Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240052

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
752 PM CDT MON MAY 23 2016

750 PM CDT

Couple tweaks to going forecast. First, removed pops western CWA
late tonight. Evening soundings from ILX/DVN both showed a very
dry air mass in place across the region. Upper ridge axis is over
the thinking the activity in western Iowa should be
shunted north into MN and not pose much threat here overnight.
PWATs are progged to increase tomorrow as moisture axis flops
eastward, so threat for some sctd showers and thunderstorms will
will slowly ramp up by afternoon, especially western CWA. Grids
for tomorrow have been updated to lower pops some and also delay
arrival. Other tweak to going forecast was to lower low temps
tonight just a smidge over mainly eastern CWA. Dewpoints remain
very low and skies largely clear besides a bit of cirrus spilling
over top the upper ridge, so given the trend of the past couple
nights with lows really tanking in the dry air felt comfortable
dropping lows just a bit.

Updated forecast has been sent.



211 PM CDT

Through Tonight...

With a dry air mass and ample sunshine, temperatures across the
region are expected to rise into the lower to middle 80s this
afternoon.  A lake breeze boundary is slowly pushing inland this
afternoon and the IL lakefront never had the opportunity to warm up,
with the temperature at Northerly Island only reaching 64 F.  High
pressure centered over the eastern Great Lakes will continue to
slowly move to the east tonight as a cold front and associated sfc
trough approach the Middle and Upper Mississippi Valleys.  Low level
synoptic scale flow will gradually veer from Sexy to SWLY
overnight...bringing slightly higher sfc dewpoints and
increasing deep layer moisture into the WRN parts of the CWA by early
tomorrow morning, with the moister air reaching the I-39 corridor.
The increase in deep layer moisture will inhibit cooling overnight
tonight and min temps tomorrow morning should range from around 60 F
along the I-39 corridor to the lower to middle 50s over NWRN IN.
Showers and thunderstorms have developed over WRN IA/MO this
afternoon and should continue to slowly move to the east this
evening. A gradual increase elevated instability in the environment
ahead of the approaching sfc trough will keep the potential for some
convective activity to reach the Rockford area during the early
morning hours.  However...some of the guidance is suggesting that
there could be a decreasing trend to the convection during the
pre- dawn hours and then increase through the day tomorrow. So,
have maintained the increasing PoP trend during the late night
hours over the WRN portions of the CWA, but will cap PoPs in the
slight chance to chance range with a slight chance of thunder



219 PM CDT

Tuesday through Monday...

Orientation of current upper level ridge pattern will generally be
maintained for several days into the extended period as a SW
subtropical jet seen on WV imagery comes onshore near the Baja
Peninsula. Left exit region dynamics of this jet will help spawn low
pressure over the Central Plains and will be the main weather player
for the middle of this week.

The end of a 10-day dry spell will come midweek as low
pressure and a frontal boundary approach from the west on Tuesday.
Column saturation appears to take place quickly in the late morning
to afternoon...although favored areas for rain will be along the
front orientated NE to SW across the CWA. There is a small chance for
thunder to accompany the rain Tuesday...but convective indices are
very low-end. Showalter and lifted indices of -1 along with weak
effective shear and CAPE values under 500 J/KG will keep the thunder
threat very small and severe threat less than marginal values.
Warmth of 80F will continue during the mid-week as H85 temps
continue to warm-advect at 13-15c both Tuesday and Wednesday.

The cold front that moves through Tuesday night will continue to
bring precip chances. That front will lift back north as a warm front
on Wednesday. This will amplify the above listed indices to provide
a much stronger threat for thunderstorms and possible severe
weather. The warm H85 conditions will now be accompanied by an
effective moisture transport thanks to the 45kt LLJ. Expect
dewpoints in the mid-60s Wednesday which will help CAPE values rise
to above 2500 J/KG. Showalter and LI improve to -4C as well
Wednesday. 0-6km bulk shear remains at there is a limiting
factor to sustaining TS activity.

Warm SW flow continues on Thursday and the 850mb temps jump
to 18C. This also looks to be the one day with lower pops and sky
cover as NE IL remains in the warm sector ahead of another cold
front. These factors led to a 3 degree increase in the max temp
forecast to the mid-80s. Would not be surprised to see our first 90F
day of the season on Thursday given several hours of sunshine and
925mb temps of 24C. Unsettled weather continues through the weekend
as dewpoints stay up in the mid 60s with max temps near 80. There
will be a chance for convection each day with the main limiting
ingredient being just a trigger in the form of outflows...lake
breezes...or surface convergence.



For the 00Z TAFS...

Lake breeze should make it through ORD shortly after 00Z with an
expected wind shift to east before winds ease late this evening
and gradually become light south. Winds should pick up a bit by
mid-late morning Tuesday from the south with no lake breeze
expected at the terminals. Very small chance of shra/tsra tomorrow
afternoon...but threat appears too low to include in Chicago area
terminals. Did keep a PROB30 for RFD in the afternoon where
chances are best...though still pretty low.



219 PM CDT

South winds at 15-25kt continue across much of Lake Michigan through
tonight. A cold front will drop southward across the lake Tuesday
night and shift winds to a northerly direction. The front may stall
across the lake on Wednesday...offering convergent winds near the
south buoy. By early Thursday...the front should lift back north
as a warm front which will shift winds back to a southerly
direction. Thunderstorms will be possible each day across the lake
with the best chance being Wednesday into Thursday along the frontal
boundary. South winds are expected for several days into the weekend
with waves not becoming particularly high.






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