Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 010136 AAA
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
736 PM CST Wed Nov 30 2016
736 PM CST
The going forecast for tonight and Thursday morning remains in
good shape with the message being occasional sprinkles trending
more patchy light snow showers/flurries overnight, mainly north,
and some flurries/sprinkles lingering into the morning commute.
Temperatures should be nearly steady with a wind chill in the mid
20s, and overall impact of precip looks to remain low.
Broad but closed upper circulation over northern Wisconsin/U.P. of
Michigan continues to funnel cyclonic flow across the area. One
embedded short wave is moving across southern Wisconsin, with
another more pronounced one dropping southeastward over the Twin
Cities. Light echoes over the CWA have mainly been sprinkles due
to a lack of cloud ice, but deeper saturation to colder than -10C
is improving per 00Z DVN raob and upstream infrared satellite
imagery. The DVN WFO was receiving flurries as of 715 pm as
verification of this.
The second disturbance still is timed to rotate over far northern
Illinois overnight as mentioned in previous discussion, providing
the best time for associated scattered snow showers. Lift remains
slightly below the DGZ in RAP and NAM guidance, and the lift is
not that great to begin with nor are lapse rates, so expecting the
showers to be mostly light and non-accumulating while pockets of
just flurries are more common. The short wave is forecast to be
east of the area by 6 a.m., with patchy flurries during the
253 PM CST
Main concern is isolated to scattered snow showers and flurries
tonight into Thursday morning.
Initial shortwave bringing isolated to scattered shower/sprinkle
activity can be seen on W/V across eastern Iowa this afternoon.
This wave will pivot northeastward, giving a glancing blow to
north central Illinois, where the highest chance range PoPs have
been focused into this evening. Farther south and east, chances
for any measurable precip will be lower, so kept PoPs in slight
chance range, though did expand them southward to account for
current radar observations. Latest guidance suggests that the
shortwave more likely to focus forcing into northeast Illinois
and northwest Indiana will swing through overnight. This is when
the highest chance PoPs have been expanded east-southeast toward
the Chicago area and northwest Indiana. Temperatures are likely
to remain primarily above freezing tonight, again with rain-snow
showers favored earlier on and then transitioning to all snow
showers with time this evening. Still not anticipating any
impacts other than perhaps brief occasional reduced visibility, as
air temps above freezing and mild road temps will prevent any
accumulations, with a dusting at most on grassy/colder surfaces.
The final trailing shortwave on the backside of the upper low will
move across the area Thursday morning, which could keep some
isolated snow showers/flurries going through mid-late AM. Any
precipitation could mix back with light rain/sprinkles before
ending. The area will remain socked in with stratus through the
day as we remain under cyclonic flow with the back edge of the
stratus way back over the Northern High Plains. Temperatures
aloft will be slightly warmer than today, but the clouds will
suppress any limited warming, so expect highs in the upper 30s to
around 40 (38-41). West winds gusting up to 20-25 mph will make
for a chilly feeling start to December and meteorological winter
despite seasonable temperatures.
241 PM CST
Thursday night through Wednesday...
A rather cloudy, and at times, active weather pattern looks to be
in place across the central CONUS through the period.
The first period of active weather could come into play on Sunday
as a northern stream mid-level disturbance tracks eastward towards
the area. There has been some questions as to the evolution of an
evolving close upper low over near the Baja of California Friday
into Saturday. Most notably, will it try to phase with the
northern stream wave. Current indications suggest that it will
not phase with the northern stream wave. As a consequence...it
appears that moisture return to the area could be modest, allowing
only light precipitation to occur over the area Sunday. P-Type
could be an issue Sunday, with the potential for snow, though with
marginal thermal profiles, we have continued the mention of rain
Following this system it does appear there will be a day or so of
quiet weather across the area as surface high pressure builds over
the area. However, forecast confidence decreases substantially
early next week. With the southwestern CONUS/northern Mexico upper
low possibly remaining removed from the northern stream wave
moving across the region Sunday, it appears possible this system
could begin to eject northeastward sometime Monday into Tuesday as
another strong upper Trough, and an attendant speed max, digs over
the western CONUS. This could set up a period inclement weather
over the area late Monday into Tuesday. While it appears this
potential period of weather will be in the form of rain at this
time, it is possible that it could begin as a light mix Monday
night. With this possibility I have mentioned a chance for a light
rain or snow mix through early Tuesday morning.
Otherwise, there is the potential for another storm system later
next week. Forecast confidence is very high during this period,
but there have been some indications of late that a storms system
could impact the central CONUS during this period. However, the
main local impacts of this potential storm system are largely
uncertain at the present, but should be watched in the come days.
For the 00Z TAFs...
* MVFR cigs through the period with the potential a short period
of IFR cigs overnight.
* Snow showers/flurries this evening and overnight.
Expansive mvfr stratus deck remains in place under upper level
low pressure. Cigs have improved to higher end mvfr and low end
vfr, but anticipate that will gradually lower back to prevailing
mvfr tonight. Upstream obs are already indicating lowering cigs,
so will just go with mvfr cigs out of the gate for the 00Z update.
Upper level disturbances rotating around the southern periphery of
the upper low will bring waves of scattered snow shower activity
to the region overnight tonight and into Thursday morning.
Temperatures currently are still marginal for snow at 00Z, so pcpn
could start out as mixed rain/snow showers, but should then change
over to all snow through the evening. Have added a tempo for mvfr
vis and 1000ft cigs at ORD for a few hours after midnight, with
some of the guidance suggesting that some heavier snow shower
activity should cross nrn IL, mostly impacting RFD/DPA/ORD, but at
this point, it looks like MDW/GYY should remain south of the
potential more moderate snow showers. But, given relatively warm
ground conditions left over from temps in the 50s for the past few
days, not expecting any accumulation. Any pcpn should come to an
end by arnd 14-15z tomorrow morning and could change back over to
rain before all pcpn is done for the day.
Winds will remain west-southwest through this evening and then
gradually veer west overnight. Gusts to arnd 20kt are likely
through the night and for much of the day tomorrow.
312 PM CST
Westerly winds will set up over Lake Michigan this evening as
surface low pressure continues to shift northeastward over the
Upper Great Lakes. Wind speeds will be the strongest over the
southern end of the lake, where some 25 to 30 KT winds are
likely. We will continue the small craft advisory for the Indiana
shores due to waves and winds. However, we have continued to hold
off on a small craft for the Illinois shores due to the offshore
component. So, while some 25 KT winds are possibly on the
Illinois side, wave heights should remain below 4 FT. High
pressure will gradually build over the region by Saturday,
resulting in abating winds later in the week.
The weather pattern later this weekend into next week looks like
it could become active again. However, forecast confidence in
regards to the local impacts remains low this far out.
LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 9 PM Thursday.
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