Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
721 PM CDT sun Aug 30 2015

issued at 720 PM CDT sun Aug 30 2015

Have gone out with a dense fog advisory for most of eastern Iowa and
adjacent Illinois counties along I-80 for late tonight through early
morning Monday. Conditions are similar to last night...with the
same airmass in place and nearly calm winds under the west-east surface
ridge overhead. Surface dewpoints are higher in many places than
last night at this time. Crossover technique...using dewpoints at
peak heating and current forecast lows supports widespread fog.
The fog could be a little later to develop due to the higher
temperatures and temperature/dewpoint departures at this time tonight vs
this time last night.


issued at 333 PM CDT sun Aug 30 2015

Surface ridge axis extends from the Great Lakes through the County Warning Area
and into the south Central Plains at middle afternoon. The fairly
widespread dense fog burned off by middle to late am leaving behind
fairly extensive stratus which has been steadily eroding from the
south and evolving into scattered stratocu deck. The increasing
peaks of sunshine has helped to bump temperatures up to near late
August normals with many areas in the upper 70s to lower 80s.
The exception being mainly portions of northeast Iowa where stratus
sill lingers and aiding in holding temperatures down in the lower 70s
attim... but with more late day sunshine should see temperatures in these
areas surge late. Pattern aloft features an amplified ridge /heat dome/
through the plains in advance of a Pacific northwest trough. The next
24 hours will see this ridge and warmer air build into the middle-
Mississippi Valley as the Pacific northwest trough shifts across the
northern rockies. This will result in continuation of dry conditions
with warmer temperatures on Monday.


Short term...(this evening through monday)
issued at 333 PM CDT sun Aug 30 2015

Main challenge remains low clouds and fog potential with impact on

Following trends per satellite and observation from late afternoon with decreasing stratus
northwest County Warning Area evolving into scattered stratocumulus then mostly
clear for a time this evening. Late evening and overnight with
continued weak winds and abundant low level moisture expect
redevelopment of fog dense in some areas... with also some stratus
possible although uncertain on extent of any stratus formation with
latest rap and NAM models showing around 15+ kts of wind just above
moist layer which may be sufficient mixing to offset widespread stratus
formation and with just diurnal cumulus across MO Don/T expect any
advection of low deck. Min temperatures will be challenging and dependent on
fog and stratus trends. As we saw this am... potential for areas that
stay clear longer to radiate down into the 50s with near calm winds.
Otherwise... with temperatures a bit warmer today for many and with middle afternoon
dewpoints in middle/upper 60s I have kept lows mainly in range of 60-67 degrees.

On Monday...any fog should burn off a bit earlier tomorrow by middle am
with a bit stronger southerly wind developing of 5-10 kts. Any
stratus should lift to fair weather cumulus during afternoon and generally
mostly sunny skies. This will allow temperatures to climb well into the 80s
with widespread middle to a few upper 80s. These temperatures coupled with
dewpoints well in the 60s to near 70 degrees will make for very warm feel
with heat indices around 90/l90s.

Long term...(monday night through sunday)
issued at 333 PM CDT sun Aug 30 2015

Overview...building and persistent upper-level ridge over the
central U.S. Will lead to above average temperatures and humid
conditions through the extended period. Overall chances for rain
are very low.

Tuesday and Wednesday...1000-500mb thickness values are forecast to
be near 576-579 dam with 850mb temperatures in the upper teens/near 20 c.
Forecast highs are in the upper 80s to near 90 f...with heat indices
in the lower to middle 90s.

A shortwave trough from remnant monsoonal convection may traverse
the Midwest on Wednesday but models track it northwest of east Iowa/west
Illinois. Consequently kept the forecast dry.

Thursday through Saturday...raw models still appear to have a warm
bias and are projecting lower to middle 90s. Humid
conditions...850mb temperatures in the upper teens to possibly lower 20s
celsius...and a lower September sun angle back a temperature forecast
slightly below the higher raw model data.


Aviation...(for the 00z tafs through 00z Monday evening)
issued at 631 PM CDT sun Aug 30 2015

A weak flow regime under high pressure...along with abundant low
level moisture will again lead to areas of fog tonight. This is
likely to develop between 04z and 06z...and have trended
conditions becoming MVFR at all sites during this period. Through
this time...thick smoke from western wildfires will produce haze
aloft. From 06z through the rest of the night...widespread IFR to
vlifr is again likely. For appears the brl site is most
likely to experience the lowest conditions. Monday morning...the
fog will again be slow to clear and have optimistically trended
all sites back to MVFR visibilities between 15z and 17z.


Dvn watches/warnings/advisories...
Iowa...dense fog advisory from 2 am to 8 am CDT Monday for Benton-Cedar-
Clinton-Des Moines-Henry Iowa-Iowa-Jackson-Jefferson-Johnson-
Jones-Keokuk-Lee-Linn-Louisa-Muscatine-Scott-Van Buren-

Illinois...dense fog advisory from 2 am to 8 am CDT Monday for Henderson-
Henry Illinois-Mercer-Rock Island-Whiteside.



short term...McClure
long term...uttech

National Weather Service Glossary of Abbreviations