Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 200531

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1231 AM CDT Thu Apr 20 2017


Issued at 326 PM CDT Wed Apr 19 2017

Latest satellite imagery shows a thinning of the cloud cover over
eastern IA and northwest IL, north of the stationary front evident
by the edge the field of cumulus across far southern IA just south
of a line from Fairfield through Burlington to Galesburg. North of
the front, temperatures were in the upper 50s to around 70, while to
the south temperatures were in the upper 70s to lower 80s with
dewpoints in the lower 60s. Surface analysis at 18z showed low
pressure connected to this boundary along the central Nebraska/north
central KS border with pressure falls over NW IA. Aloft, weak
shortwaves that were associated with thunderstorm complexes were
over NW Indiana and far SW MN. A stronger and more significant
shortwave was moving into the northern plains with a divergent upper
level flow moving into KS and Neb ahead of a 100 kt 300 mb jet
moving through the Rockies.


.SHORT TERM...(This Evening through Thursday)
ISSUED AT 326 PM CDT Wed Apr 19 2017

Challenge remains centered on the severe risk tonight as lift ahead
of the system moves out of the plains interacts with the sharp
surface boundary, deep shear and building instability over the area.
Pressure fall/rise trends this afternoon back the synoptic models`
depiction of the surface low tracking into northwest IA, while
pressure rises over northern Great Lakes and cool low level
east/northeasterlies will try to keep the surface boundary hung up
across northern IL and southeast IA. So far, a mid level cap and
lack of forcing has kept the frontal boundary inactive, which will
change this evening with the advancing upper level shortwave
forcing, which will be stronger to the north of the forecast area.
This evening, a 40 to 50 kt flow at 850 mb will focus into north
central IA, and possibly into eastern IA over the advancing surface
warm front. The extent of this occurrence may be critical as to
whether the warm front becomes active. High res and synoptic
models continue to vary in the depiction of convective trends and
rainfall areas tonight, ranging from possible heavy rainfall over
east central IA along and norht of the warm front (GFS,ECMWF,Canadian)
to nearly dry through the period (NAM and many high res models).

With below average confidence, have gone with chance pops this
evening along and north of the surface warm front, then higher pops
overnight across the north closer to the surface low track.
Developing deep shear, steep mid level lapse rates and enhanced low
level shear along the warm front will justify nearly all severe
weather modes, with damaging wind, large hail and heavy rainfall,
and isolated tornadoes. SPC maintains an enhanced risk over western
and central IA into our far western CWA with a slight risk over much
of the CWA.

Thursday: High pressure builds in with cold air advection and at
least a gradual clearing of the low clouds. Highs were kept in a
range from the mid 50s northwest to near 70 in the far southeast.

.LONG TERM...(Thursday Night through Wednesday)
ISSUED AT 326 PM CDT Wed Apr 19 2017

Mostly quiet and cooler weather will dominate the long term forecast

For Thursday night, some patchy frost is possible along the Highway
20 corridor, especially in favored drainage areas of the Wapsi River

Friday night into Saturday, the northern edge of a rain shield will
likely impact the southern counties, but the heavier precipitation
should remain well to the south. With more extensive cloud cover,
have adjusted Saturday highs down into the low 50s far south, with
slightly warmer temps to the north.

Sunday through Wednesday, expect temperatures to warm back above
normal, with highs in the 60s to low 70s. Two additional systems may
impact the forecast area, the first Monday night and the second on
Wednesday. Model inconsistency continues in these periods and have
kept pops limited to no higher than the chance category.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
ISSUED AT 1227 AM CDT Thu Apr 20 2017

Cold front will sweep through eastern Iowa and northern Illinois
through 15z/20. Ahead of the front a small organized area of
SHRA/TSRA will move through northeast Iowa into Wisconsin. Overall
VFR conditions were ahead of the front except around convection
where IFR conditions were occurring. Behind the front conditions
will deteriorate to MVFR with pockets of IFR. After 15z/20
widespread MVFR conditions will be seen with improvement to VFR
after 00z/21.


Issued at 326 PM CDT Wed Apr 19 2017

On the mainstem Mississippi, morning trends and latest flood
guidance now indicate minor flooding is likely at Burlington and
have issued a flood warning at that point. Gladstone will also rise
above flood stage today with minor flooding continuing into early
next week. Keithsburg is forecast to rise near flood stage Thursday
night, and with limited confidence, have issued a flood watch. New
Boston may reach flood stage, but not until Sunday night, so will
hold off on a flood watch there until confidence increases. From
Muscatine and points upstream, in bank rises are forecast into next

On the tributaries, latest trends suggest Wapello will stay below
flood stage and have cancelled the watch. Elsewhere, flood warnings
remain in effect at Joslin, Marengo, and DeWitt.

The heaviest rainfall overnight into this morning has occurred south
of Interstate 80, with isolated swaths up to 1.5 inches.
These amounts are not expected to have a significant impact on
rivers. Looking ahead, there is some potential for widespread storms
with heavy rain occurring north of Interstate 80 tonight, but
confidence in this happening remains low. Will be monitoring hydro
closely tonight. If widespread heavy rain materializes, then we
would likely be dealing with more points in flood and potentially
higher crests through the weekend into next week.




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