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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
637 PM CDT Monday may 4 2015

Synopsis...
issued at 330 PM CDT Monday may 4 2015

A nearly stationary frontal boundary extended from south of
Chicago to just south of the quadrant cities and then to Saint Joseph
MO. To the north of the boundary...low clouds...winds from the
northeast and temperatures were in the upper 60s to lower 70s. To
the south of the boundary winds were from the south with
temperatures in the middle to upper 70s. Dewpoints across the County Warning Area
ranged from the upper 50s to lower 60s.

At 3 PM Doppler radar indicated a few thunderstorms in Hancock
and McDonough counties in western Illinois moving due east. An area of
showers was increasing in southwest/southcentral Iowa and
thunderstorms were firing in Kansas associated with a short wave
embedded in the flow pushing into SW Kansas.

&&

Short term...(this evening through tuesday)
issued at 330 PM CDT Monday may 4 2015

Overview...visible satellite showed some breaks in the cloud
cover and MUCAPES were rising to 2000 j/kg and are forecast to
2500 j/kg later this afternoon. However...shear is weak and
freezing levels about 10k. This suggests multicell clusters
producing near severe hail around 1 inch. Precipitable waters
increase to 1.50 inches so heavy downpours will accompany the
stronger storms.

Tonight...short wave will ride into southeast Nebraska early this
evening and then across Iowa overnight. This will provide large-
scale ascent and with the boundary bisecting the County Warning Area thunderstorms
will continue to increase in coverage late this afternoon and
evening. Hrrr/arw/WRF-nmm indicate this congealing into a rather
large area of showers and embedded thunderstorms overnight. The
hail threat appears to be confined to mainly this evening with the
better instability. Heavy downpours can be expected at any time
and some locations should be able to pick up over an inch of rain.
Due to the boundary bisecting the middle to the County Warning Area there will be
a nice thermal gradient. Minimum temperatures will range from the
lower 50s from Dubuque to Freeport...to the lower 60s in our
southern County Warning Area.

Tuesday...showers and a few thunderstorms in the morning will
gradually diminish as the day progresses as forcing weakens. The
stronger forcing shifts well to our west as a deep upper level
trough edges closer to the plains. Operational models suggest the
frontal boundary will move little but the convection overnight may
have a say in that. Should be quite a thermal gradient again with
high temperatures ranging from the lower to middle 60s in far northwest Illinois
to around 80 in our south. However...depending on where the
boundary will exist...cloud trends and any lingering showers this
may impact temperatures greatly.

Long term...(tuesday night through next monday)
issued at 330 PM CDT Monday may 4 2015

Tuesday night and Wednesday...several of the 12z run models try to
migrate the main warm front to the northern County Warning Area by early Wednesday
morning...with main forcing shifting off digging l/west trough across the
Central Plains and elevated warm air advection fed ribbon of showers storms to the
north across Minnesota into WI. Thus besides a few sctrd to isolated showers/
storms across the northern County Warning Area Tuesday evening...this period may be a lull
in precipitation barrage this week. But rather low confidence in this
scenario and could see the NAM/S more sctrd precipitation coverage along
and north of i80 for much of the evening pan out especially with a
more southerly warm frontal position hanging on. Lows ranging from
the lower 50s in the northeast...to around 60 in the far south with
similar dpt variances.

Southeast Continental U.S. Ridge with southeast Atlantic Cut-off tropical looking
system making for a blocking pattern Wednesday into at least the end of
the week. Short-wave spokes scooting up the plains around stalled
northwestern High Plains l/west trough may make for much more convective
coverage to the west/northwest of the County Warning Area and a mainly dry first
half to Wednesday locally. This may allow temperatures to warm into the 80s...but
latest sref trends suggest clouds and a lead band of showers/storms
to make it into the southwestern third of more of the dvn County Warning Area by
late Wednesday afternoon. But the main forcing focus closer to the upper
wave will continue to move across the west central into northeastern
plains/MO River Valley Wednesday night...with several of the model solutions
suggesting the precipitation weakening as it moves into our area of concern
Wednesday night into Thursday morning...the more significant convection
remaining off to the west and northwest.

Thursday and Friday...as stalled southwesterlies with seasonally
robust warm moist conveyor impinging on a low level boundary continues
from the plains up into the western Great Lakes...several of the medium
range model runs are pointing to late Thursday and Thursday night for a better
chance for more outspread convection or a mesoscale convective system type event into the
local area as the main surface front draws closer from the west. Long
range shear profiles and thermodynamics suggest marginal severe
storms possible...with a better chance at locally heavy rain...
projected precipitable water/S climb to over an inch to 1.2 inches as Thursday
progresses. Thursday is a warm day with readings well up in the 80s
possible...but again convective debris and precipitation itself may
temper the high temperature potential and will play it more conservative.
Any out flow boundaries from Thursday night...or the main boundary
itself stalling out right across the County Warning Area on Friday may make for a wet
end to the week.

Saturday through next Monday...almost daily precipitation chances
continue...and several of the medium range models suggest a large
piece of the western Continental U.S. L/west trough to roll out across the middle Continental U.S.
Or upper Midwest this period...with several targeting Sunday. With
adequate heating this could be a more organized severe weather day with
synoptic scale forcing making it/S way across the MS River Valley into
the Great Lakes...it/S just timing this wave. Less severe potential if the
wave roll out delays until later at night such as Sunday night into
Monday morning. But the heavy rain threat still there...and
accumulative rainfall potential from this week and into the weekend
may range from 3-5+ inches with the mref suggesting 50 miles either
side of the i80 corridor curving back into south central Iowa. Hopefully
then a temporary reprieve from the rains and storms for the early
part of next week in the wake of the passing l/west trough. ..12..

&&

Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 633 PM CDT Monday may 4 2015

Widespread showers and thunderstorms will overspread the region
tonight with extended periods of MVFR and probably higher end IFR
conditions in rain. Conditions should improve by middle to late
morning as a warm front moves north. Winds will be from the east
at 5 to 10 kts...becoming south to southeast at 5 to 15 kts by middle
day on Tuesday.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.

&&

$$

Synopsis...haase
short term...haase
long term...12
aviation...Nichols

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