Scientific Forecaster Discussion
fxus63 klot 161143
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
643 am CDT Mon Sep 16 2019
228 am CDT
As we transition into fall, the clash of slightly cooler air from
the north with a very moist air mass brings the recipe for fog.
A weak cold front continues to slowly ooze into NE Illinois from
Wisconsin. Along and ahead of the front an area of stratus continues
to have a slightly lower footprint locally. Nighttime fog and
surface observations confirm a rapid deterioration to dense fog
behind the stratus as the cooler airmass encounters mid to upper
60s dewpoints. South of the stratus where lower level
temperatures/dewpoints are slightly higher, additional fog is
developing, but has been largely shallow and localized. Guidance
and observations suggest that a slight expansion of the dense fog
advisory southeastward is warranted as this is the more favored
area for fog, but farther south rap/hrrr vis forecast confirm that
limited dense fog elsewhere.
The upper ridge across the plains will shift overhead today, and
this will shift the center of surface high pressure off toward the
eastern Great Lakes (near Lake Huron this afternoon) and set up and
onshore wind pattern. Conditions will warm back into the lower to
maybe mid 80s near central IL, while cooler lower 70s will be found
at the NE Illinois Lakeshore. Some fog development is again possible
tonight, though guidance is less aggressive.
This setup will change very little on Tuesday as the upper ridge,
while weakening some, generally holds its own across the region. The
ridge position does allow for a shift to southeast surface winds as opposed
to east-northeast today, and it also allows upstream energy from a sharp upper
trough in Lee of The Rockies to drive through the center of the
country but north and west of the Mississippi River. There we will
have some modest warming, and in spite of some additional clouds, we
will also be precip-free.
243 am CDT
Tuesday night through Sunday...
Upper ridging will be a persistent feature across the region through
the upcoming week, resulting in above normal temperatures.
Tuesday night through friday: the fairly robust ridge axis will
align roughly from the mid-Mississippi Valley northward to southern
Hudson Bay Tuesday night. A mid-level trough will lift NE out of the
northern High Plains to Hudson Bay Tuesday night into Thursday. This
will act to weaken the northern extent of the ridge, ultimately
forcing the ridge axis to the east of the County Warning Area by Thursday.
Meanwhile, the County Warning Area will reside on the western periphery of a surface
high entrenched across the eastern Great Lakes. A fairly pronounced
Theta-E axis will develop from Lake Superior SW into the central
Great Plains by Wednesday night.
All of this sets the stage for a stagnant, but active period of
weather just west of the County Warning Area Tuesday night into Friday as subtle
impulses riding along the western portion of the ridge tap into the
unstable airmass and produce multiple rounds of convection. While
the expectation is that the County Warning Area should remain just far enough east
of this activity to remain dry through Friday, will carry generally
slight chance pops across the northwest third late Wednesday night into
Friday as decaying convection may edge into northwest Illinois.
Otherwise for this period, the upper ridge will support above-normal
temps under 800 mb temps of ~18c and h9 temps ~21c. The warmest day
looks to be Friday, where temps in the mid to potentially upper 80s
run 10f+ above normal.
Saturday through sunday: the combination of deep troughing over the
Pacific northwest mid-week and Hurricane Humberto will eventually
breakdown the upper ridge by this weekend. While the trough and
associated cold front will pass well north of the area near the
Canadian border, a cold front will move through the County Warning Area. Timing with
guidance varies considerably, owing to the forecast strength of the
front. The GFS depicts a stronger front that quickly crosses the
area on Saturday, whereas the CMC and European model (ecmwf) show a weaker front that
slowly sags S/southeast across the area Saturday night through Sunday.
Either way, there is the potential for at least some unsettled
weather this weekend as we close out astronomical Summer.
for the 12z tafs...
Low-end IFR to LIFR ceilings are prevalent across the area at 12z
taf issuance, with some LIFR visibility also present at rfd/dpa.
Ceilings will likely remain steady through 13z before daytime
heating begins to slowly lift them through the morning. Visibility
should also quickly improve by 14z. Ceilings will eventually
become MVFR and/or scatter out by this afternoon. There is
increasing confidence that IFR ceilings will advect back into
northeast Illinois and Northwest Indiana from Lake Michigan this
evening, then persist through the night. However, if this does not
occur, sky clear conditions with higher chances of lower visibility from
br/fog would become the most likely scenario.
Illinois...dense fog advisory...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012-ilz019-ilz020 until 10 am Monday.
Lm...dense fog advisory...Winthrop Harbor to Gary in until 10 am
Visit US at http://weather.Gov/Chicago (all lowercase)