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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
300 am CDT Monday may 4 2015

1130 PM CDT

Monitoring isolated shower/thunderstorm development across north
central Illinois late this well as an approaching
line of thunderstorms over eastern Iowa and southwest Wisconsin.

Broad warm air advection with low level jet oriented into northern Illinois is allowing for
isolated development at this time and with weak instability in
place...strongest cells are producing pea size hail. This will
likely continue over the next couple of hours out ahead of
additional development to the northwest of the County Warning Area. Have this
line of showers/thunderstorms to the County Warning Area around 1 am CDT...but
with some uncertainty still in place as to the overall evolution
of this precipitation after its arrival. As its already weakening and
slowing slightly with outflow pushing ahead of it at this
time...likely scenario is for this line to make it to northern
Illinois with only a marginal/isolated strong to severe threat.
Still think the more likely scenario tonight in terms of hazards
will be small hail with heavy rainfall primarily over northern



Short term...
303 PM CDT

Through Monday...

Compact but fairly vigorous mesoscale convective vortex moving into northwest Illinois
middle afternoon will continue to push across far northern Illinois
through early this evening. Convection has re-flared entering into
the diurnally favorable period and with weak destabilization
taking place in advance of the mesoscale convective vortex would anticipate showers and
thunderstorms to continue to March steadily eastward across mainly
northern half of the County Warning Area...likely moving out onto the lake early
this evening.

Should see a lull in the convection for a time in the wake of the
mesoscale convective vortex before chances of showers and thunderstorms begin to ramp back
up again later tonight with the approach of the cold front from
the northwest. Guidance continues to hit quantitative precipitation forecast pretty hard late
tonight into first half of the day Monday and am maintaining
likely probability of precipitation...but given that we will be entering a typical
convective downtime am quite Leary about about showers and
thunderstorms being as widespread and precipitation heavy as model
would suggest.

Synoptic front...potentially modified by convection tonight into
Monday morning...will be in the process of making its way
southward Monday and likely get an added boost from the lake.
Models differ on timing of the frontal passage Monday and this
timing uncertainty along with the potential for convection and/or
convective debris add a lot of uncertainty to the temperature
forecast for Monday. When the lake enhanced front moves through
look for temperatures along the shore to crash into the 50s to perhaps
40s right along the shore. Cannot rule out some marine fog as well
given the cold marine layer encountering 55f+ dewpoints.

Could see some diurnal redevelopment or re-intensification of
convection Monday afternoon along the front...but guidance suggests
better large scale ascent will be moving away from the stalling out
frontal boundary during the afternoon. Ahead of the front
temperatures should once again soar into the 70s...potentially lower
80s where rain and cloud cover hold off and allow for better



Long term...
303 PM CDT

Monday night through Sunday...

Quasi-stationary front likely to take up residence over the area
Monday night and potentially through Wednesday night depending on
your guidance of choice. Convection will likely ultimately play a
role in frontal timing...but certainly the slower northward advance
advertised by the WRF-NAM would seems reasonable based on the
expected periodic mesoscale convective system activity. One such mesoscale convective system looks to potentially
affect the area Monday night into early Tuesday morning...before
rain chances with the front dwindle a bit as upper heights rise
placing the more favored vorticity track well north and removed from the
frontal boundary.

There is the potential for fog north of the boundary Monday night
really through Wednesday...particularly at night inland and just
about anytime near the lake.

Front is prognosticated to begin heading back northward Wednesday and place
the majority of our County Warning Area in the warm and humid air mass. The one
potential exception Wednesday could be the Illinois Lakefront where
southeast wind cutting off the lake could prevent the full effects
of the warmth from reaching the beaches. By Wednesday night and
Thursday a stronger southerly flow should send the front well north
into Wisconsin with unseasonably warm and humid air mass over the
entire County Warning Area Thursday through Friday.

Front does look to sag southward into the vicinity of our County Warning Area over
the weekend bringing the threat of occasional showers/storms and
probably setting up a large temperature gradient once again with
winds off the lake north of the boundary potentially keeping
Lakeside areas rather chilling while locations south of the front
remain warm/humid.




//Ord and mdw concerns...updated 06z...

* thunderstorms and rain likely in the predawn to daybreak time for at least a
couple hour window. Scattered thunderstorms and rain could be seen both before and
after this.

* Winds turning northwest with thunderstorms and rain and then likely a period of
variable before shifting north-northeast middle to late morning.

* MVFR ceilings likely near and after the wind shift for a period
during midday and possibly the entire afternoon. Chance for IFR

* Chance of additional rain showers/thunderstorms and rain this evening and more so

//Discussion...updated 06z...

A complex of storms will continue to shift east-southeast and very
likely across taf sites prior to or near daybreak. This should
continue on a gradual weakening trend...but enough instability to
warrant thunderstorms and rain prevailing for a couple hour window at northern Illinois
taf sites. Behind the initial push of storms...some redevelopment
could occur as seen in central Iowa as of 05z...but even if that
were to occur it should be more light and maybe just rain showers.

A wind shift to the west-northwest is likely with the convection
including a few gusts over 20 knots especially at rfd where the gusts
may last close to two hours. The forecast for wind direction then
gets very challenging as a period of variable wind is looking
likely at and just after daybreak...but for exactly how long and
what speed is difficult to say at this point. A surface cool front
will then swing southward turning winds to the north-northeast at
all taf sites by midday. MVFR ceilings are likely to accompany this
middle shift with the chance for IFR....especially for taf sites not
far from the lake as lake-cooled air collides with dew points in
the middle-upper 50s.

Later tonight into early Tuesday morning will likely see more
showers and storms as an upper level disturbance advances along
and to the north of the frontal boundary that will be laying to
the south of the taf sites. At this point think better chances are
after 06z Tuesday.

//Ord and mdw confidence...updated 06z...

* medium-high in thunderstorms and rain occurring and timing.

* Low on specific wind direction during the morning rush time as a
period of variable within that time is likely.

* Low-medium on timing of wind shift to north-northeast.

* Medium-high in MVFR ceilings but low on timing and duration.

* Medium in rain showers/thunderstorms and rain Monday overnight.

//Outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday...rain showers with periods of thunderstorms and rain likely. MVFR likely at times. East-
northeast winds.

Wednesday...chance of thunderstorms and rain...though primarily VFR. Southeast winds.

Thursday...VFR. South winds.

Friday and Saturday...chance of thunderstorms and rain. South winds...becoming north late



257 am CDT

A cold front will advance southward across the region this
morning reaching the far southern part of the lake by early
afternoon. This will turn winds to the north-northeast. While the
lake has warmed considerably in the past few weeks...the cooler
air behind the front combined with the high moisture along it may
generate fog namely across the southern part of the lake. Winds
will turn easterly later tonight and into Tuesday and then back to
the southeast and south Tuesday night into Wednesday as the front
returns northward as a warm front. A warm sector will remain over
the lake during Thursday and Friday meaning stability should keep
gusts mainly in the 20-25 knots ballpark during that time.



Lot watches/warnings/advisories...


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