Scientific Forecaster Discussion
fxus63 klot 191133
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
633 am CDT Thu Sep 19 2019
240 am CDT
Satellite and radar pictures show what will be a slow but
eventual breakdown of the upper ridge across the region.
Convection which has largely been confined farther upstream is now
slowly inching its way closer to northern Illinois. The activity
across northeast Iowa is largely attributed to a weak upper wave
and some increased moisture transport with more robust
thunderstorms into western Iowa where instability is much greater.
Meanwhile the showers ahead of the stronger forcing quickly erode
with the instability gradient being fairly sharp across northwest
and into north central Illinois.
High pressure at the surface will also only slowly shift east, and
thus the lower levels across much of the local area will still be
influenced by the drier airmass to our east and will keep the
instability/Theta-E gradient fairly large across Illinois. With
the subtle forcing and weak steering flow, any activity this
morning will be tied closer to the forcing with the thunderstorm
complex that could briefly impact the I-39 corridor this morning
before hitting the instability gradient.
Following this, isentropic lift with the southwest flow may continue
to drive some showers and storms across northwest IL, northern Illinois
and southern WI, but we will still feel the influence of the surface
high to keep most of the area dry. It should become fairly warm yet
again today, but there will likely be quite a bit of higher cloud
Subtle energy will remain embedded in the upper ridge, but it is the
warm front/composite outflow boundary from this upstream
thunderstorm complex will serve as focus late tonight into early
Friday, but more focused to our south and west through the day.
321 am CDT
Friday night through Wednesday...
Friday night through sunday: deep troughing over the Pacific
northwest will quickly erode a Stout Ridge across the Great Lakes. A
cold front well removed from its parent low over Hudson Bay will
then approach the area on Saturday, slowly crossing the County Warning Area on
While an isolated shower or two is possible Friday night, the area
should remain mostly dry under shortwave ridging. This should also
continue into Saturday morning. Then, considerable moisture
transport into the region will occur from the SW by Saturday
afternoon as the tropical moisture from once tropical storm
Imelda advects into the area ahead of the front. Precipitable water values of
around 2.0" will be near the climatological Max and in the 2-3
Standard deviation/95-99 percentile for the third week of September.
Guidance hints that a convectively enhanced subtle shortwave trough
will accompany this moisture and may produce some shower/storm
activity across the area in the afternoon.
Considerable moisture transport continues Saturday night into Sunday
on 40-50kt low-level winds as mid-level troughing and the cold front
approach from the west. This sets the stage for a period of
potentially heavy rain across the area late Saturday night into
Sunday. Given persistent model signals for several days and pattern
recognition of a pre-frontal tropical moisture surge in fall, will
continue to gradually increase visibility of the message of heavy
rain. Of most concern will be a renewed flooding risk across the
area, particularly for rivers across the northern County Warning Area that are still
in flood stage form heavy rains late last week and this past weekend.
By early next week, modest mid-level ridging will be the dominate
feature across the area, producing generally quiet conditions
through at least Tuesday.
for the 12z tafs...
Chicago Metro sites: VFR conditions should continue through the
period with passing mid/upper-level clouds. Will continue to monitor
the evolution of convection currently west of the area as it will
likely approach north-central Illinois late this morning. The convection
will encounter an increasingly unfavorable environment near or east
of I-39, with a rapid decrease in intensity and coverage expected
before nearing the Chicago Metro. However there is a small chance a
few showers may survive to the western vc of Ord/mdw early to mid
afternoon. Winds will likely settle at south-southwest through much of the day,
though a lake breeze should back winds southeast or east-southeast at Ord/mdw/gyy mid
to late afternoon. However, outflow winds from the convection may
reach the Chicago Metro and keep the lake breeze east of Ord/mdw.
Rfd: as noted above, convection is slowly approaching rfd from the
west. This convection will encounter a rather unfavorable
environment near rfd, but confidence is now high enough that some ts
may come close from the west to include a period of thunderstorms in the vicinity mention
Visit US at http://weather.Gov/Chicago (all lowercase)