Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 190208 

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

807 PM CST

Latest model trends have been to shift the main precip axis a bit
farther south late tonight through Tuesday morning. Additionally,
guidance has also trended a touch cooler, with increasing chances of
near or slightly sub-freezing temps north of I-88 late tonight. This
may create a more complicated precip-type issue through at least
daybreak Tuesday for areas north of I-88 and primarily west of the
Chicago Metro. Forecast soundings and recent pilot reports indicate
a lack of ice in the low-level stratus deck, with a significantly
deep dry layer between the low stratus and mid/upper-level clouds.
As the sheared trough over western Minnesota nears, increasing weak lift
and some minor warm air advection may support an initial brief period of
drizzle/freezing drizzle in the aforementioned area before deeper
saturation and the resultant addition of ice into the column change
precip over to snow/rain. Do not think there is any major concern
for icing on surfaces at this time, but have added a slight chance
of freezing drizzle at precip onset late tonight while also
increasing the snow wording north of I-80 early to mid-morning



Short term...
216 PM CST

Through Tuesday night...

Conditions continue to be cloudy and cool across the region this
afternoon with temperatures in the 30s to around 40. Our next
weather disturbance of interest is currently noted in the GOES 16
water vapor channels shifting southeastward over the Dakotas this
afternoon. This sheared mid/upper impulse will continue to shift
southeastward over our area very late tonight and on Tuesday
morning, and will drive our next period of light precipitation.

Current indications continue to support much of the this
precipitation falling as a cold rain. However, I cant rule out
some light snow initially late tonight, particularly over
portions of far northeastern Illinois where the column could initially
be cold enough to support some snow before it changes to all rain
around daybreak Tuesday. Periods of light rain will then persist
across northern Illinois and northwestern in Tuesday morning before
abating in the afternoon. Clouds will likely persist across the
area through the afternoon, and this should result in temperatures
similar to those observed today, with readings generally around



Long term...
237 PM CST

Wednesday 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 a good
soaking rain for portions of the area Wednesday night into
Thursday morning as an area of low pressure tracks to our
northwest. Wind directions and magnitudes over Southern Lake
Michigan will then need to be watched in the wake of this area of
low pressure into Friday due to some possible risk for Lakeshore
flooding for the Northwest Indiana shore. Finally, another system
is possible Friday night into Saturday, but uncertainty remains
high in this period.

Operational and ensemble guidance is still in overall solid
agreement in resolving a pretty complex pattern Wednesday night
into Thursday. 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. 00z GFS appeared to be a bit of an outlier
with an upper 990s mb low with this system (deepening to mid 990s
over lake mi), due to an anticipated lack of phasing of the
positively tilted southern stream wave and an incoming northern
stream trough. The lack of phasing will also keep the surface low
moving along at a good clip off to the northeast and just north of
the cwa, embedded in fast southwest flow aloft.

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 east-northeast. Questions still
remain as to how this system will evolve as it tracks towards the
central Continental U.S.. for this reason, confidence in the forecast for
precipitation Friday night into Saturday remains low, so only
slight chance to low end chance pops have been maintained in the
forecast database. Dry weather looks likely for Sunday with fairly
seasonable temperatures.



for the 00z tafs...

Light southwesterly winds this evening will trend towards
southeasterly at speeds under 6 kts. Expansive MVFR stratus is
expected to gradually build down into an IFR deck through the
evening and overnight hours. Can't rule out some MVFR vsbys
developing as far east as Ord/mdw, but the main vsby restrictions
in br are expected in vicinity of dpa and rfd.

Main uncertainties this taf cycle are on the track on the
aforementioned weather disturbance. A pretty narrow corridor of
precipitation is expected to materialize very late tonight and
into Tuesday morning. Guidance continues to disagree a bit on the
ultimate location of the steadier precipitation, although a slight
southward shift has been noted in afternoon guidance. Either way,
any precipitation chances look to remain confined to mainly a 4-6
hour window Tuesday morning.

Can't discount some brief -fzdz (of -fzfg) near rfd at the
leading edge of the precipitation shield towards ths 08-11z
timeframe as temperatures dip to near or just under the freezing
mark. Closer to the Chicago-area terminals, temperatures look to
remain a degree or two above freezing, but we'll keep a close eye
on trends this evening as if temperatures fall a few more degrees
than forecast, there could also be a brief period at the front end
for some -fzdz before the column saturates and warms. Through the
mid-morning, a cold rain is anticipated, although a few wet flakes
can't entirely be ruled out. Surface winds should begin to favor
an east or northeast direction, but should remain quite light
under 5-6 kts before returning to the northwest through the
afternoon and evening hours.

Cigs look to build down quickly Tuesday evening/overnight. Stratus
may try to build down to LIFR with a potential for some dense fog.
Confidence in this is too low to carry a mention in the tafs at
this time, however.



Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations