Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Omaha/valley NE
1142 PM CDT Thursday Aug 21 2014
issued at 610 PM CDT Thursday Aug 21 2014
Thunderstorms have developed in north central Kansas as of 23z. Mean
winds would take this activity northeast into southeast Nebraska.
The atmosphere over southeast Nebraska has become moderately unstable this
afternoon with objective analysis indicating around 2500-3000 j/kg
of MLCAPE and little convective inhibition. Although effective
shear is marginal...some organization to the thunderstorms is
possible with 20-30 knots. Thus given the instability and marginal
shear...isolated severe weather will be possible over the next
several hours in this area. Going forecast has thunderstorms
increasing over the next couple hours in this area and appear
Short term...(tonight through sunday)
issued at 304 PM CDT Thursday Aug 21 2014
Sporadic thunderstorm chances through the weekend continue to be the
primary forecast concerns.
Much of middle America will remain under southwest flow aloft for the
next few days as mean trough in the west begins shifting toward the
plains. Several impulses within that flow could trigger showers
or storms from time to time...with at least one significant wave
bringing our best storm chances Friday night.
Strong shortwave trough dropping southeast through western
British Columbia will close off an upper low in the northwest U.S.
And drift into Idaho by Saturday morning. Meanwhile upper low now
moving into the northern Baja California will get kicked northeast as a
strong shortwave into the High Plains of Nebraska and Kansas
Friday night into Saturday. This system will provide our best
chance for thunderstorms along with some heavy rain
potential...generally Friday night into early Saturday.
Until then...scattered showers have continued across southeast
Nebraska this afternoon on back edge of shortwave/vorticity center that
brought thunderstorms to much of the area earlier this morning.
Showers and attendant cloud cover has kept temperatures a few
degrees cooler there...but just to the south temperatures were in the
upper 90s. Expect these showers to only slowly diminish through
the late afternoon...but additional isolated development to the
west is possible. Then a weak frontal boundary currently over
northeast Nebraska is expected to settle into southeast Nebraska
and southwest Iowa tonight. High instability in Kansas combined
with weak middle level impulse should kick off at least isolated
convection this evening...which will work northeast along frontal
boundary into parts of eastern Nebraska and western Iowa fueled by
southwesterly 35 knots low level jet. Any convection that does affect
our area should be exiting to the east Friday morning.
Frontal boundary will then begin lifting back to the north later
Friday morning and afternoon as pressures lower in the High
Plains ahead of approaching shortwave. Front could focus scattered
thunderstorms as early as middle afternoon if clearing skies allow
instability to return. Attention then turns to the west where
interaction with High Plains shortwave and axis if high
instability/shear trigger convection in western Nebraska/Kansas.
Surface low is forecast to ride along frontal boundary to the
northeast into central and eastern Nebraska by early Friday
evening and exit to the northeast Saturday morning. Threat for
large hail and damaging winds will be highest in our western County Warning Area
Friday evening when surface based instability is high. Threat
should transition to more of a heavy rain situation overnight
given weakening instability along with precipitable water values
over 2 inches. South to southwest low level jet combined with
northeast-moving storms will provide good potential for training
storms laying down potentially heavy rain amounts. Track of
surface low will be critical for highest potential...with
consensus of models suggesting northeast Nebraska northwest of a
Columbus to Wayne line is most likely to see heaviest rainfall.
Will have categorical probability of precipitation there with more scattered activity to
the southeast. Far southeast Nebraska and southwest Iowa could
remain dry Friday night.
With convection lifting northeast Saturday morning...expect most of
the afternoon to be dry. However frontal boundary will remain in
northeast Nebraska...and could see scattered afternoon or evening
regeneration of convection there...which could then leak southeast
toward Interstate 80 overnight. A similar situation presents itself
for Sunday. But by then strong middle level jet rounding base of upper
low/trough drifting into the western Dakotas will help drive front
deeper into the County Warning Area. Persistent southerly flow and deep moisture
will be more than sufficient to support scattered convection along
the front during afternoon heating...especially near and northwest
of the front. Areas to the south may be capped until Sunday
Long term...(sunday night through thursday)
issued at 304 PM CDT Thursday Aug 21 2014
Our unsettled weather pattern will continue into middle week. Upper
low/trough moving into the western Dakotas Sunday will swing a
potent middle level jet streak through the plains Sunday night. High
instability along frontal boundary in eastern Nebraska interacting
with forcing associated with jet streak means scattered
thunderstorms should develop or be ongoing Sunday evening in
northeast Nebraska...gradually working southeast overnight as front
pushes across the rest of the County Warning Area. Cooler temperatures will result
behind this front...but southwest flow aloft will remain in place
until axis of upper trough swings through the plains later Wednesday.
We will start seeing the affects of that trough as early as Monday
night when moderate warm advection/isentropic upglide regime sets up
over Nebraska and western Iowa and remains in place until at least
Wednesday morning before trough moves east. So will have at least
chance probability of precipitation Monday night through Wednesday.
Temperatures will not be as warm through middle week with clouds/rain
hindering warming. Looking for highs in the 80s on average with some
70s in the north.
Aviation...(06z tafs for kofk...klnk and koma.)
Issued at 1136 PM CDT Thursday Aug 21 2014
Isolated to scattered thunderstorms and rain have developed this evening and still
could affect the klnk/koma tafs for the next couple of hours
before the activity shifts eastward and out of the taf sites. Not
expecting heavy thunderstorms and rain...but brief MVFR conds would be possible.
Otherwise we may see some MVFR fog toward morning at koma/kofk. VFR
conds are expected through the day on Friday...but by the end of the
period some thunderstorms and rain are possible at kofk and will include a tempo
group after 03z with MVFR conds. Some threat at koma/klnk as well
but currently confidence is higher at kofk.