Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
105 am CDT sun Sep 21 2014
936 PM CDT
Latest radar imagery depicting scattered shower and thunderstorm
development persisting mainly across the far southern portions of
the County Warning Area...while isolated development continues elsewhere. This
line of showers/thunderstorms are developing along low level
convergent axis stretched across central Illinois within moderate
instability...while isolated development is forming near any
better low level convergence. Heights of these storms have not
been overly great...likely due to lack of any steep lapse rates.
Will likely observe a southward trend with this precipitation over the
next couple of hours...with isolated small hail or gusty winds
still not out of the question with any stronger storms. Large vorticity
maximum will continue to swing east/southeast across the upper Midwest
and Great Lakes region tonight...while cold front/surface trough
shift southeast through the County Warning Area. Current precipitation over southwest
Wisconsin will shift southeast through the northern third of the
County Warning Area along these features...but do expect best development to
remain in Wisconsin. A thunderstorm or two will be possible but
dont expect coverage to be overly great...as instability axis will
shift southeast with time through early Sunday morning.
329 PM...main forecast concerns are severe potential through
early this evening then cooler temperatures with rain showers for
Line of thunderstorms from Northwest Indiana back through west
central Illinois moving into rather moist and unstable air and this will
be the best location...southern/southeastern third of the County Warning Area...
for severe weather into early this evening. Further north...precipitation
is more showery with embedded thunder and the back edge of this
precipitation should be through much of the northern half/two thirds of
the County Warning Area by early evening.
An upper wave moving southeast across the upper Midwest will move
across the western lakes later this evening with precipitation with this
feature likely remaining mainly north of the County Warning Area but some showers
or an isolated thunderstorm is still possible later this evening.
A strong cold front will then move south across the lakes region
overnight reaching northern Illinois Sunday morning. This will shift the
winds north/northeast by afternoon with a period of showers
possible from middle morning through middle/late afternoon. Some
uncertainty regarding coverage but potential is high enough for
chance probability of precipitation for much of the day. Precipitation and cloud cover along with
colder air spreading into the region will keep temperatures cooler and
only have highs in the 60s.
As high pressure moves across the area Sunday and Monday...light
winds will likely allow temperatures to cool to the dewpoints with fog
possible. The rest of the period is expected to be dry with temperatures
still in the 60s Monday...then warming a few degrees through the
70s each day through the end of the week. Cms
//Ord and mdw concerns...updated 06z...
* isolated thunderstorms and rain ending by 7z.
* West winds veering northwest by daybreak. Speeds generally around
10-14kt with gusts developing after daybreak to 20-24kt.
* MVFR ceilings developing around 10z and likely persisting through early
Line of showers and thunderstorms moving through at taf time...should
quickly end by 630z for Ord/mdw...gyy will be closer to 7z.
Elsewhere storms have moved to the east with mainly VFR ceilings
pushing in. However further north across Wisconsin MVFR ceilings have
developed and will continue to slide southeast towards northern Illinois
airfields...but hold off until around daybreak. Winds have mainly
become westerly with speeds around 10-14kt...and as daybreak nears
gusts will be developing to near 20-24kt. Guidance indicates that
with the middle-level system dropping southeast towards the taf
sites...MVFR ceilings will be developing and likely persist Thursday early
afternoon. In addition it appears some vcsh or light rain showers will accompany
the low ceilings later this morning through early afternoon. Winds will also
be turning northwest then north by this afternoon. Then drier air will
arrive and scour the low clouds by sunset.
//Ord and mdw confidence...updated 06z...
* high confidence in wind dir/speed/gusts.
* Medium confidence in precise timing of ceilings...high confidence in
MVFR ceilings developing though.
* Medium confidence in timing and placement of rain showers later today.
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday and Tuesday...VFR.
Wednesday through Friday. VFR. Southeast winds.
123 PM CDT
Front through the middle Mississippi Valley. The first round gales
has subsided and the stronger...Small Craft Advisory level winds
over the nearshore waters are subsiding as well as the frontal
trough approaches the lake and the gradient weakens. As the low
lifts to the northeast through Quebec...it will drag a cold front
across the lake...turning winds northwesterly and then northerly by middle day
Sunday. The strongest push of cold advection should be over the
southern portions of the lake...so...there is still a chance for
marginal northerly gales over the southern portions of the lake. Will
maintain the going gale watch for Sunday and not upgrade to a Gale
Warning just yet since there is still some uncertainty as to the
timing...duration and extent of gales. The front will finally push
south of the lake by middle day Sunday with high pressure building over
the Central Plains...maintaining northerly winds through Sunday night or
early Monday morning. By Monday afternoon...the center of the high
will slide just south of the lake and then move to the eastern Great
Lakes Tuesday...allowing winds to turn southerly again.
However...do not expect the southerly winds to get too strong...perhaps
10-15kt...since the areal extend of the high will spread from the
East Coast to the middle Mississippi Valley...setting up a weak southerly
gradient. The next upstream trough developing in the Lee of The
Rockies Thursday and Friday is not expected to make much eastward
progress until Saturday night or Sunday...so the light southerly winds
should prevail over the lake through the weekend.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...7 am Sunday to 10
lmz876-lmz878...9 am Sunday to 7 PM Sunday.
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