Thu Feb 08 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1134 AM CST Thu Feb 8 2024

KEY MESSAGES...

- Strong south winds this afternoon/evening (gusts to 45 mph)

- Small chance (10-20%) of an isolated strong to severe storm or
two late this afternoon and early evening (Level 1 of 5 Risk)

- Unseasonably warm weather expected today and Friday, with near
record temperatures possible

- Chance of snow (30%) mainly SE of I-55 on Monday

&&

UPDATE...
Issued at 1008 AM CST Thu Feb 8 2024

The forecast appears to be on track.

A recent hand surface analysis reveals a near 990mb surface low
pressure system in far southeastern South Dakota with a surface
cold front arcing southwestward across Nebraska and far
northwestern Kansas. A subtle pre-frontal trough was located
from eastern Nebraska through central Oklahoma, ahead of which
aggressive moisture transport is occurring with surface dew
points in the upper 40s to lower 50s as far north as western
Missouri. Closer to home, partly cloudy to even pockets of
mostly sunny skies are allowing for mixing into a stout low-
level jet, with observed wind gusts ranging from 30 to 35 mph
and quickly rising surface temperatures. In fact, temperatures
in the low to mid 50s are already being reported across parts of
the area.

A leading band of showers is beginning to develop near the
Mississippi River, and will continue lifting east-northeast
across far northern Illinois this afternoon. With a pocket of
dry air remaining in place through early afternoon, the radar
returns may be largely indicative of virga. However, evaporative
cooling may allow for efficient mixing of downward momentum
leading to a "pop" of gusty winds (up to 45 mph). Outside the
shower, synoptic winds will remain gusty (also up to 45 mph),
as well. The threat for gusty winds is now covered by a Special
Weather Statement through 6 PM. Outside of clouds and showers,
temperatures are poised to rise to near or above 60 degrees,
which is nearly 30 degrees above normal for this time of year.

A quick peek at incoming model guidance and observational trends
confirms the forecast for a quickly moving area of low-topped
showers and thunderstorms this evening is on track. Coverage of
storms continues to look isolated to scattered mainly along and
north of I-80, first developing west of I-39 between 5 and 7 PM
before moving east and reaching Lake Michigan by 9 or 10 PM.
While widespread severe weather is not expected, the degree and
orientation of low-level shear (with respect to storm motion)
will support low- to mid-level rotation in the deepest cells.
Accordingly, a few storms may produce locally gusty winds (to 60
mph) and marginally severe hail (1 inch in diameter). If a
thunderstorm can develop before sunset (before the low-level
thermal profile becomes neutral to weakly stable), a brief
tornado may occur as well. Such a threat appears highest
near/west of I-39 through 6 or 7 pm and matches experimental
automated severe weather probability models. However, the threat
for a locally damaging wind gust or hailstone will continue to
Lake Michgian. These types of environments to not typically
favor widespread severe weather, though the somewhat unusual
timing with respect to the time of year does warrant heightened
awareness.

Updated products have been sent.

Borchardt

&&

DISCUSSION...
Issued at 340 AM CST Thu Feb 8 2024

All is quiet early this morning across Chicagoland with passing
high clouds and temperatures still in the 40s! The focus
continues to be on the deepening surface low currently located
in central Nebraska. This feature is expected to continue
deepening as it lifts northeast toward Ontario, which places
the region squarely in the warm sector. In response to this
system, the surface pressure gradient will tighten through the
morning with strong S to SSW wind gusts up to 40 mph developing
by late morning. Opted to hold off on issuing a Wind Advisory
with this update (criteria of 45 mph). Wind gusts on the warm
advective side of the front are often overdone in model
guidance, though sporadic higher gusts are certainly possible
given how strong winds are just above the surface. Ultimately
the upper end of our gusts will be dependent on how deeply we
mix.

As for our shower and thunderstorm chances we are still
expecting two "rounds" today, the first of which will be in the
form of a broken line of pre-frontal showers that moves through
this afternoon. QPF with this band of showers continues to favor
lighter amounts (0.05-0.10") though the convective nature of
the showers could result in locally higher amounts. Greater
attention is on how the late afternoon/early evening period
evolves (round 2). In the wake of the afternoon showers better
low-level moisture is progged to lift into the region (40-50
degree dewpoints) in addition to steepening mid-level lapse
rates. This in turn results in an increase in (elevated)
instability (500-1000 J/kg MUCAPE) across the area late this
afternoon into early evening and set the stage for thunderstorm
development.

There continues to be an array of possible outcomes among the
various available models when it comes to storm coverage and
strength. The broader consensus is that isolated to potentially
widely scattered showers and storms develop along/near the I-39
corridor between 4-5pm and very quickly lift northeast into the
Chicago metro, likely weakening with time. Given ample synoptic
support (left exit region of a strong upper jet/PVA) and very
strong deep layer shear in place, a supercell or two certainly
can`t be ruled out and would be capable of locally damaging
winds and hail. Am not too excited about the brief tornado
threat except perhaps in our far west/southwest where dewpoints
may be high enough for a storm to try to become surface based.
There also remain questions as to how much of an impact the
earlier showers have on limiting the northeastward expansion of
unstable airmass into the area, especially into the Chicago
metro, but nevertheless there remains a conditional strong to
severe thunderstorm threat (Level 1 of 5) during the 5-9pm
timeframe (earliest west, latest east).

Temperatures will respond to the strong warm advection and
mixing with April-like temperatures currently forecast with in
the 50s to near 60 possible today and tomorrow and overnight
lows still in the 40s. The extent of afternoon showers and cloud
cover may limit us from reaching record highs today but it could
be close!

Looking beyond the today`s system, a pattern change results in
cooler temperatures (albeit still near to slightly above normal
for mid-February), with highs in the upper 30s to lower 40s and
lows in the 20s to lower 30s. As far as precipitation chances
go, a subsequent shortwave ejects across the Rockies toward the
area Friday night into Saturday which will bring another chance
for showers (possibly even a few storms?) as an associated
surface reflection/low moves across the area. Depending on how
quickly it exits the area to the east some snow may try to mix
in along/north of I-88 before precipitation comes to an end
early Saturday morning. Beyond the weekend the details become a
bit less clear as differences remain in the handling of the
high amplitude upper trough moving across the central CONUS
which could bring chances for some winter precipitation to
southern parts of the region during the Monday timeframe (30%),
currently favoring areas south of I-55.

Petr

&&

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1117 AM CST Thu Feb 8 2024

Aviation Forecast Concerns:

* Strong south to south-southwest winds gusting 30+ kts this
afternoon and early evening. Sporadic higher gusts near 40 kts
possible.

* A few scattered showers early afternoon, then a couple hour
window into early this evening for scattered fast-moving TSRA.
Isolated MVFR conditions, 1" hail and locally higher wind
gusts possible.

* Breezy southwest winds linger into Friday.

Deep 992 mb surface low was tracking northeast across eastern
South Dakota approaching midday, with a windy and very mild warm
sector air mass across northern IL/IN. Have been observing
surface gusts from 30-35 kts, with a few sporadic gusts near 40
kt at times. Increasing cloud cover will likely keep those
higher gusts more limited this afternoon, though gusts in the
low 30-kt range will likely persist into early this evening
before easing a bit. Direction expected to be largely 180-200
degrees, eventually veering more 200-240 later this evening. May
see gusts drop off for a time later tonight/early Friday
morning, but should see gusts near 25 kt from the southwest
develop again after sunrise.

Low level moisture is initially somewhat lacking, though strong
south-southwest flow will increase moisture through late
afternoon. Will likely see a couple hour period of scattered
higher-based showers early this afternoon (though some of this
may not reach the ground over wide areas). Of greater impact, we
continue to monitor the expected development of scattered
thunderstorms late this afternoon into early evening across
northern IL. These will be quick-moving, with an hour or two
window at any given TAF site generally from about 23Z at KRFD,
and from around 00Z for Chicago terminals. Brief hail to 1" and
locally higher wind gusts will be possible with the strongest of
these storms, along with brief MVFR conditions and tops around
FL300. VFR conditions expected overnight into Friday.

Ratzer

&&

CLIMATE...
Unseasonably warm temperatures today into Friday may approach
or reach records for both Rockford and Chicago. The following
are the record highs and record high minimum temperatures:

Thursday, February 8th:

Chicago: Record High: 62 in 1900, 1925.
Record High Min: 54 in 1925.

Rockford: Record High: 58 in 1990.
Record High Min: 40 in 1925.

Friday, February 9th:

Chicago: Record High: 56 in 1886.
Record High Min: 47 in 1966.

Rockford: Record High: 54 in 1925.
Record High Min: 43 in 1966

Petr

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 AM CST Friday for the IL and IN
nearshore waters.

&&

$$