Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221947 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
247 PM CDT Thu Jun 22 2017

.SHORT TERM...
225 PM CDT

Through Friday...

Concerns center around isolated thunderstorm development this
afternoon, and then with a line of storms late this evening and
overnight.

Inhibition gradually continues to erode from the south generally
driven by low level moisture advection soundings. AMDAR soundings
still depict some decent capping thanks to fairly warm
temperatures in the 800-700 mb layer, at least closer to Chicago
The HRRR now has backed off a bit on the coverage for the
afternoon and early evening, consistent with very little/weak
convergence in place and a lack of a larger scale forcing
mechanism. But there is some weak upper forcing pushing in from
the south, thus we maintain an isolated shower and storm mention
for the afternoon/early evening. Shear profiles are still fairly
weak during this period thus severe threat is low.

We will hang onto instability through the evening ahead of the
cold front currently draped across the northern plains/upper
midwest. Stronger upper level winds will approach the area along
the southern fringes of the upper low across Manitoba. Clusters of
storms will continue to develop along and ahead of the front. Any
storms that area able to get going this evening ahead of the front
will form in an axis of 1000+ J/Kg of MLCAPE and shear in excess
of 30 kt, which keep the threat of hail or locally damaging winds,
mainly across north central IL this evening, especially if
anything forms upstream and develops a cold pool while pushing
ahead into the continued unstable airmass. The cold front will
pass through the Chicago area after midnight, with the line of
storms passing southeast with it. There are some waves in the
southwest flow aloft that may initially slow the progression of
the front before it gets shoved southeastward late tonight, and
may prevent a solid line, but confidence is pretty high in many
areas seeing some activity. Moisture pooling ahead of the front
would suggest at least some brief heavy downpours in this
scenario, while the more significant flood threat is largely just
to our north/northwest.

Quite the airmass change will filter air on Friday, with much
drier air across the upper Midwest (note the 40s Tds in MN). These
will not get here that quickly but eventually will get in here
this weekend. Still humidity levels will be much lower, and temps
look to top off in the upper 70s to near 80. It may be a bit
cooler along the IN shore. Showers and storms look to still be
ongoing tomorrow morning, but guidance is in good agreement of
getting the front through in the afternoon.

&&

.LONG TERM...
238 PM CDT

Friday night through Thursday...

For the longer term forecast period, general trends remain
consistent with previous runs and the short term forecast period
trends.

By Friday night, the longwave pattern will be trending toward a
higher amplitude with building ridging over the west coast and broad
troughing over the ern 2/3 of the CONUS.  The broad upper trough
will help contribute to a period of below normal temperatures for
the weekend through early next week.  The local area will be under
fast nwly flow aloft, with a series of nrn stream shortwaves
dropping through the nwly flow, sharpening the upper trough and
generating sct showers.  Chances for thunderstorms will be low,
though not out of the question for the afternoon hours.  The highest
chance for any pcpn through the weekend will likely be Sunday
afternoon into Sunday evening when a stronger shortwave is expected
to drop through the upper Mississippi Valley and across the upper
Great Lakes.  There are some strength differences noted in the
longer range guidance, with the ECMWF trending stronger than the
GFS/GEM, though all indicate similar timing.  So, will carry chance
PoPs for the nrn portions of the CWA with some scattered
thunderstorms possible.  Any pcpn should be scattered, so the day
should not be a washout.  Temperatures for the weekend through early
next week will be below normal, with highs only in the lower 70s
through Monday. With high pressure associated with a dry continental
polar air mass sliding out of Canada and across the cntrl Plains and
Mississippi Valley, dewpoints will also be anomalously low for late
June and should only be in the upper 40s to around 50 F.

By Tuesday, the upper ridge is still expected to build east across
the Rockies and out over the cntrl CONUS while sfc high pressure
shifts to the east coast.  Rising heights aloft and increasingly sly
flow at the sfc will set up a return flow of warm/moist air with
temps reaching the upper 70s for Tuesday and the lower 80s for
midweek.  Dewpoints should also be on the rise back into the upper
50s to lower 60s.

&&

.AVIATION...
For the 18Z TAFs...

Concerns center around isolated thunderstorm development this
afternoon, and then with a line of storms late this evening and
overnight. MVFR/Low VFR clouds will also spread in from the south
through the afternoon and evening. Concern regarding winds
shifting to NNW-N, to possibly NNE at ORD/MDW just behind the
front.

A continuous stream of MVFR or lower VFR clouds will advect
northward through the afternoon and evening. Guidance suggests
this may briefly come in MVFR and settle to VFR before becoming
predominant MVFR ahead of the cold front tonight.

Convection continues to weaken off to the northwest. There is a
weak convergence axis over northeast IL that may trigger a shower
or two that may become a storm as inhibition erodes. Confidence is
too low on coverage to include in the TAF set, as the forcing is
subtle, but will need to monitor. Better chances storms comes
tonight. High confidence in a line of storms moving with the cold
front, with some weakening trend likely after passing the Chicago
terminals. Current RAP/HRRR timing is just slightly later than
previous runs, but 6-7z timing in an around the Chicago terminals
seems reasonable. There could be some prefrontal showers or
thunderstorms after 3-4z as well, but confidence is low on this.

Winds may abruptly shift to N or even NE with the front at ORD/MDW
but look to settle back to NW through the morning. MVFR clouds
lift behind the front.

KMD

&&

.MARINE...
245 PM CDT

The low over western Manitoba will weaken as it crosses Ontario
tonight and then continues to Quebec Friday. An associated cold
front will sweep across the lake tonight into early Friday
morning with winds becoming west to northwest behind the front.
Westerly winds are expected to continue through early next week.

High pressure should then build in behind the exiting cold front,
spreading across the plains Sunday and the western Great Lakes
early next week, setting up a period of relatively light and
variable winds. As the high shifts to the east and new low
pressure develops over the northern plains, winds will trend more
southerly and strengthen to arnd 15 to 20 kt by Tuesday. A
strengthening southerly pressure gradient could bring winds up to
30 kt by mid week as high pressure remains parked over the sern
CONUS and the trough and associated cold front track east across
the plains.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 7 PM Thursday.

&&

$$

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