Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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649
FXUS63 KDVN 212151 AAA
AFDDVN

Area Forecast Discussion...UPDATED
National Weather Service Quad Cities IA IL
351 PM CST Sun Jan 21 2018

.SYNOPSIS...
Issued at 322 PM CST Sun Jan 21 2018

Strong low pressure system was centered over central Kansas this
afternoon. A warm front stretched east from the low, cutting
across southern Iowa. This warm front was slowly moving north
today as temperatures reached near 60 degrees in the far southern
CWA. This low pressure system will drive the weather in the short
term.

&&

.SHORT TERM...(This Evening through Monday)
ISSUED AT 322 PM CST Sun Jan 21 2018

Convergence along and just north of the warm front is leading to
patchy dense fog. This will likely continue until stronger forcing
arrives in the area later this afternoon and into the evening. At
this time, rain will likely improve visibilities. Due to the
spotty nature of the dense fog decided to go with a SPS to cover
it.

Overall, the blended models appear to be biased lower in temps and
northward progression of the warm front due to the NAM. Again the
NAM believes that there is snow where there isn`t and that melting
is not occurring today. This of course is incorrect. Trends in
hourly temps have warmer temps across the CWA than any guidance
today. This leads credence to the idea that the H85 WAA tonight
will lead to warming temps and moistening of the BL tonight across
the area. As the low approaches the area, the system begins to
occlude. The triple point is forecast to move directly over the
CWA. Then a clear slot is expected to rush across the eastern CWA
during the day and into the afternoon tomorrow. Consensus on this
evolution of the system is high. What is still of question is
exact placement of these elements. I believe that the bias in the
models is for a more cooler solution than what will actually
occur, as a result have trended fields towards a slightly more
northern, and warmer track. That latest HRRR hourly temps suggest
that I am not warm enough tonight.

As far as sensible weather goes, the drizzle will turn to rain
showers tonight as the H5 divergence and H85 LLJ moves into our
area. Withe terminus of the jet nearby and a deepening cyclone,
would not rule out isolated thunder tonight. SPC has the entire
area in a risk for thunderstorms. Into tomorrow, it appears that
the CAMs try to produce thunderstorms on the back end of the
precip shield. This would correspond to high low level shear, but
a lack of 0-3km CAPE. These cells could lead to an isolated wind
gust or small hail. The main threat for severe weather will be
associated with the clear slot. Consistent signal in the CAMs
suggests thunderstorm development tomorrow afternoon. This would
coincide with the triple point across the central CWA. Low levels
winds are backed in the lowest 1km of the atmosphere. This coupled
with 0-3km CAPE forecast to be near 200 J/kg in the 18z NAM
(320-360 J/kg in the 12Z NAM) will lead to a HSLC tornado threat
from these storms. The 0-3km CAPE drops off as the system pulls
north later in the afternoon. This may be premature as the models
still think there is snow there. Updraft helicity in the NAMnest
suggests that virtually every updraft could rotate. The question
of rotation to the ground will be tied to where the 0-3km CAPE is.
SPC has introduced a marginal risk for our far southeast
tomorrow. This event will need to be watched for potential
increased messaging as an off-season fast moving event could lead
to greater impacts. If the convection and the 0-3km CAPE do not
overlap near the triple point, then the threat for tornadoes would
be very low. That said, this event looks like an almost textbook
example for cold-core low tornadoes. This needs to be monitored,
especially if thermal trends do not hold as currently forecast.

.LONG TERM...(Monday Night through Sunday)
ISSUED AT 322 PM CST Sun Jan 21 2018

There is good overall agreement between the synoptic models on the
overall pattern and progression but some differences exist in the
finer scale details. The main forecast concerns are precipitation
type through 12z Tuesday and then again at the end of the week and
into next weekend.

00z Tuesday to 12z Tuesday morning, a deep closed 500 mb and
surface low be exiting into the southern Great Lakes. As this
shortwave moves eastward, precipitation will be dictated by the
race between cold air advection and drier air moving into the air.
Currently, the GFS is most aggressive in drying out the
atmosphere as the deformation zone rotates across eastern Iowa,
northwest Illinois, and far northwest Missouri in the wake of the
low. Other models continue to be saturated in the dendritic growth
zone (-10C to
-20C) which is key for ice production and a transition to snow or
drizzle. With the uncertainty in the timing of colder air and
saturation decided to go with a rain and snow mix and assume that
moisture. will linter aloft until precipitation come to an end.
Any freezing rain or drizzle will be dependent on the surface
temperatures but models have them dropping below freezing after 06
utc on Tuesday which is about the time precipitation will come to
an end from west to east by 12 utc on Tuesday. Snow amounts will
be light with 1 to 2 inches possible in my far northwest Illinois
counties with less than 1 inch possible in other locations.

Tuesday through Wednesday, the 500 hpa pattern will be more of a
northwesterly flow aloft with a shortwave passing to our north on
Wednesday.  The surface low will moving across northern Minnesota
and Wisconsin with limited moisture thus only overcast skies are
expected in our area. High temperatues will be closer to
climatological normals in the upper 20s north to lower to mid 30s
south. However, low temperatures will be 5 to 10 degrees above
normal resulting in a slightly above average period.

Thursday through Saturday, 500 mb ridging will move across the area
with another surge in warmer air ahead of another storm system
diving into the Desert Southwest.  As the ridge moves to the east
strong warm advection will take place with similar temperatures to
Saturday and Sunday of the past week for Friday into Saturday.  This
will bring the chance of precipitation into the region Friday night
into Saturday. Right now precipitation amounts are light despite the
fact that the Gulf of Mexico appears to be open so this forecast may
change. Precipitation type may start out as a rain snow mix before
transiting to all a rain during the day on Saturday.

In the wake of the this system, Canadian High pressure will build
into the region with a return to near normal temperatures and dry
conditions for the start of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
ISSUED AT 1208 PM CST Sun Jan 21 2018

Busy TAF period with IFR and LIFR flight rules dominating the
period. The next few hours may see some sites bouncing back to
MVFR, this should be short lived. Most sites will see a DZ then
rain through the period. Very minimal improvements to TAF sites
expected late in period.


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...Gibbs
SHORT TERM...Gibbs
LONG TERM...Cousins
AVIATION...Gibbs



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