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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
916 PM CDT Friday Oct 24 2014

Update...
issued at 913 PM CDT Friday Oct 24 2014

Fog has once again developed rapidly in the early evening hours.
As was the case last night...it is already dense at initial
development. It should not improve until dry air advection begins
behind the wind switch/weak cold front arriving around 4 to 5 am
west...and around 8 to 9 am east. With winds possibly decoupling
further...this may slow down compared to model timing thus further
delaying fog improvement. I have already issued the dense fog
advisory for all but the northeast and east...where clouds may
hold on through the wind switch.
Ervin

Update issued at 550 PM CDT Friday Oct 24 2014

A quick update sent to add drizzle for areas in the north 1/2
early this evening...as this is already spreading east through
eastern Iowa at this time. Also...dense fog appears quite likely
overnight ahead of the cold front...I have increased fog in the
grids and will consider an advisory later this evening as things
develop.
Ervin

&&

Synopsis...
issued at 259 PM CDT Friday Oct 24 2014

Widespread fog and stratus from this morning has since narrowed down
to a roughly 100 Mile wide axis from southeast Minnesota through central and
eastern Iowa to far northern MO. 18z surface analysis showed this was
a favorable area of low level moisture advection ahead of a surface
trough that reached from S central Minnesota through western Iowa to north
central Kansas. Surface dewpoints over central Iowa were in the upper 50s
to lower 60s early this afternoon...while lower to middle 50s were more
common to the east over the local forecast in the deeper mixing and
sunshine. The boundary over western Iowa will advance slowly east
tonight...with its associated tongue of moisture and convergence
likely leading to a resurgence of low clouds and fog.

&&

Short term...(this evening through saturday)
issued at 259 PM CDT Friday Oct 24 2014

Main focus is on the potential for fog tonight. Light southerly
winds ahead of the advancing surface trough over western Iowa will
continue to advect low level moisture...holding dewpoints in the middle
to upper 50s this evening. Fog will likely develop as winds decouple
from the weak gradient flow under initially mostly clear
skies...leading to radiational cooling. This fog will most likely
develop first in the west and northwest...closest to the existing stratus
field and then expand southeastward into northwest Illinois by late evening. A
thermal ridge at 850 mb...migrating through the area by 06z will
reinforce the low level inversion and assist the fog process. A
developing deep dry layer above the inversion may lead to
drizzle...which has been included across the north...timed with peak
convergence with the surface frontal passage. The big question for
tonight is how thick the fog will get and then how long before the
developing Post frontal low level cool and dry air advection leads
to dissipation Saturday morning. With low confidence on the coverage
of dense fog...will hold off on an advisory for now...but include
widespread fog from late evening through early morning. Low
temperatures were kept close to the previous forecast...from the
upper 40s northwest to the lower 50s southeast.

Saturday...a developing northwest flow resulting from an upper ridge building
out west...and advancing surface ridge will lead to mostly sunny
skies and light winds. A large thermal ridge over the plains...with
850 mb temperatures into the teens as far north as the Dakotas...
will lead warm temperatures for late October. Under this thermal
ridge...surface temperatures were in the 70s and 80s this afternoon
across Nebraska and South Dakota...with the help of deep mixing and downslope
compressional warming. However...even with relatively shallow mixing
shown in model soundings...temperatures in the local area Saturday
should reach at least the middle 60s to lower 70s.

Long term...(saturday night through friday)
issued at 259 PM CDT Friday Oct 24 2014

Mild and mainly dry into Monday before next front brings period of
wet weather followed by cooler temperatures /near normal/.

Saturday night...high pressure to settle over region with dry and
cool conditions. Ideal radiational cooling setup with light
wind...drier air and mainly clear skies supportive of staying near
to just below cooler side of guidance for mins.

Sunday and Sunday night...high pressure overhead Sunday am will
shift east during the afternoon ahead of developing system in the
plains. Strengthening pressure gradient between departing high and
plains low to result in increasing southeast winds 10-20+ miles per hour.
Kept highs close to previous forecast ranging from middle 60s north to
near 75 far south... but these temperatures could be few to several degrees
too mild given non-favorable warming southeast wind component and
potential for band of warm advection clouds. Even if lower would
still be mainly 60s to around 70 for highs which is still above normal.
Continued with small shower chances Sunday night north of I 80 with
surge of Theta-E advection on nose of 40-50 knots low level jet ahead of
northward advancing surface warm front. Lows Sunday night may actually
occur in the evening with steady to rising temperatures overnight with the
passage of warm front and southerly winds 10-20 miles per hour.

Monday through Tuesday...while still some variances on strength of
next upper level trough making inroads from the plains... the signal
overall increasing for period of wet weather by Monday night into
Tuesday with cold frontal passage. Have raised probability of precipitation into likely Cat
eastern half of County Warning Area but favorable jet structure with potential surface
wave in entrance region... upper diffluence and ample moisture suggest
likely to categorical probability of precipitation needed most if not all of County Warning Area with mainly
moderate quantitative precipitation forecast amounts 0.20 to 0.60 inch. Monday looks to be last of the
above normal warmth...with pre-frontal warm sector featuring deep mixing
on gusty south/southwest winds fueling widespread highs in the 70s.
Rain anticipated to shift east as day progresses on Tuesday with frontal
passage followed by cooler and drier conditions on gusty northwest winds.

Wednesday through Friday...cooler but only back to around normal with
quasi-zonal upper flow. Quick moving system suggested later Wednesday
and Thursday but with moisture limited probability of precipitation were kept in slight chance
category.

&&

Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 613 PM CDT Friday Oct 24 2014

The same moist air mass that produce last nights dense fog event
is overhead tonight. Once again...winds should decrease to under
5 kts...allowing fog to form...with very low IFR ceilings and
visibilities expected. Minimums will likely be reached again at
all sites in eastern Iowa and northwest Illinois...especially
between 06z and 14z/25. After 14z...northwest winds should usher
in dry air...ending this period of persistent vlifr/IFR/MVFR
weather we have dealt with the past 24 hours.
Ervin



&&

Dvn watches/warnings/advisories...
Iowa...dense fog advisory until 7 am CDT Saturday for Benton-Buchanan-
Cedar-Delaware-Dubuque-Henry Iowa-Iowa-Jackson-Jefferson-
Johnson-Jones-Keokuk-Linn-Van Buren-Washington.

Dense fog advisory until 9 am CDT Saturday for Clinton-Des
Moines-Lee-Louisa-Muscatine-Scott.

Illinois...dense fog advisory until 7 am CDT Saturday for Jo Daviess.

Dense fog advisory until 9 am CDT Saturday for Hancock-Henderson-
Henry Illinois-McDonough-Mercer-Rock Island-Warren.

MO...dense fog advisory until 7 am CDT Saturday for Scotland.

Dense fog advisory until 9 am CDT Saturday for Clark.

&&

$$

Update...Ervin
synopsis...sheets
short term...sheets
long term...McClure
aviation...Ervin

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