Scientific Forecaster Discussion
fxus63 klot 291200
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
700 am CDT Thu Jun 29 2017
330 am CDT
Main concerns are the risk for strong to severe thunderstorms,
torrential downpours and at least localized flooding later this
afternoon into this evening. In addition, a likely lull in
thunderstorms through at least the early afternoon will enable
very warm temperatures, along with dewpoints near/around 70,
yielding heat index values in the lower to mid 90s. The boundary
layer warming and rich moisture will also enable destabilization
to maximize as a plume of steeper mid-level lapse rates returns to
the area from the plains.
A deep surface low over Southwest Lake Superior will slowly
progress east across the northern Great Lakes region today, with
its trailing cold front inching toward the area but likely
remaining to the northwest. The remnant outflow from last night's
storms will likely wash out and not play a role in trends later
today. This will keep warm and humid southwest flow in place,
with gusts in the 20-25 mph range. Forecast soundings indicate
that the atmosphere will become uncapped and strongly by mid
afternoon with 2000 to 3500 j/kg of MLCAPE, so cannot rule out
isolated to widely scattered showers and thunderstorms in the
early-mid afternoon. However, the primary impetus for more
widespread convection looks to be an upper level disturbance
currently across the Central High plains. The timing of this wave
looks to be from the late afternoon through the evening.
Westerly deep layer shear of 30-35 kt and the strong instability
will certainly be supportive of strong to severe thunderstorms,
with primary risks for damaging winds and hail. The low-level jet
strength will not be what it was on Wednesday, but with high
instability and rich moisture/high low level cape, cannot rule out
a very isolated tornado. If an organized line of thunderstorms
can form as has been shown on some overnight cam guidance, the
main risk will be strong to severe wind gusts. A corridor of high
precipitable water up to or in excess of 1.75" near I-88 to south of I-80 will
also make for very efficient rain producers once again. Considered
idea of issuing another Flash Flood Watch given the excessive rain
across far northern Illinois on Wednesday evening, but with
potential for rainfall axis to be a bit south and overall
uncertainty, opted to allow day shift to assess trends.
After the probable evening round of thunderstorms, it's uncertain
whether atmosphere will be supportive of additional activity
overnight tonight, so kept pops in chance range. If most of the
thunderstorm activity can clear out, the approach of a surface low
from the southwest and diminishing winds may even be supportive of
some patchy fog, but have not added it to the grids yet.
311 am CDT
Friday through Wednesday...
The current period of active weather will start to lull at some
point Friday but it is not clear how quickly this will occur.
Surface low pressure will be tracking over the region in advance of
a broad upper trough building into the region. A cool frontal
boundary will also be approaching from the northwest. Guidance shows
a range of possibilities but eastern and southern areas would likely
be favored for a period of convection beyond what may be occurring
first thing in the morning. The speed of the front and the upper
trough will be the determining factor. Will carry highest pops in
the morning and continue low pops for the southeast into the
afternoon, with lowest pops north. Highs will be cooler than today
but dewpoints do not drop off much so it will still be humid. The
main upper trough axis swings by Friday night and may be enough to
generate a few showers. The area will be under northwest upper flow
for Saturday and 850 mb temps will cool several degrees which should
translate into highs a few degrees cooler than Friday. The pattern
will be favorable for the passage of weak waves in the mean flow
bringing a minimal chance for some isolated/scattered show
development depending on where any wave would track. Expect a
primarily dry day though.
High pressure crosses the area Sunday morning but the northwest
upper flow will persist with a decent signal for a weak surface low
to bring another frontal boundary into the area later in the day.
This will lead to another chance for showers/thunderstorms. This
front may end up getting hung up over or just south of the forecast
area into Monday. Upper ridging builds eastward into the northern
plains Monday and the upper Mississippi River valley Tuesday with an
upper trough forecast to split off over the Central Plains. Surface
low pressure would develop to the southwest and the location of the
front will be key in determining how active the weather becomes
locally. Do not have much confidence in how things play out for
Monday or Tuesday but it will certainly bear watching as
thunderstorm activity may be over the area or end up to the south.
for the 12z tafs...
Main concern is again thunderstorms and rain potential, which appears to have a most
likely focus of later today through the evening based off guidance
trends. A surface low will slowly progress east across the
northern Great Lakes region, with its trailing cold front inching
toward the area but likely remaining to the northwest. This will
keep warm and humid southwest flow in place, with gusts in the
20-25 kt range. Forecast soundings indicate that the atmosphere
will be uncapped by the afternoon, so cannot rule out isolated to
widely scattered rain showers/thunderstorms and rain in the early-mid afternoon.
However, the primary impetus for more widespread convection looks
to be an upper level disturbance currently across the Central
Plains. The timing of this wave looks to be from the late
afternoon near the MS river and progressing eastward through the
evening. Confidence is medium-high in the tempo thunderstorms and rain groups at
the taf sites, and medium in timing. If an organized line of thunderstorms and rain
can form, the time window for thunderstorms and rain will likely be shorter, along
with a greater risk for strong to severe wind gusts.
After the probable evening round of tsra, it's uncertain whether
atmosphere will be supportive of additional activity overnight
tonight through Friday morning. A surface low nearing the area
and lighter winds could also result be supportive of ceiling and visibility
reductions from the late evening through the overnight.
311 am CDT
Low pressure over Western Lake Superior will track eastward to east-
central Ontario into this evening. Gusty south to southwest winds
this morning will ease allowing waves to subside through the day.
Small Craft Advisory conditions will subside through the morning
as well. This same low will drag a frontal boundary across the
lake which will likely stall out as another trough of low pressure
moves along the front into Friday morning. The front is expected
to push south of the lake later Friday with high pressure
gradually building in by Sunday morning. Another low pressure
system will track north of Lake Michigan later Sunday bringing
another frontal boundary southward down the lake into Monday.
Lm...Small Craft Advisory...nearshore waters
until 1 PM Thursday.
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