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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
647 PM CDT sun Oct 20 2019

Short term...
300 PM CDT

Through tonight...

For the remainder of this afternoon and tonight, main forecast
concerns will be how much low stratus may spread inland from Lake
Michigan and into nern Illinois/nwrn in late tonight and the timing of
showers developing across nrn/wrn portions of the County Warning Area.

While none of the guidance has a particularly good handle on the
lake stratus this afternoon, the current set-up, with daytime
warming today eroding any of the lingering fog/stratus over land,
satellite imagery and Lakeshore web cams clearly show an area of
low stratus over the cooler Lake Michigan water. Weak high
pressure centered over srn WI is slowly moving east and will cross
Southern Lake Michigan and into Southern Lower Michigan overnight
tonight. Lgt/vrbl winds will settle to Ely overnight with a warm
front developing over cntrl Illinois overnight. A deep sfc low and
associated cold front will steadily track east overnight, but
will remain to the west of the area. Warm advection/isentropic
lift along and north of the warm front should provide adequate
lift, with some elevated instability, mainly over nrn Illinois
overnight. The warm advection and broad lift will allow for some
scattered showers to develop after midnight, or more likely,
toward the pre-dawn hours as the main system appears to be
trending a little slower in the latest guidance. Pcpn coverage
should increase from west to east toward dawn, with prevailing
rain showers likely for locations along and west of I-39 toward
dawn. In the persistent warm advection pattern and with increasing
cloud cover overnight. Temps tonight should range from the upper
40s near the Illinois/WI border to the lower 50s south of I-80.

&&

Long term...
335 PM CDT

Monday through Sunday...

Potent storm system currently taking shape across western Nebraska
and South Dakota with an impressive appearance on moisture-channel
imagery. With the left exit region of a 110+ kt upper jet streak
arcing across Nebraska, pressure falls have been ongoing within the
preferred left-exit region today. Objective analysis depicts a sub-
1000 mb surface low developing into central South Dakota with
several sites reporting wind gusts to near 50 mph at times. This
system will begin marching eastward tonight and will bring our next
round of widespread showers and thunderstorms to northeastern
Illinois and Northwest Indiana on Monday.

By Monday morning, expect that broad warm advection and attendant
strong upper-level divergence will force a fairly cohesive area of
rain and embedded thunderstorms, which should incrementally spread
across our forecast area into the mid-late morning hours. Not
surprising to see guidance trending subtly slower with the onset of
more appreciable precipitation chances given the swinging/digging
nature of the main vort lobe. Looks like the initial bout of rain
chances will arrive into our southwestern locales around or shortly
after daybreak tomorrow, and into The Heart of Chicago likely
towards the tail end of the morning rush. The character of our
airmass will change a great deal between this evening and Monday
as upper 50s to near 60 degree dewpoints will surge northward
within this system's warm conveyor. As a result, pwats look set to
head above 1.3-1.4 inches with an associated threat for at least
localized corridors of more moderate to briefly heavy rainfall.
Thankfully, we've had quite some time for soils to dry after our
late- September/early- October rounds of heavy rainfall, and this
system's warm conveyor belt looks to blast eastward quickly enough
to limit precipitation amounts and the flooding potential.
Widespread rain amounts of near half an inch appear reasonable
with some localized amounts near one inch before all is said and
done.

There may be a brief break in activity during the late-morning hours
as the initial warm advective Wing heads east and before additional
convection develops along the incoming cold front. There are some
signs that a very narrow corridor may set up immediately ahead of
the cold front where at least some surface-based parcels may decide
to join into the inflow of developing showers/storms. However,
even the most aggressive soundings (rap) reveal overall marginal
low-level lapse rates which will likely act to inhibit the
development of widespread surface-based convection ahead of the
cold front. That said, the kinematic profiles are impressive with
40+ kts of flow present above 850 mb with large, looping
hodographs in the lowest 3 km. With this in mind, can't entirely
rule out the potential for a few more over-achieving storms into
the late-morning/early-afternoon with a threat for strong/gusty
winds before the mid-level dryslot shuts off precipitation chances
quickly into the mid-afternoon.

The other concern will be the winds, which may gust into the 30-40
mph range from mid-morning on, both ahead of the cold front
with southeasterly winds, and behind it as winds sharply turn out
of the southwest during the late afternoon. Any clearing skies
should be short-lived as wraparound moisture will likely drive a
widespread area of stratus back across the area into Monday
evening and Tuesday. Can't rule out some shower activity as well
with some marginal instability developing in the steep low-level
lapse rate environment under the strengthening mid-level
inversion.

For Tuesday, nudged high temperatures down a bit as low cloud cover
looks to hold fast through much of the day with perhaps some very
late-day clearing across our far southwest. Gusty westerly winds
will be the main story on Tuesday in the cold advective regime with
profiles supporting gusts into the 30-40 mph range once again.

The next shortwave disturbance is slated to translate down the left
over baroclinic zone on Wednesday, bringing the next bout of
precipitation chances to the area. Even though warm advection looks
robust with this next wave, it will have to work hard against a very
dry atmospheric column, so we'll be capping pops in the low-end
chance category Wednesday afternoon and night. This system is then
expected to drive a strong cold front through the region to end the
week. Temperatures by Friday morning may once again drop to near
freezing across much of the area as a result. At this time, it looks
like incoming drier air will win the battle here, ending
precipitation chances before any potential phase change would occur.

Carlaw

&&

Aviation...
for the 00z tafs...

Numerous concerns for the Chicago area airports through Monday
evening. Summarizing them:

- IFR cigs this eve and early overnight with at least potential
for mdw to reach 200 ft and have visibility drop under 3sm

- period of moderate to briefly heavy rain with likely embedded
thunder during the mid-morning Monday, with the chance for
isolated storms during the early afternoon

- strong winds on Monday persisting even into Tuesday, with the
potential for 35 kt gusts anytime after 14z Monday, and a wind
direction near due south from 17z-20z

For this evening, an area of stratus and fog in far northeast
Illinois will continue to expand westward, blanketing the Chicago
airports through the remainder of the evening. Given the setup and
the after dark timing, the clouds should continue to inch lower,
and that gives concerns mdw could reach 200 ft. Confidence is low
in 200 ft happening at mdw right now, and is especially low in
how far visibility will drop. As winds turn more southeast and
speeds increase overnight, the ceilings and visibility should
improve after 07z or so.

Quite the strong weather system will continue to mature into
Monday across the Midwest. An area of moderate rain with embedded
thunder and heavier rain rates will move over the region during
the morning hours with high confidence. The confidence for at
least scattered/embedded thunder is medium-high. Behind this, the
actual cold front will have potential for scattered showers and
storms as well during the early afternoon.

The winds on Monday will be the most persistent issue. During the
morning rain, sporadic high gusts could easily be mixed down. In
the wake of that rain and before the cold front (17z-20z or so),
the wind direction will be southerly with high confidence, and
gusts could be up in the mid 30 kts. Behind the cold front from
mid afternoon into Monday night, southwest winds will be gusting
over 30 kt and seeing sporadic gusts over 35 kt, possibly even a
couple to 40+ kt, is certainly in play.

Mtf

&&

Marine...
335 PM CDT

Several periods of strong and gusty winds are expected across the
nearshore waters of Illinois and Indiana Monday and into Tuesday.
Initial winds on Monday morning and into the afternoon hours will
be out of the southeast, with some occasional gales possible.
Winds will sharply shift out of the southwest late Monday
afternoon and evening with continued occasional gale force gusts
possible into the overnight period. More frequent southwesterly to
west-southwesterly gales are then expected to develop on Tuesday
before winds subside late Tuesday night and into Wednesday.

Carlaw

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...dense fog advisory...Illinois nearshore waters until 4 am Monday.

Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Monday to 10 am Tuesday.

Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...10 am
Tuesday to 10 PM Tuesday.

&&

$$

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