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Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1147 PM CST Thu Feb 23 2017

..aviation update...

Synopsis...
issued at 345 PM CST Thu Feb 23 2017

Strong storm system taking shape across the Central Plains this
afternoon as a surface low was located over Garden City. A warm
front extended east from this low through central Illinois. Closer to
home, easterly flow was impending the northern push of warmer air
this afternoon. This should not last long as the pressure falls
suggest that the low will start to pull north.

&&

Short term...(this evening through Saturday morning) issued at
345 PM CST Thu Feb 23 2017

This evening and tonight...

The Storm Prediction Center has outlooked roughly areas along and
west of the Mississippi r. In the dvn forecast area with a
marginal risk for severe thunderstorms.

Scattered thunderstorms are forecast to develop mostly after
7 or 8 PM in response to increasing isentropic lift on nose of
850-700mb jet working to release elevated instability. MUCAPE
around 500 j/kg and effective deep layer shear of ~50 kts will
cause updrafts to rotate, increasing the threat for isolated
severe hail around 1" in diameter. However, pea to dime size
reports will be more common. Rainfall amounts of 0.25 to 0.75
inches are likely, highest north of I-80.

Friday and Friday night...

Strong Spring-like storm system will impact portions of the upper
Midwest. Since models are in agreement tracking the sfc low southeast of
the Quad Cities, and in coordination with surrounding wfos,
adjusted Max temps down below superblend using the much colder
consraw data. Impressive forecast high temp gradient ranges from
lower 30s northwest to near 60 southeast. The colder daytime temps on Friday
will cause periods of wintry precip across the northwest forecast area
through the day, as opposed to first beginning Friday
evening/night.

Winter weather returns:

Winter Weather Advisory has been issued for Buchanan, Delaware,
Dubuque, Benton, Linn, and Jones counties. A wintry mix of sleet
and freezing rain changing to snow is expected to result in
hazardous travel from icy roads. Please see the advisory product
for location specific details. Be prepared for slippery roads
outside of the advisory as well, just doesn't look like the
impacts will be as significant.

Even though temps have been anomalously mild the past 1-2 weeks,
sfc temps are forecast to fall into the 20s/10s by Friday night
into Saturday morning - cold enough for accumulating snow and icy
conditions. Have a general 1 to 4 inches of wet snow forecast in
the advisory area. Elsewhere, a half inch to around 2 inches. In
addition to the snow, minor ice accumulations around 0.10 inches
are possible, most likely in the advisory counties.

After a brief lull in the precipitation mid Friday morning, the
occluding 850mb low will ride NE along the baroclinic zone,
crossing through east Iowa/northwest Illinois. Strong 850mb frontogenesis
juxtaposed immediately to the west/northwest of the aforementioned low is
forecast to redevelop wintry precip eventually changing to all
snow. On a larger-scale, the mid-level vorticity Max/upper-low
will bring a period of DCVA/deep ascent from the afternoon through
Friday night - reinvigorating a band of light to at times
moderate snow. This deformation zone snow will slowly work from
west to east through the forecast area.

Saturday morning...

Light snow ends quickly but temps in the 20s and 10s may keep
roads slippery until the sun can warm pavement temps. Uttech

Long term...(saturday through thursday) issued at 345 PM CST Thu
Feb 23 2017

Long term forecast challenges revolve around a brief cool down for
the weekend before the flow turns to become more southerly and
warmer temps occur. A series of waves through the flow bring with
them the chance for precip in the extended.

Cooler, more seasonable temps will warm into the 40s and 50s the
beginning of next as the flow turns zonal to southwest. Good
agreement on the flow pattern between guidance leads to a higher
than average confidence in temperatures. Past the flow regime,
models start to diverge in the actual vort fields and subsequent
surface weather. There will be systems through the flow, however,
the placement and timing of these systems is still uncertain.
Therefore precipitation forecasts will likely change.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1144 PM CST Thu Feb 23 2017

Periods of rain and isolated storms overnight with mostly IFR and
intervals of LIFR conditions with a brisk northeast wind of 15 to
25+ miles per hour. As a surface low moves east on Friday the winds will
shift to the north and northwest at 10 to 20+ miles per hour by afternoon
with a chance of rain...changing to snow at dbq and Cid terminals
by 25/00z. After 25/00z...mostly IFR/LIFR conditions to continue
with light snow likely at all terminals through 25/06z with
northwest winds of 15 to 25+ miles per hour.

&&

Hydrology...
issued at 307 PM CST Thu Feb 23 2017

Forecast rainfall from the event today and tomorrow along with
routed flow north of the area will lead to rises on area streams
and rivers. The Iowa and Cedar rivers are forecast to see rises
that are dependent on forecast rainfall through this event.
Conesville and Marengo are forecast to reach flood stage this
weekend. With low confidence in total rainfall amounts, these
sites have been placed in a Flood Watch. Columbus Junction,
Wapello, Oakville and DeWitt are forecast to reach flood late this
weekend and into next week. With the low confidence in rainfall
totals and these rivers forecast to reach flood past 48 hours,
flood watches were not issued at this time.

Forecast rises on the Mississippi River are expected through the
period as routed flow moves through the system. Some of this flow
in contingent on rainfall through Saturday. Regardless of this,
the river is going up. Some sites could see flood warnings in the
next week or two.

As confidence increases in the potential for river flooding,
additional watches and warnings could be issued.

&&

Dvn watches/warnings/advisories...
Iowa...Winter Weather Advisory from 6 am Friday to 6 am CST Saturday
for Benton-Buchanan-Delaware-Dubuque.

Winter Weather Advisory from noon Friday to 6 am CST Saturday
for Jones-Linn.

Illinois...none.
MO...none.
&&

$$

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