Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
609 am CDT Thursday may 28 2015
Short term...(today and tonight)
issued at 307 am CDT Thursday may 28 2015
Forecast confidence is low today due to significant uncertainty in
thunderstorm potential. When/where/if they develop are problems...along
with associated cloud cover and temperatures.
Considerable adjustments may be necessary on the day shift...and
possibly before this night shift is over.
Aloft: 00z analysis and WV imagery showed a trough over the western
USA. This trough will gradually advance east through tonight and will be
located just west of the forecast area by daybreak Friday. Several shortwave
troughs were embedded and this makes thunderstorm potential much less clear.
Surface: the remnants of a weak front were dissipating over Kansas.
The region will essentially be in the warm sector through tonight.
Daytime heating and the approach of the upper trough will deepen the
Lee-side trough today. This trough will advance east into Nebraska/Kansas tonight
in advance of a strong cool front that will drop into the northern
Early this morning: we are monitoring potential for thunderstorms to
erupt in low level jet-driven warm air advection.
6 hour quantitative precipitation forecast verification ending 06z has been abysmal...which only
furthers the lack of confidence. A quick look at the 03z sref quantitative precipitation forecast
and its quantitative precipitation forecast probabilities suggests it may have the best handle on
what might transpire this morning.
Thunderstorms have been on-going over central/southern Kansas overnight and were
moving into eastern Kansas at 08z. New thunderstorms have erupted over SW Kansas. Sref
suggests this activity may remain all S and east of the forecast area and
our forecast may be too high on probability of precipitation this morning. If nothing develops
by the end of this shift...we will be making substantial changes
to lower probability of precipitation.
Today: clouds will be determined by thunderstorm develop...and at 08z the
thunderstorms near ddc are sending a cirrostratus canopy rapidly north into
the forecast area. So expect most areas to be mostly cloudy through
midday... and areas from the Tri-Cities S and east may be cloud most
of the day.
Whatever thunderstorms develop this morning should be east of the forecast area
this afternoon...probably over eastern neb/KS. Probability of precipitation were lowered this
afternoon. Believe the forecast area will be between that activity to
the east and new thunderstorm development to the west within the Lee trough.
Forecast temperatures could be too warm by several degrees where cloud cover
remains thick/opaque the longest.
Tonight: whatever thunderstorms develop to the west should survive
propagate/move into the forecast area due to the advancement of the
Lee trough. Temperatures in the middle-upper 70s with dewpoints in the low-middle 60s
and middle-level lapse rates of 7-7.5 degree c/km will result in MLCAPE of
1500-2000 j/kg over the western fringe of the forecast area. So some of
this activity that moves in early this evening could be severe over
Dawson/Gosper/ Furnas counties. Deep layer shear will be 20-25
kts...suggestive of multicell convective Mode.
The threat for severe will rapidly decrease as instability
diminishes with sunset.
Long term...(friday through wednesday)
issued at 307 am CDT Thursday may 28 2015
The forecast looks wet to start...dries out over the weekend and
into the first part of next week...with chances for precipitation returning
around the middle of next week.
Convection may be ongoing starting the day on Friday ahead of an
upper trough poised to cross the northern and Central Plains.
Models differ on the position of the surface low and cold frontal
positions with the GFS/sref faster with surface features than the NAM.
Hard to tell at this point and will go with a blend for the early
morning features. The front is slow to depart our southeast zones
in the afternoon which will provide some chance for redevelopment of
storms along the baroclinic zone...in line with the Storm Prediction Center day two
marginal risk area. Instability averages around 1500 j/kg in our
southeast but NAM suggests this may reach 2000 j/kg or so. Shear remains
under 30kts and not looking for much in the way of severe weather
but will have a small window of opportunity for a strong storm in
our southeast County Warning Area especially if frontal boundary slows further.
Post frontal showers and isolated thunderstorms remain possible into
Friday evening while the upper system crosses the plains...then
precipitation winds down pretty quickly as moisture is scoured south with
the front and much drier advects in from the north.
Friday night is looking chilly with 850 mb temperatures dropping into the
single digits across our northern zone and to around 10c in the
south as a 1030 mb surface high builds south from the Dakotas. The
center of the high...which is two Standard deviations above normal
for this time of year...remains to our north and models differ on
whether or not clearing will reach our northern zones prior to
daybreak. Will continue with the chilly morning lows mainly in the
40s for Saturday.
Over the weekend...conditions remain dry. Saturday is looking
cool under the influence of the surface high then in return
flow/warm air advection by Sunday temperatures will trend up again. Heights rise aloft
heading into next week and the warming trend continues. The upper
ridge axis breaks down with the flow transitioning southwesterly
by the middle of the week as the next trough moves into the
Pacific northwest. This next system will set up a front across the
plains by the Wednesday time frame and did not deviate much from
ensembles/extended init. Will need to keep an eye on a meandering
trough/low in the vicinity of the Gulf states or Southern Plains but
for now models keep this system to our south.
Aviation...(for the 12z kgri/kear tafs through 12z Friday morning)
issued at 608 am CDT Thursday may 28 2015
Today: VFR cirrostratus ceilings around 20k feet. A few cumulus will develop
around 4k this afternoon. S-south-southeast winds 10-15 kts. Confidence: low
Thursday eve: probably VFR to start...but another period of thunderstorms is
possible after 03z. Southeast winds 10-15 kts. Wind shift with cool
fronal passage around 09z. Confidence: low