Scientific Forecaster Discussion
fxus63 klot 221126
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
626 am CDT Thu Jun 22 2017
319 am CDT
Primary short term forecast challenges include temps today then
convective chances tonight.
Warm front is lifting north across the County Warning Area early this morning with
warmer and especially more humid air spreading into the area. In
the past 6 hours have seen dewpoints jump about 4-7f with a pool
of near 70f dewpoints now into southern County Warning Area. As warm front
continues north into WI today, the pooling tds along the front
should also shift north with dewpoints in our area probably mixing
out a bit this afternoon, but likely holding well into the 60s.
Temps are tricky today with temps already rising behind the warm
front in the middle of the night, however cloud cover could make
the difference between upper 80s and what otherwise could be
low/mid 90s given the 925mb temps.
Small mesoscale convective system is currently ongoing over northern Iowa and moving
generally east to east-southeast in our general direction, however Storm Prediction Center rap based
mesoanalysis suggests that this activity should run into strong
convective inhibition before arriving which should result in a
pretty quick decay early this morning. None the less, debris
cloudiness from this convection as well as cirrus fanning north
from land-falling Cindy opted to not making any changes to our
going high temps even though they are a bit below what is
supported by 925mb climo.
Some moisture from Cindy could be enough to allow for a shower or
storm to develop later this afternoon over central Illinois and did keep
a slight chance pop down south to cover that potential. More
organized, intense convection is likely to Blossom farther north
across WI west-southwest into north Iowa this afternoon in advance of the
approaching cold front. This activity is likely to move generally
eastward with a gradual southward propagation allowing it to
spread into our County Warning Area from the north later this evening and
particularly overnight. By the time the storms arrive, instability
would be weakening/inhibition strengthening, which favors storms
to be in a weakening phase as they arrive, especially give shear
profiles are pretty marginal to maintain organized severe threat.
Cannot rule out an isolated severe threat in our northwest County Warning Area late this
evening, but overall am quite unimpressed with the prospects for
severe weather tonight. High pwats do support the idea of
efficient rain producers, but not seeing any strong signals
favoring training convection and a significant flash flood threat.
319 am CDT
Friday through Wednesday...
cold front should be in the process of moving across the area
daybreak Friday with showers and T-storms probably lingering into
the morning hours over about the southeast 1/3 to 1/2 of the area.
In the wake of the front, look for noticeably less humid air to
begin advecting from the northwest in Friday afternoon. As is typical
with Summer cold front, the cooler air lags pretty far behind the
front, so temps should still be seasonably warm Friday with many
areas likely still reaching into the 80s, albeit mostly lower 80s.
Over the weekend, an anomalously deep/chilly upper trough is
forecast to dig into the Great Lakes region. The cold air aloft
should allow for a very healthy afternoon cumulus field to Blossom
both Saturday and Sunday, but rain prospects don't look too
impressive given the drier air mass in place. The latest version
of the GFS convective parameterization has a tendency to break out
widespread light convective precip in these type of scenarios and
am favoring the drier European model (ecmwf) solution. Certainly a brief light
shower or sprinkle is possible this weekend, but vast majority of
the weekend looks to be dry and seasonably cool with highs only in
the lower end of the 70s spectrum.
Upper trough is forecast to eventually loosen its grip and allow
for a moderation trend next week, but latest European model (ecmwf) is less
progressive that prior runs, so we could be in store for a couple
more days with sub-80f highs in late June to kick off next week.
Overnight low overs the weekend and into next week should fall
into the 50s outside of the Chicago urban heat island, so some
very comfortable temps/humidity levels are coming after today's
brief taste of moderate heat/humidity.
for the 12z tafs...
Scattered showers are blossoming over northern Illinois. There is very
little instability to support the showers, so thinking they will
only last an hour or two. Gusty southwest winds continue with
Max gusts around 25 kt. A stable environment should prevent precip
from occurring through this evening.
However, convection will be ongoing across Iowa and Wisconsin
today. A line of showers and storms will shift southeast into
northern Illinois late this evening or around midnight. I have high
confidence in a convective line impacting the terminals, but still
only have medium confidence in exact timing. Right now it looks as
if storms will reach Rockford around midnight. Thunderstorms are
expected along the leading edge and may have a few additional
lightning strikes embedded in the area of rain following the
line of storms. Guidance still suggests MVFR or IFR cigs will
spread north this evening. My confidence is still low in IFR cigs,
but MVFR cigs are possible after midnight.
A cold front swings through Friday morning bringing drier air to
the region. Cigs should lift to VFR and winds shift to northwest
around 10 kt.
217 am CDT
Southwest winds will be gusting around 25 kt today so issued a Small
Craft Advisory through this afternoon.
The low over western Manitoba will weaken as it reaches Ontario
tonight and then continues to Quebec. Its cold front will pass over
the lake tonight into early Friday morning with winds becoming west
to northwest behind the front. Westerly winds are then forecast
through early next week.
The remnants of Tropical Storm Cindy will move over the southern
Mississippi Valley today and then the low will fill as it shifts
east over the Ohio and Tennessee River valleys late this week. High
pressure builds over the plains Sunday and then builds over the
western Great Lakes early next week. Winds become southerly behind
the high Wednesday.
Lm...Small Craft Advisory...nearshore waters
until 7 PM Thursday.
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