Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 010525
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 AM CDT FRI JUL 1 2016
909 PM CDT
THE MAIN UPDATES THIS EVENING WAS TO TRIM THUNDER CHANCES TONIGHT.
WATER VAPOR Imagery THIS EVENING SHOWS A WELL DEFINED UPPER-LEVEL
IMPULSE DIGGING EAST-SOUTHEASTWARD OVER THE UPPER MISSISSIPPI
VALLEY. AN ASSOCIATED STRENGTHENING NORTHWESTERLY UPPER-LEVEL JET
(~90 KT) IS EXPECTED TO NOSE INTO NORTHERN ILLINOIS LATE THIS
EVENING AND INTO THE OVERNIGHT HOURS...WHICH LOOKS TO PLACE THE
AREA IN THE FAVORABLE LEFT EXIT REGION OF THIS UPPER JET. BECAUSE
OF THIS...IT APPEARS LIKELY THAT SHOWERS WILL CONTINUE TO FESTER
OVER THE AREA LATER THIS EVENING THROUGH THE EARLY MORNING HOURS.
WHILE AN ISOLATED LIGHTENING STRIKE OR TWO CANT BE RULED OUT...IT
APPEARS THE BEST CHANCE OF THIS WOULD BE WEST OF I-39 IN CLOSER
PROXIMITY TO INSTABILITY AXIS. SOME OF THE LATEST AMDAR SOUNDINGS
OUT OF KMDW INDICATE VERY LITTLE TO NO INSTABILITY OVER
NORTHEASTERN ILLINOIS...SO WE HAVE DROPPED THE THUNDER THREAT OVER
THIS AREA...GOING WITH JUST SHOWER WORDING.
THE SURFACE COLD FRONT WILL GRADUALLY SHIFT OVER THE AREA
OVERNIGHT...WITH WINDS BECOMING NORTHERLY BY DAYBREAK. SOME LOWER
CLOUDS COULD MOVE OVER THE AREA BY EARLY MORNING IN THE WAKE OF
THIS FRONT...BUT THESE CLOUDS WOULD LIKELY GRADUALLY DIMINISH
234 PM CDT
Lead impulse with decaying convection looks to perhaps bring some
light showers to far southern portions of the CWA rest of this
afternoon. Otherwise, attention will be focused to the northwest
on the developing convection over IA this afternoon in advance of
sharp mid-upper level trough digging into the upper Mississippi
Valley. Debris cloudiness over much of the CWA in association with
the decaying convection is limiting instability, though some late
day clearing could allow for some modest destabilization over our
western CWA. Deep layer shear on the order of 30-40kt could be
enough to support a marginal severe wind/hail risk despite MLCAPE
of less than 1000 j/kg. Better threat of a stronger storm is west
of our CWA, where more sfc heating is taking place this afternoon.
Even with the weak and weakening instability as the evening wears
on, still think the magnitude of the forcing with the strong
trough could be enough to allow showers and isolated thunderstorms
to march across much of the CWA.
Weak pre-frontal trough should shift winds to the northwest behind
the convection, but more substantial synoptic cold front will come
charging down the lake late after midnight with a wind shift to
north. Though expect the lake to get involved, probably leading to
an acceleration of the front down the lake with winds near the
lake probably veering a bit more northeasterly with a period of
gusts of 30+ mph possible late tonight/early Friday near the lake.
Some post-frontal stratocumulus appears likely, perhaps enhanced a
bit from the lake before land-lake stability flips in the
afternoon and skies clear over the lake and behind the expected
afternoon lake breeze.
246 PM CDT
Friday night through Thursday...
Friday night through Thursday... To start out the period, strong
upper level ridging will be in place across the Northern Rockies
and Canadian Rockies with northwest flow into the Great Lakes.
At the surface, high pressure will be centered directly over
northern Illinois and northwest Indiana providing favorable
conditions for radiational cooling with clear skies and very
light/calm winds. Expect overnight lows to bottom out near 50 in
some of the typical low lying cool spots of the CWA while the
Chicago Metro area holds onto mid to upper 50s. Quiet weather
along with seasonably cool conditions will continue through the
day Saturday under the influence of the high. Light flow will
allow an afternoon lake breeze to develop. Temperatures inland
should top out in the mid to upper 70s while immediately along the
lake front we will only see low to mid 70s.
Meanwhile, over the weekend, the baroclinic zone is expected to
serve as the focus for convection from the Central Plains east
across the Mid-Mississippi Valley. Models continue to struggle
with the evolution of a wave that is progged to develop along
the baroclinic zone Sunday into Monday, though have in general
been trending north with the axis of heavy precip over the past
couple days. Central and southern Illinois still appear more
favored to see strong thunderstorms with the potential for heavy
rain Sunday into Sunday night though the NAM for instance still
tries to bring convection up through roughly the I-80 corridor.
Models are struggling with convective feedback and might be over-
deepening the surface low along the boundary as well as the
northward push of the baroclinic zone. Convective outflow may
actually end up reinforcing the surface boundary south and lack of
any significant upper level support would not be conducive of the
boundary pushing north. Continue to favor a more southerly
solution, but given the model spread and uncertainty, will remain
conservative and maintain chance PoPs mainly south of the I-80
corridor. Wherever it does end up setting up, NAM/GFS both
suggest PWATs of around two inches will be in place along with the
threat for heavy rainfall.
The region will undergo a warming trend from Tuesday on as the
upper ridge slides east to the mid section of the country and
low/mid level southwesterly flow develops across the Midwest.
Latest GFS and ECMWF continue to suggest we might see low to mid
20C 850mb temps mid to late next week which would support
temperatures hotter than currently advertised by the general model
consensus blend we use as a starting point. Given uncertainty in
convective chances under an increasingly unstable air mass and
some run-to-run inconsistency, opted to not make any changes from
the blend, but there remains some concern we may see a heat wave
locally by late in the week. The 12Z ECMWF continues to call for
low to mid 90s Thursday and Friday, but the 00Z run was cooler.
For the 06Z TAFs...
1225 am...Forecast concerns this period are wind directions/speeds
and mvfr cigs. Low pressure over southern Lake Michigan will move
east early this morning. A cold front will push across the
terminals in the next few hours shifting winds northerly. Winds
have turned north/northeast across southeast WI and the front is
moving rather slowly down the west side of the lake. Guidance in
decent agreement with a wind shift around 09z so maintained
current timing but in the 08z-10z time window looks on track. Once
winds shift to the north/northeast there will likely be a period
of gusts into the high teen/lower 20kts range which will slowly
diminish by late morning or early afternoon. Northeast winds will
turn more east/northeast later this afternoon and become light and
variable this evening as high pressure builds across the region.
There is currently quite a bit of bkn/ovc mvfr cigs across
southeast WI...which then scatters back across far northern IL.
Once winds shift...a period of prevailing mvfr cigs looks
reasonable and these will likely persist through at least mid
morning but gradually lift by late morning with clouds expected to
scatter out this afternoon.
Thunderstorms have developed from far southeast Lake Michigan back
across eastern IL. All of this convection is expected to remain
south and east of the terminals with perhaps a passing shower in
the next few hours. But no mention with expected isolated coverage
and short duration. cms
246 PM CDT
A weak frontal trough stretches from an area of low pressure near
Hudson Bay across Wisconsin and Iowa this afternoon and is
providing modest southerly flow across Lake Michigan this
afternoon. The front will push across Lake Michigan this evening
and tonight allowing a strong northerly breeze to overspread Lake
Michigan. A small craft advisory will be issued for the Indiana
and Illinois nearshore waters very early Friday through Friday
evening as winds and waves increase. High pressure will build
across the region Friday night into Saturday allowing conditions
to subside. Lake breezes should be able to develop across the
southern end of the lake Saturday afternoon. A weak low is
expected to move east along the Ohio Valley early next week
resulting in fresh east to northeast flow across southern Lake
Michigan. There is quite a bit of variance between models on the
strength of the low, though, which means lower confidence in the
wind speeds Monday.
IL...Beach Hazards Statement...ILZ006-ILZ014...6 AM Friday TO 6 PM
IN...Beach Hazards Statement...INZ001-INZ002...6 AM Friday TO
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...5 AM Friday TO 10
Small Craft Advisory...LMZ743-LMZ744-LMZ745...5 AM Friday TO 4
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