Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 190344

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1044 PM CDT Tue Apr 18 2017

Issued at 320 PM CDT Tue Apr 18 2017

At 3 pm CDT, surface low pressure was located across eastern
Minnesota, with a trailing cold front stretching through far
western Iowa into Kansas. Ahead of the front, a south breeze
gusting over 20 mph helped boost afternoon temperatures into the
upper 60s to mid 70s.


.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 320 PM CDT Tue Apr 18 2017

The main forecast issues in the short term period are rainfall
trends/timing/intensity tonight into Wednesday.

For this evening, a front will sag into the northwest with
increasing chances of showers and thunderstorms. Coverage expected
to increase somewhat by late evening and overnight. The big
question is how far south the front will make it before stalling
out. The various CAMS and synoptic models have varying solutions
but seem to be converging on the Highway 34 corridor. The bulk of
late night activity will likely be confined north of the boundary.
Mid-level lapse rates around 7C/km and sufficient MUCAPE indicate
some hail threat with the strongest updrafts, but the overall
severe threat is low.

This marginal hail threat will likely continue into Wednesday
morning, as shower and thunderstorm coverage increases northward
along with the warm front. Much of the afternoon should be dry
except for the exiting showers/storms across the northeast.
Wednesday high temperatures will be quite tricky with a very large
north to south spread anticipated. Have stayed close to the
blended guidance (mid 60s north to low 80s south) as later
adjustments will be inevitable.

Regarding short term flooding issues tonight into Wednesday, areas
north of Interstate 80 are the most primed in terms of soil
moisture with recent heavy rain. These areas are expected to be
spared another round of widespread heavy rain tonight, but
localized higher amounts are possible Wednesday morning. I do not
see this initial round as being problematic. Any additional heavy
rain Wednesday night would be another story.

.LONG TERM...(Wednesday Night through Tuesday)
ISSUED AT 320 PM CDT Tue Apr 18 2017

Main focus is convection that will likely be on-going at the
beginning of the forecast period. Beyond...a change to cooler
weather follows for late week into the weekend. The system for the
weekend has now shifted further south in the model consensus,
leading to a dry forecast for most of the forecast area.

Wednesday night: scattered thunderstorms may be underway along and
north of the surface warm front, which is depicted north of
interstate 80. The associated surface low spins up over southwest
IA, lifting east-northeast with the wave of low pressure aloft
moving across the Dakotas into MN. The low, and associated warm
front and trailing cold front will be the primary focus for
thunderstorms overnight. With PW values around 1.25 inches in the
airmass in advance of the system, heavy rain will be threat, along
with locally severe storms due to adequate instability likely
developing in the warm sector and sufficient shear, especially near
the warm front and track of the surface low. Model QPF fields
generally target the northwest half of the forecast area with .5 to
over 1 inch, where the system will interact with the best diurnal
instability early on. While the amounts look reasonable, the
coverage may be overdone and have stayed closer to a blended QPF of
along I-80 to .50 along the Iowa highway 20 corridor for the
evening, then less than a quarter inch after midnight over mainly
the northeast. The most intense storms may have a threat of flash
flooding, which will need to be monitored as much of eastern IA is
already saturated from the last storm system. Cooler, drier air that
follows the cold front will drive low temperatures into the upper
40s to upper 50s by morning.

The back edge of the showers may affect the far east Thursday
morning, otherwise high pressure building into the region will
provide cooler weather Thursday through Saturday. Highs in the upper
50s to mid 60s will be closer to late April normals. Overnight lows
are expected in the 40s and may drop into the 30s with the
initial surface ridge passage Friday morning.

Slight chances for rain are mentioned only across the far south over
the weekend as the consensus now keeps the next low pressure system
tracking well to the south across the mid MS Valley. Sunday begins a
slight warming trend into early next week as a more zonal upper
level flow resumes.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening)
ISSUED AT 654 PM CDT Tue Apr 18 2017

Elevated band of showers, with possible embedded thunderstorms,
will work from south to north through E Iowa/NW Illinois between
07Z-19Z/Wednesday. Confidence is low on thunder occurring, so only
mentioned at KBRL where instability is highest. Confidence on
widespread coverage of showers/storms is also low because this
activity has yet to form over S Iowa. As sfc winds turn to the
ENE late tonight, ceilings will deteriorate to MVFR. Brief periods
of IFR are possible, but did not include in TAFs at this time. Uttech


Issued at 1030 PM CDT Tue Apr 18 2017

Minor flooding continues along the Rock River at Joslin, the Iowa
River at Marengo, and the Wapsipinicon River at De Witt 4S. These
sites are past their forecast crest levels and continue to fall.

Model runs from late this afternoon, and hi-res model runs from
this evening and tonight, are trending less aggressive on a
widespread threat for heavy rainfall tonight and Wednesday night.
For this reason, the Flood Watch at Moline on the Rock River has
been cancelled.

On the Iowa River at Wapello, a Flood Watch continues. While this
site is not forecast to reach flood stage, the river level
continues to rise. Any additional rainfall could cause the river
level to reach minor flood stage.

On the Mississippi River, a Flood Watch also continues for
Gladstone, where the latest forecast has it reaching flood stage
by Wednesday night. However, there remains limited confidence in
both the routed flow and effects of possible rainfall during this




LONG TERM...Sheets
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