Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 201749

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 PM CDT Wed Sep 20 2017

1246 PM CDT

Concern is on convective development late this afternoon/early
evening and how far east/south it will make it, along with hot and
humid conditions that look to persist for the coming days.

It is indeed hot and humid for mid September, and ORD/MDW are
already almost at 90 degrees today. Warm south winds and little
cloud cover will allow most locations to get close to 90 today. Convective
temperatures on the RAP are in the 90s across IA, which looks to
delay any initial convective development near the cold front until
late this afternoon. There is pretty good agreement though that
thunderstorms will develop between 5 and 6 pm near the Mississippi
River. The question will be with the upper forcing being a bit
weaker farther east, and with the convection arriving around
sunset or so, that should limit the eastward thunder concerns with
this initial development. There could be some sprinkles that get
closer to Chicago but the forcing does not appear to overcome dry
mid levels.

While west/northwest IL is favored at this time, the low level
jet will ramp back up in this high PWAT environment, which could
lead to an uptick in showers and storms late this evening/early
overnight hours. PWATs near the front remain elevated and do
increase, so if showers and storms do get a bit closer, some
modest rain rates could occur in the far northwest.

Upper level heights rebuild tomorrow, and continued deep layer
southerly flow will lift the effective frontal boundary back
northward. The lower level airmass will be on par with today`s if
not a bit warmer, and thus many areas will likely tag 90 or even
the lower 90s. These are near record levels for mid to late
September. With continued upper 60s to around 70 dewpoint, this
places heat indices in the middle or even upper 90s.



242 AM CDT

Thursday through Tuesday...

A closed upper low is expected to dig from the Pacific NW into the
Intermountain West Thursday and Friday with downstream
amplification of the upper ridge occurring over the eastern half
of the U.S. The pattern becomes very blocky late in the week
through the weekend which will result in fairly stagnant
conditions allowing a prolonged period of above normal heat and
humidity along with primarily dry conditions. For now, both the
ECMWF and GFS are in reasonable agreement that this pattern will
begin to break down around midweek next week allowing cooler air
back into the region moderating temperatures closer to seasonal
norms. In the meantime, expect afternoon highs area-wide in the
mid to upper 80s and a few spots may tag the 90 mark Thursday
through Saturday. In addition, dew points in the mid to upper 60s
will be in place through this time frame.



For the 18Z TAFs...

Near term concerns are initially low and limited to gusty
south-southeast winds, but will need to monitor the western
flying area for later afternoon and evening convection.

South-southeast winds will continue to gust to near 20 kt this
afternoon as a cold front approaches from the northwest.
Convective initiation looks to not occur near the front across IA
into WI until after 22z and possibly a tad later. The window for
thunder potential is really pretty short and confined mainly to
north central IL during the evening time frame. There could be a
few showers that get closer to ORD/MDW as the front gets pretty
close, but there is general agreement that the bulk of the
showers, and most of the storms, will remain west of ORD/MDW.

There could be some fog late tonight with the lighter winds and
some high dewpoints, especially in any areas that get rain and
clear a bit. Otherwise the front will lift back north on Thursday
for breezy south winds.



242 AM CDT

Southerly winds will gust to 25 kt this afternoon. High pressure
passes over Lake Superior tonight, and winds become north to
northeast over the northern half of the lake. Southerly flow
returns across the lake Thursday night due to a blocky pattern. A
low pressure trough will develop from the northern Mississippi
Valley through the southern Plains Friday while a high pressure
ridge remains parked over the eastern U.S. This pattern will hold
and result in southerly winds through early next week.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.