Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1234 PM CDT Monday Aug 31 2015

issued at 1223 PM CDT Monday Aug 31 2015

Satellite trends are finally showing some very weak diurnal cumulus
developing at middle day. Maximum temperatures have been raised slightly
based on trends this morning. The increased temperatures now
bring into question the potential for any diurnal convection late
this afternoon and into the evening hours.

Based on trends of the diurnal cumulus that has developed and how it
looks...the potential does not look promising for any late
afternoon convection. If convection would develop...areal coverage
would be less than 5 percent.

Update issued at 648 am CDT Monday Aug 31 2015

Most areas of dense fog have improved the last hour or so. This
was due to increased winds across the area. As such have cancelled
the dense fog advection. Some areas may still see isolated dense fog
through at least 9am. Also added haze to grids as it appears that we
may see hazy skies not expect this to affect visibilities at
airports...but will be noticeable due to the wildfire smoke in our


issued at 300 am CDT Monday Aug 31 2015

Another foggy morning is shaping up across the area. The fog has
been slower to develop than last night as slightly warmer temperatures
have kept the temperature dewpoint spread larger than yesterday. Surface high
pressure is still entrenched across the area this morning. The
upper level ridge across the area is shifting slightly to the
east. Smoke associated with wildfires in the west made for a hazy
day yesterday. This smoke could continue to affect the area today
as well. A stalled out surface boundary is across central Illinois and
central MO. This Boundary....Ridge and smoke are the main
contributors to the forecast in the short term.


Short term...(today and tonight)
issued at 300 am CDT Monday Aug 31 2015

Main forecast concern in the short term forecast are the
temperatures and possible isolated showers today across the area.
Smoke associated with wildfires in the western US will make for a
tricky temperature forecast today.

With the models too warm to begin with...the GFS seems to be
unnaturally warm for the 850 mb and h92 appears that MOS
seems to have the best handle on high temperatures today. The met yesterday
suggested cooler highs for Sunday and again for today. So I
started with the met across the area. I then lowered temperatures across
the north by a degree or two...trying to account for the smoke
across the area today. The smoke and possible lingering fog...the
high temperatures today have a high likelihood of busting.

Another potential forecast bust is the chance for probability of precipitation today. With
the surface boundary to the south and fog across our area today.
Differential heating could aid in bringing that boundary closer to
our south. Convergence along this boundary along with daytime
heating could pop isolated showers and storms. A couple of ifs are
involved in this. First...temperatures need to get high enough to reach
convective temperatures...this may be hard with the smoke...second the
boundary pushes closer to US. If this occurs...we could see
isolated showers and even some thunder this afternoon. As such
went with a 14 pop across that area. Some of the cams are popping
storms as well. Will pass this on to the day shift.

Long term...(tuesday through sunday)
issued at 300 am CDT Monday Aug 31 2015

Return to late Summer heat and dry or mostly dry this week with
possible isolated afternoon storms...then rain likely by late in the
weekend and turning cooler.

Long term forecast confidence assessment...fair or average due to
stagnant ridging pattern providing little sensible weather issues.
As have mentioned past 3 nights...ongoing issues are the same which
is persistence continues to show most periods may be a degree or two
too warm for highs and possibly 2 to 4 plus degrees too mild for
mins most periods and locations. Second issue is the risk of
isolated showers or storms as convective temperatures may be reached
middle to late PM into early evening hours.

Overview...initialization and verification at 06z once again
straight-forward with limited trends and impacts in stagnant late
Summer pattern until late Saturday. Besides bl moisture a bit too
moist...and thus dewpoints too high by a few degrees. Run to run and
forcing tools continue to suggest a even mix of all solutions.

Tuesday through Friday...once again...essentially no change in
forecast. The stagnant ridging pattern continues with above normal
temperatures and humid. Highs should be 85 to 90 degrees most
locations each day with heat index values rising in PM into the
lower 90s. Persistence suggests highs may a degree or two too high
with limited winds and mixing and persistence. Lows in the middle
to upper 60s very possibly 2+ up to 5 degrees too mild for later
shifts to reassess. Analysis continues to suggest one or more days
of isolated storms seem probable as convective temperatures will be
very close to being reached. Any storms should be non-severe but
gusty winds over 35 miles per hour are possible for later shifts to consider.
Heat indices into Saturday for several hours should reach the lower
90s...though forecast dewpoints based on persistence likely a few
degrees too high.

Saturday and Sunday...approaching cool front have chance probability of precipitation with
lots of moisture streaming into the region from an active SW monsoon
flow. Local techniques suggest most areas will see decent rains
with locally heavy rainfall and possibly a low risk of isolated
strong or lower end severe storms in the PM hours. In other words a
typical early September convective weather event. Timing and extent
of this system should be clarified next 24 hours with amounts better
known in next 72 hours. Local rain tools suggests mostly moderate
amounts with risk of 2 to 3 inches which may produce some river
rises but unlikely to result in significant hydrologic issues at this time.


Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1223 PM CDT Monday Aug 31 2015

VFR conditions will be seen through 06z/01 as an upper level high
remains across the Midwest. There may or may not be some very
isolated diurnal convection within visual distance of kbrl after
21z/31 but areal coverage would be under 5 percent. After 06z/01
MVFR conditions should develop due to light winds and high
moisture content of the lower atmosphere. Conditions would then
again improve to VFR after 12z/01.


Dvn watches/warnings/advisories...


short term...Gibbs
long term...Nichols

National Weather Service Glossary of Abbreviations