Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 261141 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
541 am CST sun Feb 26 2017

Short term...
309 am CST

Through tonight...

Main forecast concerns are with scattered flurries mainly across
northern Illinois this morning, and gusty southwest winds later
this morning into the afternoon.

Surface ridge axis pushing east through the region early this
morning, with dry conditions and mainly partly cloudy skies in
place. However, mid/high level cloud cover is increasing across
northern Illinois ahead of approaching mid level energy and warm air advection.
Do think most locations will remain dry today, but am monitoring
weak returns pushing east over Iowa at this time. The bulk of
these returns are not likely reaching the ground given the amount
of drier air currently present in the lower levels. However, some
isolated reports of flurries have been observed across Iowa. As
the approaching mid level wave catches up to the stronger warm air advection,
while encountering steeper lapse rates, do think some further
blossoming of this precip and lowering ceilings could occur as it
reaches northern Illinois over the next one to two hours. So have
made mention of scattered flurries through mid morning, mainly for
areas north of I-80. At this time only expecting flurries, and so
don't anticipate any snow accumulation.

Overall cloud trends may be tricky through the day, especially
with some breaks in this progressive mid/high level cloud cover.
Continued partly cloudy to partly sunny skies for this morning,
but do have clearing occurring this afternoon. This will likely
affect high temps for today, but with the advection of warmer
temps, still anticipate temps well into the 40s. While surface
ridge shifts to the east this morning, gradient and flow aloft
really increase throughout today. Thicker cloud initially in place
this morning may limit some daytime mixing, but as this departs
later in the day, should begin to really tap into the stronger
flow aloft. Highest gusts should stay in the 35-40 miles per hour range, just
below Wind Advisory criteria today. However, higher speeds at the
top of the mixed layer could reach the surface, providing
slightly higher gusts the currently forecast. Will need to
continue to monitor trends with the potential for stronger speeds
later this morning into the afternoon.



Long term...
338 am CST

Monday through Saturday...

Conditions begin dry on Monday, with southerly flow in place and
while continued warming occurs ahead of approaching mid level
energy. More defined shortwave trough is forecast to lift
northeast through the central Continental U.S. Monday night into Tuesday,
with associated surface low lifting northeast into Iowa. Fairly
energetic flow and stronger warm air advection should allow showers to develop
ahead of this system, and so have continued shower chances Monday
night. This precip shield likely exits by Tuesday morning, but as
this low approaches northern Illinois later in the day Tuesday,
will likely see showers Blossom across the region once again. With
increasing moisture and instability, do have some concerns for
thunderstorms and have now included mention of thunder. At this
time, this appears to stay confined to the southern County Warning Area and likely
occurring in the afternoon and evening. As this low then departs
to the northeast, larger mid/upper level trough pushing through
the Central Plains will allow for additional cyclogenesis to
occur. Additional precip then should develop and quickly lift
northeast across the region. Still some model variability, but am
monitoring for a colder solution to be in place with the arrival
of this precip. If this colder air pushes in quicker as the upper
level trough moves overhead, then could see more snow than
currently forecast. Pattern still appearing active on the backside
of this system, with guidance still indicating a fast moving
system to push southeast through the region Thursday into Thursday
night. With colder air in place, precip would be snow.



for the 12z tafs...

Concerns today center on strong southwest winds which will likely
have gusts at or a little above 35 kt at times late this morning
into the afternoon.

The area will be between low pressure well to the north and high
pressure well to the south, but both features will be on the move
so it is a recipe for a tightened pressure gradient and gusty
winds, especially with clearing later this morning/early this
afternoon. At 2000 ft, a channel of 40 kt winds are being
observed on National Weather Service dmx and dvn VAD wind profilers as of 1130z and
this will shift over the area this morning and early afternoon.
Expect that as the back edge of the clouds moves over, gusts will
noticeably come up during the late morning as those winds are
tapped into. While the maximum of highest winds aloft moves
slightly past US this afternoon, the mixing will be deeper, likely
continuing some of the higher gusts.

At this time, confidence in specific wind gust magnitude and
timing is medium, as this afternoon if mixing is deeper than we
have forecast, gusts could conceivably be more 37-39 knots. For
specific wind direction, there will be a slight veering during
the late morning and early afternoon as deeper mixing ensues. A
wind direction of 180-190 degrees this early morning should shift
to 220-230 degrees by 18z. It is likely to stay right around that
range all afternoon.

Gusts will drop off fairly steadily near and shortly after
Sundown. Confidence on cloud cover is low for tonight, although if
it occurs, bases should be at or above 4000 ft.

Otherwise, there could be some non-restrictive flurries early this
morning, but that should not cause any impact.



153 am CST

The lake will find itself within a tight surface pressure gradient
today resulting in strong southwest winds. Low pressure just to
the north of the Great Lakes will be moving quickly east driving
these southwest winds with temperature profiles appearing
unstable enough to favor gale gusts. Some occasional 40 kts look
likely over the southern part of the Lake. A cold front will move
across the northern part of the lake this evening turning winds
west-northwest, but speeds will be on their way down fairly
quickly later this evening.

The active pattern continues into midweek with a pair of low
pressures approaching from the southwest and merging into one
deepening low near Lake Huron by midday Wednesday. There is a high
amount of model spread on how strong the merged low will be and
thus the magnitude of north to northwest wind speeds behind it
over Lake Michigan on Wednesday afternoon and night. Gales are
certainly possible given the pattern and influx of cold air



Lot watches/warnings/advisories...
Lm...Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...9 am
Sunday to 6 PM Sunday.

Gale Warning...lmz777-lmz779 until 6 PM Sunday.



Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations