Area Forecast Discussion

FXUS63 KLOT 191117
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
517 AM CST Mon Feb 19 2018
326 AM CST
Today through Tuesday night...
Several forecast concerns through the near term of the forecast 
this morning, including dense fog potential today and heavy 
rainfall tonight through Tuesday night. A flood watch remains in 
effect for the entire forecast area tonight through Tuesday night. 
Also, could be a period of wintry mix with some glazing potential 
Tuesday night into early Wednesday morning.
Deep south-southwest lower and middle tropospheric flow was in 
place across the Midwest early this morning, ahead of an area of 
surface low pressure over Kansas. Unseasonably warm and moist air 
was spreading north across Missouri and downstate IL in 
association with a developing warm front, and this air mass will 
continue to spread north into the forecast area today. Surface dew
point temps in the low-mid 40's were as close as central IL at 
this hour, and as this moist low level air spreads north over our 
colder ground and lingering snow pack, fog is expected to develop 
and may become dense in spots later this morning.
Increasing deep moisture, persistent southwesterly flow aloft, 
and a series of low-amplitude mid-level short waves will bring 
several rounds of precipitation to the area today, with the first 
round of showers and a few thunderstorms having traversed the 
central and southern parts of the cwa overnight. High-res guidance
depicts the next surge arriving this morning, from northeastern 
Missouri where showers thunderstorms were already developing per 
regional radars. Another mid-level speed max emerges from the 
western CONUS trough tonight, with another surface wave lifting 
along the front across the region into Tuesday. Guidance remains 
in good agreement in developing relatively heavy rainfall amounts 
of between 1.50" and 3.00"-4.00" tonight through Tuesday evening, 
before the front to our west eventually moves across the area. 
Heaviest amounts are progged across the southeast third of the 
forecast area, though frozen ground and runoff from melting of the
lingering snow pack across the north/northwest will all aid in 
presenting the potential for flooding and rises on area rivers and
creeks. Thus have maintained the flood watch, with no changes to 
timing or aerial extent.
With the warm front lifting across the area today, temperatures 
will warm substantially from the south. Forecast highs range from 
the upper 40's along the IL/WI border, to the lower 60's across 
the south. Not much fall-off expected tonight as south winds 
persist in this warm and moist air mass, and temps on Tuesday are 
again expected to be in the 50's and 60's. The cold front begins 
to slowly push across the area Tuesday afternoon, and gets more of
a push Tuesday night as high pressure builds across the Northern 
Plains. Temps will fall below freezing Tuesday night from 
northwest to southeast behind the front, and will likely support 
precip becoming a mix of rain/sleet and snow, and possibly a 
little freezing rain before ending. The bulk of the QPF looks to 
occur prior to temperatures becoming cold enough for mixed precip,
though there could be some issues with light icing or sleet/snow 
accumulation Tuesday night with this wintry mix. Will have to 
continue to monitor this situation with future model runs, but 
could foresee needing some sort of winter weather advisory Tuesday
night into early Wednesday morning if freezing rain were to 
producing some glazing.
326 AM CST
Wednesday through Sunday...
Pattern looks to remain somewhat active through the latter part 
of the week and the weekend. Temperatures are expected to run 
above average, except for Wednesday behind the cold front, though 
the frontal zone lingers off to our south through the period 
beneath persistent southwest flow aloft. Periodic low-amplitude 
short waves translating along the low level baroclinic zone will 
have the potential to produce a few periods of light precip for 
the area, with guidance focusing the best chances Friday into the 
The cold front clears the forecast area Wednesday morning, with 
any lingering precipitation limited to the far southeastern part 
of the cwa early in the morning. The front then settles south of 
the Ohio River, but becomes nearly stationary as it is parallel to
the southwesterly flow aloft. Weak surface high pressure spreads 
across the upper Midwest to our north through Wednesday night, 
with seasonably colder, average temperatures. While most of the 
guidance is dry, the 00Z ECMWF does indicate some very light QPF 
along the ILX/LOT border, and have held on to some low chance pops
over the far south tier of counties late Wednesday night. Thermal
profiles are marginal for snow, with soundings supporting the 
potential for a light wintry mix if it does indeed precipitate 
Guidance depicts another short wave lifting northeast from the 
southwestern U.S. Thursday, renewing warm advection across the 
frontal zone to our south. GFS/ECMWF both develop light precip in 
a region of isentropic ascent across IA/western WI and northwest 
and north central IL by afternoon, with precipitation eventually 
becoming more widespread across the area later Thursday night. 
Thermal profiles across our northwest counties are most conducive 
for snow during the night, though thicknesses increase during the 
day Friday. This would support snow or a mix which would change to
all rain during the day Friday as temps warm back into the 40's 
in most areas.
Similar conditions persist into the weekend, as another slightly 
more amplified upper wave is depicted in guidance. Daytime temps 
remain in the 40's both Saturday and Sunday supporting primarily 
rain with this system, though a brief period of rain/snow mix is 
possible later Friday night and early Saturday if the slightly 
cooler ECMWF thermal field were to verify.
326 AM CST
The combination of snowmelt (0.85" of liquid in a core sample from 
here in Romeoville), seasonally frozen ground limiting absorption
of moisture, and heavy rain, will likely lead to sharp rises 
over area rivers and streams in the next few days. Most areas will
see at least an inch of rain, with 2 to 4 inches possible in some
locations. Expect several rivers to get into flood, and numerous 
rivers to get above bankfull. Ice jam breakup may also contribute 
to flooding in the coming days. While the location of the axis of
heavy rain is not a certainty, there is a strong signal for 
potential flooding concerns. Current forecasts have the Kankakee 
River at Shelby getting close to major flood stage. 
For the 12Z TAFs...
Tricky forecast today with several concerns as a warm front lifts 
across the region ushering in unseasonably moist air mass that 
will result in low ceilings, fog, along with showers and an off 
chance for a thunderstorm or two.
Ceilings and Fog: No big changes in thinking with respect to  
ceilings, though confidence in dense fog is diminishing. As warm 
frontal boundary slowly lifts across the region today, conditions 
will continue to deteriorate and bottom out within the frontal 
trough. Southeast to south-southeast winds are expected in the 
vicinity of the warm front and will occur with the lowest 
conditions, then expect winds to veer to the south-southwest once 
the front lifts north of the terminals. Conceptually, there 
should be at least modest improvement in the low ceilings and fog 
Unfortunately, even within 12-hours of expected frontal passage  
there are still relatively large timing difference for the wind 
shift between the models as well as how dense fog may become this 
afternoon. Think that a large player in the discrepancies is the 
poor handling of the snowpack between the HRRR/RAP/NAM models, all 
of which have considerably too much snowpack and tend to bring in 
the densest fog and the slowest fropa. Leaning more towards 
GFS/GLAMP guidance with faster fropa and hopefully are more 
realistic (faster) temperature rise through the day. These models 
also do not bring down visibility as much, thus have opted to 
cautiously trend a little higher with the forecast, except for 
DPA/RFD where snowpack persists.
Precipitation: largely looks on track with best chances still 
expected mid/late morning through early/mid afternoon within the 
vicinity of the warm front. Guidance still hinting at some weak 
elevated instability so would likely expect a few thunderstorms, 
though coverage not anticipated to be high enough to be TAF worthy  
at this time. Deep moisture lifts to our north post fropa so 
should taper to a light rain or drizzle with the possibility of a 
lull in precip for several hours. Guidance in general agreement 
showing coverage increase again this evening, but vary with 
respect to the placement of the best coverage. Think there will 
be at least some scattered showers through the evening/night, but 
not terribly confident on where the axis of more persistent and 
heavier precip will set up just yet, though RFD remains favored.
Low Level Wind Shear: low level jet ramps up in the warm sector
this afternoon and evening with guidance suggesting winds at 020 
increasing to around 50 kt out of the southwest, with stronger 
winds just above this. Would expect some gustiness to reach the 
surface given the strength of the winds just aloft, but should 
still be close for LLWS of 30+ kts difference.
303 AM CST
A series of lows will move across the region through midweek.
Early this morning, one area of low pressure is located over the
Central Plains with an elongated trough extending northeast 
across far northern Lake Michigan towards Quebec. This first low 
will move across central Lake Michigan this evening and fill, and
the elongated trough will stall over central portions of Lake 
Michigan. Expect north to northeast flow gusting to 30 kt across 
the north half of the lake while south winds of 25 kt or so will 
be in place across the south. Meanwhile, an unseasonably warm and
moist airmass will be in place south of the trough. Areas of fog 
appear possible within the moist air mass through Tuesday along 
with the potential for a few thunderstorms.
Another low will form over the mid Mississippi Valley Tuesday and 
lift across central Lake Michigan Tuesday evening. This low will 
consolidate and deepen with a trailing cold front eventually 
pushing south across the remainder of Lake Michigan by late 
Tuesday evening with moderate north winds gusting 25 to possibly 
30 kt behind. Strong high pressure will build across the Northern 
Plains in the wake of the front and spread across the western and 
central Great Lakes by Thursday allowing winds to diminish late in
the week.
IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ032-ILZ033-ILZ039...6 PM Monday to 6 AM Wednesday.
IN...Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019...6 PM Monday 
     to 6 AM Wednesday.