Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240831

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 AM CDT TUE MAY 24 2016

329 AM CDT

Through Tonight...

Today and tonight will feature a gradual transition to a more
unsettled and humid pattern while continued warm. Forecast
adjustments for the short term were to refine hourly timing of
chances of rain/thunder...confining that mainly west of I-55 for
today and this evening. These are mainly low
anticipating a majority of the CWA to remain dry through midnight.

The positively-titled upper trough across the western U.S. is
inching east with the upper ridging over the area to be nudged
eastward by tonight. The associated surface cold front has been
just as slow...draped from the eastern Dakotas to the western
Plains and will remain the primary focus for developing convection
today as is creeps southeastward. As is often the case with semi-
blocked patterns such as this and no presence of significant
forcing...convection has a difficult time advancing much further
east than its trigger areas. Early this area of
showers and some storms associated with an MCV in central IA is
moving east-northeast. This should continue to fade as it outruns
its forcing...though a 40-45 kt low-level jet as sampled on the
DVN VAD profiler probably will continue some showers into far
northwest IL after daybreak...with a lesser chance of reaching
north central IL.

A warm day is again on tap with 850mb temps again of 13C to 15C.
Surface winds will be more southwest today and steer in higher dew
points likely to reach the lower 60s by late today in north
central IL and elsewhere later this evening. This may make the
air mass slightly more difficult to warm and some clouds west of
I-55 will likely keep readings a few degrees shy of yesterdays.
Eastern areas should peak similar to yesterday with mid 80s.

While the front itself will remain well northwest of the CWA
today...gradually increasing convergence across eastern IA into
southern WI might be enough to spark late day/evening isolated
storms that could move into north central IL. Have maintained low
chances for that. While instability may peak in excess of 1000
J/KG...bulk shear remains low in the absence of any noteworthy
short the chance for anything severe is low.

Later tonight as a surface low strengthens in the western
Plains...warm and moist advection aloft will gradually increase.
It is possible some storms may develop in this regime...again
more favored across the western forecast area. It will be a mild
night with mid 60s forecast for lows...about 15 degrees above



329 AM CDT

Wednesday through Monday...

Southwest upper flow will prevail through this period keeping
temperatures above normal as well as multiple chances for storms.
Timing of these chances is fairly difficult...though it appears on
Wednesday that the afternoon and/or evening look to have the best
storm chances in nearly two weeks. The severe threat continues
low...greater than today/tonight...but still low. Have also
increased temperatures on Thursday which could see upper 80s to an
isolated 90 if sunshine prevails.

By Wednesday morning model solutions are in decent agreement of
the surface boundary being over southern WI or so...possibly lake
enhanced into far northeast IL. This is expected to lift northward
as a warm front with dew points in the mid 60s areawide...allowing
for MLCAPE values to peak around 1500 J/kg depending on just how
warm we reach /low to mid 80s expected/. The upper flow is
unsettled with a modest agreement in guidance of a low-amplitude
shortwave tracking over the area during the mid-afternoon to mid-
evening. This would be an ideal time for convective development if
it were to pan out. Shear values are much better further west of
the CWA and thus storm organization/intensity over our area may
struggle...but coverage still looks enough to warrant likely PoPs
in most of the CWA. Isolated severe storms with a brief wind/hail
threat could occur.

There appears to be a lull in much forcing during the day Thursday
with a capped environment likely temporarily be in place. Given
the nature of Wednesdays system...not expecting much for
lingering cloud Thursday could be quite mild. The 925mb
temperatures are forecast to be 20C to 23C. In the past 30 years
when 925mb temperatures have been this warm in northern Illinois
during late May...Chicago has had a mean high of 86 degrees. This
is why could see an isolated 90 or two for highs mainly in north
central IL if uncontaminated warming can occur.

Thursday evening and overnight the forcing for convection beings
to re-increase as the southern upper low of the western over CA/NV...progresses into the Plains. A lot of
uncertainty on how far east forcing will be Thursday Night but
a few of the storms could be stout if they do form over the area.
Chances for rain continue through Friday and again may be higher
west depending on the track of the upper low and its forcing.
While chances for storms are in the forecast daily through the
Holiday is a pattern that resembles a summer
one...i.e. it is not a pattern for widespread rain and refinement
in timing/details will improve as the week moves along.




1210 am...Main concern this period is potential for thunderstorms
which appears to be mainly for rfd and far northwest IL. High
pressure remains over portions of the Ohio Valley and has slowed
the progression of thunderstorms over the plains. Models continue
with this trend suggesting that much of the area will be dry
through late tonight with the exception of possible convection
firing from eastern IA into southern WI later this afternoon into
this evening. If this were to timing for rfd will
likely be centered around 00z. Still fairly low confidence but
opted to maintain prob30 mention and adjust hours to currently
expected best timing. For the rest of the terminals...confidence
is low and expected coverage of anything that may develop is also
low and have continued dry forecast. There may be additional
isolated/scattered activity late tonight into Wednesday morning
but confidence here remains low.

Light south/southeasterly winds will turn more southerly or south/
southwesterly by morning. Wind speeds then increase into the
10-15kt range by mid/late morning and there could be a few higher
gusts at times. Low level winds currently in the 30-40kt range but
forecast soundings suggest these will diminish some by sunrise and
shortened duration of low level wind shear. cms



125 am...A weak cold front will slowly move southeast across
northern Lake Michigan late tonight. This front is expected to
become stationary across central portions of the lake by midday
Wednesday as low pressure begins moving north across the plains.
This front will then lift back north as a warm front Wednesday
night. As low level moisture increases across the region and
spreads over the cooler water...some fog is possible. A large
trough of low pressure is expected to persist over the plains
through early next week with prevailing southerly winds over the
lake. cms





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