Scientific Forecaster Discussion
fxus63 klot 260025
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
725 PM CDT Thu Apr 25 2019
715 PM CDT
Only changes to the forecast for this evening/tonight, were to
make some minor adjustments to pop trends and to limit thunder.
Dry conditions in place for much of the County Warning Area at this time, with the
large precip shield to the south exiting, and with precip along
and behind the front slowly entering northwest Illinois. Expect the
front to continue moving through this evening, with northerly
winds increasing and with precip chances increasing over the
entire County Warning Area. Current precip shield expected to hold together, as
it slides southeast this evening through early Friday morning.
Thunder has not really been present over the last couple of hours
and with the weak instability in place across northwest Illinois further
weakening and shifting south, don't think there will be much
thunder this evening/tonight. So have removed thunder from the
135 PM CDT
Main concern for the period will be timing of a cold front tracking
across the region this evening.
For the remainder of the afternoon, in a weak pressure gradient, a
lake breeze has formed and pushed inland. While much of the area
will see temperatures hit or s;lightly exceed the 70 degree mark,
following the passage of the lake breeze, temperatures along the
lake front have dropped into the lower 50s. There is some
opportunity for temperatures along the lake to rebound as there is
still some filtered sunshine over nern Illinois. The other area that has
remained a bit cooler is the far sern portions of the cwa, where
sole lingering light rain and cloud cover associated with an upper
low centered over the lower Ohio Valley. This system is expected to
very slowly lift newd, so additional rain is likely across that area
into the evening.
As for the cold front expected to sweep across the region this
evening, the latest guidance is trending and hour or 2 slower with
the progression of the front and the onset of associated pcpn and a
rapid wind shift. Expect that the bulk of the pcpn will be Post-
frontal as the mid-level deformation axis focuses the pcpn along and
behind the front. Latest guidance still suggests some modest
elevated instability along with a northern stream shortwave tracking
across the region. There may be a chance for some isolated/embedded
thunder with the main area of showers, but given the relatively weak
instability and a diurnally unfavorable timing, have limited any
thunder mention to slight chance invof the front before midnight.
Following the passage of the cold front, winds will quickly shift to
nly-nnely, gusting to 20-25 mph. Overnight lows tonight should be
in the lower to middle 40s.
330 PM CDT
Friday through Saturday night...
Multiple concerns through Saturday night, most notably the risk
for ultra rare significant late season snow for portions of the
area Saturday afternoon through Saturday evening. Currently, the
area of greatest concern for significant wet snow accums is for
parts of the Wisconsin state line counties. Given uncertainty
inherent to this setup, opted to hold off from issuing any winter
headlines with this package. However, if trends hold or
significant snowfall risk zone shifts south more, we could
certainly end up needing to issue one for at least the state line
counties. In addition to the snow Saturday PM for areas that
could receive it, strong northerly winds (gusts to 30-40 mph)
could plaster trees and power lines with snow and result may be
minor tree damage and sporadic power outages in addition to
further visibility reductions. Finally, this short surge of very
strong northerly winds could result in Lakeshore flooding. More
details can be found below.
On Friday, the main concern is windy and dry/very dry conditions.
Behind tonight's cold front passage, upstream trends suggest that
clouds will quickly clear out. Soundings suggest deep mixing to
over 800 mb during the afternoon, which will likely mix out dew
points to the upper 20s-lower 30s. Expecting highs to around 60
north to low 60s much of the rest of the area except some mid 60s
well south. Exception could be 50s for portions of Northwest
Indiana shore due to brisk northwest winds. Speaking of the winds,
the deep mixing mentioned above will likely yield frequent gusts
to 30-35 mph. Combo of low humidity and gusty winds will likely
yield an elevated risk for the spread of brush fires. For more on
this, see fire weather discussion below. Brief surface high
pressure ridging Friday night and clear skies for a good chunk of
the night will enable mid to upper 30s lows (low 40s rest of area)
in parts of northern Illinois and Northwest Indiana. Quickly
diminishing winds going calm could support patchy/areas of frost
for some areas and can't rule out needing an advisory for some
counties if mid/high clouds look to hold off through the night.
For the main event on Saturday-Sat. Night, models continue to be
in good agreement on fast moving, compact, potent surface low
tracking across the region. Despite it almost being may, air mass
in the cold sector of the cyclone will be cold enough for snow.
The million dollar question is the exact track of the surface low,
which has been wobbling run to run. Trend on 12z operational
guidance for models that had been farthest north, was a bit of a
southward shift. This is important because a consensus track far
enough south of the surface low will place more of the area in a
risk for significant snow accumulations and travel impacts. Like
the April 14th event, saturday's system will have ingredients in
place to produce heavy snowfall rates where thermal profiles are
supportive. This includes: strong synoptic lift; very strong low
and mid level frontogenesis; high moisture content for a snow
system and steep mid-level lapse rates (instability for at times
convective snowfall rates).
Noted difference as things stand now is that it appears the area
of intense snowfall could be quite narrow due to a very strong
signal for a narrow, intense, and persistent frontogenetic
circulation. While the April 14th event certainly had narrow areas
of more intense banding, the swath of noteworthy accums was fairly
broad for an April snow event. Saturday's system could end up
having a sharper cutoff south to north cutoff from no snow to
several inches of wet snow. This makes the surface low track (and
track of 850-700 mb lows) extremely important to who gets what. As
alluded to in intro paragraph, there had been 2 "camps" with
respect to the surface low track, the GFS farther south and the
European model (ecmwf)/NAM/Canadian farther north. Trend on 12z suite was for a
nudge south in the farther north guidance and GFS roughly holding
steady from previous run. Mean of 51-member European ensemble also
nudged south from previous run. These trends put the northern County Warning Area
precariously close to being in the zone for heavy wet snow accums,
but forecast soundings really do show a razor thin margin.
System rain will overspread the area during the day Saturday and
then dynamic cooling driven by very strong upward motion will cool
the column a distance north of the low track, particularly in The
Heart of the low level f-gen driven banding, which model average
places along or just north of the Wisconsin state line. As we saw
in the April 14th event, heavy enough snow rates can easily
overcome the strong April sun and warm antecedent ground, but once
again, exactly where this likely moderate-heavy banding sets up
will dictate magnitude of accums and travel impacts within the
County Warning Area. As the gradually weakening surface low quickly tracks east
across Indiana by late Saturday evening, cold advection aloft will
likely support a mix or change to wet snow down to near the I-80
corridor in Illinois or so before precip quickly comes to an end.
While the system deformation axis is over northern Illinois
during the evening, could see lingering moderate-heavy rates into
the evening, important because after dark this could increase
snow accum efficiency some. Deterministically, have amounts in 3-7
inch range for approximately northern halves of WI state line
counties, with very minor accums south of I-90. Anyone living from
about I-88 northward should follow the latest forecasts closely,
as a further southward trend would bring more of that area into a
risk for several inches of very wet snow.
232 PM CDT
Sunday through Thursday...
A fairly active weather pattern is forecast to persist through next
week as broad cyclonic flow develops across the western Continental U.S..
several mid-level perturbations embedded within the resultant
southwesterly flow across the the central US will deliver at least
several appreciable chances for rain and perhaps some
thunderstorms, especially towards the end of the week.
By the start of the long term period on Sunday morning, low-level
moisture will be rapidly scoured out as Saturday night's impulse
departs off to our east. This reprieve will be short-lived,
however, as the next shortwave in line will begin translating
eastward across North Dakota on Monday evening. Warm advection along
the eastern and southern fringes of the developing surface low will
Foster the next round of showers late Sunday night and towards
Monday morning. Given the orientation of the 850-700 mb flow, it
looks like the core of the steeper mid-level lapse rate plume will
remain sufficiently displaced to our south to limit the potential
for thunder on Monday. That said, forecast soundings across our far
southeastern counties (mainly south and east of I-57) show just
enough elevated instability to warrant an introduction of isolated
thunder wording into the gridded forecast, primarily Monday
afternoon and evening.
Precipitation chances may temporarily decrease Monday night and into
Tuesday as a low-level baroclinic zone begins to sharpen and set up
immediately to our south and east, and as we await the arrival of
the next set of shortwaves. Guidance remains split on the temporal
evolution of the large scale upper-level pattern Tuesday and beyond,
which casts some doubt on the how pops will ramp back up on
Wednesday. For the time being, opted not to deviate from the multi-
model consensus at this range, and indicate the next wave of higher
(60-70%) pops by Wednesday morning. Based on today's guidance, it
appears as if we may remain on the cool-side of the aforementioned
frontal zone through Wednesday. With limited lapse rates aloft, it
looks like the thunder potential may remain low, at least until
Thursday or Friday when there are signs that the warm sector may
attempt to build northward some.
414 PM CDT
Northwest winds will gust to 30-35 mph on Friday, along with full
sun and minimum relative humidity values during the afternoon down into the
25-30% range. While there will be a period of wetting rain
tonight, the expected windy and dry conditions could quickly dry
out 10-hour fuel moisture. How quickly fuels dry out in the wake
of tonight's rain is the main uncertainty with respect to risk
for more rapid spread of brush fires on Friday. If fuels dry out
quickly enough, some locations could get close to red flag
criteria. In addition, for the minimum relative humidity values, there's some
potential for dew points to drop a bit lower than forecast during
the afternoon due to deep mixing over 6kft. Possibility is also
there for slightly warmer temperatures in spots. Should one or
both of the above conditions be met (drier/warmer), minimum relative humidity
values could drop below 25%. Thus, if confidence increases that
fuels will dry out quickly enough and in attaining widespread min
relative humidity values of 25% or less, then fire weather headlines would be
Portions of Northwest Indiana near Lake Michigan (portions of
Northern Lake and Porter counties) will likely have lower mixing
heights and cooler temperatures due to the onshore northwest
winds from the lake. There is less concern for these areas for a
significant fire weather risk.
for the 00z tafs...
Aviation items of interest for the Chicago area airports are:
- wind shift to north-northwest with a cold front prior to 03z,
then becoming more gusty from the due north for several hours
- 3-5 hour period of showers beginning around 04z with MVFR
ceiling and visibility possible at times during
- winds re-increasing from the northwest Friday morning and
persisting through the afternoon, with gusts around 30 kt
Winds have become light and at times even variable over Chicago
early this evening. A cold front is approaching though and this
will arrive at Ord and mdw prior to 03z, possible even as early as
just after 01z at Ord. This will shift winds to 340-350 degrees,
with some gusts possible, before a stronger surge from 360-010
degrees beginning 03z-04z. Gusts are likely to be between 20-28
kt for a period after that stronger surge. Confidence in wind
trends this evening is medium-high.
With this front, scattered showers will occur behind the front.
Confidence in these lasting 3-5 hours is high, and any MVFR should
be temporary and above 1500 ft.
As a weather system deepens to our east on Friday, it will drive
gusty northwest winds. Confidence is high in gusts reaching at
least the upper 20 kt and sporadic gusts in the mid 30s are
possible. Confidence is medium-high in a 300-330 degrees during
the strongest winds.
414 PM CDT
Brisk northerly winds behind a cold front later this evening
through the overnight will likely gust to 25-30 kt. There is a
non-zero chance for brief near gale force gusts later tonight,
particularly along the Indiana nearshore. After a brief lull,
northwest winds will quickly pick back up with sunrise on Friday
and likely gust up to 25 kt through the late afternoon, with 30 kt
gusts possible within a few miles of shore during the afternoon.
It's possible that winds will diminish to 20 kt or less during the
afternoon east of Gary Indiana, but confidence is lower with this.
The primary marine concern over the next few days is with a
significant low pressure system that will affect the area on
Saturday through Saturday night. Depending on the exact track of
the low, northeast winds will strengthen north of the low track
and become hazardous to small craft during the afternoon. As the
low tracks into Indiana on Saturday evening, a sharp increase in
speeds of the northeast to north winds will occur as colder air
surges down the lake. During this time, a period of gale force
gusts is probable and possibly higher end gales for a time. The
lingering high waves will subside on Sunday as winds quickly
diminish with high pressure moving overhead.
Lm...Small Craft Advisory...nearshore waters
until 7 PM Friday.
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