Scientific Forecaster Discussion
fxus63 klot 221123
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
623 am CDT sun Jul 22 2018
253 am CDT
Main focus for the short term forecast period will continue to be
the impacts of an unseasonable upper low slowly tracking to the
upper Ohio Valley.
The low pressure system was located over Southern Lake Michigan.
Upper low that has been responsible for the unseasonably cool and
showery conditions in now east of the local area and dropping into
the upper Ohio Valley. The upper level pattern continues to be
slowly progressive with upper ridging building toward the
Mississippi Valley while the upper low remains nearly stationary,
while becoming more expansive as a series of weak shortwaves are
absorbed into the main circulation. The local area will remain
under deep layer northerly flow through tonight, keeping the chances
for occasional light showers across the region. While there will
likely be more dry time than wet, scattered light showers will be
possible through the day. As the upper ridge gradually builds ewd,
the pcpn potential will end from west to east overnight with
decreasing cloud cover as well. High temperatures will remain
unseasonably low, with highs in the upper 70s. Lowest temperatures
will continue to be near Lake Michigan, where north winds off of the
lake will keep highs only in the lower to middle 70s. Lows tonight
will be in the lower 60s.
253 am CDT
Monday through Saturday...
The long term forecast period will, generally, be quiet, with dry
weather and more seasonable temperatures. Upper ridging will
continue to build across the region through mid-week. Temperatures
for Monday through mid-week should be closer to normal, with highs
in the lower to middle 80s. An upper low is expected to move into
the upper Great Lakes region by Thursday, setting up broad upper
troughing over the ern 2/3 of the Continental U.S.. this will likely lead to a
cooling trend toward the end of the week and into next weekend.
for the 12z tafs...
Forecast concerns how long IFR cigs will last this morning and
continued scattered showers.
Showers continue to stream southeastward into NE Illinois and northwest in on
the back side of low pressure across the Ohio Valley. Showers do
reduce vis to as low as 3sm, but these reductions do not last
very long. The best convergence is across northwest in where a better
chance exists for stronger showers, though expect a continued
expansion of rain and IFR cigs in the 11z-15z period. There could
be a few lightning strikes over the lake this morning, then
across northwest in in afternoon. Ceilings are the lowest at the shore
and inland to Ord/mdw. Ceilings have generally been at 800 feet or
higher, and while there is a low chance that ceilings dip a bit
more briefly. Confidence on timing the lifting to MVFR is medium.
Guidance still contends the farther inland sites will generally
hold MVFR though still feel a period of IFR is possible farther
inland too. Better chances for showers at Ord/mdw will be through
midday. Expect this afternoon most locations will lift to VFR.
North winds will continue to drift to more northeast, maintaining
speeds of 10-15 kt and gusts to around 20 kt through the daytime
hours. Expect a decent ramp down tonight.
335 am CDT
Main concerns center around continued northeast winds
today and how long waves will take to subside across Southern Lake
Michigan. Low pressure over the Ohio River valley will linger
there today and then gradually sink south and weaken tonight.
Northeast winds will continue on the back side of the low through
the day. These winds will maintain higher waves, and thus the
Small Craft Advisory will remain in effect through the afternoon
for all of the nearshore waters. Scattered showers and possibly a
thunderstorm or two will continue through tonight. There is a
slight chance for waterspouts through Sunday morning, particularly
over the southern half of the lake. Winds will begin to gradually
diminish through the day as high pressure builds into the upper
Midwest. Winds will slowly ease Monday into Tuesday as the high
shifts into the western Great Lakes.
A weak wind pattern will then be in place through midweek, before
a cold front arrives on Wednesday night. Expect some storms ahead
of the front, then gusty west winds behind the front for the
latter half of the week.
Illinois...beach hazards statement...ilz006-ilz014 until 10 PM Sunday.
In...beach hazards statement...inz001-inz002 until 10 PM Sunday.
Lm...Small Craft Advisory...nearshore waters
until 5 PM Sunday.
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