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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
347 PM CST Wednesday Nov 25 2015

issued at 330 PM CST Wednesday Nov 25 2015

12z upper air analysis has a low level jet running from Texas into the western Great
Lakes. This low level jet was helping to transport higher moisture from the
Gulf Coast into the Midwest. Satellite trends through middle afternoon
show plenty of low and middle level clouds across the Midwest.

18z surface data indicates high pressure over the East Coast with a cold
front running from The Arrowhead of Minnesota into southwest Kansas.
Dew points were in the 30s across the Great Lakes and northern
plains with 40s and higher from the middle Mississippi Valley and
Central Plains on south.


Short term...(this evening through thursday)
issued at 330 PM CST Wednesday Nov 25 2015

Radar continues to indicate no returns other than some lower clouds
across the area through middle afternoon. Thus it is quite possible
that dry conditions will continue through sunset.

As the atmosphere continues to saturate tonight...light rain/drizzle
will slowly break out across the area. Where winds drop off closer
to the approaching front...patchy fog should also develop as
moisture increases.

Overall tonight will be quite mild with lows more typical of the
highs for late November. Rainfall tonight will be quite light with
amounts under 0.20 inches.

On Thursday light rain/drizzle will become more widespread during
the morning as the overall forcing/moisture increases. Patchy fog
should also be seen around the frontal boundary as it slowly moves
across the western third of the area.

During the afternoon hours...rainfall will slowly become more heavy
over time. The patchy fog should continue around the frontal
boundary with the cold front along the Mississippi River by sunset.
Precipitation through sunset will be in the form of rain.

The amount of forcing/moisture surging into the area raises the
possibility of a couple of isolated thunderstorms on Thursday. There
are subtle signals within the models suggesting that this is
possible. However...this was not included in the forecast as the
actual number of storms would probably be five or less.

Temperatures ahead of the cold front Thursday will be quite mild
with readings well above normal. The models are indicating that
there should be a fairly quick 10 degree temperature drop in about 2
or 3 hours once the front passes any one location.

Long term...(thursday night through next wednesday)
issued at 330 PM CST Wednesday Nov 25 2015

Thursday night and Friday...latest model runs suggest deep lift/
omegas occurring along and Post-frontally in a saturated airmass of
precipitable water/S 1.2 to 1.4+ inches for ongoing main precipitation window from 00z-
08z. Long fetch linear precipitation band will push from west-to-east across
the County Warning Area Thursday night as does the front...and will keep areas of fog
along the low level moisture convergence in the vicinity of the front itself
especially during lulls in heavier rain. Will go with a general 0.75
to around 1 inch additional rainfall during 12 hours through 12z
Friday...but could see several areas getting an inch or more Thursday night
if frontal push temporarily stalls. Just some lingering lighter
amounts from 12z-18z Friday of a tenth of an inch or less in the
southeastern third to half of the dvn County Warning Area...and prefer the 12z GFS
suggesting a mainly dry Friday afternoon and evening except across the
extreme southern County Warning Area. Thus storm totals still look to be 1-2 inches
in general...with a few areas of 2 to 2.5 inches possible. Also
continue to see marginal in-wrapping low level cold conveyor behind the
front while rain is still occurring aloft for much of any ice/
freezing rain threat in the northwestern third of the dvn County Warning Area late
Friday night. There may be a period of sleet...but for now will
continue to go a rain-snow mix transition to all snow west of the
Cid to dbq areas into early Friday morning. Just a wet dusting up to a
half inch expected currently in these areas by dawn Friday.

Temperatures to continue to fall through the period in Post-frontal cold
air advection...with Friday highs occurring during the morning. Feel
the 12z GFS and to some extent the NAM...taking into account Post-
frontal system surface and upper ridging more reasonably and shunt
most of secondary overrunning precipitation generated off Southern Plains
low pressure system Friday the south of the dvn County Warning Area into Sat
morning. The 12z European model (ecmwf) is much further north and would have snow or
a wintry mix spreading up across the south half of the County Warning Area Friday night
into Sat morning. With warming temperatures aloft and boundary layer
north/northeasterly flow...will just clip the far south with a
slight chance of rain or freezing rain for Friday night for now.

Saturday and Sunday...with a more dominant western Great Lakes low level
ridge as well as a ridge lobe aloft...the 12z GFS is mainly dry
through Sunday night for the local area with all the action to the
south across the Southern Plains...MO and up the Ohio River valley. This
in contrast to the new European model (ecmwf) which again brings overrunning rain or
a wintry mix up across the County Warning Area late Sat night and for Sunday. As a
compromise...will continue to advertise low chance probability of precipitation mainly in the
south. Areas that manage some sunshine this weekend should warm up
into the 40s.

Early next week...the latest suite of medium range models continue
to advertise a large slow-rolling cut-off upper low moving east across
the Midwest...with an active southern stream south of it possibly
taking brunt of ingest of both Pacific and Gulf moisture feeds.
Thus the more substantial precipitation events may occur to the south and
southeast of the County Warning Area...with just wrap around precipitation itself affecting
the local area under the upper cyclone late Monday through Tuesday.
It/the low will look to become more cold core as it pushes through
the region...setting the stage for mainly snow Monday night into Tuesday in
whatever precipitation bands it manages to generate over the local forecast
area. But model inconsistentcies with this system make for low
confidence at this juncture.


Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1139 am CST Wednesday Nov 25 2015

VFR conditions will continue through 00z/26 as a storm system
organizes in the plains. After 00z/26...conditions will slowly
deteriorate to MVFR with areas of IFR conditions through 12z/26.
After 12z/26 conditions are expected to deteriorate to IFR/LIFR with
rain as the storm system moves into the western Great Lakes.


issued at 330 PM CST Wednesday Nov 25 2015

Long duration rainfall event through Thanksgiving and into Friday
morning still on track to bring rainfall amounts from 1 to 2+ inches
across the area. With some snowpack still on the ground and
saturated soils...a large percentage of this steady rain could go
straight into runoff. Ensemble river forecasts suggest many area
rivers making flood stage when the highest quantitative precipitation forecast potential is used.
But using the latest forecast amounts...the Iowa river at
Marengo...English River at Kalona...and north Skunk River at
Sigourney make flood stage by the weekend. Several other rivers or
river sections make action stage...such as the rock...Pecatonica...
wapsi...and lower sections of the main Stem Mississippi from
Keithsburg on south by Sunday or early next week. Will hold off any
early issuance of river flood warnings for now to better see how
rainfall and amount potential will lay out over the next 24-36 hours.


Dvn watches/warnings/advisories...


short term...08
long term...12

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