Area forecast discussion...updated
National Weather Service Chicago/Romeoville Illinois
710 PM CDT sun may 19 2013
Mesoscale discussion...
709 PM CDT
Continuing to monitor strong convection in northwest Illinois and
environmental trends ahead of it. There is a likelihood for some
of this to reach the northwestern and western forecast area...but
severe potential may be limited.
The convection that developed near the Mississippi River late this
afternoon was and continues to be on the front edge of an
increasing middle-level jet. This is also on a surface warm front
and high T/dew point tongue with associated large cape of 3500+ j/kg
verified by the 00z dvn radiosonde observation data. Radar and observation trends
show that the storm area near Galesburg has an organized structure
and is moving more east-northeast and deviant from the mean steering
flow...with Galesburg having gusted to 39 knots. Bunkers motion
vectors indicate this should continue to move northeast likely
around 30 knots and be the primary storm area that could impact
western LaSalle/Lee/Ogle/Winnebago counties between 830 and 1100
PM. Deep layer shear of around 40 knots should increase to 50-55 knots
by late evening and support organized and regenerating storms in northwest
Illinois...which is supported by a portion of the convective allowing
guidance. These look to maintain at least a wind and hail threat
into the middle to late evening and a watch may be needed...per Storm Prediction Center
sels mesoscale discussion.
Mtf/krein
&&
Discussion...
308 PM...convective/precip potential remains the main forecast
concern/challenge...through middle week.
Thunderstorms that developed across western Illinois late this morning
dissipated as they moved northeast into a very capped atmosphere
while thunderstorms that developed a bit further north and are now
moving across far southern WI appear to be associated with a wave
moving across Iowa. This activity has also struggled to shift east
due to the capped environment. This wave lifts north this evening
and it appears any additional thunderstorms will spread across the
area later this evening and more likely overnight from convection
firing across the plains. Short term guidance continues to suggest
additional development across northern Missouri late this
afternoon which would arrive in the western County Warning Area by sunset but its
difficult to see what wave would help kick off this activity but
have maintained low chance probability of precipitation through middle evening across the
western County Warning Area.
With the expectation that overnight convection will be spreading
northeast and dissipating Monday morning...this should allow for a
period of dry conditions into early Monday afternoon. Cap Monday
afternoon is much weaker than today and even appears to be gone
across western areas by middle/late afternoon. Little change to the
airmass over the next 24 hours...perhaps dewpoints slightly higher
thus while thunderstorm coverage/timing remains uncertain...
anything that develops will have the potential to become severe
with damaging winds the main threat and large hail the secondary
threat.
From Monday night through Wednesday morning...there again appears
to be some consensus...especially with the European model (ecmwf)/Gem for a series
of waves to move across the western/southern lakes region as the
upper low slowly shifts east through midweek and eventually opens
into a trough with a period of thunderstorms/heavy rain Monday
night into Tuesday morning and the again Tuesday night into
Wednesday morning. With precipitable water values approaching 1.5
inches and plenty of level moisture...locally heavy...perhaps even
torrential rainfall will be possible. Storm total model quantitative precipitation forecast
amounts are still somewhat low ranging in the 1 to 2 inches but
should convection train over the same areas...locally higher
amounts are certainly possible. The main surface low moves east
across the southern lakes Wednesday into Wednesday night with a
trailing cold front pushing south across the area. This will usher
in much cooler and drier conditions for end of the week.
Highs today have reached in the middle/upper 80s most locations with
several locations tagging 90 across the western County Warning Area. Depending on
clouds and convective trends...upper 80s/lower 90s look reasonable
again for Monday. Southerly winds should be strong enough to
prevent any lake cooling but outflow from any thunderstorms could
turn winds onshore with much cooler temperatures. Highs on Tuesday
still potentially into the 80s but again dependent on cloud cover/
precipitation trends. Cms
&&
Aviation...
//Ord and mdw concerns...updated 00z...
* a lake breeze with a wind shift to easterly in close proximity
to both airports through 02z.
* Chance of thunderstorms and rain late tonight and through all of Monday...with the
most favored times highlighted in the tafs.
* Gusty south winds through Monday.
//Discussion...updated 00z...
A complex aviation forecast with chances of thunderstorms for much
of Monday and Tuesday. Initially...a lake breeze which developed
earlier had retreated due to a warm front influence...but now has
pushed back. Per coordination with Ord tower this is basically
sitting over the airfield and should through 01z or so. Wind gusts
at times will be between 10 to 15 knots with speeds fluctuating
between 100 and 170 degrees. This feature should remain just east
of mdw. The only other wind note for tonight is that aloft a low-
level jet will Orient itself over the area with SW winds of 35-40
knots above 1500 feet. This does not look to present true low level wind shear...but
may be close...namely at rfd.
As for convection...ongoing storms in western Illinois have a favorable
environment and radar trends to evolve northeast into part of the
far western tracon area and near rfd. Some of these could have
some gusty winds near rfd. The better chance for storms at
chicagoland taf sites arrives on Monday and Monday
night...although it is tough to time the best window. It is
expected that the numerous storm complexs over the plains/MO/IA
will fade late tonight into the morning as they had east...but the
remnants could bring scattered rain showers/thunderstorms and rain to the area in the morning.
Monday will present less of a cap in the atmosphere so at least
scattered convection will probably be seen in northern Illinois by late
afternoon/eve.
With a tighter pressure gradient and continued pressure falls
across the area on Monday...southerly winds should see gusts to
the 25 knots ballpark during the late morning and afternoon across
all taf sites.
//Ord and mdw confidence...updated 00z...
* high confidence that the lake breeze will remain over/very
close to Ord through 01z. Medium confidence on speeds. High
confidence on wind trends after 02z.
* Medium confidence on morning rain showers chances.
* Medium confidence in at least having convection in the region on
Monday afternoon and evening but low confidence on specific timing.
//Outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...periods of thunderstorms and rain. Gusty SW winds.
Wednesday...likely thunderstorms and rain during the day. Chance during the night.
Potential for MVFR ceilings.
Thursday...chance of rain showers.
Friday...VFR likely. Gusty NE winds.
Saturday...chance of thunderstorms and rain during the night.
Mtf
&&
Marine...
159 PM CDT
The main weather maker for the next few days will be the surface
low...currently across the plains. This storm system is expected to
move east-northeastward towards the middle Missouri Valley tonight into
Monday morning. As it does so...an associated surface warm
front...extending eastward across northern Illinois and
Indiana...will move northward across at the southern half to two
thirds of the lake later tonight and on Monday. In the wake of this
front...the winds will shift more southerly and should increase up
to 15 to 25 knots by Monday. However...to the north of this
front...winds will primarily remain easterly at around the same
magnitude.
The other concern will be the threat for periods of dense marine fog
later tonight and into at least the first part of the week. As
this warm front lifts over the lake...much higher dew points will
move over the relatively cold water...possibly resulting in some fog
development.
It appears that the surface low will slowly move eastward across the
middle Mississippi Valley and to near Southern Lake Michigan by middle
week and then continue eastward across the lower Great Lakes region
later in the week. This should allow warm front to shift back
southward with time during the week. During this time...the wind
directions will primarily become east northeastward 15 to 25 knots
north of the front. Later in the week...as the surface
low passes east of Lake Michigan...the winds still look to be rather
Stout up to 30 knots out of the north for a period as a good area of
high pressure builds eastward across Ontario and the upper Great
Lakes region.
Kjb
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&
$$
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