Scientific Forecaster Discussion
fxus63 klot 230523
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1223 am CDT Thu Mar 23 2017
158 PM CDT
Warm advection will kick in tonight as cool high pressure exits to
the eastern Great Lakes region. Mid and high clouds from low
pressure off the California coast will stream east-southeast over the area
tonight over a flattening ridge across the plains. With the
increasing clouds expect temps to hold a tad warmer than last
The developing warm front will shift northeast across the area
later Thursday and Thursday night. Lower level warm advection with
this front will likely kick off some scattered afternoon showers.
Weak upper ridging and southeasterly flow maintaining drier lower
level conditions will preclude more widespread shower activity.
In spite of cloudier conditions, highs will recover closer to
normal, except away from the lake in northeast Illinois where onshore
330 PM CDT
Thursday night through Wednesday...
Theme of the extended is a jump to much warmer temperatures Friday.
A cold front brings temps back down into the 50s and 60s Saturday
and temps stay relatively steady through the remainder of the
forecast. Periods of rain and storms are expected for much of the
A surface low takes shape over the plains Thursday night, and we'll
be in the warm sector Friday. Still looking at above normal highs
around 70, but have questions regarding how well we will warm with
thick cloud cover overhead. Typical gusty southwest winds are
forecast, and continue to think the majority of precip will stay in
WI, north of the forecast area.
Showers spread across the area from northwest to southeast Friday
evening through Saturday morning as a cold front surges down the
lake and upper level vorticity lobes move over the forecast area. I
have medium-high confidence in embedded thunderstorms Friday night,
but less confidence in storms Saturday morning. Cape and the better
forcing look to be tied to low itself, so if the low is closer to
the forecast area Saturday, could see more thunder than forecast.
However, if the low is slower and further from the forecast area,
there could be less thunder. I have high confidence in measurable
precip Saturday, but there could be brief dry periods.
The low moves over the forecast area Sunday and continues northeast
to the eastern Great Lakes Monday. Showers associated with the
first low come to an end Sunday night. Long range models differ on
how the next low will evolve, but they do agree that we may be on
the northern fringe of its precip. High pressure then builds in mid
Extended temps past Friday will generally be in the 50s or low 60s
away from the lake. Onshore flow will keep the lake front around 10
for the 06z tafs...
A surface area of high pressure over the eastern Great Lakes will
shift eastward over the central Atlantic coast by late afternoon.
As this occurs, expect the current light easterly winds to become
southeasterly 10 to 15 knots during the day. By late this evening, a
warm front, associated with low pressure developing over the
Central High plains, is expected to shift northward over the area.
Prior to its passage, a period of MVFR cigs and some scattered
showers will be possible, particularly during the evening.
However, once the front lifts north of the area, expect south-
southwesterly winds with the main precip chances and lower ceilings to
shift northward into Wisconsin.
Low level wind sheer may also become an issue tonight as winds
1500-2000 feet above ground level increase to 50-55 knots. In spite of the fact that
surface winds could be up around 10 to 15 knots (with some slightly
higher gusts) late tonight with the passage of the warm front, the
actual magnitude of the winds off the surface are high enough to
justify a mention of such in the latest tafs.
330 PM CDT
High pressure over the lake will continue to shift southeast
reaching the mid Atlantic coast Thursday. 30 kt south winds set up
on the back side of the high Thursday. Do not have the confidence
to issue a Small Craft Advisory at this time, but winds and waves
may be hazardous to small craft Thursday. A cold front sinks south
across the lake Friday and Friday night, and winds become north
behind it. Meanwhile, the next surface low forms over the plains
Thursday night into Friday. The low weakens as it moves over
Illinois Sunday morning and then dissipates over the eastern Great
Lakes Monday. Guidance then differs on how the pattern will evolve
early next week. The European model (ecmwf) features a baggy pattern with light
winds while the GFS has another low move up the Ohio Valley with
strong northerly winds over the lake.
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