Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
857 am CDT Tuesday Jun 2 2015
310 am CDT
Fairly active flow in place across the plains and upper
Midwest...but much quieter in our region early this morning. Surface
high pressure remains in control...largely centered to our
north...with the ridge axis stretched from northern lower Michigan
into the lower Mississippi Valley.
Active convection in the plains driven by vorticity maxes riding along
the northern edge of the upper ridge will remain to our west for a
couple of days as the ridge axis shifts eastward and upper level
heights continue gradually increasing. As this occurs...debris/high
cloud cover will drift closer each day. We may also have some fair
weather cumulus as well. Therefore today will feature mostly
sunny skies...then Wednesday mostly sunny to partly cloudy
conditions. Surface high pressure will drift east some today...and
more so on Wednesday...but still influencing the area to keep any
moisture return from Gulf well west. The net effect of this to
allow a warmer air mass to invade the region...and also allow
surface flow to veer from northeasterly to southeasterly.
Therefore the general warming trend continues each day...with
typical lake cooling (of some 6-8f from interior Cook county)
given the synoptic easterly flow over the Cold Lake.
310 am CDT
Wednesday night through Monday...
A bit more complicated evolution to unfold for the weekend as
shortwave energy propagating ahead of the cutoff low off the
Pacific...and with guidance mixed on handling smaller scale vorticity
maxes and how/where they will progress through the region has
prevented any definitive decisions on best timing of precipitation
chances. Generally chances will be increasing Friday and into the
weekend. Thursday morning starts off with decaying convection once
again from the plains...but as compared to days past...some of the
lift...both aloft and from level isentropic lift/moisture
transport...may sneak into our western areas Thursday morning.
Upper level heights remain high on Thursday to preclude much
farther eastward progression of any showers/storms as the NAM
A similar scenario unfolds into Friday...with the precipitation
shield getting closer to the area but still likely in a decaying
state. Friday afternoon shortwave energy will likely be in the
region...and with an upper low moving east through
Ontario/Quebec...high pressure behind it will be sending a cold
front toward the area which will help to focus precipitation chances
Friday and for the weekend. These combined factors may be enough
to trigger some thunderstorms as well combined with afternoon
instability. All of this dependent on cloud cover from any nearby
morning convection. Therefore had to hang onto wishy-washy 50 probability of precipitation
for Friday from the smart blend initialization...though this seems
a decent day/evening to have a chance for storms.
Things get more muddled for Saturday as we need to see how quickly
the cold frontal boundary sinks through and if it remains in the
area. The same GOES for Sunday as northern Pacific energy attempts
to phase with some of the energy from the still cutoff low in the
west...and bring a a cold front with it. So have higher probability of precipitation for
Sunday than Saturday at this point in time...but confidence on the
weekend forecast certainly on the lower side until timing becomes a
bit more clear.
//Ord and mdw concerns...updated 14z...
* easterly winds near 10kts this afternoon.
High pressure over the Great Lakes region will slowly move east
tonight into Wednesday. Light winds to start will become more
easterly later this morning with speeds increasing to 6-9kt by
early afternoon. Its possible that a lake breeze may develop and
move west this afternoon. If this were to occur...there could be
a period of speeds of 10-12kts. Continued with prevailing 9kt at
Ord/mdw and trends will need to be monitored. Cms
//Ord and mdw confidence...updated 14z...
* high in wind direction and medium in speeds this afternoon.
//Outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
* Wednesday night and Thursday...dry/VFR. Southeast winds.
* Friday...chance of shra/tsra. MVFR likely. Northeast winds.
* Saturday...slight chance of rain showers. MVFR possible. Northeast
* Sunday...chance of shra/tsra. South winds.
* Monday...slight chance of shra/tsra. Gusty west winds.
330 am...high pressure centered over the Great Lakes will slowly
move east tonight and Wednesday. One area of weak low pressure
will move from the Dakotas tonight to Western Lake Superior
Wednesday as it dissipates. Another weak low will move across the
Ohio Valley Thursday night and Friday. Winds will remain rather
light during this time period. But as stronger high pressure
builds across Ontario on Friday...a cold front will likely move
south across Lake Michigan Friday into Friday night. Stronger
northerly winds are expected but uncertainty regarding how tight
the gradient will become leads to low confidence for speeds. Cms
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