Scientific Forecaster Discussion
fxus63 klot 240815
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
315 am CDT Wed may 24 2017
315 am CDT
Unseasonably strong upper low over Iowa is moving steadily south
this morning with GOES-16 low (7.34 micron) and mid (6.95 micron)
water valor imagery depicting the upper low nicely and also of
note is a strong shortwave over Arkansas. As the Iowa upper low
moves south today, the strong shortwave trough over Arkansas is forecast
to pivot around the southeast and then eastern flanks of the upper
low. GOES-16 imagery confirms there is a healthy supply of
moisture with this feature which should spread north today across
Indiana as the shortwave lifts north and surface low pressure
deepens in response. A healthy trowal is likely to develop with
the northwestern extent of this precipitation probably making it
into at least our southeast third of our County Warning Area later this
afternoon, with a pretty sharp cut off to the northwest.
Other than this precip, anticipate the slug of rain over NE Illinois
currently to decrease in coverage and intensity as the upper low
drops southward. A good deal of the County Warning Area will spend most of the day
with little or no precipitation, with some isolated instability
showers probably developing western County Warning Area this afternoon to the west
of the shadow of more stable air spreading inland from Lake
Michigan. Any spotty diurnal showers west will quickly fade at
sunset with guidance now in pretty good agreement in shifting the
precip with the secondary S/W east of our County Warning Area during the early
evening, with our County Warning Area likely rain-free by midnight, if not sooner.
Some partial clearing is expected tonight, but guidance is
currently suggests winds should stay up enough to prevent fog from
becoming a big problem tonight.
Temps will remain cool for this time of year today, especially
near the lake where a brisk wind blowing down the full fetch of
the lake will probably hold temps mostly in the low to mid 50s
near the shore, while inland areas likely climb into the low to
Thursday, temps will moderate and we should see at least partly
cloudy skies. Gradient will favor north-northwest winds, which will
undoubtedly allow a healthy lake breeze to form in the afternoon
and penetrate inland. Behind the lake breeze, temps along the
shore should drop into the low to mid 50s, while inland areas
moderate into the mid 60s to low 70s (warmest west).
315 am CDT
Friday through Tuesday...
Medium range forecast gets a little tricky with generally lower
confidence in the details. In the wake of the departing upper low,
we should transition to largely a zonal flow over much of the
Continental U.S. Late in the week into the first part of the weekend. Due to
the zonal flow, shortwave troughs moving across the country will
be low amplitude and because of this models are struggling with
the timing of shortwaves and subsequent rain chances late in the
week into the weekend.
Unfortunately, blending model solutions together results in
precip chances in the forecast almost every period from Friday
through Monday because the widely varying model solutions. This is
likely misleading as the majority of the weekend will be dry. Due
to the lower than average confidence in patterns like this, made
very few changes to blended model initialization grids.
GFS 7 day running mean skill scores have been very low recently,
so leaning on the European model (ecmwf) and its 51 member ensembles suggests that
the baroclinic zone will likely set up pretty far to the south of
our County Warning Area Friday into Saturday, with precip threat Friday, Friday
night, through early Saturday looking very low over most of our
County Warning Area. Best chances (which still appear low) would be our southern
counties Friday night.
By Saturday night through Monday, look for another unseasonably
strong large deep tropospheric closed low to dig southeast into
the Great Lakes region. A fair number of eps members and the
operational European model (ecmwf) support a shortwave trough zipping east along
the southern flank of this digging long wave trough. This
shortwave could be close enough to bring some threat of showers
and thunderstorms north into our County Warning Area Sat night, mainly southern
County Warning Area and looks to be one of the better chances of seeing
widespread-ish rain this weekend.
Sunday and Monday look mainly dry, though depending on the
positioning the coldest air aloft, couldn't rule out some isolated
to widely scattered instability showers and perhaps a storm
during mid afternoon through early evening hours. At this point,
neither sun nor Mon look to be a washout and could very well still
end up being mostly dry.
Temps should be closer to normal this weekend with lake cooling
likely Saturday, but looking less likely Sunday and Monday with
westerly surface winds most likely to bring the 70 degree warmth
right up to the Lakefront.
for the 06z tafs...
Latest radar imagery depicting an area of showers steadily moving
northeast across northeast Illinois and northwest in. With its
orientation, expect several hours of additional showers early this
morning. This precip axis will then slowly work back west and
southwest later this morning into midday, with drier conditions
returning this afternoon through the remainder of the forecast
period. Rfd and now dpa are observing LIFR/IFR ceilings and do
think the remaining terminals have a few more hours of VFR
conditions, before these lower clouds spread south/southeast. Then
anticipate these lower ceilings to be in place through mid
morning, but with an improving trend expected into midday. VFR
conditions likely returning by the afternoon, but will need to
monitor lower ceilings returning later tonight into early Thursday
morning. Northeast winds will be in place for much of today, with
speeds steadily increasing later today.
309 PM CDT
Low pressure dropping from Wisconsin into Indiana tonight and then
moving to Ohio Wednesday evening will support northeast winds
across the lake through Thursday night. Winds are lighter and more
variable Friday and Saturday under the influence of a ridge of
weak high pressure moving over the lake.
Lm...dense fog advisory...lmz740-lmz777-lmz779 until 9 am Wednesday.
Small Craft Advisory...lmz740-lmz741-lmz742...1 PM Wednesday to
10 am Thursday.
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