Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/04/20
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
909 PM CST Mon Feb 3 2020
.UPDATE...
900 PM CST
Only minor changes needed to the inherited forecast this evening,
mainly to trim and slightly reduce precipitation (drizzle)
chances. Evening RAOBs indicate that the broad, persistent area of
modest isentropic upglide has resulted in some moistening in the
low- levels, but recent VAPOR AMDAR soundings out of Milwaukee and
Midway do show that things have not fully saturated, with
lingering pockets of dry air present in the 900-800 mb layer. This
also seems to be corroborated by recent ceiling observations,
with only a small sliver of 2000-3000 foot bases creeping into far
southeastern Iowa. Additional/strengthening isentropic ascent on
the 285 K theta surface through the rest of the evening does look
to occur northeast of us across the mid lake which should
encourage lowering cloud bases which look to drift into parts of
the CWA after midnight. For really solid drizzle events, we`re
typically looking for cloud bases down around or under 1000 feet
with deeply saturated profiles and cloud depths into the 8-10 kft
range and forecast soundings indicate we won`t be getting
anywhere near that overnight into Tuesday morning.
There does, however, appear to be perhaps a 4-5 hour window where
some light drizzle could get squeezed out of the saturating low-
level columns, centered most likely straddling I-88 and in the
immediate vicinity of the lake. These locales look to remain
generally above freezing, limiting our concern for icing potential
due to freezing drizzle. Farther to the north and west--where it
will be cooler and likely more below freezing--the depth of the
dry sub-cloud layer does now appear to be just too deep to allow
any small droplets to survive the fall to the surface. As a
result, have pared PoPs back north and west of a Mendota to
Crystal Lake line. While we can`t discount a brief period of some
light freezing drizzle, think the very limited duration combined
with still warm road surfaces (36-38 degrees per recent
observations across McHenry County) will greatly curtail the
potential for travel impacts.
Evening guidance is hinting at the potential for a small
frontogenetic circulation possibly poking a bit more into our
southeastern counties Tuesday afternoon with a potential for some
light snow accumulations, but will defer any alterations to this
part of the forecast to the midnight shift.
Carlaw
&&
.SHORT TERM...
242 PM CST
Through Tuesday night...
The primary forecast concerns in the short term period center
around the potential for a short period of drizzle/freezing
drizzle late tonight into early Tuesday, along with the lake shore
flooding threat on Tuesday.
Temperatures have warmed nicely across much of the area under
partly cloudy skies early this afternoon in spite of the switch
to northeasterly winds. This is especially the case inland from
the lake and outside of the lingering snow field over north
central IL. In these areas, temperatures have warmed into the
upper 40s, to even the mid 50s far south closer to a surface
frontal boundary over central IL. Colder air is on the way,
however, for tonight and on Tuesday as a surface high over the
eastern Dakotas continues to shift eastward across the Upper
Midwest and into the western Great Lakes region. This will result
in temperatures on Tuesday remaining in the 30s.
There continue to be signs that some lower clouds and possible
light precipitation will develop across parts of northern IL
overnight in response to a developing band of frontogenesis along
the 850 MB frontal boundary. Unfortunately, rather low mid-level
moisture is expected, which looks to support rather warm (above -10C)
cloud top temperatures, likely not supportive of heterogeneous
ice nucleation. For this reason, any light precipitation that does
fall late tonight into early Tuesday morning will likely be in
the form of drizzle or freezing drizzle, depending on surface
temperatures.
At the present time, surface temperatures are expected to drop
into the lower 30s over northern IL late tonight, especially west
and north of Chicago. These areas will hence have the best chance
to experience a short period of freezing precipitation through
around daybreak Tuesday. Confidence in this becoming a
significant problem into the morning commute remains on the low
side at this time, so no headlines are planned at this time.
Aside from some small chance for some light precipitation over my
far southern counties Tuesday into Tuesday night, it appears most
of the area will remain on the dry side as we await Wednesdays
storm system. The main weather story for Tuesday will be the gusty
northeasterly winds likely producing high waves up around 10 feet on
southern Lake Michigan. With the continued high lake levels, this
is likely to result in at least some minor lakeshore flooding and
erosion problems along the southern Lake Michigan shores through
Tuesday night.
KJB
&&
.LONG TERM...
330 PM CST
Wednesday through Monday...
The main forecast concern is an increasing likelihood of a
plowable accumulating snowfall area wide Wednesday PM into
Thursday. Latest thinking would place timing of this snowfall in
line with causing travel impacts to both the Wednesday evening and
Thursday morning commute. At this time, this looks to be an all
snow event for most of or the entire CWA.
Prior to today, much of the guidance (especially non ECMWF) had
been struggling handling the pattern for this potential snow
system, with poor run to run continuity. That changed substantially
with the 12z cycle, as global operational and ensemble guidance
came into much better agreement in bringing accumulating snowfall
to the entire CWA. Should today`s trends continue, we will likely
need winter headlines for most of if not the entire CWA.
Short-wave energy and a strong mid and upper level jet will round
the base of a longwave trough of the western CONUS this evening,
with the trough digging across the southwest and then into Texas
tomorrow into Wednesday. Latest trends suggest a coupling of the
strong upper level jet associated with the longwave trough and
another strong jet streak over the northeastern 1/3 of the CONUS.
This development entails surface cyclogenesis along the Texas Gulf
Coast by Wednesday morning. The low will then track quickly
northeast to the middle Ohio Valley by Wednesday night and near
Lake Erie by early Thursday while steadily deepening to the lower
990s mb. This non-NAM consensus surface low pressure track is a
climatologically favorable one for accumulating snow in much of
the local area.
In addition to favorable large scale ascent from the neutrally
tilted upper trough and upper jet divergence, this system will
have a good moisture feed of above normal PWATs due to open Gulf
trajectories in the system warm sector. The strengthening synoptic
system will interact with and strengthen an already tight thermal
gradient/baroclinic zone over the region, yielding good mesoscale
forcing from stout lower and mid-level frontogenesis. With all
this in mind, expect snow to break out from south/southwest to
north/northeast Wednesday afternoon and evening. Assuming no major
changes in the favorable aspects of this set-up, the warm
conveyor belt Wednesday evening and then deformation area
precipitation later Wednesday night into Thursday morning should
support steady and occasionally moderate snowfall rates.
As to a possible fly in the ointment, the system warm sector will
likely have plentiful convection, which in *some* cases (ie. Jan
10-11 2020) can cut off some of the moisture transport and modify
mass fields. This system will also be moving rather quickly, which
may limit relatively speaking the QPF and snowfall amounts. That
said, think that the favorable aspects to this set-up could
potentially lead to higher QPF and snowfall than in the official
forecast. Finally, depending on the exact track of the system,
including 850 mb low, a farther north/northwest track could
possibly bring mixing issues into the far southern CWA, and dry
slot impingement Wednesday night. At this time, am a bit less
concerned with possibility guidance shifts back southeast again.
With the poor guidance agreement until today, the explicit QPF and
snowfall has been trended a good deal higher than previous
forecasts. Do not want to put too much emphasis on the exact
amounts yet, as we may not be done trending as alluded to above.
However, what is available on the probabilistic snow page gives a
2 to 5" range. The main message is that confidence is increasing
in a plowable snowfall for the entire area and the potential for
significant impacts to the Wednesday PM and Thursday AM commute.
With temperatures in the 20s to around 30 and lift favorably
positioned within the DGZ on forecast soundings, snow ratios
should be near to a bit above climatological average, in the
12-16:1 range. With a farther north surface low track now favored,
it will be breezy along with the snow, which could yield some
blowing/drifting issues. Keeping winds in check somewhat though
will be the lack of a strong surface high north or northwest of
our area. Northeast to north winds will be strongest over southern
Lake Michigan (gusts to 25 to perhaps 30 mph), so lakeshore
flooding concerns may return quickly on the heels of tonight
through Tuesday night event.
The system snow will come to an end by Thursday mid day or early
afternoon east. Thursday evening and night (and possibly Friday
AM), we`ll have to watch for developing lake effect snow showers.
It`s too early to say how the lower level convergence will set-up
and thus which areas will be most favored, but favorable
convergence, thermodynamics, and inversion heights of 9-10kft
could add to the system snow totals somewhere near/along IL and IN
shore. Expected snow cover over the region will keep temperatures
near seasonable through Saturday, with a modest warm-up Sunday as
a clipper system passes north, though it too likely kept in check
by snow cover. This may be followed by a cold front passage on
Sunday night into Monday. Have some low chance PoPs during this
time, but confidence is low out in that range.
Castro
&&
.AVIATION...
For the 00Z TAFs...
The concerns and confidence for the Chicago area airports are:
- High confidence in winds being between 010-030 degrees for most
of the TAF, most frequent 020 degrees, with medium confidence in
peak gusts 28-33 kt during the Tuesday morning rush
- Low confidence in any IFR and brief freezing drizzle overnight
and early Tuesday morning, which if were to occur would most
likely be between 10Z- 13Z
- Chance for light snow Tuesday evening mainly over the southern
TRACON area
Saturation has been slow to occur in the low-levels going into
this evening, but should happen more readily into overnight
resulting in areas of lower MVFR ceilings. The profiles are
supportive of patchy drizzle even though IFR ceilings should be
somewhat spotty. Without IFR ceilings, it may keep the drizzle
notably light. With temperatures inching downward and dew points
dropping below freezing early this evening, there is the potential
for freezing drizzle. Following a mild day, pavement temperatures
are likely to lag milder so with any drizzle expected to be both
light and short duration, impacts should be limited. Any low
clouds look to clear out between mid-late morning.
North-northeast winds will continue through the TAF period, with
any wobble in direction forecast to be small. Confidence is high
in the peak gust time being late tonight through early afternoon
Tuesday, with a gradual diminishing thereafter. A few gusts higher
than the 31 kt forecast in the ORD TAF are likely. Gusts look
unlikely to top 35 kt.
Wind speeds will continue to gradually ease Tuesday night, but
being off the lake with slightly colder air, there is potential
for lake effect flurries/patchy light snow into far northeast
Illinois, but that looks mainly overnight Tuesday night (beyond
current TAF). Also Tuesday night should see light snow across
central Illinois that may reach up into the southern TRACON area,
with medium-high confidence in this staying south of ORD and MDW.
MTF
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Lakeshore Flood Advisory...ILZ006...midnight Tuesday to 6 PM
Tuesday.
Lakeshore Flood Advisory...ILZ014...3 AM Tuesday to 6 AM
Wednesday.
IN...Lakeshore Flood Advisory...INZ001-INZ002...3 AM Tuesday to 6 AM
Wednesday.
LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor
until 3 AM Wednesday.
Small Craft Advisory...LMZ741-LMZ742 until 6 AM Wednesday.
Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 AM Tuesday to 9
AM Wednesday.
&&
$$
Visit us at weather.gov/chicago
Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago