Scientific Forecaster Discussion
fxus63 klot 251957
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
257 PM CDT Thu may 25 2017
238 PM CDT
The 19z experimental GOES-r imagery shows clearing skies
associated with a lake breeze across the WI/IL/in shorelines of
Lake Michigan this afternoon. Thickness of the cloud cover is also
decreasing as the bases lift. Some peeks of sunshine will then be
likely through the evening with a scattering trend of the clouds.
Clouds are also taking on a more cumulus-nature as compared to
the stratus observed across the area earlier today. Churning low
pressure over the Ohio Valley will push further east through the
evening, allowing weak high pressure to influence the region. This
will bring lighter winds as the pressure gradient relaxes.
Moisture content remains quite low, but there will still be a
small chance for patchy fog in outlying rural areas tonight. This
is dependent on current cloud cover clearing out to allow for
radiational cooling and fog formation.
The 500mb pattern shows a departing trough in the eastern Continental U.S.
As a ridge builds in from the Central Plains. This will allow for
warming temperatures the next couple days, and the flow will
become zonal for Friday. Satellite imagery is showing a southern
Pacific jet that continues to pump in moisture to the southwest
US. A weak embedded wave near nm/Texas will quickly shoot northeast
to help spark some showers for Friday. This wave will work in
association with developing low pressure and a warm front. There
is a marked increase in 850 mb Theta-east and total totals index as
warmer and unstable air works into the region from the southwest.
While model guidance is quite variable with the probabilities for
convection or rain showers Friday, there remains a chance for
development from afternoon through evening.
253 PM CDT
Friday night through Thursday...
Zonal flow aloft early in the weekend will gradually become more
meridional through the period with longwave troughing expected to
result in active and seasonably cool weather much of the upcoming
Low amplitude shortwave and weak surface reflection will shift east
of the County Warning Area after midnight Friday night with precipitation ending
behind the wave as mid level height rises overspread the region.
Most of the day Saturday should be dry under the influence of a
transient ridge building across the region. Northeast flow will keep
temperatures cooler along the lake front, in the upper 50s and low
60s, but inland expect the warmest day of the upcoming week with
highs in the upper 70s to around 80.
Attention will turn to the west later in the day as another
shortwave begins to carve out an increasingly amplified upper
trough over the northern Great Plains. At the surface, low
pressure will move across northern Missouri into central Illinois
Saturday evening. Showers and thunderstorms will increase in
advance of the low with the greatest focus along the warm front
which is expected to be draped across central Illinois. There
remains some uncertainty in the northward extent of precipitation
which may become cut off from the better moisture feed due to
ongoing convection to our south and for now have confined likely
pops to areas south of I-80 with just chance pops north.
There should be a lull in any precipitation starting Sunday morning
as the main 500mb vort passes overhead. Cold air advection into the
region will help steepen low level lapse rates during the day Sunday
which will help set the stage for isolated diurnal showers and
possibly a thunderstorm. Forcing on Sunday is fairly negligible
though there continue to be mid level height falls as an upper low
gradually meanders from the Canadian prairies into the upper
Midwest. Lack of any appreciable forcing or focus for ascent will
keep any precip coverage low on Sunday.
Upper low/trough axis will gradually shift east across the Great
Lakes early through the middle of next week which will keep the
local area under cooler northwest flow aloft. There will be periodic
chances for showers and possibly a thunderstorm or two as several
low amplitude waves rotate over the area. Steep low level lapse rate
and fairly minimal cin will persist through the middle of next week
and precipitation will be diurnally favored.
for the 18z tafs...
Stratus clouds associated with low pressure over the in/Ohio border
continue to slowly lift and scatter out through the afternoon today.
Visible satellite imagery shows decent clearing over Lake Michigan
which should expand over land east to west. Expect the trend of VFR
to spread across the terminals in the next couple hours. There is a
weak lake breeze visible via surface observation as well as
terminal Doppler radar. As a result, a slight shift to north-northeast winds
near 10kt will occur 20-21z today. As low pressure departs further
to the east this evening, improving conditions will result. High
pressure overhead tonight will lead to variable winds, but there
is not much of a fog threat as dewpoint temps remain well below
surface temps. There is a small chance for rain or thunderstorms
Friday afternoon/evening which will be updated in future
253 PM CDT
A moderate northerly gradient in place over Lake Michigan this
afternoon will gradually ease as low pressure over the upper Ohio
Valley departs towards New England Friday. As winds ease, the
Small Craft Advisory will be allowed to expire this afternoon for
the Illinois nearshore waters, but will continue a few more hours
into the evening as waves gradually diminish in the Indiana zones.
A transient ridge will pass over the lake Friday which will allow
for generally light flow across the lake and for afternoon lake
breezes to develop. Flow should remain relatively light through
the remainder of the weekend as the western lakes fall between two
areas of low pressure -- one moving east across the Canadian
prairies and another lifting from the Southern Plains to the
central Great Lakes. Fresh west flow will develop early next week
as these two lows merge over Ontario and slowly lift to near James
Illinois...beach hazards statement...ilz006-ilz014 until 7 PM Thursday.
In...beach hazards statement...inz001-inz002 until 10 PM Thursday.
Lm...Small Craft Advisory...Illinois nearshore waters until 4 PM Thursday.
Small Craft Advisory...in nearshore waters until 10 PM Thursday.
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