Area forecast discussion...updated aviation
National Weather Service St Louis MO
706 PM CDT Sat may 25 2013
Short term...(through late tonight)
issued at 323 PM CDT Sat may 25 2013
An mesoscale convective system remnant will continue to weaken this afternoon as it moves across
the area. Schc-chance probability of precipitation look reasonable for the late afternoon period based
on radar trends.
Attention then turns to the overnight period. Late afternoon surface observation revealed a
sharp warm front stretching across western MO and southwestern Iowa into the northern
plains. This boundary is expected to provide the focus for additional
areas of precipitation tonight however it is not yet clear whether precipitation will
take the form of another mesoscale convective system /possibly more than one/...a broad area
of precipitation anchored near the 850 mb warm front...or some combination of the
two. These scenarios seem equally plausible given the current boundary
location...forecast low level jet strength and orientation...instability
forecasts...and observed trends over the previous 18 hours. At this time the
best precipitation chances appear to be over the northern and northestern County Warning Area...but it is
equally possible that a broad area of precipitation could remain north and
east of the County Warning Area for most of the night or that an mesoscale convective system could form and
bring widespread rain to a large portion of the County Warning Area. In
short...unsettled weather will continue through tonight.
Kanofsky
Long term...(sunday through next saturday)
issued at 323 PM CDT Sat may 25 2013
(tonight-tuesday)
Given how difficult determining todays convective trend has
been...pinning down this threat over the remainder of the Holiday
weekend will certainly remain very challenging since there is
essentially no change in the overall synoptic pattern.
Synoptic and hi-res expicit thunderstorm models are in agreement in
a very broad sense that an elevated thunderstorm threat will persist
across the region as weak shortwaves propagate through the middle level
ridging...interacting with and occaionally intensifying broad zone
of low level warm advection along 850mb baroclinic zone that is
prognosticated from the middle MO valley into Illinois. However...there is almost
no agreement on where and when storm initiation will occur since low
level jet and forcing seems much less focused than what occurred
last night and today.
For now will maintain a broad brush approach to forecast through
Monday...and fine tuning both probability of precipitation and temperatures will become a
Nowcast issue once convective trends become better defined. Highest
probability of precipitation will remain over our north/NE counties near the 850mb thermal
gradient where lift should be maximized and/or southeast propagation of mesoscale convective system
clusters will be most likely. However...as was proven today
certainly cannot rule out thunderstorm chances further S due to weak
shortwave energy...a more unstable airmass in the warm sector...and
possible boundary interactions...so will maintain chance probability of precipitation
elsewhere.
In the Monday night-Tuesday time frame upper ridge will push east
and SW flow will become established over area as upper trough deepends
over the western U.S. With time this should push the 850 mb
thermal gradient and mesoscale convective system threat further north and east...with a
corresponding drop in probability of precipitation from S to north across the County Warning Area during this
time.
(Wednesday-saturday)
SW flow aloft will dominate the middle Mississippi Valley for the
end of the week as upper level low slowly forms over the northern
rockies. Primary question is how much upper level dynamics will be
able to work east into our County Warning Area as large blocking ridge will also be
forming over the southeast Continental U.S.. consensus of medium range
guidance suggests lead trough initially over the SW U.S. Should lift
into the plains Wednesday night and clip our area on
Thursday...followed by several additional shortwaves rotating
around the upper low. Throughout the period...it appears that main
dynamics will stretch from plains into upper Mississippi
Valley...suggesting that main thunderstorm threat will be over west/north
sections of our forecast area...with the storms struggling to work into southeast MO
and S Illinois.
Since region will be well entrenched in the warm sector...
temperatures should be above average with daytime highs primarily in
the 80s.
Truett
&&
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 643 PM CDT Sat may 25 2013
Thunderstorm potential continues to be the primary forecast issue
for the next 24 hours. Am expecting VFR flight conditions and
light southeast flow to prevail through the period...with
primarily 2 periods of thunderstorm potential. The first is
expected tonight after 06z as the low level jet interacts with an
upper level disturbance. Think the best area to expect these
storms will stretch from northeast Missouri and west central
Illinois down along and east of the Mississippi River into parts
of southwest and south central Illinois. Scattered storms could
develop further south and west as well...but the coverage looks to
be much less widespread in these areas. These overnight
thunderstorms should weaken and decrease in areal coverage by middle
Sunday morning. By Sunday afternoon we could see another round of
scattered afternoon storms.
Specifics for kstl...
Thunderstorm potential continues to be the primary forecast issue
for the next 30 hours. Expect VFR flight conditions and light
southeast flow to prevail through the period...with primarily 2
periods of thunderstorm potential. The first looks like tonight
after 09z as the low level jet interacts with an upper level
disturbance. The majority of the storms may be further east than
Lambert...across southwest Illinois. However I can't rule out
thunder at the Airport at this time. These overnight
thunderstorms should weaken and decrease in areal coverage by middle
Sunday morning. By Sunday afternoon we could see another round of
scattered afternoon storms. Expect any thunderstorm activity to
die out pretty quickly after 00z.
Carney
&&
Lsx watches/warnings/advisories...
MO...none.
Illinois...none.
&&
$$
Weather forecast office lsx
