Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270830

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 AM CDT Sat May 27 2017

.SHORT TERM...Through tonight...
329 AM CDT

Near term concerns center around cool and moist conditions with
pockets of clouds and even some fog and then switch back to
shower and thunderstorm chances overnight into Sunday.

A relatively moist air mass was left in the wake of yesterday`s
rainfall. With high pressure moving in briefly behind this system,
clouds have been slowly clearing, but given the low dewpoint
depressions, even lower clouds have and should continue to fill in
for many locations that lose the initial cloud deck given the
lighter winds. In the cloud breaks fog has been occasionally near
dense in spots, but generally after period of visibility
reductions the light ne wind component congeals this into a low
cloud deck.

Forecast soundings from the RAP/HRRR depict this moisture to be on
the shallow side and generally consistent with obs, and both
indeed show somewhat fast clearing/lifting by mid morning or so.
After the clouds and slow temperature climb, a seasonally mild
airmass aloft should translate to temperatures near or a tad above
normal, with 70s and even some low 80s south. Afternoon clouds
may hamper things a tad from upstream convection, and onshore flow
will keep the lake areas cooler yet again.

Eyes then turn to the endless parade of shower/embedded
thunderstorm complexes, the next one across KS/NE. While the
precipitation shield is fairly expansive, the better instability
lies along the southern portion of this activity. There is some
disagreement with how the northern portion of this complex will
behave between the hi-resolution cams keeping most of the action
attached to the higher instability axis as the warm front will
remain draped across our southern area. Meanwhile the larger
scale global guidance while consistent in keeping the
thunderstorms largely focused along and south of I-80, is in line
with the large scale pattern of warm moist ascent ahead of the
low and upper height falls suggests at least a period of showers
and some embedded thunder for many areas overnight. Confidence
given these inconsistencies gets lowered some, but the timing of
occurrence would be mostly after midnight with higher confidence.
Increased PWATs again suggest brief moderate to heavy rainfall.
There is a marginal risk in our southern areas for severe hazards.
mainly wind/hail given the proximity of the warm front.



307 AM CDT

Sunday through Friday...

Medium range guidance continues to depict that deep upper
troughing will dominate much of the Great Lakes and Northeast
through most of the period. A series of mid-level disturbances
are indicated from Sunday through about Wednesday night which will
present the potential for primarily diurnally driven shower and
thunderstorms through mid-week, before a weakening upper level
ridge shifts east into the region Thursday/Friday. Overall, the
pattern will support slightly lower than average temperatures
heading into the start of June.

Remainder of Memorial Day Holiday weekend...

Lingering precipitation/weak convection may be ongoing early
Sunday morning, especially across the south/southeastern parts of
the cwa in association with a low-amplitude short wave and area of
surface low pressure which develops into the area overnight.
Coverage and intensity should be on the decrease as this wave
moves out of the area during the morning, though a broad area of
height falls and large scale ascent is expected to be developing
in advance of the main upper trough axis which approaches the area
from the west/northwest during the day. Forecast soundings depict
fairly benign lapse rates initially during the morning in the
wake of the overnight action, though the combination of diurnal
warming and cooling of mid-levels through ascent associated with
the approaching trough should result in new convective
shower/thunderstorm development as we head into the afternoon. The
greatest threat of organized storms should be focused along/ahead
of the cold front moving out of the eastern parts of the forecast
area through early afternoon, where stronger instability will
exist. Forecast soundings from the WRF depict MLCAPE values
approaching 1000 J/kg in the far southeast during the afternoon,
tapering to 300-500 J/kg across north central IL with slightly
lower surface dew point temps behind the front. Deep layer bulk
shear values of 50-60 kts also shift across the southeastern cwa
during the afternoon, where SPC has maintained a marginal severe
risk for day 2/Sunday. Best focus for strong/severe convection
could end up southeast of the cwa defendant upon effective
convergent zone and outflow boundaries from Saturday night MCS
activity. Guidance generally brings the upper trough axis across
the area by/during the evening, which combined with loss of
diurnal instability should allow precip to dissipate after sunset.

Pattern persists Monday, with guidance indicating another short
wave propagating through the west-northwest mid-level flow south
of the upper low north of the Lakes. Forecast soundings depict
skinny cape profiles of a few hundred J/kg developing during the
afternoon, within a fairly deeply mixed boundary layer to 800-750
mb. In addition to likely supporting at least some scattered
afternoon showers and thunderstorms, this should produce breezy
westerly surface winds as wind speeds of 30-40 kts are evident at
the top of this mixed layer. Gradient itself is not overly strong,
so have held wind gusts to 30 mph at this time, but this may be
worth monitoring for the potential of more gusty conditions over
the next few model runs. In any case, breezy and mild conditions
are anticipated, with highs both Sunday and Monday expected in the
low-mid 70`s and with offshore flow negating lake cooling
especially Monday.

Mid and late next week...

Story remains the same as most head back to work Tuesday, though
with slightly warmer mid-level temps limiting the degree of
diurnal instability somewhat. Thus any diurnal showers and
isolated thunderstorms are expected to be limited to northern
portions of the cwa during the afternoon. Breezy westerly surface
winds should still hold off lake breeze development, with highs
from around 70 north to the lower 70`s south supported by model
thermal fields.

The upper trough then drifts further east on Wednesday, allowing
surface high pressure to gradually build across the region into
Thursday. Mainly dry weather is anticipated, with weakening west-
northwest winds eventually resulting in the return of lake breeze
cooling for the latter part of the week. Differences develop in
the various guidance by this part of the extended, particularly
with the amplitude of the upper ridge nudging east across the
Plains into the Mississippi Valley. GFS maintains a little sharper
feature, while the ECMWF flattens the ridge considerable and the
GEM erodes it and develops a trough by 12Z Friday. Thus confidence
in details does lower by the end of the period.



For the 06Z TAFs...

Concerns are on existing MVFR and IFR clouds, and then on
IFR/LIFR clouds and or fog expanding once the lingering clouds
erode. The later periods will then need to address showers and
embedded storms.

An area of lower clouds has begun to erode from the northwest, but
may slow its progression in the overnight hours and likely expand
now that we are in the overnight hours with moist conditions and
light northeast winds under the ridge. In addition where clearing
has occurred, dewpoint depressions are low and not expected to
climb, thus IFR or even LIFR clouds will likely return and/or low
to very low visibilities. Confidence is not high on how low and
how long conditions stay down but at least occasional reductions
if not longer and the pattern is favorable for low conditions of
some sort for many areas to start the day. What does form may be
initially slow to clear with the light profile, but then just
expect east winds with a lake breeze likely.

An area of showers and thunderstorms across the central and
southern plains will lift northeast tonight. Expect south of the
main terminals where the more active thunderstorms will be given a
significant gradient in instability from south to north, but
expect a period of showers and embedded thunder area wide as the
forcing with the wave is decent. More details with this in the
future TAFs and just hinted at for now in ORD 30 hr. General timing
is medium-high confidence.



315 AM CDT

Overall, weak pressure gradient will linger across Lake Michigan
early in the period, until low pressure lifts out of the Plains
and moves across the south end of the lake early Sunday. This low
is expected to deepen as it pulls away to the east-northeast
Sunday night and Monday, and merging with a deeper low which
remains nearly stationary north of the Lakes across Ontario
through early next week. As this occurs, winds will shift westerly
and increase, with the strongest winds expected Monday and Monday
night. Relatively warm air over the still cool lake waters will
make for stable conditions out away from the shore, and should
limit mixing of stronger winds aloft. Still, westerly wind gusts
of 25-30 kts are possible during the Monday-Tuesday period. The
nearly stationary low will eventually weaken and drift east during
the mid-late week period, with the gradient relaxing again across
Lake Michigan as a broad area of high pressure spreads southeast
from the northern Plains to the Midwest.






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