Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240521

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1221 AM CDT Thu May 24 2018

242 PM CDT

Through Thursday...

High pressure has made for nice afternoon across much of the region,
with readings away from the lake into the upper 70s to low 80s. The
lake breeze boundary which has been somewhat diffuse, is still
noticeable on KLOT radar and is making inland progress, with
noticeable cooling to around 60 degrees at the lakefront, and around
70 in the Chicago loop. Expect some additional cooling inland with
the lake breeze before it washes out tonight. Winds ease and go
variable tonight, with some fog possible closer to shore and
possibly inland around any river valley locations.

The warming trend will commence a bit more on Thursday as the
upper level ridge will shift overhead and surface high pressure
will move to the eastern Great Lakes. This will enable more
southerly flow across the area, allowing temperatures in most
areas to get into the 80s. Southeast surface winds will keep the
IL lakefront cooler again, with areas in northwest Indiana warming
at least a longer period of offshore winds. Warm advection and
forcing remain west of the area during the day, but we will still
see some passing higher clouds.



222 PM CDT

Thursday night through Wednesday...

The very warm conditions will be the main weather story going
into the holiday weekend as a much warmer airmass shifts over the
area in the wake of the eastward departing surface high.

Overall, it appears that temperatures will warm well into the 80s
Friday through Sunday, with temperatures on Sunday possibly
getting into the 90s. Additionally, considering surface dew
points will likely be into the middle 60s, heat indices into the
mid 90s will be possible, especially on Sunday. Cooler, but
pleasant conditions then look to arrive for Monday as a cold front
shifts down the lake Sunday night. This should result in the
coolest weather (highs in the 60s) near the lake, and low to mid
80s inland on Monday.

While it appears there will be a lot of dry time across the area
through the weekend, there will be some chances for
thunderstorms, particularly late Friday and Saturday. While the
upper-level pattern will feature a building ridge and warming
atmosphere across much of the central CONUS on Thursday, guidance
is in good agreement with forecasting height falls aloft and
flatting the upper ridge across the north central CONUS Friday
into Saturday. This is in response to a disturbance expected to
shift eastward across the Northern High Plains Thursday afternoon
and evening. The disturbance is likely to spawn showers and
thunderstorms across the Upper Midwest on Friday as it interacts
with a diurnally destabilizing atmosphere. While much of this
activity looks to be to our north and west, I cant rule out some
of these storms impacting portions of northern IL late Friday.
Some of these storms could also be strong, but again the main
focus may remain just out of our area. Some additional isolated
storms will also be possible on Saturday.

Next week it appears there will be some quiet weather for a couple
of days as another surface high sets up over the Great Lakes
region. This should set up onshore northeasterly flow and cooler
conditions early next week. The next chance of storms after this
weekend will be late in the period (by the middle of next week) as
the pattern looks to get a bit more active over the region again.



For the 06Z TAFs...

High pressure centered over the Great Lakes will result in light
south to southeasterly flow and VFR conditions through the day
Thursday. Expect mostly clear skies with only some passing high
cirrus. Only area of uncertainty is with winds associated with a
lake breeze Thursday afternoon. Some models show a lake breeze
forming with winds backing to the SSW (around 200) ahead of the
boundary. Other models maintain a light S/SSE winds through the
afternoon. Winds will back to the SE behind the lake breeze during
the late afternoon/evening hours, then gradually veer to the SSW



222 PM CDT

Dense fog continues to be the main weather story over the lake at
this time as surface high pressure dominates across the lake.
Unlike yesterday, we can see the fog rather well on visible
satellite imagery. While it is not over the entire lake, there is
quite a bit over the open waters of the lake, including the
northern half of the lake. For this reason, we have including the
north half of the lake in the advisory and are running it through
Thursday morning. Forecasting the ending of this dense fog is
always difficult, since the water temps in the open waters is
still so cold. However, it does appear that southerly winds will
begin to increase on Thursday (10 to 20 kt) as the surface high
shifts east. As a result, this may end up pushing any lingering
fog to the far northern portion of the lake on Thursday.

Southerly winds are then expected to continue into the weekend,
albeit light. A cold front then looks to shift southward down the
lake sometime late Sunday through Sunday night. Northerly winds
are then expected early next week in the wake of this front. This
may result in a bit higher waves on the southern end of the lake
by Monday.



LM...Dense Fog Advisory...LMZ777-LMZ779 until 10 AM Thursday.




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