Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
1238 PM CDT Sat Aug 23 2014
issued at 532 am CDT Sat Aug 23 2014
Stratus is being to develop early this morning and the hrrr is
doing a decent job with the low cloud depiction. The model
indicates the more widespread low cloud potential primarily along
and north of the Platte River...but ceilings are also starting to
lower farther south into north central Kansas. Confidence is not
high on the development of fog as winds are farily steady from the
southeast. Models suggest the low clouds will persist for around
three hours or so...breaking up by middle morning.
Short term...(today and tonight)
issued at 402 am CDT Sat Aug 23 2014
Early morning convection was lifting out of our County Warning Area on the nose
of the low level and as the warm frontal boundary lifts northward ahead
of an upper low edging east from Idaho. Severe weather reports from
the evening convection were limited however rainfall was
widespread with several locations receiving amounts near or above
Winds will transition southerly as the warm frontal boundary lifts
into the Dakotas today and a period of breezy conditions are
forecast during the afternoon with gusts of 25 to 30 miles per hour
possible...especially for our southeastern zones. Afternoon temperatures
are looking similar to the last few days with highs ranging from the
upper 80s to the middle/upper 90s...aided by deeper mixing. Cannot rule
out an isolated thunderstorm in the afternoon in warm air advection ahead of the upper trough
moving into Mt/WY.
Uncertainty exists in the convective potential tonight as the upper
low edges into the western Dakotas and a cold frontal boundary
advances eastward from the High Plains and reaches our western County Warning Area
prior to daybreak. Warm air advection ahead of the front and convergence/dynamics
along the baroclinic zone may lead to isolated/scattered storms. If
storms develop...lack of good shear and marginal instability should
limit storms from becoming severe. As the cold front arrives late
tonight...drier air advects southeast behind the boundary with dps
dropping off to the 50s by Sunday morning in our western zones.
Long term...(sunday through friday)
issued at 402 am CDT Sat Aug 23 2014
Aloft: a fairly amplified eastern USA ridge/western USA trough sun will
gradually deamplify next week and become progressive. A potent
closed low will will be ejecting out of the trough...crossing the
Dakotas sun. This will leave a +tilt trough behind over the western USA
Mon-Tue. This trough will advance into the plains Wednesday and the last 2-3
runs of the ec/GFS/Gem/UKMET are in decent agreement through this time
frame. Agreement breaks down Thursday and we saw hints of this 24 hours
ago. The GFS keeps the trough progressive while the UKMET/ec and some
of the past 4 Gem cycles break the base of the trough off from the
westerlies which become re-established over southern Canada. The GFS
develops northwest flow over the plains while most other models have the
trough axis overhead.
Surface: a cool front will be crossing the forecast area Sun morning.
This front will become stationary across northern Kansas Monday as it becomes
parallel with the flow aloft. The southern fringe of Canadian high pressure
will build in Mon-Tue. However...with the trough remaining to the west
the front will be convectively active
Hazards: multiple rounds of thunderstorms appear likely with Monday night
continuing as the highest confidence. Excessive rainfall could be a
problem if these thunderstorm clusters track over similar areas and we may
see Hydro/flooding concerns.
The daily details...
Sun: while the best low-level dewpoints will be whisked away by this
time frame...there is a narrow tongue of 10-12c 850 mb dewpoints along
the cool front. A couple of showers/thunderstorms may be on-going just northwest of
the forecast area at daybreak. Present indications are that they will
dissipate. By afternoon this moist tongue will be over the southeast fringe
of the forecast area...and could support development a thunderstorm or two.
Sref MLCAPE 2000-3000 j/kg. Deep layer shear around 25 kts will
support severe multi-cell convective Mode.
Sun night: a Lee-side low will become established near pub in the
right rear quadrant of an 80 knots ulj. The result will be upslope
flow...the development of a low level jet and enhanced Theta-E advection. Rich
850 mb dewpoints will advect back into the region leading to
destabilization and thunderstorm development over the High Plains. Some of
this should filter east into the forecast area after midnight.
Mon: lots of uncertainty on temperatures. Dependent on the amount of cloud
cover and lingering thunderstorm activity from Sun night. This will act to
reinforce the front via differential heating. Areas north of I-80 could
stay in the 70s while north-central Kansas will bake into the middle 90s. Any
leftover thunderstorm activity should wane during the day. Cant rule out an
isolated late day thunderstorm or two.
Monday night: the models continue to advertise mesoscale convective system development at the nose
of the low level jet. High probability of precipitation are in order especially north of Highway 6.
Tue: probably a lull in thunderstorm activity in the wake of the mesoscale convective system. Again
temperatures are highly uncertain. With convectively reinforced front still
near the Kansas-Nebraska border...late day thunderstorm development cant be ruled
Wednesday-Thu: for what it's Worth potential exists for more thunderstorms and if
mesoscale convective system activity is placed just right...multiple episodes could result
in excessively heavy rainfall.
The 18z/22 GFS ensembles show above normal uncertainty in temperatures Wednesday-
Thursday. If the ec turns out correct...we could see substantial cloud
cover/stratus trapped beneath the front as the upper trough moves in.
The Gem/ec/GFS/dgex all indicate potential for 1 or both days with
daytime temperatures jammed wayyyy below normal /as cool as upper 60s to
Fri: dry with seasonable temperatures.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1235 PM CDT Sat Aug 23 2014
Slowly eroding low clouds out of south central Nebraska. Grand
Island pretty much in the clear it appears...while Kearney may
skirt with some scattered afternoon clouds but overall trend
should be clearing out. Winds will continue to increase ahead
of deepening low level trough with gusts over 20 miles per hour not uncommon.
Precipitation which develops to the west tonight could move into
the area late in the forecast...but was not confident in its
coverage to add to forecast at this time.