Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KLOT 300535

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 AM CDT Thu Mar 30 2017


A closed off upper low continues to spin northeastward across
south-central Kansas this afternoon with an associated surface low
following suit. An initial wave of forcing is moving northeastward
across the forecast area with bringing areas of light rain with
it. A persistent feed of lower level dry air has been helping to
erode this band of light rain, but some coverage has been
maintained. Expect this to continue across the central and eastern
part of the forecast area over the next couple hours with some
areas seeing a little light rain or even just sprinkles where
something does fall. 

Radar mosaic also shows another area of ascent, oriented more
west to east, in conjunction with a corridor of mid level warm 
advection. Scattered to numerous showers have developed with this 
with the best coverage across southeastern Iowa and northeast 
Missouri at 240 pm CDT. This forcing will cross the area a little
later this afternoon and very early this evening bringing another
window for scattered shower activity. It appears coverage will 
wane for a short time after this activity lifts north but the 
primary area of forcing will arrive in the western forecast area 
later this evening and spread east tonight. Expect that this will
bring a much more widespread area of showers/rain with it. 
Elevated instability spreads northward overnight and thunder is 
possible just about anywhere, though areas along and south of 
Interstate 80 have the best chance. East to northeast winds will 
increase this evening and gusts may push 30 mph through the
overnight hours. 



222 PM CDT

Thursday through Wednesday...

Main concerns will for the long term forecast period will center 
around pcpn/TS chances associated with 2 srn stream upper closed 
lows lifting out of the swrn CONUS.

The models are coming into better alignment with the timing and 
strength of the first in the series of srn stream closed lows,  By 
Thursday morning, the upper low center should lift into the srn 
plains with the associated sfc low in ncntrl OK and a warm front 
extending east into the Ohio Valley.  Strong high pressure over ern 
Canada will keep generally east to northeast flow over the region. 
The sfc pressure gradient should strengthen through the day 
Thursday, with wind gusts arnd 30 mph developing.  As the system 
continues to lift to the northeast, pcpn will continue through the 
day.  Latest guidance suggests that the instability axis should be 
suppressed south in the morning, with lingering TS chances generally 
south of I-80 in the morning and gradually spreading north to the 
Wisconsin border through the day.  With deep layer moisture also 
increasing through the day, with pwats in excess of 1 inch over the 
area, periods of moderate to heavy rainfall will be possible.  Some 
area rivers already running high from previous rainfall, additional 
rises will be likely, and some river flooding will be possible.  The 
temperature forecast for Thursday also presents some challenges as 
it will be highly dependent on how far north the warm front will 
lift.  At this time, expect that the warm front will get hung up 
near the srn tip of Lake Michigan.  The nrn portions of the CWA 
should be in the cool ely flow, north of the warm front and temps 
should top out in the lower to middle 40s.  South of the front, 
temps should be able to reach into the lower 60s.  Latest guidance 
would suggest that the front will lay out near the I-80 corridor, 
but there could be some bust potential to the temperature forecast 
if the front can surge a little farther north or is suppressed more 
to the south.  Expect rain to continue through Thursday night and 
possibly linger into Friday morning as the main sfc low lifts newd 
through nrn IL/IN.  Any lingering rain should end by Friday 
afternoon.  As the sfc low passes northeast of the CWA, winds will 
shift from generally ely to more northerly, allowing for cooler 
conditions to overspread the entire CWA, with highs Friday only in 
the lower to middle 40s.  The far sern portions of the CWA should 
see temps peg 50 F.  Should the low take a slightly slower track, 
the warmer air could linger over the ern portions of the CWA a 
little longer.

There should be a lull in the pcpn Friday night through Saturday 
night as shortwave upper ridging builds across the midwest as the 
first upper low exits to the northeast and the next srn stream 
system begins to lift out of the swrn CONUS.  While confidence is 
relatively high in another round of pcpn associated with this next 
system, timing of onset is still questionable as the longer range 
models diverge somewhat on the speed with which the upper low lifts 
newd.  Generally favor the slower ECMWF solution with this system as 
the GFS has a bias of lifting these types of systems too fast.  So, 
will carry chance PoPs spreading into nern IL by Sunday morning, 
there is a strong chance that the onset of this pcpn could be 
delayed into Sunday afternoon or evening.  The general trend with 
this system should be very similar to the first system, though there 
are indications that is should take a more southerly track.  The GFS 
is not only trending faster, but is also trending farther north, 
through central IL/IN, while the ECMWF is trending farther south, 
through the Ohio Valley.  Again, favor the more southerly track of 
the sfc low as well, which would limit pcpn potential to lighter 
rain and any associated TS potential to the far srn portions of the 
CWA, if any TS can even spread that far north.


For the 06Z TAFs...

An upper low rotating over northeastern KS late this evening will 
continue to pump moisture rich and weakly unstable air into
northern Illinois and northwest Indiana over the next 36 hrs or 
so providing off and on showers along with a few thunderstorms 
through much of the TAF valid period. Have identified two favored 
time frames for thunderstorms at the Chicago terminals. The first 
will be overnight into the predawn hours and is currently ongoing 
over portions of central Illinois. The second time frame is less 
certain, with timing differences in the models, but would either 
be late morning or early to mid afternoon Thursday. Leaning on the
latter period for now as it is supported by both NAM and RAP 
guidance. Have tempos in for TSRA to cover these two time frames 
where the forcing is stronger and more focused. For RFD, guidance 
indicates that instability will not overspread that area 
overnight, so only have one tempo for TSRA during a better period 
of forcing late Thursday morning. Outside of the two 
aforementioned periods of better forcing, there will be continuous
more nebulous forcing throughout the period which will support 
off and on showers with no strong signal for any breaks in the 
precip at this time. Overall forcing weakens some after 00Z 
tomorrow evening and we begin to lose some of the deeper moisture,
so would expect precipitation to taper to drizzle.

ENE winds will stay moderately strong and gusty overnight with
gusts into the low to mid 20 kt range. Strong winds in excess of
40 kt off the deck may be able to occasionally mix down, 
especially with showers resulting in a few sporadic gusts of 
around 30 kt. Winds should taper some through the morning and 
afternoon tomorrow, and slowly back towards the north and 
eventually north-northwest tomorrow evening/overnight as the 
surface low lifts across central and downstate Illinois.



253 PM CDT

Northeast to east flow will increase through the remainder of the 
afternoon and this evening as low pressure lifts from the southern 
to central Plains. The low is expected to track east from Missouri 
tonight to southern lower Michigan by Friday morning. East winds
are expected to increase to 30kt tonight into Thursday with 
occasional gale force gusts to 35 kt possible, mainly across the 
south half of the lake. The low will continue east to the eastern 
Great Lakes Friday night and then merge with another low off the 
mid Atlantic coast Saturday. Winds over Lake Michigan will back to 
the northerly, and remaining moderately strong. There is an 
increasing chance for more widespread gale force winds Friday into 
Friday night in the northerly flow. This pattern will keep Small 
Craft Advisory conditions in place for the Illinois nearshore 
waters into Saturday, with the likelihood that a Small Craft 
Advisory will be needed for the indiana waters as well when winds 
turn northerly on Friday. High pressure will begin to build over 
the region on Saturday allowing winds to relax some over the 
weekend before another low tracks through the Ohio Valley early 
next week.


LM...Small Craft Advisory...Winthrop Harbor to Burns Harbor until 
     4 AM Saturday.

     Small Craft Advisory...Burns Harbor to Michigan City IN until 3 PM Thursday.




National Weather Service Glossary of Abbreviations