Again do not see this being a big deal if it were to develop.

MTF

&&

.LONG TERM...
230 PM CDT

Monday through Saturday...

Following the cold frontal passage Sunday afternoon flow is
expected to quickly shift back out of the west-southwest for
Monday in response to a strong area of low pressure shifting
eastward across Ontario. Winds will likely be breeze again during
the day with some gusts up around 30 MPH looking likely.
Temperatures should once again top out around 60 degrees, and this
warmth should make it all the way to the lake shore with offshore
winds expected.

Another cold frontal boundary is forecast to shift over the area
Monday night as a potent shortwave digs over the Great Lakes into
Tuesday morning. This feature may result in some scattered light
showers over parts of the area late Monday afternoon and Monday
night, though most of the precipitation with this system is
expected to remain to our northeast in closer proximate to the
better upper level support. For this reason we continue to carry
some low end pops for this light shower potential.

A fast moving surface high will build in over the area in the wake
of this cold front for Tuesday. The main result will be dry but
cool weather across our area for the day Tuesday. Expect high
temperatures to be held down into the low to mid 50s inland, and
in the 40s near the lake.

Later in the week from late Wednesday onward we will have more
chances for showers from time to time as a few additional low
amplitude disturbances shift across the region. The first one of
note is a southern stream system, which looks to track eastward
across the south central CONUS on Wednesday night into Thursday.
While this system looks to remain mainly south of the area, there
will be some low threat of rain on its northern periphery getting
close to my southern CWA on Thursday. Additionally, there are also
some chances for showers, and possibly some thunderstorms, on
Thursday farther north into southern WI in association with a
weak frontal trough of low pressure. While our official forecast
continues to carry some low end POPs areawide for this period,
most of the area could fall right in between these two areas of
precipitation on Thursday. For this reason, I have continued the
trend of leaving the threat of thunder out of the forecast at this
time.

Another low amplitude disturbance looks to follow quickly on the
heels of Thursdays system for the end of the week. For this
reason, the chances for showers will continue into the end of the
forecast period.

KJB

&&

.AVIATION...
For the 00Z TAFs...

Aviation forecast concerns:

* Strong southwest winds gusting 30+ kts diminishing slowly this
  evening, but remaining gusty overnight.

* Non-convective LLWS developing later this evening, as surface
  gusts decrease, but winds increase to around 45 kts.

* Winds shift to northeast off lake midday Sunday behind a cold
  front.

Tight surface pressure gradient was in place across the Midwest
this afternoon, between high pressure south of the Ohio River and
deep low pressure pressing east across Canada. Tight gradient
combined with full sunshine and strong mixing has allowed for
strong southwest winds, gusting 30-35 kt. Though remaining gusty
overnight, winds speeds will decrease slowly after sunset this
evening. As this occurs however, winds in the 1500-2000 foot agl
level will increase to around 45 kt from the west-southwest and
will produce non-convective LLWS conditions above the somewhat
weaker surface winds. Winds will diminish during the predawn
hours surface and aloft as a cold frontal trough approaches from
the northwest.

The cold front will push southeast across the terminals
uneventfully Sunday morning, shifting winds to the northwest.
Moisture is pretty limited across the region, with VFR conditions
expected to prevail. Winds will likely shift northeast off of
Lake Michigan across the Chicago terminals midday. A period of
10-12 kt northeast flow is likely early afternoon behind the lake
breeze, before easing later in the afternoon and evening.

Ratzer

&&

.MARINE...
245 PM CDT

A Gale Warning is in effect to 10pm CDT for the entire Illinois
and Indiana nearshore. Fairly regular southwesterly gale force
gusts to around 35 kt should continue into early evening. Some
sporadic 40 kt gales are possible along the Illinois nearshore.
With strong winds aloft through the evening, there`s some chance
that the gale warning headline may need to be extended for a few
hours. At the least, expect hazardous winds for small craft
through much of tonight ahead of a cold front passage that will
shift winds to north and northeast on Sunday.

Castro/MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...nearshore waters until
     10 PM Saturday.

&&

$$

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