Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 202101

Area Forecast Discussion
National Weather Service Quad Cities IA IL
301 PM CST Sat Jan 20 2018

Issued at 300 PM CST Sat Jan 20 2018

An area of low clouds advanced north to Interstate 80 during the
afternoon hours. Temperatures were mostly in the 40s area-wide,
and actually warmer over some areas north of I-80 where the sky
was clear.


.SHORT TERM...(This Evening through Sunday)
ISSUED AT 300 PM CST Sat Jan 20 2018

The main forecast issues in the short term period involve
prospects for dense fog and freezing drizzle overnight.

Low level moisture and synoptic lift will be on the increase from
south to north tonight. Expect widespread drizzle and some fog to
develop. Surface temperatures near freezing are mostly likely
along Highway 20 in Iowa, and cannot rule out a light glaze of ice
in this area early Sunday morning. Will likely have to wait until
closer to event time to get a good handle on impacts, if any.
Various high resolution model guidance develops fog overnight as
well. Highway 20 has the most lingering snow cover, albeit light,
and would be a favored area for dense fog. Confidence in
coverage/timing is low and will pass along for later shifts to

Rain will increase in coverage through the day Sunday, with
persistent patchy fog. Expect highs near 40 north to around 50
degrees south.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 300 PM CST Sat Jan 20 2018

Long term forecast focuses on the system ongoing at the start of the
period.  After this system moves out of the area a more normal to
slightly warmer weather pattern is expected.  Next weekend, another
system seems likely.  Current therm profiles suggest mainly rain at
this time.

Strong H5 trof expected to be just west and south of Iowa.  H5
divergence will aid in large scale vertical motion across the area.
Strong H85 WAA will also lead to rising motion on a largescale. The
overall jet structure does not support additional lift via a jet
streak until Monday morning.   With the warm draw today, it appears
that the blended models are too low with temperature and moisture
return tomorrow. The NAM still believe there is snow where there is
no snow and is bring air temps down to low.  For temps, went with a
superblend/ECMWF 50/50 blend.  This brings the 50s into the area.

As far as convection goes, there is a chance for elevated convection
associated with the terminus of the H85 LLJ.  This is why isolated
thunder has been kept in the forecast.  What does appear possible
now is clear slot convection Monday morning across the eastern CWA.
At this time, upper level divergence and a jet streak will lead to
chances for rain.  During this period, all of the guidance suggests
that 0-3km CAPE will be around 100 - 125 J/kg.  Looking at the low
level wind field, strong 0-1km shear also exists.  The sfc winds are
backed and the 0-1km shear vector is out of the SW.  This appears to
be a good set-up for HSLC tornadoes.  If the CAMs develop cells on
the clear slot, or even a broken line, there will be a threat for
HSLC tornadoes, especially as the 0-3km shear increases from 12z to
18z on Monday.  This will need to be monitored by later shifts.

As the colder air wraps in, a transition to snow seems likely.
Currently the forcing in the dendritic layer is short-lived so it
appears that at the transition time the heaviest snow will occur
before the drier air scours out remaining moisture.  Overall
accumulations are very light save the NE CWA with a pivoting nature
of the system will result in longer chances for snow.  Still
accumulation in this area is less than 2 inches at this time.


.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
ISSUED AT 1213 PM CST Sat Jan 20 2018

Low stratus will continue to advance north this afternoon and
evening, eventually impacting all TAF sites. DZ also developing
late this afternoon/tonight as ceilings and vsbys drop into the
LIFR and VLIFR categories. Surface temperatures early Sunday
morning will be borderline for FZDZ at KDBQ and will not introduce
at this time. Precipitation will then transition to rain Sunday.




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