Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | 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 271943

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
243 PM CDT WED JUL 27 2016

335 AM CDT

Through Thursday...

Satellite imagery this morning depicts quiet conditions
across the Midwest as surface high pressure is remains in
control. Several smaller scale disturbances are embedded
in west/northwest flow aloft across the Northern Plains and Upper
Midwest ahead of a more organized shortwave in the positively
tilted upper trough axis in North Dakota. The most noticeable area
of thunderstorms is a convective complex across the nation`s
heartland, near the intersection of a cold front stretch from the
Black Hills of South Dakota northeast to Lake Superior, though
this area has and will continue to weaken as evidenced by warming
IR cloud tops. Ahead of the cloud shield patchy fog has formed.

Today:  The upper trough will continue its southeastward
trajectory, while surface high pressure shifts only slightly
eastward to the southern tip of Lake Michigan. The main forcing
this afternoon will be west and north of the area, but the myriad
of small scale disturbances will pass through our region. Expect
increased high clouds and another round of afternoon cumulus
development, most notably along the lake breeze. A weakly unstable
air mass will develop ahead of this wave. Forecast soundings,
similar to upstream observed soundings depict a fairly dry column
today. Convergence other than the lake breeze is weak as is shear,
but there could be some shallow showers/storms this afternoon if
diurnal timing of any waves is favorable. If there are an that do
form, which short term guidance is not too excited about, the lake
breeze could be a trigger point, possibly favored along the IL/WI
border or northern tier of counties where several guidance sources
paint some weak QPF. High temperatures today will be similar to
Monday, mid to upper 80s inland and near 80 at the lake.

Tonight and Thursday: The coverage of storms and some storms may
increase tonight as the leading more organized shortwave moves
through, but coverage is also not certain. Guidance depicts
scattered coverage given the less than ideal forcing. The core of
the upper trough axis will move overhead on Thursday. This is the
day that models suggest would be the best chance of shower
activity, with highest chances across along a nw-se axis from
Rockford to Lansing, IL.



335 AM CDT

Friday through Tuesday...

Broad upper troughing remains in place Friday and Saturday, while
surface high pressure remains positioned north of the area with
drier northeast flow in place across WI and into IL. Have
maintained chances of precip Friday, still on the lower end.

Upper heights increase some Saturday in spite of a weak trough
lingering, and then increasing more significantly on Sunday.
Cannot rule out showers a a weak theta-e ridge will be focused
into NE IL/NW IN Saturday and less so Sunday, but no washouts
either day as dry northeast low- level flow will continue.

Warm and more humid conditions will arrive to start the work week
as southwest flow increases ahead an upper ridge/hot dome that
will expand eastward. CPC has the region pegged with fairly high
probabilities of above average temperatures during this time
frame. 850-925 Temps support readings well into the 90s, while a
max of blended guidance gives temperatures in the 90-95. Dewpoints
will again be elevated in this pattern. GFS Ensemble guidance has
the highest confidence on the Tuesday-Wednesday time frame, though
of note the ECMWF keeps us more on the edge of the heat dome an in
a more favorable position for impacts from both nearby and
possibly local convection. Blended guidance keeps slight chance to
eventually chance pops M-W.



For the 18Z TAFs...

Surface high pressure over the area continues to result in light
winds across the area early this afternoon. A lake breeze is
beginning to shift inland over the northeastern IL near shore
areas. This lake breeze should approach the eastern terminals later
this afternoon, with light (5 to 10 KT) easterly winds likely in
its wake. Prior to its passage, it appears the winds will remain
south-southwest up to 6 KT. Otherwise, similar to yesterday, VFR
CU with bases around 4-6,000 FT will continue to develop over the
area, and they could result in periods of a BKN CIG this

A cold front will drop southward over the area on Thursday
morning. This will result in more of a northeasterly wind on
Thursday. It also appears the threat of some scattered showers
and some thunderstorms will increase during the day. At this time,
though it appears the main focus for thunderstorms could end up
in the vicinity of the cold front late Thursday morning and
afternoon, which could end up near I-80 by early afternoon. As a
result, it is possible most of the thunderstorm active could miss
the terminals just to the south. However, given uncertainties this
far out, I have opted to hold onto the PROB30 for thunder, but
have pushed it off a couple hours.



228 PM CDT

Today`s somewhat weak and variable wind regime will evolve to a
stronger northeasterly fetch tonight and long northerly fetch by
Thursday evening. It is not yet clear whether wave heights at the
south end of this fetch will prompt a small craft advisory, but
they should at least be in the 3 to 5 ft range by late Thursday
into Thursday night. This north to northeasterly fetch will
persist into the weekend. Monday will be a time of transition to
southerly flow again, which then will persist into the middle of
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.