Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
1222 PM CDT Sat may 30 2015
Short term...(today and tonight)
issued at 405 am CDT Sat may 30 2015
Light rain developed during the overnight hours associated with
an upper trough crossing the plains...but this activity is
expected to come to an end by daybreak or shortly thereafter as
the system passes. Otherwise...not a whole lot to say about the
short term periods with clearing skies during the day in drier
air/subsidence behind the departing upper wave. Steady north winds
will decrease throughout the day as the surface gradient relaxes as
the surface ridge axis reaches central Nebraska and Kansas. In the
cooler airmass following the frontal passage on Friday...look for
temperatures to be below seasonal normals in the 60s to around 70
Heading into tonight the surface ridge axis migrates east with return
flow gradually setting up. In the east/southeast low level flow in our
eastern zones some models suggest low cloud cover may work into our
eastern counties and this will be something to keep an eye on. An
upper jet nosing southeast from the western Dakotas may also bring in
some cloud cover for our northern zones. Ultimately temperatures will be
dependent on extent of cloud cover and have went with lows in the
40s in the still cool airmass.
Long term...(sunday through friday)
issued at 405 am CDT Sat may 30 2015
Aloft: the ridge over the western USA sun will progress east over the
plains Monday-Tuesday as the next trough moves onshore in the west. Passage of
the ridge axis late Tuesday will result in the flow backing to west-southwest. The
evolving trough over the west will become broad with multiple low-
amplitude shortwave troughs with low-predictability moving through from
time to time.
Surface: the influence of cool Canadian high pressure will decrease sun
as it continues moving east. Return flow will develop in its wake with a
Lee trough redeveloping. Much of the upcoming week the forecast area will
be in the warm sector. It remains uncertain when/if the Pacific front
that moves onshore with the initial trough will even move through here.
The main polar front will stretch out from west-east over the northern USA.
That will be the primary focus for thunderstorms. The past 2 runs of the ec
and Gem try to take a low NE into the Great Lakes...with the front
sagging S through the forecast area. Believe this is due to convective
feedback and this idea has been discounted.
Some daily details...
Sun: variable multi-layered clouds invade as a weak shortwave trough
moves into the northern plains. The forecast reads p/cloudy but it could be
M/cloudy for a time...and the forecast is probably not cloudy enough.
Scattered showers will be over the Dakotas. There is a chance the southern fringe
could drop into Nebraska as some sprinkles...and this is now in the forecast
north of Highway 6.
Monday-Tue: we could have a major snag in the forecast. Multi-model cross
sections all show a layer of stratus overcast. That means there is
considerable bust potential to the downside where stratus occurs...
and to the upside where it does not occur. Daytime highs in the 60s
Gulf moisture will rapidly return north sun-Mon...with an eml advecting
overtop of this moist plume as middle-level winds back to the SW. Expect
thunderstorms will be a daily occurrence within the Lee trough...some of which
will consolidate into thunderstorm clusters/mcs's and propagate east into or
near the forecast area. This activity will be enhanced and/or joined by
new development within the diurnal pulses of the low-level jet.
Determining when/where/if thunderstorms will occur is impossible with any
degree of confidence due to weak synoptic forcing. So the forecast has a
chance of thunderstorms pretty much every day.
Rainfall: this pattern has potential to deliver some very heavy
and/or excessive rainfall...dependent on the tracks of this mesoscale convective system
activity. The past two runs of GFS/ec have run-total quantitative precipitation forecast in the 3-7"
range over parts of the forecast area. This will need to be monitored.
Severe: please see the day 4-8 discussion from Storm Prediction Center. There will be
potential for severe thunderstorms. However...it is too early to provide any
specifics given the overall lack of forcing. The potential will
need to be determined on a daily basis as we see how details
evolve. One thing does seem clear: weak wind fields and a lack of
shear will limit the severe threat to primarily wind and hail.
Aviation...(for the 18z kgri/kear tafs through 18z Sunday afternoon)
issued at 1216 PM CDT Sat may 30 2015
The stratocumulus from this morning has been slow to move to the
east and will keep a little in through the afternoon...but skies
should clear out as the surface high pushes into the area. North
winds will diminish later in the afternoon and be light through
the night before turning towards the south on Sunday.