Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 klot 141901 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
201 PM CDT Mon Oct 14 2019

Short term...
155 PM CDT

Through Tuesday...

No significant concerns through evening, just pleasant but
seasonally cool conditions, lighter winds and plentiful sunshine
through the remainder of the afternoon.

Temperatures will be able to dip fairly quickly this evening with
the lack of cloud cover and lighter winds, but expect clouds to
increase in conjunction with increased southwest flow warm
advection. This is in response to a fairly sharp trough across the
northern rockies and adjacent plains. Moisture return has been
limited given a fairly expansive surface high to our south that
extends to the Gulf of Mexico.

A significant surface low will develop and strengthen as it will
track from the Nebraska Panhandle later today to Northern Lake
Michigan by late Tuesday afternoon. There is a narrow and
concentrated period of forcing in conjunction with the warm
advection Wing and moisture axis Tuesday morning. Steepening lapse
rates aloft should promote some showers and maybe an isolated
storm or too given presence of a bit of instability above 700 mb.

The low will still be strengthening Tuesday afternoon and a pre-
frontal surface trough will promote additional showers/isolated
storm development ahead of it in the afternoon. It still appears
these would be favored for I-57 eastward, and likely even farther



Long term...
155 PM CDT

Tuesday evening through the weekend...

The strong surface low will be crossing Northern Lake Michigan by
Tuesday evening with the attendant cold front expected to be
clearing or just clear of the far eastern forecast area by early
evening. Will carry some low pops in the east for the evening
until timing can be better refined. Much of the deeper moisture
looks to be scoured out by the front for when the main upper
trough pivots across the area from late Tuesday evening through
the overnight. Cannot rule out some sprinkles or a shower with the
upper trough passage with favored areas being far northeast Illinois
and northern portions of northwest in. Winds will increase from the
northwest and gust into the 30 or maybe 35 mph range Tuesday night
and much of Wednesday. Expect a fair amount of stratus through at
least early Wednesday afternoon with some scattering possible
later in the day from west to east. A Few Lake effect rain showers
are possible over Eastern Lake and Porter counties in Indiana.
Highs will probably only warm to around 50.

The upper trough departs to the east as upper ridging builds west and
moves Wednesday night and moves toward the area. A surface ridge
crests the area during the day Thursday bringing sunny skies. A
light west to southwest wind and abundant sunshine will help highs
to reach the mid to upper 50s. The upper ridge moves overhead
Friday while the surface high is off to the east. Breezy south
winds will keep the warming trend going with highs in the 60s. The
ridging departs with an upper trough rapidly moving across the
central and northern plains Saturday. The main surface low is well
to the north but a weak cold front does sweep across the area
Saturday bringing the next chance for some showers and possibly
some thunderstorms. Well see how quickly the front arrives
Saturday, with a later arrival supporting a pretty mild day for
most areas with mid 60s to perhaps lower 70s but an earlier
passage would temper highs a bit. A more active southwesterly flow
sets up for Sunday into early next week which may bring more
showers Sunday. Longer term guidance generally agrees that an
upper trough will amplify over the central U.S. Early next week
but vary markedly on surface and other features.



for the 18z tafs...

No concerns in the near term other than some breezy west winds
that will drift southwest this afternoon. Focus will be on an area
of showers and possibly a T-storm or two Tuesday morning over/near
the terminals, then possibly additional redevelopment southeast of the
terminals in the afternoon.

Confidence has increased for at least a period of showers Tuesday
morning, and guidance has trended a tad earlier, therefore will
target the early/mid portion of the morning for shower tempo
mention. There is some weak instability aloft that could result in
a few taller showers or storms, but confidence on coverage and
length of occurrence too low for taf inclusion at this distance.
Expect a period of MVFR cigs behind the front. Some hi-res
guidance points at pockets of IFR, but again low confidence at
this distance and it could hinge on the degree of precip at each

Winds back to southeast this evening, then trend back to SW Tuesday
behind the initial round of precipitation, and then toward west late



updated 155 PM CDT

Northwesterly gales are possible on Southern Lake Michigan on
Tuesday night into Wednesday. A strong cold front will sweep
across the lake Tuesday afternoon. The parent low pressure system
will strengthen as it tracks eastward from the northern lakes to
the eastern lakes Tuesday night into Wednesday morning, tightening
the pressure gradient. In addition, a seasonably cold air mass
moving over the relatively mild lake waters will result in unstable
conditions and effective mixing. At this time, confidence is highest
in gales for the Indiana nearshore, but they are certainly possible
for the Illinois nearshore as well. The northwesterly wind direction
means that waves will be largest along the Indiana nearshore. Will
go ahead and issue a gale watch from late Tuesday night through
Wednesday afternoon for the in and Illinois nearshore waters. May end up
needing a Small Craft Advisory Tuesday evening, prior to the gale
watch, especially across the in nearshore waters as waves will
beginning to build.



Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

National Weather Service Glossary of Abbreviations