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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
604 am CST sun Jan 21 2018

Short term...
240 am CST

Through Monday...

Lots to talk about in the short term as powerful storm system
traverses the region the next 48-60 hours. Potential threats
include dense fog, minor flooding, thunderstorms, a non-zero
severe weather risk, and potential for some accumulating snow.

GOES-16 water vapor imagery early this morning nicely depicts a
sharp trough, with most recent imagery suggesting system closing
off a mid-upper level circulation now over AZ/nm. This system is
progged to continue deepening with guidance suggesting 500mb
heights greater than 2 Standard deviations below normal with this
system as it tracks across the plains and into the mid-Mississippi
Valley by Monday.

Lee side surface cyclogenesis already underway over the southern
High Plains and as strong mid-upper level height falls spread
east, look for the surface cyclone to deepen and organize further
today. Warm frontogenesis is expected to take place east of the
sfc low into central IL, with majority of our County Warning Area likely remaining
on the cool side of the warm front today and through much of
tonight. As the cyclone and warm front intensify, look for
easterly sfc flow to develop north of the warm front, which should
result in cooling temps this afternoon downwind of the chilly and
partially Frozen Lake. As that cooler marine air spreads in it
could accelerate/accentuate the development of fog, possibly
dense, that is likely to take place to the north of the warm
front. Given how effectively the snow cover has been wiped out of
the past couple of day, confidence in how dense the fog gets is
low and while dense fog is possible, confidence is not high enough
to consider any sort of dense fog headline.

Areas of drizzle and perhaps some light showers expected to
increase in coverage this afternoon as the warm front sharpens and
low level warm air advection (waa) strengthens. Lead shortwave
emanating from the upper low is progged to track northeast toward
northern Illinois by late tonight. In addition to this weak shortwave,
very strong warm air advection is progged to strengthen markedly as 900mb low
level jet strengthens to 50-60kt+, with the nose of this jet
nosing into northern and central Illinois. In addition to the strong
forcing, guidance suggests MUCAPES of several hundred j/kg
tonight, which should be plenty for a decent coverage of
thunderstorms tonight over our County Warning Area. Any thunderstorms will only
act to increase rainfall amounts, which look to be quite healthy
given precipitable waters progged to increase to 200-250% of normal late
tonight into early Monday. The greatest coverage of showers and
thunderstorms over our County Warning Area looks to be between 06z and 15z
Monday. See hydrology section below for more on the heavy
rain/flooding threat. As rain increase Monday night, look for it
to help mix out the fog and lead to some improvements in
visibility, particularly if any dense fog develops.

Surface warm front is forecast to lift slowly north tonight,
especially after midnight. Temperatures and dewpoints will rise at
least into the mid-upper 40s south of the warm front, and perhaps
rise into the 50s tonight south of the warm front. Warm front is
expected to approach and probably clear the Illinois/WI border Monday
morning, with all of our County Warning Area getting into the unseasonably warm
and moist air mass. NAM and WRF-nmm continue to exhibit a
significant cool bias over what it perceives to be a quarter to
half inch of snow remaining on the ground through the day Monday.
This is adversely affecting the model blended temperatures and
have trended high temps Monday toward the GFS/European model (ecmwf) which have
highs climbing well into the 50s across the County Warning Area.

GFS, ECMWF, and NAM all suggest a pronounce dry intrusion will
rotate around the strong upper low and over-top the warm sector
over central and northern Illinois during the day Monday. Steep mid
level lapse rates within the dry intrusion becoming juxtaposed
over-top the unseasonably warm/moist air mass in the warm sector
could allow for a swath of several hundred j/kg of SBCAPE to
develop ahead of approaching cold front. Instability should be
shallow, but guidance is in good agreement in breaking out an arc
of convection along the cold front within the dry intrusion
Monday afternoon across Illinois. Generally speaking, bulk shear
shouldn't be overly impressive close to the upper low, but ambient
atmospheric vorticity in the vicinity of the upper low could be
enough for some non-zero brief tornado threat with this convection
Monday afternoon/early evening.

Monday night, the upper low is forecast to move across northern
IL, with an impressive trowal forecast to develop and wrap around
the north and western side of the upper low. Could see a band of
intense precipitation within the trowal as strong f-gen and
presence of very steep mid-level lapse rates sets the stage for
high precip rates and possibly even isolated convection. At this
time, it looks like just our far northern County Warning Area could be scraped by
this trowal, with the majority of the intense precip remaining
north of the Illinois/WI border. To the south of the trowal, more
showery, lighter precipitation is likely to continue across our
County Warning Area overnight into early Tuesday morning. This southern stream
system is not expected to phase with the polar front jet and
should have a somewhat limited supply of cold air, despite the
time of year. This should result in only a gradual change over to
rain/snow mix and then all snow, with the best chances of
accumulation being near the WI border where precip rates stand the
best chance of helping augment the otherwise marginal surface
temps to support accumulating snow. At this time, the majority of
guidance keeps the better chance of accumulating snow near and
north of the WI border, though over the past several days there
has been a slight southward shift in the forecast track of this
system, any farther deviation south in the track could bring the
potential for a brief period of heavy snow farther south into
northern Illinois late Monday night into early tues morning.

- Izzi


Long term...
240 am CST

Monday through Saturday...

After all that excitement in the short term, the long term
portion of the forecast is decidedly quieter. A modest push of
colder air (in reality, just close to average temps) is expected
to filter into the area Tuesday into Wednesday. By Wednesday
night, mid-upper level heights begin rising as more anomalous
upper ridging builds east over the region. Strong northern stream
cyclone is forecast to develop late in the week over the northern
plains which should help strengthen warm air advection over the
area late this week. Assuming no surprises Monday night in terms
of laying down a respectable snow cover, the stage should be set
for much above average temperatures Thursday and particularly
Friday. For now, stuck with the model blended consensus forecast
with highs well into the 40s to near 50 Friday, but models suggest
the southerly flow through Friday should largely be lacking low
level moisture, so stratus shouldn't impede the warm up thurs or
Friday and highs could easily climb well into the 50s by Friday.

Saturday looks to start very mild before a cold front moves
through with rain and then colder temperatures for mainly the
second half of the weekend. Cold snap looks fairly strong, but
progressive with indications of another warm up heading into next
week, though admittedly confidence decreases substantially in the
forecast beyond this weekend.

- Izzi


240 am CST

Majority of the snow cover in the area has melted and there has
been some small response on some area rivers. Significant, and
potentially locally heavy, rainfall is expected late tonight into
Monday morning. This rain will be falling on largely frozen
ground, resulting in the majority of the rain going straight into
runoff. Given the potential for rainfall totals to exceed an inch,
should see significant rises on many area rivers, with some mostly
minor flooding possible on main Stem rivers. In addition, expect
ponding of water to be a problem, particularly in poor drainage
areas. Generally expect that any flooding will be more of the
nuisance variety and probably could be handled with flood
advisories. Given the expectation of mainly minor flooding, opted
to hold off on a Flood Watch and will just issue an esf and
continue to highlight the threat in the severe weather potential statement.

The quickly increased stream flow on frozen rivers and rising
temperatures will likely dislodge ice on frozen and partially
frozen rivers. The flowing ice could jam up and result in an
increased ice jam flood threat in addition to the potential for
minor areal flooding. Ice jams tend to be favored at bends
in area rivers and at choke-points and obstructions to free water
flow such as bridges.

- Izzi


for the 12z tafs...

Significant concerns for today and tonight regarding cigs and
vsbys in the unseasonably moist airmass ahead of an approaching
low pressure system. Nearby obs this morning are already showing
500 ft cigs and these have been working their way toward the
Ord/mdw terminals as seen in the few/scattered 600 ft group in the
12z mdw obs. Vsbys from 1/2 to 1/4 sm have been lagging the lower
ceiling trends by about 90 minutes at upstream locations.

While confidence is high about these lower cigs getting into the
mdw/Ord terminals, there is less certainty that the very low vsbys
will make it into the city. Opted to carry a tempo 1 sm as a heads
up that lower conditions could develop. Also have very low
confidence regarding the duration of the lowest conditions this
morning. There is a pocket of better conditions to our west and
southwest which could push this first round of IFR/LIFR off to the
east as the low moves closer.

Overall the trends are toward lower conditions as the day
progresses. Most forecast guidance would support going with LIFR
conditions in both vsbys and cigs from mid afternoon into the
evening. Have opted to start slightly more optimistic but would
not be surprised to see later updates nudge toward lower values.



326 am CST

Low pressure moving out of the Southern Plains will cross the lake
Monday night, and ahead of this low a warm front will lift into
the southern part of the lake. This will produce both variable
wind conditions on either side of the front as well as increasing
chances for fog formation as the day GOES along and better
moisture moves over the Cold Lake along with the front.

A gale watch remains in effect for the north half starting late
Sunday night and continuing into early Monday evening. After what
appears to be a short lull Monday night, winds across the entire
lake likely increase to gales again by early Tuesday morning and
persist into at least Tuesday afternoon. To avoid confusion with
multiple watches or watches mixed with warnings, opted to let the
day shift look at latest trends and decide how to handle upgrade
to warnings.



Lot watches/warnings/advisories...
Illinois...dense fog advisory...ilz023-ilz032-ilz033-ilz039 until noon

In...dense fog advisory...inz010-inz011-inz019 until noon Sunday.



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