Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 202300

Area Forecast Discussion
National Weather Service Quad Cities IA IL
600 PM CDT Tue Mar 20 2018


Issued at 304 PM CDT Tue Mar 20 2018

18Z surface data has low pressure over the Tennessee Valley with
high pressure across the upper Midwest. Dew points were in the teens
and 20s across the Great Lakes and upper Midwest with 20s and 30s
across the Plains.


.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 304 PM CDT Tue Mar 20 2018

Late this afternoon through Wednesday
Assessment...high confidence

Quiet and dry conditions will be seen from late this afternoon
through Wednesday as high pressure builds into the Midwest. The
precipitation across central and western Iowa will slowly dissipate
as the overall forcing weakens late this afternoon and evening.

.LONG TERM...(Wednesday Night through Tuesday)
ISSUED AT 304 PM CDT Tue Mar 20 2018

A significant system will impact the Midwest late this week.

Wednesday night
Assessment...high confidence

Quiet and dry conditions will be seen as high pressure moves into
the Great Lakes. There is a low risk of some patchy fog developing
Wednesday night in river valleys and low lying areas due to the
light winds.

Thursday into Thursday night
Assessment...medium confidence

A weak system will move from southern Iowa into the southern half of
Illinois Thursday into Thursday night. From a conceptual picture the
initially dry atmosphere will need to moisten before precipitation
reaches the ground. Additionally, the better moisture inflow will be
across Missouri so the potential is there for the more significant
precipitation to remain south of the area. If the dry air holds off
precipitation until later in the morning or afternoon, the
precipitation would be in the form of rain. If precipitation
develops shortly after sunrise, the potential is there for sleet to
develop before changing over to rain.

Right now chance pops are south of a Vinton, IA to Aledo, IL line
during the morning with chance to likely pops south of an
Independence, IA to Kewanee, IL line in the afternoon. Thursday
evening slight chance to chance pops are south of an Ottumwa, IA to
Galesburg, IL line with dry conditions after midnight.

Assessment...low confidence

A significant storm system will affect the area Friday into
Saturday. However, an overall lack of run-to-run continuity with the
models raises questions.

First, high pressure will be over the Great Lakes helping to feed a
dry easterly flow into the area. Thus it will take time for the
atmosphere to saturation. Some model solutions show surface dew
points dropping during the day on Friday indicating the overall
depth of the dry air is increasing. Once saturation occurs, the
track of the system becomes important in regards to how much cool
and dry air will be pulled into the area.

The ECMWF has been somewhat consistent in wanting to keep the
daylight hours of Friday dry. The CMC global brings precipitation
into the area during the afternoon hours but dprog/DT trends are
just starting to show a weak drying trend. Since this trend with the
CMC global is new it will need to be watched to see if it continues.
The GFS brings a considerable amount of precipitation into the area
on Friday. Given the known moist bias with this model, precipitation
moving into the area during the morning is questionable.

As a result of these differences, the model consensus has dry
conditions Friday morning with slight chance to chance pops over the
entire area Friday afternoon. Precipitation would be in the form of

Friday night on...

Friday night through Saturday night
Assessment...low confidence

The main impacts of the storm system will be seen Friday night into
Saturday with any lingering precipitation ending Saturday evening.
There is agreement among the models that precipitation will be seen
across the area. However, the track of the storm system becomes
important in regards to precipitation type.

The model consensus has likely to categorical pops Friday night into
Saturday. As mentioned earlier the track of the storm is important
in regards to precipitation type. Right now there are differences
between the models regarding track and where forcing will be
maximized across the area.

Right now precipitation will start out as all rain Friday evening
with a change to all snow to just south of the I-80 corridor. The
snow would change back to all rain on Saturday. The snow would be
quite wet so slushy accumulations are possible.

South of the I-80 corridor down to an Ottumwa to Macomb line a
rain/snow mix would be seen late Friday night and Saturday morning.
The far southern areas would see a cold rain for the entire event.

Assessment...medium to high confidence

The model consensus has quiet and dry conditions on Sunday as high
pressure moves into the western Great Lakes with near normal

Sunday night through Tuesday
Assessment...low confidence

The models have another storm system moving through the Midwest
Sunday night through Tuesday. However, there are significant
differences between the global models on where this storm system
will track.

The ECMWF has dry conditions Sunday night through Monday with the
storm system arriving late Monday night into Tuesday. The CMC global
keeps the entire area dry Sunday night through Tuesday with a strong
high pressure remaining over the Great Lakes. The GFS brings a slow
moving storm system through the area Sunday night through Tuesday.
The slow movement of the system by the GFS is due to a very strong
high pressure from the Great Lakes to the east coast.

Right now the model consensus has slight chance to chance pops
Sunday night and Monday with chance pops Monday night into Tuesday.
Precipitation would be in the form of snow at night and rain during
the day.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening)
ISSUED AT 558 PM CDT Tue Mar 20 2018

VFR conditions expected to dominate the TAF cycle, as high pressure
brings about clearing skies later tonight into Wednesday. Winds will
remain northerly at around 10 kts.




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