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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
344 am CDT Sat Aug 1 2015

issued at 343 am CDT Sat Aug 1 2015

High pressure remained in place across the Central Plains and
southern Mississippi River valley this morning. Closer to the
area...a well defined moisture gradient was located across our
area where dewpoints in the SW were middle to upper 60s. While across
our NE zones...dewpoints were in the middle 50s. At 850 mb a thetae
push was leading to the development of showers and thunderstorms
across southeast Minnesota and SW WI. These showers should drift to the S and
east throughout the morning with the 850 mb push. Thunderstorms were
also forming in our SW zones. This was due to a weakly convergent
surface flow and surface moisture boundary.


Short term...(today and tonight)
issued at 343 am CDT Sat Aug 1 2015

Main forecast concern for the short term are the chances of rain
and thunderstorms late this evening and into the overnight hours.
A fast moving vorticity in the flow will lead to the development of
showers and thunderstorms across South Dakota and NE. These storms will then
track east or southeast through the late evening.

Today...surface winds should turn to the west SW and usher in more moist
air into the area. The development of afternoon cumulus should be the
only impediment to the sun today. Later in the afternoon the
vorticity maximum should approach the area from the northwest.

Rather unimpressive wave on WV should be rounding the ridge this
am and then marching to the SW. 00z observation should no 500 mb height falls
in the vicinity of this wave. Regardless...models have heights
dropping later today as the wave approaches the area. This wave
should help to trigger convection to the north and west of US.
Guidance and conceptual models suggest that this convection
should grow upscale to a mesoscale convective system or two and move to the east tonight.
Without a well defined front in the area...the movement of the
mesoscale convective system will be dictated by steering level winds and 850 mb jet. Mass
convergence on the 850 mb level suggests that convection will move
east across north Iowa and then after 6z start to dive S as well. This
would bring the mesoscale convective system into our area. The 850 mb jet is not that
impressive but would lead to decent speed and directional shear in
the low levels. This would lead to a potential severe wind threat
across the area. Lapse rates in the model soundings...especially
the NAM...are impressive and suggest a hail threat as well with
these storms.

That being said...I still have doubt in this forecast. For one...deep
layer shear is unimpressive until the wave gets closer to the area
near 12z. Convection growing upscale would be cold pool dominant
and may not be able to maintain itself in such weakly sheared
environment. Even more disconcerning is the fact that the cams do
not produce much in the way of convection overnight. While the
cams may not always nail the area where storms form....they
usually do a good job of nailing whether or not things will fire
across an area. As a result....decided to stay in the high chance
category for probability of precipitation.

Long term...(sunday through friday)
issued at 343 am CDT Sat Aug 1 2015

Sunday and Sunday least scattered storms should be found
Sunday morning within zone of warm advection/isentropic ascent
attendant to veering low level jet above surface warm frontal zone
with activity slowly decaying as jet weakens. In wake expect
atmosphere to recharge during afternoon with enough solar insolation
and gusty southwest winds boosting temperatures well into the 80s to
possibly around 90 degrees in some locations depending on amount of
sunshine. These temperatures coupled with surface dew points well into the
60s to lower 70s may yield strong instability with SBCAPE of
3000-4000 j/kg which along with prognosticated bulk shear 0-6km of 30-40
kts yields favorable environment for organized severe potential.
Attim targeting possibly far eastern Iowa but especially northern Illinois
being on fringe of warm eml. Main threat would appear to be damaging
winds... with large hail a secondary threat. Would have to watch
for any localized backing of winds which coupled with warm front
or outflow boundary interaction would enhance helicity and yield some
tornado risk. Storm chances would be diminishing by middle Sunday evening
with passage of cold front attendant to shortwave moving through the
Great Lakes.

Monday and Monday night...have removed probability of precipitation Monday with general model
consensus of building in bout of surface ridging in wake of cold
front providing slightly cooler and less humid day. However... Monday
night surface ridge quickly shifts east and return flow develops
which will bring attendant chance of showers and storms by late evening
and overnight...mainly southern County Warning Area.

Tuesday through Wednesday...have shifted rain chances to roughly the
southern 2/3rds of County Warning Area in scenario closer to GFS of boundary stalled in
or near southern County Warning Area and upper level disturbances shifting eastward in
flattening flow. However...dprog/dt of several models show trend
of stronger incoming surface high which results in front settling
further south and a mainly dry scenario... which will need to be
re-evaluated. With gradual draw down on 850 mb temperatures from middle teens to
around 12c to 13c and north/northeast wind should yield temperatures near to
below normal... with highs middle 70s to lower 80s and lows in the 50s to
lower 60s.

Thursday through Friday...heat dome looks to build back into the plains
while cooler temperatures reside around Great Lakes with upper low. The County Warning Area may
reside close enough to battle zone for another round of rain chances...with
temperatures remaining near to below normal.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1143 PM CDT Friday Jul 31 2015

VFR conditions will be seen through 00z/02 as high pressure moves
into the Great Lakes. There may or may not be some very patchy
MVFR visibilities due to light winds through sunrise. After 00z/02 a
thunderstorm complex is expected to develop in western Iowa and
eventually move east. This thunderstorm complex might affect
kcid/kdbq/kmli/kbrl after 05z/02.


Dvn watches/warnings/advisories...


short term...Gibbs
long term...05

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