Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221746

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM CDT Mon Apr 22 2019

330 AM CDT

Through Tuesday...

Somewhat of tricky forecast today with a fair amount of
uncertainty regarding extent of cloud cover and the subsequent
impact on temps/dewpoints/winds for today.

Upstream evening ROAB observations from OMA, SGF, and TOP all
confirm what models have advertising and what SPC mesoanalysis
data shows, which is a large reservoir of very steep mid level
lapse rates, approaching dry adiabatic in some areas. IR imagery
early this morning shows quite a bit of convection developing over
MO up into eastern IA, with surface observations indicating cloud
bases generally above 12kft. This mid-level convection is
developing in an area where the very steep mid level lapse rates
are juxtaposed with the more moisture laden mid level air mass as
seen on GOES-16 6.95 micrometer channel (mid level water vapor).
This area of mid level convection will likely move across our CWA
through the early morning hours with very dry low levels likely
resulting primarily quite a bit of virga, but did add some
sprinkles to the grids in case some of the more robust convection
actually manages to penetrate the sub-cloud dry air.

Fast on the heels of this mid-level convection, GOES simple water
vapor RGB imagery shows a well defined shortwave trough
approaching NW MO early this morning. The RAP and SREF both have
this shortwave moving across the CWA this afternoon, which will
probably keep cloud cover fairly prevalent with perhaps some more
mid-level convection/virga early-mid afternoon. Based on
observational trends which tend to suggest more cloud cover today,
did nudge high temps downward a couple degrees from blended model
guidance and previous forecast. If there are sizable breaks in the
cloud cover, then high temps will likely approach 80F in those
areas with better low level mixing likely resulting in stronger
winds and lower dewpoints (ref upcoming fire wx section in AFD for
more details). Even without the breaks in cloudiness, still
anticipate highs into the mid 70s which would still allow for
enough mixing for strong/gusty winds. By late afternoon/very
early evening could see gusts around 40 mph.

Getting a decent signal in the last several runs of the HRRR and
HRRRx that there could be some likely elevated convection move
into the northwest portions of our CWA later this afternoon.
Given steep lapse rates and moderate shear, hard to rule out a
strong to isolated severe storm capable of producing some large
hail. In the wake of this lead shortwave trough, high-res models
show a narrow ribbon of surface based instability developing just
ahead of the approaching cold front. By 00z, the HRRR and
experimental HRRR have that north- south axis of better sfc based
CAPE near to locally over 1000 j/Kg from near KDBQ south into west
central IL. To realized this magnitude of instability, the HRRRs
have Tds pooling in the low- mid 60s in a narrow ribbon just ahead
of the cold front. Given upstream observations, this may be a bit
aggressive, so instability could end up being weaker than progged
with more inhibition. Forcing may still be enough to overcome
this and allow for some sfc based convection to develop over
eastern IA later this afternoon.

Nocturnal cooling of the boundary layer should result in weakening
instability and increasing inhibition across our CWA this evening,
suggesting that storms will either already be weakening as they
enter into our western CWA and if they aren`t weakening as they
arrive, they should soon after begin weakening. Given the steep
lapse rates and modest shear, can`t totally rule out a stronger
storm making into northwest CWA early this evening, but threat at
this point appears limited.

Pair of cold fronts with this system, with the first moving across
the area this evening into the early part of the overnight hours
with only some modest cooling, secondary cold front will move
across the area Tuesday morning with a wind shift to the north and
more noteworthy cooling expected behind that boundary. This front
will likely be augmented by the chilly lake with more pronounced
cooling expected near and downwind of Lake Michigan Tuesday. Can`t
rule out a shower or two overnight tonight into Tuesday morning,
especially eastern CWA, but chances look low and any impacts
probably lower.

- Izzi


338 AM CDT

Tuesday night through Sunday...

Generally looks to start off as a rather tranquil period with
somewhat below average temperatures. While GFS/NAM are in
disagreement with the ECMWF in handling of some southern stream
shortwave energy mid-week, all the guidance suggests that any
meaningful rain chances with that wave will stay south of the
area. While GFS and ECMWF aren`t in great agreement with the
southern stream energy, both agree in digging a trough into the
Northeast later this week with associated sfc cold front moving
across our area in the Thursday-ish time frame. Moisture looks
limited (at best), so the low chance/slight chance POPs for
showers looks reasonable, if not overkill for Thursday.

Timing differences exists late in the week into next weekend with
respect to the next shortwave trough that will impact the area.
For now, given the low confidence, so no compelling evidence to
suggest any changes were needed to the blended model
initialization for temps or precip chances.

- Izzi


402 AM CDT

As mentioned in the short term discussion above, there is a great
deal of uncertainty regarding the extent of cloud cover today. If
cloud cover does indeed stay fairly extensive today, then fire
weather risk will remain somewhat muted with RH not likely to drop
below 35-40%. However, if there is a sizable break in the cloud
cover, then chances for conditions approaching or even meeting red
flag criteria would increase significantly, especially eastern CWA
(northeast IL into NW IN). The 10-hour fuel stick moisture values
at the Indiana Dunes dropped to 7% yesterday, so the very dry
conditions the past day or two has resulted in a significant
drying of fuels. Sunshine would likely allow temps to approach 80F
and dewpoints to mix out into the 40s which would result in RH
values potentially dropping below 30%. Winds by later this
afternoon look to become quite strong and any sunshine would
likely only aid in mixing down stronger winds, with 10 meter
winds potentially gusting to around 40 mph.

Oncoming day shift will need to closely monitor satellite trends.
If it becomes apparent that there will be more sunshine than
currently expected and dewpoints mix out, then day shift may need
to coordinate with fire wx managers about potential short fused
red flag warning.


For the 18Z TAFs...

1246 PM...Forecast concerns include...

Strong/gusty south/southwest winds into this evening.
Wind shift to northwest Tuesday morning.
Wind shift to northeast Tuesday afternoon.
Chance of showers this afternoon.
Chance of showers...possibly a few thunderstorms this evening.
Mvfr cigs overnight through early Tuesday afternoon.

Southerly winds are expected to turn a bit more to the south/
southwest this afternoon with speeds/gusts increasing. Forecast
soundings show fairly strong winds in the low levels and if these
were to mix to the surface...gusts into the lower/mid 30kt range
would be possible but confidence is low so will continue with
upper 20kt gusts which will continue into this evening as the low
levels remain mixed. Winds will turn more southwest later this
evening then shift northwest before daybreak Tuesday as a cold
front moves across the area. Winds will turn more northerly by
mid/late Tuesday morning. A lake breeze/wind shift is possible
Tuesday afternoon as high pressure approaches the area.

Earlier shower/thunderstorm activity has weakened/dissipated as
it moves east of the current instability. Much of the short term
guidance continues to show a small chance for showers across the
Chicago area this afternoon. Confidence is fairly low and these
should be low impact...perhaps just sprinkles...but opted to carry
tempo vfr showers given the consensus of the models. Scattered
thunderstorms are expected to form over northeast IA/far northwest
IL late this afternoon and then spread across northern IL this a weakening phase. Best chance for thunder will be at
rfd and while thunder can/t be ruled out further east...coverage
looks isolated so maintained just shower mention for now.

Cigs will lower to mvfr behind the cold front overnight with mvfr
cigs continuing into late Tuesday morning/early Tuesday afternoon.
Some brief ifr cigs are possible but prevailing cigs look to
remain low mvfr. cms


LM...Small Craft Advisory...nearshore waters until 3 AM Tuesday.




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