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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
225 am CST Mon Nov 18 2019

Short term...
112 am CST

Through Tuesday...

Extensive stratus blankets the region and indications are that it
should be largely here to stay for a while with just a chance of
it clearing out this evening western County Warning Area. There is drizzle and
areas of mostly light fog over most of northern Illinois as of 07z. GOES
nighttime microphysics rgb shows and area of thicker stratus east
of a rfd to pia line with thinner stratus and some improvement in
visibility to the west of this area where stratus isn't as thick and
likely not drizzling. Simple extrapolation would have the thicker
stratus, and probably the steadier drizzle and fog, over mainly
just our northwest in counties by 13z.

While ceilings will likely gradually lift today, generally
expecting a cloudy day which should allow for only a minimal temp
rise through the day and have nudged highs down a bit closer to
the colder guidance. With the expectation of cloud cover and most
likely stratus hanging around tonight and Tuesday, have gone near
the top of the spectrum of guidance for temps tonight and closer
toward the coolest guidance for Tuesday. If cloudiness were to
unexpectedly clear out, look for a larger diurnal range in temps.

Other concern is with sheared out clipper system zipping southeast
across the region tonight. There is a distinct bifurcation in
guidance solutions tonight with the domestic models (nam, GFS,
hires arw and nmm) tracking this wave farther northeast than the
foreign model guidance (ecmwf and gem). The domestic models would
result in a period of mostly light precip moving across most of
the County Warning Area late tonight into Tuesday morning, while the foreign
models would keep most of the precip south and west of our County Warning Area.
Unfortunately, each model's respective ensemble suite generally
support its operational run, so not much help there picking a
model Camp to go with tonight. Given the uncertainty, broad-
brushed medium to high end chance pops across most of the area,
though if domestic models end up being correct then pops would
likely need to be increased substantially.

Precip type is tricky with several competing factors. First, NAM
is exhibiting it typical cold bias near the boundary layer due in
at least part to misrepresentation of snow cover in the model. If
stratus sticks around and keeps temps up, then most of any precip
that falls would probably end up rain. Forecast thermal profiles
remain precariously close to the rain/snow line for this event and
given just the inherent uncertainty in precise temp forecasting
its prudent to include mention of both rain or snow given how
close it will be to snowing. If cloud cover keeps sfc temps above
freezing, impacts of whatever falls would be largely mitigated,
but should temps be colder than forecast then that would open the
door to perhaps some light freezing rain/drizzle and even a touch
of snow accumulation. Kept the grids and official forecast skewed
slightly toward the more optimistic/warmer/less impactful
solution, but with a footnote that it wouldn't take much to nudge
temps down a bit and raise the prospects of some impact from
wintry precip.

- Izzi

&&

Long term...
225 am CST

Tuesday night through Sunday...

There continue to be a couple periods of note in the long term
part of the forecast, most notably a high likelihood of the first
soaking rain of the month for portions of the area Wednesday night
into Thursday morning. Depending on the wind direction and
magnitude behind the rain producing system, there could be some
risk for Lakeshore flooding for portions of the Northwest Indiana
shore. Finally, another system is possible Friday night into
Saturday, but uncertainty is high in this period.

The main question on Tuesday night into Wednesday is whether
stratus will continue to linger. If it does, then low temps will
likely be a bit milder than the official forecast of around 30 to
lower 30s (mid 30s downtown chicago). There's better support
during the day on Wednesday for breaks in the clouds, so with
partial sunshine, mid-level height rises, and warm advection
aloft, high temps have a good chance to rise above normal for
essentially the first time this month.

Attention then turns to our weather maker for Wednesday night into
Thursday. Operational and ensemble guidance is still in overall
solid agreement in resolving a pretty complex pattern. A
positively tilted southern stream wave will eject from an upper
low off the baja that itself will have absorbed the remnants of
east-pac Tropical Storm Raymond. This wave will then quickly track
northeast across the plains and into the western Great Lakes,
with consensus for attendant surface low being in low 1000s mb.
The lack of phasing will also keep the surface low moving along at
a good clip off to the northeast, embedded in fast southwest flow
aloft. The low will likely track just to the north of the County Warning Area.

Ingredients continue to be in place for a period of moderate to
perhaps briefly heavy rain rates with this system, particularly in
portions of northern Illinois. The injection of tropical moisture,
given origins of the southern stream wave, entails a plume of
upwards of 1" pwat, in the 99th percentile for this time of year.
A robust 850 mb low level jet pushing 50 kt is also progged,
suggesting strong moisture transport, and also can't rule out
embedded thunder somewhere in the region. Do not have any
explicit thunder mention yet due to lacking instability and weak
lapse rates. At this time, track of system and focus of moisture
transport favors the northwest 1/3 or 1/2 of the County Warning Area for the
highest rain amounts by early afternoon Thursday, with totals up
around 3/4" possible in parts of northwest and north central Illinois.

Models continue to indicate that quick drying on the back side of
the system will shut off the rain before any transition to wintry
p-type. Temperatures will rise after midnight Wednesday night
through early Thursday afternoon prior to cold front passage,
driven by Stout low-level warm advection in the warm sector of the
surface low with gusty south to southwest winds. The warmest
temperatures of the entire month are quite possible for highs on
Thursday, with mid to upper 50s forecast and pushing 60 for some
locations, particularly south of I-80. Winds will flip to
west-northwest and gust to 25-30 mph behind the cold front
passage early Thursday PM, along with falling temps. Wind
direction will become northwesterly enough later Thursday night
into Friday over Lake Michigan that there is still some concern
for a lower end Lakeshore flooding risk for northwest in shore,
particularly Porter County.

After a cooler and dry Friday, forecast confidence lowers Friday
night into Saturday. The mid and upper level pattern over the
Continental U.S. Will continue to be quite complex, so would expect some
large changes in the modeling in the coming days. Yet another
closed low in the southwest will likely eject out and eventually
get sheared out as the wave tracks northeast. The operational
European model (ecmwf) indicates a decent surface low with this system tracking
along the Ohio Valley. The idea of a system tracking somewhere
near the Ohio Valley has decent European model (ecmwf) ensemble support, as well as
the 00z Canadian, but very little support in the GFS suite. Still
plenty of time to resolve things, such as whether precip would
make it this far north, so slight to low chance pops in grids
appear reasonable. With air mass in place not that cold, thermal
profiles would likely be marginal re. Rain/snow/mix if precip does
occur. Sunday will then likely be dry with near seasonable
temperatures.

Castro

&&

Aviation...
for the 06z tafs...

IFR/LIFR ceilings should linger through much of the night with areas
of dz and light fog (br). Some potential that there could be some
improvement in conditions toward sunrise as winds lock in more
from the NW, but the more likely scenario is any noteworthy
improvement will be delayed until a couple/few hours after
sunrise. Should see a continued gradual lifting of ceilings through
the day Monday lifting to higher end MVFR by late in the
afternoon. Cannot rule out ceilings scattering out late in the day or
Monday evening, but confidence is low so opted to maintain ceilings
through the taf cycle. Next fast moving upper level disturbance
could bring some precip to Ord and mdw during the last hour or two
of this taf cycle. Will access this threat further overnight and
likely introduce some precip into the tafs for early Tuesday
morning with the 12z issuance this morning.

- Izzi

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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