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Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1030 PM CST Wed Dec 11 2019

..06z aviation update...

Synopsis...
issued at 205 PM CST Wed Dec 11 2019

A clipper that brought a light and fluffy accumulating snow to much
of eastern Iowa and northwest Illinois moved out of the area late
this morning. Low-level frontogenetical forcing and high slrs
allowed accumulations of one to around two inches along a line
from Cedar Rapids to the Quad Cities, leading to some difficult
travel condtions during the morning.

Once the system passed, skies cleared and allowed for plentiful
sunshine early this afternoon. Temperatures as of 2 PM ranged from
17 in Cedar Rapids, to 32 in Keokuk. Temperatures inside the snow
cover were generally in the upper teens to near 20, while outside of
that temperatures were warmer.

&&

Short term...(this evening through Thursday night)
issued at 205 PM CST Wed Dec 11 2019

Key messages:

1) quiet and dry weather expected for most over the next 36
hours.

2) chance of mixed precipitation along and north of the Highway 20
corridor late tonight through Thursday afternoon.

3) temperatures rebounding back above normal Thursday.

This evening and tonight...

Will be quiet and dry this evening and early tonight as high
pressure quickly moves through the area. Winds will die down briefly
as the pressure gradient weakens over the high. With clear skies
overhead, this will help temperatures quickly bottom out as the sun
sets, especially over areas with snow cover. I have cooled
overnight lows a little further than guidance as it doesn't
handle the effects on temperatures extremely well. Expecting lows
in the low to mid teens roughly along a line from Vinton, to Cedar
Rapids, to the Quad Cities. Temperatures to the south and
northeast will be warmer by a few degrees, especially south of
I-80.

As a surface low pressure develops over western Kansas and Nebraska
late this evening, winds will begin to pick up around midnight from
the southeast. With mixing renewed, this will result in overnight
lows occurring during the evening as temperatures rise beginning
around midnight. This low will quickly move northeast and bring a
strong 925-850 hpa low level jet across the Mississippi River
valley.

Thursday and Thursday night...

Precipitation chances will begin to increase around 6-9 am for areas
along and north of the Highway 20 corridor. Forecast sounding
thermal profiles suggest it will be cold enough for snow to start,
so there is potential for a brief window of light snow accumulations
of a few tenths of an inch. Heavier snow amounts are expected
further into northeast Iowa and southwest Wisconsin where quantitative precipitation forecast is
higher and colder temperatures are expected to be more prolonged.
Speaking of temperatures, continued elevated warm air advection with
the jet will allow snow to begin mixing with rain, with a complete
changeover expected to all rain expected around midday. This will
continue before exiting by the evening, leaving mainly quiet
condtions in place for Thursday night. The NAM continues to
advertise potential for some areas of drizzle, but this appears to
be an outlier as ensembles and other deterministic guidance favor
dry condtions.

Some snow cover will persist into tomorrow morning, but it should
all be gone by the afternoon as gusty south winds usher mild air
back across the region. While the snow may temper afternoon highs
slightly, they are expected to rise into the upper 30s to low 40s by
the afternoon. Thursday overnight lows will range from the lower to
middle 20s over eastern Iowa, to low 30s in west central Illinois
and northeast Missouri.

Long term...(friday through wednesday)
issued at 205 PM CST Wed Dec 11 2019

Key points:

1. Light wintry mix of rain / freezing rain/drizzle / snow is
possible from midday Friday through Saturday am, ending Saturday
afternoon and evening as snow as the system exits.

2. A larger all snow system is slated for Sunday night through
Monday night for parts of the mid to upper MS valley.

Friday and Saturday, a cold front will be poised just northwest of
the area early Friday which will move through the area by Saturday
morning. Meanwhile, a clipper-type system will sweep down across the
Iowa-Missouri border region Friday night through Saturday morning.
The model blend has come in slightly warmer, which lends to more of
a rain/snow mix type event and less on accumulating snow.
Regardless, it appears to be a light quantitative precipitation forecast event with better moisture
north of the County Warning Area. That said, the GFS does break out some much higher
quantitative precipitation forecast for Saturday. It remains an outlier at this time.

Sunday evening through Monday night, models bring a larger system
across the southern Mississippi Valley region. There remains
considerable uncertainty with this system with models disagreeing on
timing, track, and quantitative precipitation forecast placement. The GFS seems to be the driver for
the local area, bringing a swath of snow up across northern MO and
central Illinois region. It is also the furthest north with the quantitative precipitation forecast. As
this remains in the day 5-7 timeframe, there is plenty of time for
refinement.

Overall, high temperatures will be in the 20s through much of the
period, with lows in the single digits and teens.

&&

Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1020 PM CST Wed Dec 11 2019

Low level wind shear will impact the terminals overnight and early
Thursday morning, as southwesterly low level jet increases to around
50 kts above a lifting warm front and attendant east/southeasterly
surface wind. The low level shear will abate mid to late morning
as stronger winds aloft mix down and lead to prevalent gusty
southerly winds 15-30+ kts. A low pressure system will track across
far northern Iowa and into Wisconsin on Thursday. Much of any
precipitation will be found to our north near this low track, but
there is a chance for a rain/snow mix mid morning through afternoon
at mainly dbq and Cid. And will have to monitor for potential of
needing to expand this further south to include mli. Ceilings
will drop into lower VFR to MVFR by mid morning through afternoon
across the region in the wake of the warm front and ahead of an
approaching weak cold front. Winds will diminish with the weak
cold frontal passage and turn northerly by late afternoon through
Thursday evening.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

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