Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KLOT 160520

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 PM CST Fri Dec 15 2017

817 PM CST

Latest radar imagery depicting blossoming snow across far 
northeast Illinois, but primarily over the lake and areas 
eastward. Locations in far northeast Illinois and even portions 
of northwest Indiana may see the western edge of this development 
for the next hour or so before this focuses more to the east. 
However, am monitoring upstream development over southeast MN to 
western WI where additional snow development is occurring. This is
in response to isentropic ascent moving east/southeast across the
region, with short term guidance fairly consistent with this 
forcing reaching far northeast Illinois in the 4-6Z time frame. 
This would allow another period of either flurries or light snow 
across the area during that time, mainly for locations along and 
northeast of a Roscoe to Skokie IL line. Given upstream trends, 
it's quite possible for more light snow to be observed, and have 
included low chance pops to the forecast. If this were to occur, 
it could be possible for isolated areas to observe a light 
dusting. Expect any snow chances to end just after the midnight 
time frame tonight. 

Did increase low temps tonight to around 30, as WAA, cloud cover,
and steady winds should keep temps steadier. Locations that could
still drop more into the upper 20s would be the southwest CWA 
where low stratus is departing. However upstream mid clouds should
soon fill any clearing skies.



1202 PM CST

Through Saturday...

There are several short lived periods for snow chances 
today, first with a weak disturbance moving through early this 
afternoon. Flurries have been reported fora few hours or less at 
any one location upstream in northeast Iowa. This evening a bit 
stronger NW flow disturbance will interact with a a northward 
moving warm frontal feature. Isentropic lift over this front will 
lead to some snow or more likely flurries, mainly northeast of a 
RFD-MDW-VPZ line during the 0z-5z time frame, or mid to late 
evening. This would also be brief, only a few hours or so. After 
this, the final push with this clipper will come overnight, but by
this point the forcing will have shifted north of the Wisconsin 
border. Some guidance tries to break out snow farther south, but 
at this point the main frontal boundary will have shifted into 
Wisconsin and the air temperatures in the cloud layer will be less
than optimal for snow production.

The story for Saturday will be the milder air mass that will advect 
into the area from the southwest as we at least temporarily break 
the cycle of northwest flow aloft. There is a closed positively 
tilted trough well south into the Baja peninsula.  Surface high 
pressure to our south will also shift into the southeast on 
Saturday, which will promote deep layer southwesterly flow. With the 
front to the north, we will be in the dry and largely cloud free 
warm sector. The surface front may not actually get north of the
Wisconsin border, so cooler southeast winds will keep readings
closer to 40 in these areas, where across central Illinois highs 
will approach 50. The gulf moisture is still far enough away to 
make for a pleasant and not too breezy day than typically occurs 
when we get into southerly winds in the winter time. 



215 PM CST

Saturday night through Friday...

Main concern for the long term forecast period remains precipitation 
type Sunday morning.

The upper level pattern is trending toward a higher amplitude, 
slowly progressive longwave pattern.  By Saturday night, broad upper 
ridging will build over the ern 2/3 CONUS with deep troughing over 
the Rockies, with upper ridging building onto the West Coast.  This 
will shift the pattern from cooler nwly flow aloft to more moderate 
swly flow. Southern stream shortwave energy will be the primary 
weather driving systems for the long term period.  In general, 
temperatures for the long term forecast period will trend toward 
above normal for much of next week as the area remains under upper 
ridging, with highs in the upper 30s to upper 40s with more 
seasonable lows generally in the mid 20s to mid 30s until late next 

The longer range models continue to come into a little better 
agreement on the timing of the closed upper closed low located over 
the southern end of the Sea of Cortez Saturday morning lifting 
northeast through the lower Missouri Valley Sunday morning.  The 
longer term model consensus is a little better in both timing and 
strength of the system as it opens up into an open wave, with timing 
that is trending slightly slower than yesterday's guidance.  By 
Sunday morning, Expect that there will still be sub-freezing surface 
temperatures over nrn IL/IN as far south as the southern tier 
counties from Livingston to Benton Counties.  The mid-level pattern 
will be transitioning to strong warm advection, with the warmer air 
overrunning the colder surface air.  for example, the 925mb 
temperatures indicated by the ECMWF will be above 0 C by Saturday 
evening and climbing through the night as sfc temperatures drop below 
32 F, with persistent easterly flow locked in over nrn IL/IN/srn 
lake Michigan by strong high pressure parked sern Canada and the ern 
Great Lakes.  The warm front, induced by the srn stream shortwave is 
expected to still be just south of the CWA Sunday morning as 
associated pcpn begins to spread in from the southwest. GFS is 
advertising a relatively similar solution, but is trending a little 
faster in lifting the warm front to the north, likely due to a 
slightly faster progression of the associated shortwave.  At this 
point, would tend to favor the slower solution which is supported by 
the NAM, GEM and SREF guidance.  So, while the faster timing 
indicated by the GFS may only be on the order of 3 to 6 hours, this 
would have a significant impact on precipitation type.  With the sfc 
colder air more likely to be scoured out by the warm advection 
aloft.  Thermal profiles should be a shallow sub-freezing layer at 
the surface with the warmer air aloft.  The shallow cold air would 
be more supportive of freezing rain, and have carried that scenario 
for early Sunday morning.  While there is still a chance that the 
warm air aloft could mix down to the surface, setting up a rain 
scenario, there is also a chance that the initial pcpn could also 
generate some evaporative cooling near the surface, delaying sfc 
temperatures from rising above freezing.  So, to make a short story 
long, will carry a short period of freezing rain. possibly reaching 
as far north as the I-88 corridor.  However, the window of 
opportunity for freezing rain will be relatively brief, quickly 
changing over to rain as the warmer air spread north.  Given that 
pcpn amounts are likely to be light and the period of freezing rain 
brief, any glazing of surfaces should be brief.

Otherwise, have made little change to the going forecast for the 
remainder of the period.  The relatively mild should continue 
through much of next week.  the longer term models are still 
advertising another pattern shift by late next week with an upper 
trough carving out over the Upper Mississippi Valley and Great Lakes 
regions as a series of northern stream shortwaves break down the 
upper ridging and bringing a return to more seasonable


For the 06Z TAFs...

VFR conditions through the day Sunday. Southwest winds will
gradually ease the next few hours, but likely to maintain a
southwest to south-southwest flow over the area Sunday. Later in
the afternoon, look for winds to flip to light easterly and likely
remain easterly Sunday night. Some potential exists for some
patchy fog to develop after midnight, mainly at the more rural

- Izzi


215 PM CST

A relatively quiet period of weather is setting up for Lake 
Michigan for the next several days as a series of weak clipper- 
type lows track across the upper Great lakes region. The next 
chance for impactful weather and Gale Force winds will not be 
until early next week when the passage of a cold front. There is 
still some uncertainty as to the timing of the cold front passage,
but the latest model guidance continues to suggest Monday night 
or early Tuesday for the frontal passage, with west to northwest 
gales for Tuesday into Wednesday. The active pattern looks to 
persist late next week with another system bringing another round 
of strong winds to the lake.





National Weather Service Glossary of Abbreviations