Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 150837
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 AM CST Sun Jan 15 2017
237 AM CST
Through Monday night...
Going forecast remains largely on track with the biggest change
being slowing the onset of the wintry precip. Strong Canadian high
over the area will move off to the east today as a storm system
organizes over the southern plains. As is typical with closed
off upper system, model guidance has been gradually trending a bit
slower each run, and it now appears that precip will hold off
until after midnight tonight.
Dry low levels will need to be overcome via top-down saturation
late tonight into Monday morning and its possible a lot of the
initial band of precip will be lost to virga, especially eastern
CWA. Typically, top-down saturation events like this start out as
sleet once precip does penetrate the dry layer, though as column
saturates and gradually warms would look for precip to transition
to light freezing rain showers. Temperatures should be slowly
climbing Monday with the rain vs freezing rain line likely
steadily moving north through the morning, likely making it to the
WI border by early afternoon. Moderately confident that a winter
wx advisory will eventually be needed, particularly northern and
western CWA, but given we`re looking at mainly a 3rd period event
will allow day shift to get another look at things and hash out
details like which counties to include and timing.
Precip looks to continue Monday night as system closes in on the
area. Guidance generally a bit slower weakening the system
compared with previous runs, so given the fairly strong forcing
and model depiction of a couple hundred j/kg of MUCAPE, thanks
largely to colder mid levels, have opted to introduce a slight
chance of thunder Monday night. Bigger story Monday night in
addition to the showers, could end up being the areas of fog,
potentially dense, that develop as higher dewpoints surge north
into the area ahead of the sfc low.
237 AM CST
Tuesday through Saturday...
Showers look to have ended by Tuesday morning, but a weaker
upstream short wave could be enough to squeeze out a couple light
showers Tuesday and certainly should keep the cloud cover around.
Temps Tuesday into Wed look to remain modestly above average for
what is climatologically the coldest time of the year.
By late in the week and through the weekend, medium range models
develop a strong/high amplitude northern stream upper level ridge
over eastern NOAM. ECMWF and GFS have flip flopped in their
handling of southern stream shortwave potentially affecting the
area in the Friday time frame bringing rain and keeping temps
"only" solidly above normal rather than WAY above normal. Until
medium range guidance is more consistent with each other and run
to run, no changes made to blended model solution for late week
into the weekend. Any day (Fri-Sun) that is dry without fog or
low stratus being a big factor could see highs soar well into the
50s, if rain, stratus, or fog are prominent then highs may "only"
reach into the upper 40s/low 50s. Normal highs are near 30 and
lows in the teens, so temps during this period should generally be
20F+ above average during the day and >25F above average at night.
For the 06Z TAFs...
Two main forecast concerns this period...fog through Sunday
morning and then the potential for mixed wintry precip for the 30
hour ord taf...early Monday morning.
High pressure will settle across the region tonight and slowly
move east Sunday afternoon. This will result in mainly calm winds
overnight into Sunday morning that will eventually become light
southeasterly Sunday afternoon with speeds around 5kt.
Patchy fog has developed over much of the area and latest guidance
suggests at least mvfr vis with fog will continue across much of
the area through Sunday morning. However...confidence is low
regarding how low visibilities may become and its possible that
some brief/patchy ifr vis may develop. Also low confidence
regarding how much...if any fog may develop at ord/mdw and trends
will need to be monitored.
Precipitation is expected to spread north across the region late
Sunday night into early Monday morning but forecast soundings show
a large dry layer that will need to saturate before precip can
reach the surface. How fast this occurs is uncertain but will
likely occur before sunrise Monday morning. Precip type also is
somewhat uncertain as it may begin as light snow/sleet but quickly
changed over to light freezing rain. Despite this uncertainty...
prob mention for the end of the 30 hour ord taf appears
reasonable but changes can be expected with later forecasts. cms
200 PM CST
Northwest winds across the lake tonight will diminish tomorrow
morning before turning southwest, as high pressure moves over and
then east of the lake. Southerly flow will then prevail over the
lake through Tuesday morning. Low pressure is expected to track
northeast over or near the central part of the lake during midday
Tuesday into Tuesday evening. Westerly winds south of this center
should be at least 30 kt if the forecast low track and strength
pans out...so this is presently most favored across the southern
part of the lake Tuesday night. General southerly flow continues
to prevail over the lake from Wednesday through next weekend.
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