Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
327 PM CDT Sat may 18 2013
Synopsis...
issued at 208 PM CDT Sat may 18 2013
An elongated area of low pressure extended from the Southern
Plains into the the western Dakotas this afternoon. An ill defined
warm front arced southeast from the low over the Dakotas through
the middle Mississippi Valley. The region was rain free this
afternoon with the closed upstream precipitation located over Kansas and
Oklahoma. This area of showers and isolated thunderstorms was
occurring ahead of short wave moving northeast through western Kansas per
recent water vapor imagery.
&&
Short term...(this evening through sunday)
issued at 208 PM CDT Sat may 18 2013
the overall synoptic pattern remains unchanged through Sunday
morning with the upper trough remain over The Rockies and the plains
surface low moving very little. During the day Sunday a short wave ejects
out of the base of the trough and becomes negatively tilted as lifts
into MO by late Sunday afternoon.
For tonight the main concern will be thunderstorm chances late
tonight as the SW Kansas short wave lifts northeast through western Iowa.
Expect storms to develop late this afternoon or early this evening
over the central and Southern Plains as the western Kansas short wave interacts
with the developing low level jet and elevated convective available potential energy around 2000j/kg. As the
short wave moves through western Iowa late this evening/early Sunday
morning the storms should move northeast reaching east central
Iowa early Sunday morning. Models show convective available potential energy waning by this time as
well as a weakening low level jet suggesting any storms will be weakening as
they reach east central Iowa. Since the storms are expected to be
dissipating as The Reach eastern Iowa...have kept late night probability of precipitation
limited to the western third and in the low chance or slight
chance category. Given the moist air mass already in place and clouds
blowing off of convection approaching from the west we should see
lows tonight in the 60s.
For Sunday have gone with broad brush chance probability of precipitation during the
afternoon. The low levels will destabilize with low to middle 60
dewpoints in place and temperatures in the 80s. However with the
main upper trough well to the southwest and no coherent surface
boundary in place...the threat of organized storms will be
low...making it difficult to Pin Point any one area for higher
probability of precipitation. The lack of an obvious trigger and weak deep layer shear will
also mean the threat for severe storms during the afternoon will
be low.
Long term...(sunday night through saturday)
issued at 208 PM CDT Sat may 18 2013
Main focus is on thunderstorm chances and potential for severe
storms Sunday night through Monday. Slow moving upper low over the
plains will keep the potential for showers and thunderstorms...along
with above normal temperatures...going until it finally exits middle to
late week. High pressure at the surface should bring dry
condition and temperatures closer to normal late week with rain
chances increasing over the weekend.
Sunday night...developing upper level difluence and a middle level vorticity
maximum ahead of the upper level jet diving into the western U.S.
Trough...is shown by model consensus rounding the upper trough and
lifting northeast...overspreading especially the western half of the
forecast area in the evening. Even compensating for model dewpoints
likely trending a few degrees too high...surface based convective available potential energy will
likely be in excess of 2000 j/kg in the early evening...dependent on
where any afternoon convection fires. At the same time...a middle level
50 knots plus speed maximum will provide at least moderate levels of deep
shear. Uncertainties for convective development include the lack of
a strong surface boundary and the strength of the capping elevated
mixed layer. Overall...signals point toward a complex developing in
the far west or just northwest of the forecast area in the
evening...spreading NE...then east-NE following corfidi vectors across
at least the northern half of the forecast area overnight. There may
also be additional development away from any complex as the
nocturnal jet...shown around 40 knots kicks in...feeding Gulf moisture
into the region while steepening middle level thermal and thetae
lapse rates. As mentioned in the previous discussion...this setup
supports large hail...damaging wind and locally heavy rainfall. Storm Prediction Center
has a slight risk over the entire area in the current day 2 outlook
which runs through 12z Monday. Have increased probability of precipitation to categorical in
the far northwest with likelies elsewhere. High dewpoints will keep
temperatures in the 60s through the night.
Monday and Monday night...subsequent shortwaves in the southwest flow
and low level boundaries lingering from overnight convection will
trigger additional thunderstorms. Depending on the forecast
model...surface based convective available potential energy may again reach well above 2000 and
possibly higher than 4000 j/kg. This will be greatly dependent how
the overnight convection dissipates and the coverage of associated
cloud cover. Latest models continue to suggest deep layer shear will
be stronger as the upper jet approaches from the southwest. At the
same time...the deepening upper low over Nebraska will cool temperatures
aloft...providing a weaker cap than Sunday...and even steeper middle
level lapse rates. This will be more supportive of supercell
thunderstorms especially in the afternoon and early evening hours.
These supercells will continue the severe weather threats of large
hail and damaging wind...and introduce an increased risk for
tornadoes. Storm Prediction Center maintains a slight risk encompassing the entire
forecast area for day 3...which covers Monday through Monday night.
Will keep probability of precipitation in the high chance range in the daytime due to more
limited coverage...then increase to likely probability of precipitation for overnight when
more widespread coverage of showers and thunderstorms is expected.
Temperatures remain close to persistence in a largely unmodified
environment with highs in the 80s and lows in the 60s.
The upper low gradually weakens and sweeps east of the area by
Thursday. This will keep chances for showers and at least isolated
thunderstorms going through Thursday with temperatures trending back
closer to late may normals along with lower humidity. High pressure
that follows will provide dry weather through for at least Friday.
Saturday...low confidence slight chances for rain are introduced
based on the more progressive European model (ecmwf) that brings a warm front into
the area. The GFS maintains a dry forecast with the area under the
influence of high pressure retreating slowly toward the Great
Lakes.
&&
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 208 PM CDT Sat may 18 2013
VFR conditions are expected through late tonight with some MVFR
visibilities possible late tonight and early Sunday morning. A warm
front extending southeast from low pressure over the northern
plains will remain over the area through Sunday morning. We could
see some late night fog again tonight as moisture pools along the
front. Visibilities were kept in the 3-5sm range since we should
have at least broken middle or high clouds blown off thunderstorms to
our southwest...which should prevent visibilities from dropping
much lower. A mention of vcsh was maintained to cover the storms
approaching from the southwest. These storms should be weakening
as the move into the forecast area well after midnight.
&&
Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&
$$
Synopsis...dlf
short term...dlf
long term...sheets
aviation...dlf
