Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1233 PM CDT Mon Oct 28 2024
KEY MESSAGES...
- Elevated fire danger today and Tuesday, with potential for
Tuesday to be a critical fire weather day due to strong
southerly winds.
- Near-record warmth and windy conditions Tuesday- Wednesday
(southerly gusts up to 45 mph), followed by cooler
temperatures on Halloween.
- Widespread rain/showers are expected Wednesday evening into
early Thursday, likely clearing out in time for trick or
treating.
&&
DISCUSSION...
Issued at 335 AM CDT Mon Oct 28 2024
Through Thursday:
Similar to previous forecast issuances, and given another cycle of
model continuity/consistency in this period, there are no notable
changes to the forecast thinking through Thursday. Low-level
moisture trajectories will remain blocked off on today given the
strong surface high pressure ridging extending all the way to the
central and eastern Gulf Coast. Accordingly, the low-level
airmass is expected to remain parched on Monday as a warmer
airmass begins to shift into the region. With this in mind, we
continued with the previous forecast trend of going below the NBM
dew points toward the lower, better verifying ECMWF, Canadian, and
HREF mean guidance. This entails mid 30s F dew points at peak
heating for the southeast 1/2 or so of the CWA, and upper 30s to
lower-mid 40s farther northwest.
A moisture starved short-wave will increase high cloud cover
this afternoon. However, prior to the arrival of thicker
clouds, temperatures in this dry and warming airmass should thus
have no problem quickly rising into the mid 60s to around 70F
by the early afternoon. Paired with southerly gusts up to 25-30
mph (strongest NW 1/2 of the CWA), RHs dropping into the 28-35%
range should translate to a limited danger for brush and grass
fire spread. The strongest winds are currently forecast in
northwest and far north central Illinois, where dew points will
(probably) be higher, so all in all, it continues to appear
we`ll remain safely shy of critical fire weather thresholds.
That said, the day shift will need to keep an eye on
observational trends today.
Tonight, a notable upper-level impulse, emanating from the
larger scale upper trough currently residing off the Pacific
coast, will dig into the Desert Southwest into Tuesday morning.
As this occurs, it will induce the rather impressive southerly
mass response (low-level jet in excess of 50 kt at 850 mb, and
40-45 kt down to 950 mb) across much of the central CONUS from
the Plains north-northeastward into the western Great Lakes
tonight into Wednesday. As result, gusty southerly winds will
strengthen overnight as the low level jet steadily intensifies
The winds plus warm air advection will keeping overnight temps
from falling below the normal high temperatures (upper 50s-lower
60s) for this time of year.
The windy and mild conditions tonight will help set the stage
for potential record breaking warmth on Tuesday. In addition,
mixing after sunrise into the intense low level jet should
result in a rather sudden "pop" of winds up into the 40-45 mph
range, if not higher. With only some high-level cirrus expected
during the day, the summer-like 850/925 mb temps (~16C and
~20C, respectively) and strong south-southwest winds will
support highs topping out in the low 80s F on Tuesday. Assuming
this occurs, this will set a new record high for the 29th,
especially at Chicago (see Climate section below).
Continuing on the theme of the winds and fire danger on Tuesday,
we remain fairly confident that dew points will run solidly
lower than much of guidance (which are indicating Tds in the
upper 50s) as moisture trajectories will remain mostly blocked
and delaying the influx of better low-level moisture. Our
forecast is featuring lower to mid 50s dew points at peak
heating. We`ve also noted multiple runs of some of the high
resolution guidance showing a more substantial dew point drop
off (40s Td) by the mid afternoon over the southeast half or so
of the CWA.
Though there may be a temporary "weakening" of the wind field
after the overnight low-level jet eases, forecast soundings
suggest 40+ kt of flow available in the mixed layer by the mid
Tuesday afternoon. With the morning pop of winds and the
continued windy conditions in the afternoon, we may need to
consider issuing a wind advisory today depending on guidance
trends. Even with the dew points ticking up to keep RH (30-40%
forecast) which are above critical thresholds (25% or less), the
high winds could still enable brush fires to grow out of
control. For this reason, we`ll continue to message an elevated
fire danger for Tuesday in the HWO and WxStory. If confidence in
less than 30% RH grows for portions of the area (i.e., high res
guidance ends up correct on a pocket of even drier air coming in
during the afternoon), a Fire Weather Watch and eventual Red
Flag Warning may need to be considered on the day shift as
well, in coordination with local fire weather partners.
The winds will only come down slightly Tuesday night and keep
temps incredibly mild for this time of year in the mid to upper
60s, if not warmer! The low-level jet will reintensify and
result in frequent gusts to at least 30-40 mph through the
night. Wednesday`s temps will reach the upper 70s to lower 80s
again following the exceptionally mild start, though cloudy
skies may keep them slightly lower. Winds will again be pushing
advisory criteria, with some ensemble data, particularly the
ECMWF ensemble (EPS) still suggesting an even stronger signal
for 40-45+ mph gusts than compared to Tuesday.
Confidence remains high in a period of widespread beneficial
rain and isolated embedded thunderstorms Wednesday evening and
overnight as a cold front moves across the area (80-90% PoPs).
Some isolated to widely scattered showers may develop in the
vicinity of the frontal zone over the far NW CWA late Wednesday
afternoon, though most locales should remain dry until after
dark. There could be some threat for a strongly forced line of
showers and storms right on the cold front depending on how much
instability can be realized. However, with the surface cold
front eventually outrunning the elevated front, much of the
appreciable rain will likely be anafrontal (post-frontal) in
nature overnight into early Thursday. The tightening thermal
gradient aloft should also yield at least transient
frontogenetical forcing, which could result in pockets of
downpours with locally enhanced rainfall totals.
Following the cold front clearing the area Halloween morning,
much cooler (50s, dropping into the 40s after dark) and breezy
conditions are in store for trick or treaters. Any lingering
cold air advection driven showers in the morning should come to
an end by the late morning to mid day, latest in our southeast.
Thursday night will feature clearing skies, diminishing winds,
and seasonably chilly 30s-40s lows as high pressure overspreads
the area.
Castro/KJB
Friday onward:
On Friday, mid-level subsidence and the passage of a quickly-moving
surface high pressure system will deliver sunny skies and light
surface winds. With our area on the back edge of the post-
frontal pocket of cool air (850mb temperatures of -1 to +1C
centered over Lower Michigan Friday afternoon), afternoon high
temperatures in our area will be limited to the low to mid 50s.
While such temperatures will feel noticeably cooler compared to
the near-record warmth earlier in the week, average high
temperatures for November 1 are in the low to mid 50s.
The period of seasonable temperatures will be very short as the
surface high settles to our east and deep troughing develops in
the western United States this weekend. The net result will be
the reestablishment of broad south to southwesterly low-level
flow into the Great Lakes with associated increases in
temperatures and eventually moisture. With ensemble median
850mb temperatures soring toward +10 to +12C (which is above the
90th percentile of 850mb temperatures for this time of year at
both DVN and ILX per SPC sounding climatology), high
temperatures may make a run toward the mid 60s Saturday and
especially Sunday. The degree of cloud cover may ultimately
modulate the achievement of high temperatures, so for now, will
offer a conservative forecast with highs in the lower 60s both
days.
This time of year, above average temperatures often comes with
periods of rainfall. Indeed, even amongst vast spread in exactly how
the upper-level pattern will evolve early next week, ensemble
meteograms of QPF paint a clear picture that periods of rainfall
will return to the general region, particularly from Sunday
onward.
Borchardt
&&
AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1232 PM CDT Mon Oct 28 2024
Key Aviation Impacts for the current TAF period:
- Extended period of gusty south to southwesterly winds through
the forecast period
- Potential for low level wind shear concerns (even with strong
surface gusts) after midnight tonight through mid morning
Tuesday as winds aloft are at their strongest
VFR conditions and no rain is projected through the forecast
period. The main issue impacting area terminals will be an
extended period of strong and gusty winds.
Currently, winds are just east of true south this afternoon and,
with better afternoon mixing, more persistent wind gusts 20 to
25 knots are expected.
A strong low-level jet is expected to ramp up after nightfall
and will support continued breezy southwest winds gusts of 20
to 25 kt at the surface. However, the jet is expected to ramp
up after midnight to around 50 to 55 knots just 2000 feet off
the deck. Even with the strong winds at the surface, LLWS
thresholds will potentially still be met at all terminals from
about 06 to 12Z.
As the jet weakens, there could be a "pop" of strong
southwesterly winds at the surface where gusts could be in
excess of 30 knots and potentially as high as 40 knots mid
Tuesday morning. Gusts have the potential to taper off slightly
late morning as the jet ramps down. As better boundary layer
mixing develops in the afternoon, gusts could once again
increase depending on how deep the mixing can reach aloft with
better confidence in stronger gusts later in the afternoon.
While there may be periods of gusts below 35 knots, given the
risk for potential crosswind issues it was decided to prevail 35
knot gusts through the end of the 30-hour TAF window.
DK
&&
MARINE...
Issued at 335 AM CDT Mon Oct 28 2024
A prolonged period of south-southwesterly gales to 40 kt are
likely starting Tuesday morning and lasting through at least
Wednesday evening as an elongated area of low pressure gradually
shifts from the Plains into the Upper Midwest. With high
confidence (>90% chance) that gale-force winds will be observed
starting in 24 to 30 hours (daybreak Tuesday), opted to convert
the inherited Gale Watch into a Gale Warning. The strongest
gales, which may exceed 40 kt at times, are expected closest to
the Illinois and Indiana shorelines of Lake Michigan.
Borchardt
&&
CLIMATE...
Issued at 335 AM CDT Mon Oct 28 2024
Record Warmth Possible This Week
Near record to record high temperatures are in the forecast on
Tuesday October 29th. In addition, record warm low temperatures
are also in jeopardy on Tuesday October 29th and Wednesday
October 30th.
Here are the current daily records at Chicago and Rockford:
Chicago:
10/29: Record High: 78F in 1999; Record Warm Low: 65F in 1946
10/30: Record High: 85F in 1950; Record Warm Low: 62F in 1974
Rockford:
10/29: Record High: 81F in 1922; Record Warm Low: 61F in 2004
10/30: Record High: 83F in 1950; Record Warm Low: 56F in 1924
NWS Chicago
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 AM CDT Tuesday for the IL
nearshore waters.
Gale Warning from 9 AM Tuesday to 9 PM CDT Wednesday for the
IL and IN nearshore waters.
Small Craft Advisory from 10 PM this evening to 9 AM CDT
Tuesday for the IN nearshore waters.
&&
$$
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