Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
510 am CST Friday Dec 20 2013
355 am CST
Several issues ongoing this morning as well through the
weekend...with dense fog and freezing rain issues this morning
with the freezing rain now lingering into the afternoon and
possibly tonight...continued flooding concerns...and lastly
potential snowfall which the axis of heaviest snow still appears
to be across the north and northwest areas of the County Warning Area.
Addressing the more important issues in the near term...light
freezing rain or freezing drizzle continues across the advisory
area this morning...with light rain or drizzle ongoing for the
remainder of the County Warning Area. Broad southwesterly flow aloft continues to
usher several waves of energy...with elongated trough of low
pressure extending from the Central Plains up through southern
Great Lakes. As these waves of energy/speed maxes lift northeast
across the region this morning...expect this trough of low
pressure to lift east northeast to central Illinois and Northwest
Indiana through the early afternoon. Present low/middle level
convergence will continue to be a persistent through this
period...with current upstream wave to aid in tightening low/middle
level baroclinic zone. This will enhance frontogentic forcing
across the advisory area...all aiding in continued precipitation
development at least through early afternoon for the entire County Warning Area.
Have increased probability of precipitation this morning and early afternoon...but wonder
if afternoon probability of precipitation will need to be raised even further with short
term guidance hinting at continued to development even into the
late afternoon. Although current associated boundary draped across
east central Illinois and Northwest Indiana did make some
northward progression early this morning...feel that this
northward movement has ended and will remain situated until
dropping south/southeast of the County Warning Area this afternoon. Guidance
hinting at some weak cold air advection to begin occurring over the next several
hours and with latest rap analysis already indicating this across
western Illinois...feel pretty confident with this trend. This
will help temperatures in the advisory area to hover just below or at the
freezing mark through most of today. With this continued precipitation
and temperature trends...have decided to extend the Freezing Rain
Advisory through the afternoon and have also included Kane County
in the advisory as well. Will need to also monitor the afternoon
period for the remaining County Warning Area...as cold advection should help temperatures
lower around freezing into the late afternoon period. With precipitation
possibly lingering...could see some possible light icing in areas
outside of the advisory. Although temperatures should be marginal...and
am not overly concerned at this time.
With regards to the dense fog...with increasing moisture in the
boundary layer this past evening dense fog developed along to the
north of this boundary. Many reports of dense for were received
this past evening across Northwest Indiana...primarily for lake
and Porter counties in Indiana. With the boundary situated and
expected to remain this way through the remainder of the
morning...feel current dense fog advisory for these locations
remains valid. Although...southern areas of Cook County and
eastern areas of Will County will need to be monitored over the
next couple of hours as visible in these locations continues to bounce
around 1/2 mile. End timing for this dense fog advisory remains 9
am CST...and seems reasonable. This sagging boundary and slightly
drier air should help with the more dense fog...but with patchy
dense fog still not out of the question at least through middle day.
Confidence with this fog remains low at this time and will need to
monitor...possibly extending this advisory or maybe even expanding
it later this morning.
With surface low exiting to the east this evening and the arrival
of drier air...should observe overall precipitation chances to wind down
for the time being. Although with guidance continuing to indicate
broad scale lift with the approaching longwave trough to the west
it does appear as if areas across the southern County Warning Area could see
additional precipitation tonight. Although light precipitation...thermal
profiles would indicate that this precipitation would be in the form of
light freezing rain or light freezing drizzle. Another period I am
concerned about at this time is late tonight into early Saturday
morning...once again for areas across the southern County Warning Area. Will need
to monitor surface temperature trends across east central Illinois during
this time...with freezing rain once again possibly being an issue.
Expanding precipitation to the south is expected to inch further north
towards the southern County Warning Area towards the 12z Saturday time
frame...with southwest flow aloft persisting out ahead of the
approaching longwave trough. With a slow temperature trend expected
Saturday morning...a period of light freezing rain could cause
some issues primarily south of I-80.
With regards to winter storm Saturday through Sunday...latest
thinking/updates to forecast continue to support current Winter
Storm Watch across northern portions of the County Warning Area. Expected varying
precipitation types including almost all winter precipitation types could be
observed Saturday into Saturday night...with a varying thermal
profile expected to change through the period for all areas. Will
need to monitor the Saturday morning time frame into the day
Saturday for the varying preicp type...possibly adding yet another
headline to accommodate for the wintry mix. Expected surface low
track still supportive for best chances for snow or all snow
across northwest and north central Illinois with snowfall totals
in excess of 6 inches still appearing possible. The expected rain
snow line has varied slightly...but the general idea remaining the
same. Ohare to Peru Illinois line are the dividing line at this
time...with current snow forecast indicating rapid increase in
snow amounts right to the north of this line and with areas to the
south observing light snow accumulations.
Although latest trends are indicating that the axis of heaviest
rainfall is beginning to shift south of the County Warning Area...decent rainfall
amounts of up to an inch still support the possibility of
flooding...and have made no changes to the current Flood Watch.
Medium and long range...Monday through Thursday...
Main concerns beyond this weekend are the return of Arctic air and
much colder temperatures Monday-Tuesday...and the potential for a
little light snow Christmas evening-Christmas day.
Deep upper trough traversing the region Monday with fairly
impressive short wave energy passing overhead. Forecast soundings
indicate saturation in low levels...with temperatures near -15c as boundary
layer cools in blustery cold advection behind departing weekend low
pressure system. This should support flurries/light snow showers
across the County Warning Area into the afternoon hours. Of more importance than the
potential for light snow showers or flurries however will be the
much colder temperatures and wind chills as Arctic air plunges back into
the region. Temperatures at 800 mb fall to around -16 c by late afternoon with
h9 temperatures nearly so...suggesting surface temperatures may struggle to rise
above 10-15f and will likely fall into the single digits across
north central Illinois and wind chills in the negative single digits.
Surface high pressure ridge will build across the Mississippi Valley
and into Illinois Monday night in response to subsidence behind the
departing upper trough...with clearing skies...new snow cover
especially far northern Illinois...and decreasing winds expected to result
in strong radiational cooling. Both GFS and European model (ecmwf) MOS depict
sub-zero lows across much of northern Illinois by sunrise Tuesday. Surface
high pressure ridge moves across the area during the day Tuesday...
with winds backing to the south-southeast during the afternoon as
warm advection develops (aloft at least) in advance of a short wave
emerging from the plains. Moderation in temperatures limited at the surface
however with very cold start to day and strong inversion limiting
warming at the surface with highs in the teens/low 20s.
Long range guidance similar in bringing short wave trough across
the region during the Tuesday night/Wednesday period and another
Wednesday night/Thursday...though differences do exist with
amplitude of these individual waves and associated surface
reflections. The European model (ecmwf) is stronger/more amplified with both...
though at this distance have focused mainly on similarities between
guidance. This yields a little temperature moderation Christmas evening
and Christmas day with a chance of some light snow...perhaps
0.5-1.0" based on rough quantitative precipitation forecast output. Light snow may linger across
the eastern County Warning Area Christmas night into Thursday with surface cold
frontal trough. As mentioned...brief moderation produces highs in
the 20s/around 30 for Christmas day then colder air looks to return
Thursday behind the front with temperatures in the teens/low 20s.
//Ord and mdw concerns...updated 09z...
* ceiling/visibility trends through the day with IFR continuing. Dense fog
in close proximity to mdw with 1/4-1/2sm visibility still possible
* Periodic -ra/dz through midday with low chance for some
-fzra/fzdz this afternoon...mainly a Ord.
* North to north-northeast winds increasing to around 10 knots then
turning northwest this evening.
Warm frontal boundary has been oscillating north and south over a
narrow corridor from roughly kgbg to kgyy and Southern Lower
Michigan the past few hours. This has resulted in a somewhat
transient band of dense fog which as been affecting mdw and gyy at
times. A look at area webcams and surface observation shows that there has
been some slight improvement in visibility right along the boundary but
this may be due to the band of dense fog moving back and forth vs.
Any change in lowest visibilities. Expect that the boundary will meander
through daybreak leaving mdw/gyy in a variable visibility situation and
cannot rule out a return to 1/4-1/2sm visibility for at least a short
time. The boundary does look to push south later this morning as
surface low pressure moves northeast along it. This should allow
northerly winds to set up with cooler air spreading south which
should help visibility. May be tough to lose the IFR ceilings but MVFR visibility
looks to occur toward/during the afternoon at some point.
Otherwise periods of -ra/dz will continue through the morning.
Messy conditions continue across the terminals tonight with a
frontal boundary stretching from low pressure over Oklahoma to
roughly a keok to kc75 to kvpz line. Abundant moisture and lift
continue over the boundary bringing rain showers and areas of
drizzle to the terminals. Visibility is lowest right along the boundary
with gyy around 1/4sm and this will likely persist as long as the
boundary is nearby...which at this point looks to be for much of
the overnight. Areas north have seen visibility generally around a mile
and this should be the trend through the night...though there may
be some downward fluctuations at times. Ceilings should stay steady
in the 002-004 range through the night as well. Rfd may see some
freezing rain/freezing drizzle overnight as temperatures cool slightly.
Things become less certain after daybreak. The low will track
northeast to North-Central Indiana by Friday afternoon turning
winds from northeast to north-northwest through the afternoon. The
main question will be how quickly and to what extent ceilings/visibility
improve. With the boundary still in the area through the morning
will only have very minor increases in ceilings/visibility with better
increases during the afternoon as colder air filters into the
region. Showers/drizzle will continue periodically before tapering
off during the afternoon. Rfd will have the best chance of seeing
freezing rain through the morning...with a low chance that some
light freezing rain or freezing drizzle will make it into the
Chicago terminals during the early afternoon as colder air
arrives. MVFR and possibly IFR look to be in place for Friday
evening with north-northwest winds.
//Ord and mdw confidence...updated 09z...
* high confidence in IFR continuing with lowest conditions through
daybreak. Low confidence with speed/magnitude of ceiling/visibility
improvement through the day.
* High confidence in precipitation winding down this afternoon...medium-
high confidence that threat for -fzra/fzdz at Ord is low.
* High confidence in wind trends.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
Sat...wintry mix of precipitation from midday on...with a chance of a
period of freezing rain during the night. MVFR to IFR.
Sun...snow. MVFR to IFR.
Monday...MVFR ceilings. Chance of snow showers with IFR.
Wednesday...VFR becoming MVFR ceilings.
Thursday...MVFR ceilings improving to VFR.
301 am CST
Low pressure centered over north-central Oklahoma will track
northeast along a frontal boundary which extends northeast into
northern Indiana and eastward across Lake Erie and Ontario.
Meanwhile high pressure over the northern plains will move little
through the day. The strongest pressure gradient is starting to set
up early this morning and will likely result in an increase in
northeast winds across much of the lake through middle morning before
the pressure gradient eases. Speeds may push 30 knots for a time
through middle or late morning...especially across central sections of
the lake. The low will pass south of the lake later this afternoon
and begin to weaken with the high weakening as well. Another surface
low will develop over the arklatex Saturday and lift northeast to
Northern Ohio by Sunday afternoon. A very strong high will then
develop southward into the northern plains as the low moves into New
England Sunday night. This will leave Lake Michigan in the pressure
gradient between the two systems Saturday night and Sunday with
colder air spilling across the lake as well. This will set the stage
for another round of strong winds which will start out northeast
then shift north-northwest and ease up late Sunday afternoon. The
high will pass south of the lake Tuesday before low pressure tracks
southeast across the Canadian prairies and northern plains. This
should lead to another period of increased winds...this time from
the south and southwest Wednesday. Will keep speeds below gale force
through the period but will need to monitor for short windows of
gale force winds Saturday night and Sunday morning...and again early
Illinois...Freezing Rain Advisory...ilz003-ilz004-ilz005-ilz006-ilz008-
ilz010-ilz011-ilz012 until 3 PM Friday.
Winter Storm Watch...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012...6 PM Saturday to 6 PM Sunday.
Flood Watch...ilz033-ilz039...9 am Saturday to 6 am Sunday.
In...dense fog advisory...inz001-inz002 until 9 am Friday.
Flood Watch...inz010-inz011-inz019...9 am Saturday to 6 am
Lm...Small Craft Advisory...lmz740...9 am Friday to 9 PM Friday.
Dense fog advisory...Indiana nearshore waters until 9 am Friday.
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