Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
630 am CDT Sat Apr 18 2015
337 am CDT
Some changes are in the air for the northern half of the area today
and locations closer to the Lakeshore...while southern areas will
enjoy Summer like conditions that prevailed on Friday.
The closed upper low that was over The Four Corners as of this time
yesterday is now only centered over south central Colorado. The
cloud shield in the warm moist advection sector ahead of the low is
getting closer to the area...but still largely some middle and high
clouds drifting through. Very mild conditions are in place with
dewpoints holding in the middle 40s north to lower 50s south keeping
temperatures in the 50s outlying areas to near 65 downtown in the heat
island due to the mild SW offshore flow.
Today and tonight...the first changes come in the form of a shallow
cold frontal boundary that will sink southward down the lake in the
coming hours...with northerly winds shifting NE this morning. Expect
a rapid temperature drop downtown just after daybreak as this occurs
with temperatures holding steady in the 50s much of the day. This boundary
will continue to ooze southward but will be halted by the incoming
system and warm south-southeast flow. Therefore expect quite a contrast in
temperatures today as highs in the tier of counties close to the Lake Peak
this morning in the 60s then cool...while areas south like Pontiac
realize full mixing again and push the upper 70s. Expect more clouds
to filter in through the day...not only along the front but in warm
sector as well. Some hi-res guidance tries to break out some showers
along the boundary as well...but have maintained a day forecast at this
time with the upper level ridge in place likely keeping clouds
shallow in nature. With the system continuing its slow movement
tonight..the surface low well to the south and moisture advection
confined there as well...have continued previous shifts trend to
slow probability of precipitation and gone dry overnight as well.
Sunday...surface low pressure downstate intensifies and lifts NE
ahead of the incoming Colorado upper level low. Models disagree on
the strength of the individual lows that form...but many do lift the
first one through eastern Illinois/western in. As this occurs...a warm
front will lift northward and become the focus for shower activity
with a 30-40 knots low level southeast flow driving into the front. With
precipitable water values up at 1.25 inches or so...some 2 S.D.
Above normal as noted in the Storm Prediction Center precipitation discussion...along
with a deep moist column and high freezing levels should promote
efficient rain producers. The focus of this activity at this time
will be along the eastern half of our area as it lifts
north...though this a complicated evolution with several small scale
disturbances in place. Instability is weak and very high
based...but decent forcing and parcels likely undergoing charge
separation...isolated thunder mention seems a reasonable course of
352 am CDT
Sunday night through Friday...
Precipitation will continue lifting north and east ahead of this
first wave. With a modest cyclone...expect some deformation
precipitation to continue during the evening hours. Some remnant
energy associated with the Colorado low will largely drag by to the
south...but our focus will be on the northern stream upper low that
will dominate our weather pattern for a few days. A cold front on
the back side of what appears to be a secondary low will allow rain
showers to continue through the night and into Monday morning before
diminishing in the afternoon.
The upper level pattern become more amplified next week as ridging
rebuilds in the west and a strong upper ridge in the Atlantic with
the upper low stuck with nowhere to go. Expect several cool days
with small chances for rain showers as we remain on the south side
of the now slow moving northern stream low and elognated vorticity maxes
pivot southeast across NE Illinois/northwest in. With the growing season
underway...expect frost conditions may become a concern in
//Ord and mdw concerns...updated 12z...
* light and vrb winds around 4-6kt generally from the 330-360 direction
quickly become 010-030 just after 12z.
* Winds quickly increase from the northeast with gusts up to 20kt
after 14z and lasting until sunset.
* Winds turn east after 00z.
Vrb wind direction and speeds generally around 4kt or less. Winds
will turn north just after 12z then quickly turn north/northeast by
14z. A frontal boundary will slide south across Lake
Michigan...allowing an increasing wind speed and gusts to
develop by middle- morning through late afternoon with gusts to 20kt at
times. The duration of gusts doesn/T appear to be
lengthy...however the direction will be turning northeast between
020-050 degree then more towards a 090 direction by late afternoon/early
evening. Clouds should thicken...however remain VFR with bases around
4-6kft above ground level. Any precipitation should hold off until after daybreak
sun...possibly until midday sun.
//Ord and mdw confidence...updated 12z...
* high confidence in ceilings/visibility forecast.
* High confidence in winds remaining light/vrb through
12-13z...medium/high confidence in timing of winds turning
northeast with gusts developing.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...shra/isolated thunderstorms and rain south of taf sites. MVFR
likely/IFR possible. East winds becoming northeast.
Monday...chance of rain showers/MVFR early...improving to VFR. Gusty west
Tuesday...slight chance rain showers...but mainly VFR. Gusty west winds.
Wednesday...slight chance rain showers. Mainly VFR. West-northwest winds.
Thursday...VFR. North-northeast winds.
Friday...mainly VFR. Chance rain showers.
310 am CDT
Main concern for today is the potential for marginal winds for
small craft conditions. High pressure remains over the western
Great Lakes...although steadily pushing east. This will allow a
frontal boundary to energize and slide south along the lake today
and reach the southern tip of the lake by late this afternoon.
This will allow winds to turn west to north/northeast and increase
to 15 to 25 knots. It is possible that a tighter gradient will occur
along the central/southern portions of the lake...with winds more
in the 20 to 25 knots range. This may result in small craft winds for
the Illinois nearshore through this afternoon. Winds will be slightly
less robust for the Indiana shore...with waves generally only
building to 3 to 5 feet with up to 7 feet waves at times. Given the
air is slightly warmer than the lake surface...it is possible
waves may not maximize. But this is a low confidence at this time.
The gradient briefly relaxes before increasing ahead of a
developing low pressure system late in the weekend into early Monday.
Winds turn east to northeast...with speeds increasing yet again to
15 to 25 knots. A few guidance members indicate winds could briefly
touch 30 knots ahead of the low pressure system. With the low now
poised to remain south of the lake...the best gradient may be
along the southern tip of the lake Sunday night/Mon.
Lm...Small Craft Advisory...Illinois nearshore waters until 4 PM Saturday.
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