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FXUS63 KDVN 210231
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
931 PM CDT Mon May 20 2019

...EVENING HYDROLOGY UPDATE...

.SYNOPSIS...
Issued at 336 PM CDT Mon May 20 2019

While our CWA deals with a short period of high pressure moving in
today, which has resulted in one of the slowest periods of late May
stratus clearing I can remember, the Plains are seeing intense warm
frontal development from Oklahoma through southwest MO. Deep
moisture return is aiding a widespread rain and thunderstorm event
covering all of Kansas and adjacent parts of SE NE, and Western MO.
While the attention is appropriately focused on the severe weather
event to the southwest, our forecast will hing initially on how this
first wave of rain evolves.

&&

.SHORT TERM...(This Evening through Tuesday)
ISSUED AT 336 PM CDT Mon May 20 2019

Tonight should see the rain spreading in from west so east over our
southern 2/3 to 3/4 during the mid to late evening. To this idea,
there is considerable support for holding the stratiform region of
the rains already taking place together as moisture transport
increases into Iowa, and synoptic lift increases with the upper flow
becoming southwest, with lots of energy perturbations coming from
the plains. While QPF is not forecast too high tonight, with the
model descrepancies, it seems a 0.50 to 1 inch rain is south of
Highway 30, thus flash flooding is unlikely. Run off will still
occur, and this will also serve to keep our area 100% saturated
heading into Tuesday evening, which may require a flash flood watch.

Tuesday will see the warm front stay south of the CWA through the
day, with strong east winds and rain. It will not be a nice May day!
Though deep layer shear is very strong, the lack of any instability
should preclude any severe weather threat until early evening, and
even there, the squall line should be stratifying out as it enters
the CWA from the west. Winds gusting to 40-60 mph are the main
threat in the extreme southern counties, as the boundary layer
seems confidently stable. Rain on other hand may be efficient and
heavy for a hour or two as the line passes by, again pending
tonight's rain, this may require a flash flood watch.

.LONG TERM...(Tuesday Night through Monday)
ISSUED AT 336 PM CDT Mon May 20 2019

An active weather pattern continues through next weekend, providing
nearly daily chances for rain, along with a return of warmer
temperatures.

Tuesday night, a linear MCS will likely sweep through the area
along the occluded front associated with the surface low over the
central plains. With high precipitable water, from 1.25 to 1.5 inches
wrapping into the deep occluded system over the region, heavy
rainfall will be the main concern. QPF looks heaviest in the
evening, where our forecast has a model blended, middle of the road
approach with around .7 north to 1.25 south before midnight then
less than .10 east afterwards as the line exits eastward. With
saturated surface conditions, this amount of rain will impact
rivers, not currently accounted for in river forecasts (see hydro
discussion) and possibly cause some flash flooding due to the
heavy rainfall rates. The progressive nature of the system
should preclude any high end or widespread flooding issues.

A secondary threat will be the potential for severe storms. Strong
low level shear and proximity of higher CAPE to the south may lead
to damaging wind and possible spin up QLCS type tornadoes,
especially over the far south early. SPC has a day 2 slight risk
for south of the highway 34 corridor and marginal up to interstate
80. This threat would be greatest in the evening, where a triple
point of low pressure most models have passing through our
southern tier of counties may interact with favorable MUCAPEs of
1000 to 1500 j/kg an 0-3 km shear values over 50 kts.

Wednesday looks breezy and much warmer with southerly winds, higher
humidity and rain-free for 12 to 18 hours as the area gets on the
warm side of the synoptic boundary. Highs should reach the upper
70s to lower 80s and possibly warmer.

Rain chances return Wednesday night, and then follow nearly every
period for at least portions of the forecast area through next
Monday as the frontal boundary gets reestablished over the area,
aligned under an active southwest to westerly flow aloft with
continued bouts of available gulf moisture. There are no clear
signals at this time suggesting that any particular system will
bring heavy rainfall, but the continued day to day rainfall will
aggravate the on-going wet conditions and river flooding. The latest
GFS and ECMWF have widespread QPF totals for the next 7 days in a
widespread 3 to 5 inch range over the forecast area. Forecast
temperatures are likely to remain near to above normal throughout,
but there is below average confidence as due to uncertainties with
critical surface boundary locations.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
ISSUED AT 712 PM CDT Mon May 20 2019

MVFR conditions are expected to return to the area as rain moves
into the area late today into the overnight. Tomorrow, expect
some short break in the rain before a chance for thunderstorms
returns later tomorrow across the southern TAF sites.


&&

.HYDROLOGY...
Issued at 929 PM CDT Mon May 20 2019

The latest forecasts are beginning to take into account more
predicted rainfall for Tuesday. As expected, river levels at a few
sites are starting to trend a little higher than what was shown in
this morning's update. However, no categorical changes were made
with this evening's package.


Previous Discussion...


Runoff from heavy rainfall over the past several days is causing
significant rises and flooding on some tributary rivers, while
flooding continues along the Mississippi River with some renewed
rises. After a break today, another round of rain, possibly heavy,
is expected Tuesday, followed by an active weather pattern that
will bring rain chances nearly each day beginning Thursday through
next weekend. As current river forecasts only include an initial
24 hours of forecast rainfall, Tuesday's potential heavy rainfall
is not included in this morning's forecasts. Thus, additional
rises beyond what is depicted can be expected from Tuesday onward
along the tributary rivers. This will be better reflected in
forecasts issued this evening. This will also affect later
forecasts for Mississippi River, where there is also great
uncertainty with trends further out due to the active, wet weather
pattern.

Overall, the forecasts issued this morning for both the tributary
rivers and mainstem Mississippi had mostly minor adjustments
resulting from observed amounts and coverage from yesterday's
rainfall. Flooding is currently on-going or forecast for portions
of the Cedar, Iowa, English, and Skunk rivers. The Des Moines
River at St Francisville is expected to drop below flood stage
today. Levels continue to fall along the Rock River, which is now
within its banks. On the Mississippi River, Minimum to Moderate
flooding continues along most of the stretch from Dubuque to New
Muscatine. Moderate to Major flooding continues from New Boston
south to Gregory Landing, where levels were currently rising from
crest moving downstream enhanced by input from the tributary
rivers.

&&

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

$$

SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...Sheets
AVIATION...Gibbs
HYDROLOGY...Speck/Sheets
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