Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
631 am CDT sun Mar 9 2014
325 am...forecast concerns include the chance for rain changing
to snow Tuesday afternoon into Tuesday night with some snow
accumulation possible across the southern County Warning Area...and temperatures
through the period.
A ridge of high pressure extends from the Southern Plains into
western Illinois early this morning and will shift southeast today.
While light winds have shifted west/southwest with weak warming
aloft. Despite the light winds...temperatures have remained in the middle
teens or warmer. Still expect some oscillation in temperatures through
sunrise but overall trend will likely be near steady temperatures.
Increasing cloud cover later this morning into this afternoon
expected to be primarily high clouds with perhaps more broken middle
clouds toward sunset. So cloud cover not expected to inhibit
temperatures warming into the Lower/Middle 40s with a few upper 30s
possible over some of the deeper snowpack. It will also become
breezy especially across the northern County Warning Area today with breezy
conditions continuing into tonight. These winds combined with
continued warming aloft will allow temperatures to only slowly drop
through the evening and into the overnight hours. Bumped lows up a
few degrees into the middle 30s with upper 30s in Chicago. Confidence
regarding just how warm temperatures remain overnight is low but its
possible temperatures could stay in the lower 40s in the downtown area.
With even warmer temperatures expected overnight...highs should easily
reach into the 50s on Monday. With dewpoints reaching into the
lower 40s across the southern areas...rapid snow melt and emerging
bare soil/ground will likely allow many areas to make a run at 60
and 60 isn/T out of the question for the southern suburbs and Cook
County especially with sunshine expected but for now have held
temperatures in the middle/upper 50s for much of the metropolitan area.
A frontal boundary will sag south across the area Tuesday into
Tuesday evening. Timing differences for wind shift...cloud cover
and arrival of precipitation could have large impacts on temperatures but have
made no changes...lower 40s north to Lower/Middle 50s far south.
Light rain appears to hold off until the afternoon and then just
across the northern half/third of the County Warning Area before expanding south
Tuesday evening with cooling aloft leading to a changeover to
A developing storm system over the plains Tuesday will move east
across the Ohio Valley Tuesday night into Wednesday. The gradient
will tighten as the low deepens with a swath of heavier precipitation
north of the track of the low which comes across central Illinois
and central/Northwest Indiana. Still some uncertainty regarding
where the heaviest quantitative precipitation forecast falls. In addition...how fast precipitation
changes over to snow and possible rather wet snow/water ratios.
But at least a few inches of snow accumulation is possible along
and south of the I-80 corridor...potentially more but too early
for any specific numbers. Precipitation would be ending from northwest to
southeast Wednesday morning though there has been a trend of a
weak wave dropping across the area Wednesday afternoon/evening
that could produce a few snow showers.
Wednesday looking rather chilly compared to the next few days as
highs will likely remain in the Lower/Middle 30s. But flow quickly
shifts back southwesterly Thursday with highs rebounding into the
Lower/Middle 40s. GFS/European model (ecmwf) have sped up the arrival of a weak cold
front Friday afternoon which may keep temperatures from reaching the
advertised 50s but Gem is slower and have made no changes to
//Ord and mdw concerns...updated 12z...
* gusty south southwest to southwest winds this afternoon and evening
* low level wind shear probable this evening
The area of high pressure centered from the Southern Plains
through central Illinois this morning will continue progressing
south. As the southwest winds increase in response this
morning...gusts will become more frequent by 16z or so with
24-28kt gusts probable during the afternoon. As a clipper warm
sector spreads over much of the Great Lakes this evening...mixing
looks sufficient enough for still some gustiness through 07z or
so. A west southwest wind maximum aloft at 1500-4000 feet is
expected across the region by this evening...so continue low level wind shear in the
forecast for that. Clouds should thicken some tonight but no
impacting sensible weather expected.
//Ord and mdw confidence...updated 12z...
* high in wind direction remaining between 200-230 degrees during
this afternoon and evening
* high in 45-55 knots southwest flow between 1500-2500 feet during this
evening and medium in true low level wind shear developing within that time.
//Outlook for Ord/mdw for 00z Tuesday-12z Sunday...updated 12z...
Monday night...VFR likely.
Tuesday...chance of rain during the day with a chance of snow during the
night. IFR possible.
Wednesday...slight chance of snow. Gusty north-northeast winds.
118 am CST
While the pattern over the lake has been quietly as of late...as
reflected by the over 90 percent ice cover...a more active
pattern is forecast through this week. The first system will be a
quickly-moving low pressure from Lake Winnipeg early this morning
to just north of Lake Superior by late in the day. South southwest
winds will be on the increase this morning with the pressure
gradient between this incoming low and the departing high pressure
to the south. The going Gale Warning for 40 knots gusts over the
north looks sound from noon into early evening...which is when
the maximum pressure falls move across the lake. Along the
Illinois and Indiana near shore...Small Craft Advisory gusts of 25
to probably a few 30 knoters look good and these may even
continue into early overnight.
Another low pressure on Monday will take a similar path on the heels
of todays low. Not as strong of winds are expected just simply due
to less of an evolving pressure gradient. However would still expect
20-25 knots winds for a better part of the day. A cold front will drop
southward over the lake on Monday night but initially the cold push
behind this front will be minimal. As another low pressure moves
up the Ohio River valley during midweek...this should provide
enhanced northerly flow down the lake. At this point it looks
below gale force but if the southern low were to track further
north then conceivably the southern part of the lake would see a
Some ice break-up is likely in the next 48 hours which will play
into the wave model projections for waves. For the time being will
continue to omit waves from the northern half based on still solid
ice coverage seen on yesterdays visible satellite imagery.
lmz868...noon Sunday to 9 PM Sunday.
Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...noon Sunday to 10 PM Sunday.
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