Scientific Forecaster Discussion
fxus63 klot 250530
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1230 am CDT Thu may 25 2017
836 PM CDT
Rain showers are still retrograding westward across the eastern
and central forecast area early this evening, but are beginning
to show a coverage fade. These showers are associated with the
mid-level circulation across eastern Indiana/western Ohio. This
wave will move slowly north-northeast overnight with wraparound
700-925mb moisture over the area gradually easing. Showers should
have an associated slow dissipation through tonight.
With the surface reflection deepening to around 992 mb by daybreak
near Cincinnati, the pressure gradient will actually tighten
overnight over the area. The northerly wind component should
preclude much for fog across most of the area, with rap
hydrolapses also favoring stratus more than fog. Confidence in
clearing during the day Thursday remains low, though building
heights across north central Illinois should at least allow for
some holes to develop there, similar to seen in eastern Iowa this
215 PM CDT
Sharp upper level trough with pinwheeling lows will continue to
impact the region in the near term bringing overcast skies
areawide and scattered showers. Water vapor imagery this
afternoon reveals two upper lows embedded within a broader
longwave trough centered over the Mississippi Valley. A western
low is digging south across Missouri into Arkansas while another
low is lifting north across the Ohio Valley. The western low
resulted in some scattered showers over the western County Warning Area earlier
today but is otherwise exiting the area with mid level height
rises building across the mid and upper Mississippi Valley. More
widespread precipitation is in place with the eastern low and some
of the showery activity with this low will impact areas mainly
east of the I-55 corridor through the remainder of the afternoon
and evening. There is some very modest instability over
northwestern Indiana, 100-200 j/kg MLCAPE which may result in an
isolated embedded thunderstorm or two, but overall lightning
potential is low for the County Warning Area.
The western upper low is progged to dig into the deep south late
this evening and overnight which will help kick the eastern low
farther east out of the County Warning Area. This will result in diminishing
precip chances through the late evening and overnight hours.
Lingering low level cyclonic flow through mid afternoon Thursday
will likely keep much of the County Warning Area locked under cloud cover, but
ridging building into our west mid to late in the afternoon should
result in some clearing west and perhaps a few peeks of sunshine
east. Temperatures are expected to be several degrees warmer
tomorrow, with highs in the upper 60s to around 70 inland, but
only in the 50s and low 60s along the lake front.
318 PM CDT
Thursday night through Wednesday...
There are not many periods without a mention of rain from late in
the week through next Wednesday, but the window from late Thursday
into early Friday appears to be one exception. Other breaks appear
likely early Saturday and again Sunday night. This is not to say
the other periods are likely to see widespread rainfall, but chances
increase for most afternoons in response to diurnal heating, the
ongoing presence of longwave cyclonic energy aloft, and the timing
of shortwaves moving across the area.
The axis of a surface and upper level ridge positioned over the
plains this afternoon will push into the western lakes by early
Friday but by midday Friday already appears to be east of the area.
Deterministic guidance from the European model (ecmwf)/Gem/GFS solutions all show an
embedded shortwave over the area by this time in the more zonal flow
upstream from the departing ridge. The surface reflection of this
upper shortwave is handled quite differently by the different
models, but all suggest a wave will develop along a baroclinic zone
running south of the area. Given model differences it is difficult
to pin down specifics, but generally the better chances of showers
and thunderstorms would be focused across the southern half of the
forecast area Friday afternoon and evening.
This baroclinic zone drops south and allows a weak ridge to move
into the area early Saturday, but then the next surface wave
develops upstream and lifts toward the area late Saturday along with
the next upper shortwave. Models again show poor agreement with the
handling of these features, so we have not included more than chance
pops for Saturday afternoon and evening.
By late in the weekend the next upper low drops from central Canada
into the Great Lakes and remains overhead through late Tuesday.
Chance pops persist each day in this period, especially in the
afternoons and evenings, and then drop to slight chances by
Wednesday as the upper low begins to lift off to the east.
for the 06z tafs...
Expect drier conditions this forecast period, outside of a few
passing sprinkles over the next couple of hours. However, the bulk
of any remaining precip is to the south of the terminals and will
remain this way. Ceilings varying significantly across the
terminals early this morning, from VFR to IFR/LIFR. The lowest
ceilings are near gyy, with mdw and rfd at MVFR, and dpa and Ord
at VFR. These similar ceiling heights may continue, but do think
all terminals will become MVFR later this morning. Confidence with
overall trends early this morning is low though. North northwest
wind likely to persist, until lake influence shifts winds to the
northeast later this afternoon.
318 PM CDT
Low pressure over Kentucky today will move into Ohio by Thursday
and then quickly off the New England coast by Friday. This low
will support northerly flow across the lake through late
Thursday. Winds will be lighter and more variable Friday and
Saturday as a weak ridge builds over the lake. Winds eventually
increase from the southwest later in the weekend as the next low
pressure system moves slowly across Ontario Saturday and toward
James Bay by Monday afternoon.
Illinois...beach hazards statement...ilz006-ilz014 until 7 PM Thursday.
In...beach hazards statement...inz001-inz002 until 7 PM Thursday.
Lm...Small Craft Advisory...Illinois nearshore waters until 1 PM Thursday.
Small Craft Advisory...in nearshore waters until 7 PM Thursday.
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