Area forecast discussion...corrected
National Weather Service Hastings NE
426 PM CDT Sat may 25 2013
..sct severe thunderstorms to erupt in the next 3-6 hours with potential
for excessive rainfall into tonight...
Short term...(this evening through sunday)
issued at 426 PM CDT Sat may 25 2013
Corrected typos.
The remnants of last night/S mesoscale convective system was exiting MO into Illinois. The rain-
cooled air and associated differential heating put the effective
warm front from central Missouri northwest into eastern neb/SD. Strong heating
over S-central Nebraska has resulted in the loss of an apparent thermal
boundary. However...surface observation do show a pressure trough oriented west-northwest-east-southeast
across southern Nebraska. 20z msas also indicated pressure falls along this
trough. The strongest confluence was north-NE of the Tri-Cities...where
satellite shows extensive cumulus.
Present indications are that thunderstorms could erupt in that confluence
zone as the cap erodes. We/ve noted surface dewpoints are in the middle-upper 60s
due to the extensive rainfall last night. This was resulting in MLCAPE
of 3000-4000 j/kg.
Elsewhere...multiple models /including the hrrr/ develop thunderstorms
within the Lee trough over western Nebraska/eastern Colorado/northwest Kansas. These storms head east.
Not sure if they remain scattered in nature or grow upscale with Theta-E
advection and convergence at the nose of the low level jet.
The mesoscale convective vortex we referenced in earlier afd/S was moving into NE Kansas and
will not be a player in our forecast.
Heavy rain: models are not giving the same signals as last night. For
what it/S Worth...the general indication is that the heaviest
rainfall/mesoscale convective system activity will be over the northern half of Nebraska or even South Dakota
into IA/MO. However...we still have an intensifying low level jet tonight...with
its nose in vicinity of I-80. Plus the upper-level ridge is overhead.
Satellite-derived precipitable water shows a plume of subtropical moisture extending north
from the Caribbean into the Central Plains. Lbf/oax/top 12z soundings
were all 150-170% of normal. Any clustering of thunderstorms will threaten
flash flooding...especially here heavy rain occurred last night.
Flash flood watch: it was contemplated but decided not to issue for
now. A swath of 2-3" /isolated 4"/ occurred from Sherman-Hall-
Hamilton-Clay-Thayer counties. Just too much uncertainty. No
guarantees it occurs in the same area again tonight or that rfall is as
heavy.
Severe: given that magnitude of the instability...excessively large
hail and wind damage are possible...especially before 9 PM.
Cannot rule out a tornado if storms do indeed develop north and east of
the Tri-Cities. Local heights are lowest there.
Sun: probably a repeat of today. Any leftover rain departs early.
Then becoming p/cloudy. Dry most of the day. Then thunderstorm redevelopment
with threat of severe again late.
Long term...(sunday night through saturday)
issued at 337 PM CDT Sat may 25 2013
Primary concern will
be off and on chances for thunderstorms...along with high
temperatures each day.
Starting with the big picture in the upper levels of the
atmosphere...there should be a low amplitude long-wave trough over
the western half of the country...with ridging over the Missouri to
Ohio River...and a closed low over the far northeast Atlantic
Seaboard. At the surface...an area of low pressure is prognosticated to be
situated over the Kansas/Colorado border with another one over eastern Wyoming.
Should be a warm front draped east to west with the question
being...is it sitting across our County Warning Area or a little bit further north.
As the week rolls along...the upper level trough over the west
amplifies a bit and moves just a little to the east. By middle week the
eastern half of the U.S. Should see minor ridging. For our
area...this pattern should allow various middle level disturbances to
rotate through...giving US enough forcing to trigger thunderstorms.
The GFS and the ec models are in basic agreement by the end of the
workweek in taking the western trough...closing it off...and pushing
it northwest into the northern plains. This will also drag a cold
front across the area which will be another focusing mechanism for
thunderstorms.
Difficulties in trying to pinpoint where and when thunderstorms may
erupt result from several factors including...the timing of the
aforementioned short wave troughs...locations of any synoptic scale
front...and location of any mesoscale boundaries laid out from the
previous days convection...available moisture...any low level jet
development and its location. Primary factors in possible severe
include...degree of instability due to surface heating...deep layer
shear...along with any capping inversion.
Checking the factors above for Sunday evening...difficult to say if
we are going to see any middle level support with an approaching short
wave as models all over the place. Should be warm front in place
although location could range from Kansas/NE border to well north of
there. Of course any convection tonight could generate outflow
boundary that may hang around. Precipitable water values have been
well above the average and expected to remain that way for several
days. 12z NAM Cranks up the low level jet sun evening with best
convergence just north of the Kansas/NE state line at 00z...while GFS
run places better convergence associated with jet well north or east
of the County Warning Area at the same time. As far as severe weather GOES...decent
instability should linger a while into the evening while 0-6 km bulk
shear looks to be between 40 and 50 knots. So if we can get convection
to fire...there is a good chance of seeing some severe weather.
Worked through the same process for Monday/Monday night and Tuesday/Tuesday night
and assigned probability of precipitation accordingly. Starting Wednesday...we have to deal with
a new set of issues as the main low finally kicks out of the west.
Will it stay the course with the current predicted track...which is
to keep the middle level low pressure center a little north of our
County Warning Area...or will it change solutions. This will help dictate heavy rain
potential vs straight severe for Wednesday and Thursday. Would like to get
a look at a few more runs before making that call.
One other note that I forgot to mention above. While thunderstorms
wont be impossible during the daytime...this kind of set up usually
gives US the best chance for rain during the overnight hours.
Previous forecast took this track and I see no reason to change.
Temperatures have climbed to the lower to middle 80 this afternoon and
see no reason they should not hit similar values the next few
days...before cooling off just a tad by next weekend as the upper
low moves past and we get into the wrap around flow.
&&
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1244 PM CDT Sat may 25 2013
Uncertainty is above average due to thunderstorm potential.
This aftn: VFR with a slight chance of an IFR thunderstorm after 21z. South-southeast winds
will gust to around 22 kts.
Tngt: VFR much of the night but there is a slight chance of a thunderstorm. Just
cannot pinpoint it at this time. Gustiness of south-southeast winds will diminish
around sunset...but there could be some occasional gusts to near
20 kts.
Sun through 18z: VFR. South-southeast winds gust up to 25 kts.
Ceiling confidence: medium
visibility confidence: medium
wind confidence: high except near thunderstorms
weather confidence: low due to thunderstorm potential
&&
Gid watches/warnings/advisories...
NE...none.
Kansas...none.
&&
$$
Short term...halblaub
long term...ewald
aviation...halblaub
