Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
1238 am CDT Sat may 25 2013
Update...
issued at 1222 am CDT Sat may 25 2013
A trio of mcss are expected to affect the area in the next 12-18
hours. The current decaying feature began out in western Iowa
around 4pm and is encountering a lot of dry air as it moves east
into the surface ridge. Thus...it is not putting out much precipitation
and is quite elevated in nature. The next mesoscale convective system has just gotten
started in the last few hours over western Iowa...and is expected
to drift east in the mean flow just like the last one. The models
have not explicitly depicted this one...but this is where the low
level jet is impinging on the 850mb jet...according to the 00z
upper air analysis. This feature too should weaken somewhat as it
moves east...and have kept probability of precipitation still in the chance category
overnight and into the early morning hours. The next mesoscale convective system...which
began as strong surface based convection is slowly moving east and
is moving into the best plume of moisture at 850mb...and
approaching the area where we could say that the low level jet is
impinging upon a boundary...though the western portions of the
boundary helping fire the Iowa convection. This convection should
also move east...but is not currently expected to get into the
forecast area until almost middle morning Saturday...and is likely to
be in a somewhat decaying Mode as well. Have kept probability of precipitation high during
the daytime.
See updated aviation section below as well.
&&
Synopsis...
issued at 242 PM CDT Friday may 24 2013
Latest surface analysis was indicating large cool core ridge starting
to push eastward across the western Great Lakes......while return flow
and pressure falls increase across the plains into western Iowa. 12z
upper air analysis also indicating elevated moisture feed at 850 mb
mb and warm moist conveyor also becoming more pronounced up the
Western Plains. Diffluent southwesterlies with an embedded vorticity
across the northwestern High Plains pushing into weakening Omega
ridge and inducing and increasing elevated convective region across
the eastern Dakotas.
&&
Short term...(this evening through saturday)
issued at 242 PM CDT Friday may 24 2013
Tonight and Saturday...various short range hi-res models continue to pick
up on the eastern Dakotas activity...but still most are
lagging/trying to catch up in speed...timing and areal coverage of
what is occurring. Feel the lead warm air advection Wing off this ongoing process
will induce high based showers and possibly a few thunderstorms
into the western County Warning Area after 02z this evening...better chance of
even spread eastward to the MS river through 05z-06z if even in a
weakening Mode. But low level jet with thta-east feed from Central Plains
source region will be increasing...nothing very strong but still
enough to converge on slower flow at the same level and enhance of
probably mesoscale convective system development generally from northwestern Iowa
southeastward into the west central to southwestern dvn County Warning Area after
midnight and into Sat morning. The 12z run UKMET and nam80 mesoscale convective system
forcing tools both support this train of thought and after the
intial Wing of elevated showers/storms...will Blossom probability of precipitation from
west to east after midnight and through at least 15z Sat morning.
Precipitable water feed of 1-1.3 inches and convergence suggest a few locations
that get hit by the main complex could get at least an inch of
rain by middle Sat morning. Before southeasterly flow increases
overnight...temperatures to drop to low values of lower 40s in the far
east...to the upper 40s or near 50 in the west closer to faster increase
in surface/return flow. After morning mesoscale convective system decreases...mesoscale convective system spawning
parameters start to refocus back on northwestern Iowa Sat
afternoon...thus hopefully some lull in the activity for midday
and into the afternoon especially north of i80. But will keep chance
probability of precipitation for any renewed convection along out flow boundaries or warm
front trying to retreat out of MO into the southern County Warning Area. Will play
highs on the cool side...but if sun breaks out and more dry
periods occur Sat afternoon than currently expected...advertised
temperatures will be too cold in many spots. ..12..
Long term...(saturday night through friday)
issued at 242 PM CDT Friday may 24 2013
Prolific rainfall is expected through Tuesday...as a series of warm
advection driven showers/thunderstorms...possible full mesoscale convective system activity
moves across Iowa and Illinois. There is very high probability of
this occurring...as strong low level jet convergence is indicated overhead on
every model...and the past 2 days have seen quite a bit of elevated
activity...in Oklahoma yesterday...and in South Dakota today. This
elevated boundary looks to remain over the County Warning Area through
Tuesday...thus a growing threat for significant rainfall is
expected. A Hydro section will be added to this afd.
It is hard to argue against the model output probability of precipitation for Saturday night
through Monday...which solidly indicated likely or higher. Since
this stretches over many period...I will not go categorical on any
period...as there should be some breaks in the rain/mesoscale convective system activity.
Thus...a prolonged period of likely probability of precipitation should cover the
uncertainty on timing rounds of rainfall. In generally it appears a
maximum in coverage in rainfall should occur in the overnight and
morning hours...which is typical of mesoscale convective system patterns.
Temperatures will continue to cool through Sunday...with highs in
the 60s north...lower 70s south...but by Monday we should see higher
dewpoint air surge north...thus highs in the upper 60s to upper 70s
should be more widespread. The remainder of the week...we should be
in the warm sector...and should see highs in the upper 70s to middle
80s. Lows in the 60s can be anticipated much of the extended after
Saturday night...which will bottom out in the upper 40s to middle 50s
from northeast to southwest.
Ervin
&&
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1222 am CDT Sat may 25 2013
A series of convective complexes that will move across the area in
the next 24 hours will keep the taf forecasts complex. Current mesoscale convective system
has no thunder and ceilings and visibilities are VFR...and is only bringing a
little light rain. The next mesoscale convective system is not expected to move into the
area until after 11z...bringing potential for MVFR ceilings and visibilities
in thunderstorms and rain...though with anticipated weakening of this mesoscale convective system have left
thunder out of the first few hours. After this...the next mesoscale convective system
should move in after 15z...bringing a bit more certainty in
thunder potential and have put in a 3-4 hour period of thunderstorms and rain with
MVFR ceilings and visibilities. This should become more stratiform with
time...and have pulled out thunder but kept rain going for a few
more hours as this band moves through. By late afternoon we should
get a lull in storm activity but still MVFR conditions and
scattered showers. New storms should be forming out to our west
during the late afternoon and evening...but given the
uncertainties earlier in the forecast cannot pin down timing or
location and have left out of forecast entirely for now. Le
&&
Hydrology...
issued at 242 PM CDT Friday may 24 2013
Widespread rainfall of 1 to 4 inches is possible through early this
upcoming week. In some cases where training takes place...amounts
over 4 inches are certainly possible. While the heaviest axis of
rainfall is far from certain...it does appear that this rainfall
will support significant new rises on most rivers...and may cause
new flooding or prolonged flooding. This is especially true in
eastern Iowa...where the Cedar...Iowa...and wapsipinicon rivers are
already running high from rainfall upstream in north central Iowa
last week.
Ervin
&&
Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&
$$
Update...le
synopsis...12
short term...12
long term...Ervin
aviation...le
hydrology...Ervin
