Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
625 am CDT Sat Apr 25 2015
306 am CDT
Through Saturday night...
Main forecast concerns this morning will be dealing with precipitation
chances through the day today.
Low pressure is moving across eastern Kansas/NE this morning is prognosticated
to drift east-southeast across the middle Mississippi Valley this evening. Warm
advection Wing is in place across much of Illinois this morning and
will result in scattered showers through around daybreak before the
forcing shifts off to the east. However...more widespread rain will
move into the region from the southwest middle morning into the
afternoon as f-general band sets up across portions of northern Illinois
and Northwest Indiana. There has been a notable shift south in the
guidance over the past 24 hours with where this band will set up
and have adjusted probability of precipitation accordingly. NAM in particular has a sharp
cuttoff between the I-88 and I-90 corridors...as compared to the
GFS/European model (ecmwf) which still bring precipitation up past the Stateline. Have
highest probability of precipitation south of I-90 but am not as bullish as the NAM as far
as the northern cuttoff on precipitation and maintain high chance-likely
probability of precipitation up through the state line. Instability is pretty
meager...mainly south of a Rockford to Rensselaer line this
morning...then mainly along the southern tier of counties late
morning into early afternoon. Given the weak thermodynamics expect
any lightning activity to remain isolated. F-general shifts east of
the region after around 00z this evening with middle level height
rises building in behind shutting down precipitation.
In addition to the precipitation...moderate northeast winds will be in
place today as the gradient tightens north of the low track with
gusts of 25 to 30 miles per hour at times. Cold advection...cloud cover...and
rain will not allow much warming today with temperatures only
expected to top out in the middle to upper 40s. Northeast winds remain
breezy overnight with temperatures falling into the middle to upper 30s.
306 am CDT
Sunday through Friday...
Overall a relatively quiet stretch of weather is expected Sunday
through the upcoming week. High pressure builds from the Canadian
prairies into the upper Midwest Sunday driving 850 mb temperatures down to
around 0c to -2c though these temperatures will moderate to +3 to 4c by
midweek by late April insolation. This warming trend will
translate down to the surface as well with temperatures only in the
middle/upper 50s away from the lake Sunday to middle/upper 60s by
Wednesday. Persistent north/NE flow will keep conditions cooler near
the lake front all of next week and allow lake breezes to push
Surface pattern throughout much of the week has weak ridging in
place across the upper Midwest/Great Lakes which will greatly limit
precipitation chances. Both the European model (ecmwf) and GFS show an upper level
disturbance digging into the region Wednesday though with precipitable waters of
only around half an inch and soundings indicating negligible
instability...prefer to lean on the dry GFS solution...though will
maintain some slight chance probability of precipitation. The track of the upper wave diverges
after midweek resulting in some decent disagreement in the thermal
fields of the models Thursday and Friday resulting in lower than
average confidence in temperatures those days.
//Ord and mdw concerns...updated 12z...
* fairly Stout easterly winds today.
* Potential for some rain and a period of lower MVFR ceilings
possible later this morning into the early afternoon. Conditions
improve later this afternoon and evening.
* IFR ceilings looking less likely.
Low pressure over eastern Kansas will shift eastward across the
lower Missouri Valley today...and to the Ohio Valley by this
evening. This system will continue to produce gusty easterly winds
today...then becoming more north-northeast by later this evening
into Sunday. Confidence in the wind forecast continues to be
Light rain showers are gradually inching their way northward
towards the terminals early this morning. These showers have been
dissipating a bit as they move northward out of the more favorable
moisture. However...periods of light rain still look to be a
decent bet across the area into middle afternoon. We expected the
better focus for more moderate rain to remain mainly south of the
terminals. With the farther south focus of the better rain...ceilings
no longer appear that they will fall into the IFR or LIFR category
due to the presence of a continued easterly lower level flow of
slightly drier air.
Any lower ceilings that develop over the area today should improve
either by later this afternoon...or by early this evening as drier
air works back over the region.
//Ord and mdw confidence...updated 12z...
* medium-high with wind forecast.
* Medium with rain extent over the terminals.
* Medium with ceiling forecast.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...VFR. North-northeast winds.
Monday...VFR. Northeast winds.
Tuesday...VFR. Northeast winds becoming east.
Wednesday...slight chance -ra. Brief MVFR possible. North-northwest
winds becoming northeast.
Thursday...VFR. North-northeast winds.
210 am CDT
The main weather concerns over Lake Michigan will be today through
Sunday. An area of low pressure will shift eastward from the Central
Plains across the central Mississippi Valley today...then over the
the Ohio Valley tonight. Overall...this will result in some stronger
east-northeasterly winds today...followed by northerly winds later
tonight into Sunday. Wind speeds look to generally be up to around
25 knots through the period. However...the onshore component to these
winds could result in larger waves up to around or a bit above 4
feet along the Illinois and Indiana near shore waters through the
The weather pattern for much of next week looks to feature a rather
weak pressure gradient over the region as high pressure dominates.
This should result in a very weak wind regime for much of the
period...and hence wind speeds and directions will likely be largely
driven by lake and land breeze influences.
Lm...Small Craft Advisory...nearshore waters
until 10 am Monday.
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