Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
306 PM CDT Tuesday Sep 23 2014
Short term...(this evening through wednesday)
issued at 306 PM CDT Tuesday Sep 23 2014
Map analysis indicates upper tropospheric ridging trying to
establish itself from the southwestern Continental U.S. Into the northern
plains...with troughing over the eastern Continental U.S.. an upper
tropospheric short wave is also note over the Central Plains and
Central High plains. Enhanced upper tropospheric flow extends from
the northwestern Continental U.S....northeast into central Canada...and then
southeast into southeastern Canada and the northeastern Continental U.S..
upper tropospheric jet energy over our area maxes out at around
45kts near 37000ft above ground level per 12z sounding data from klbf and koax.
At the surface...an area of high barometric pressure is noted over
the middle-Mississippi Valley...with a trough extending from western
South Dakota into western Nebraska and northeastern Colorado. The
boundary-layer wind field across our area remains from the
south/southeast as a result.
Guidance from the NAM...operational GFS and ec all suggest the
upper tropospheric short wave will move east/northeast through the
next 24 hours...moving into the upper Mississippi Valley by 12z
Wednesday and nearing the Great Lakes region by 00z Thursday. At
the surface...the trough to our west will likely make a push to
the east...along with the upper tropospheric short wave...and near
our western County Warning Area this evening...but this trough should separate
from the upper tropospheric short wave and retrograde back west
closer to the Front Range late tonight into the day Wednesday. As
a result...the boundary-layer wind field may become very weak and
attempt to turn west/northwest across far western portions of the
County Warning Area for a short time this evening and tonight...but for the most
part the wind field should remain from the south/southeast through
the next 24 hours.
Thermal advection ahead of the aforementioned upper tropospheric
short wave has cleared our area to the east and as a result...the
stratiform precipitation from last night and this morning has also
moved east of our area. In addition...satellite imagery indicates
clearing skies over our western County Warning Area and moving east. Dry
conditions and the clearing trend will likely continue for at
least another couple hours...but increasing upper tropospheric
lapse rates underneath the short wave trough axis clearing our
area now...along with increasing lower tropospheric lapse rates
due to increased diabatic heating under clearing skies...could
allow convective temperature readings to be reached thus resulting
in isolated convection across western portions of our County Warning Area late
this afternoon through this evening. This is a solution supported
by quantitative precipitation forecast fields from the NAM...operational GFS and ec...as well as
simulated reflectivity from the hrrr. Given all this...went ahead
with 20% probability of precipitation generally west of Highway 281 21-00z.
Heading into the overnight hours...guidance suggests the axis of
a 35-40kt jet streak will develop just above the boundary layer
from western Kansas into central Nebraska. Thermal advection along
this jet axis appears as though it will be sufficient for
scattered precipitation production across the County Warning Area tonight and as a
result...went ahead with 20-40% probability of precipitation across much of the County Warning Area
There may be some lingering precipitation across our
east/southeast Post-sunrise Wednesday as the aforementioned lower
tropospheric jet axis shifts east...but this should be short-lived
as the jet axis diminishes due to increased diabatic heating /
boundary-layer mixing and as a result...only went 20-30% probability of precipitation
across our east/southeast 12-15z Wednesday. An overall lack in
Omega should then present dry conditions through the rest of the
day on Wednesday.
Forecast soundings from our western County Warning Area...per the NAM and
operational GFS...suggest parcels rooted near the surface this
afternoon and evening will likely have ~1500j/kg of potential
energy to work with...along with ~30kts of deep-layer bulk shear.
Contemplated going ahead with a mention of severe hail in the
severe weather potential statement...but the overall threat for convective activity seems so
limited this afternoon...felt it more prudent to keep the sub-
severe hail mention going in the severe weather potential statement. The same forecast soundings
also suggest a modest inverted-v sounding could be realized
through about 800mb and given this...will go ahead with a mention
of brief thunderstorm wind gusts to around 50 miles per hour in the severe weather potential statement for
this afternoon and evening. Heading into the overnight hours
tonight...forecast soundings from across the County Warning Area...per the NAM and
operational GFS...suggest parcels rooted to around 800mb will have
~1000j/kg with cin of ~-5j/kg. With a little forcing from the
aforementioned lower tropospheric jet axis...parcels rooted a
little lower to around 850mb could have ~1500j/kg to work with
while perhaps overcoming ~-30j/kg of cin. Deep-layer bulk shear
values on the order of ~30kts are also expected tonight and given
this...will go ahead with a mention of sub-severe hail in the severe weather potential statement
for tonight as well.
Long term...(wednesday night through tuesday)
issued at 306 PM CDT Tuesday Sep 23 2014
Forecast reasoning: no significant change from 24 hours ago. The main
belt of westerlies will curve around a deep east Pacific trough and
then turn NE and become zonal over Canada. As the Pacific trough
approaches the West Coast Thu-Fri...a closed low will form over California
Sat and heights will build downstream with anomalous /+2 South Dakota/
subtropical high pressure closing off and migrating across the Great
Lakes. This will cause the trough currently exiting the Central Plains
to stall Thursday and cut-off just S of the region. A portion of this
is likely to perform a loop and cycle back SW into the area. This
low will remain cut-off through sun...until the California low advances through
the intermountain west and acts as a kicker sun-Mon. This western low
will open up and swing through here Monday- Tuesday.
Overall the past 2 cycles of the Gem/ec/UKMET/GFS and the GFS
ensemble mean are in unison on this scenario...suggesting above
average predictability in the pattern. This scenario maintains
continuity from yesterday as well.
Aloft: the trough axis/pv ribbon will extend from WI-IA-KS-OK Wednesday
night into Thursday and it will pivot anticyclonically. Friday-Sat there
is considerable uncertainty amongst the determinstic models and
GFS ensemble members on where the upper low will reside. Anywhere
from OK to Iowa...NE to MO is on The Table. The current mean is near
Kansas City. A shortwave ridge should move through Sun night or Monday
quickly followed by a low-amplitude shortwave trough.
Surface: high pressure will remain parked over the eastern USA through
sun... maintaining return flow here on the plains. The polar front
will remain far to the north of the forecast area...with part of it slowly
inching through the intermountain west. That portion of it will probably
not move through here until Tuesday.
The daily details...
Wednesday night: quiet and dry. Low temperatures above normal...but MOS and
isobars suggest winds could turn calm in some areas . So used a
blend of the NAM 2m temperatures and met MOS to take Lexington/Ord cooler
than most guidance.
Thu: the pv ribbon/shear zone aloft will sink S of I-70. The
subsidence inversion and warming middle-level temperatures will restrict or
eliminate potential instability. Looks like a very nice day with
temperatures 5f warmer than normal.
Friday-Sun: uncertainty. Forecast dependent on position of the upper
low/cold pool aloft. Cooling temperatures aloft could lead to an isolated
diurnally driven shwr/tstm.
It's not in the bag yet...but it's possible this could turn out
to be a very nice stretch of weather with temperatures at or above normal.
That said the forecast is dry and the models are mostly dry. While we
cannot completely rule out an isolated afternoon/early evening
shower or thunderstorm...overall believe odds favor a mostly dry stretch of
pleasant Fall weather. We may eventually need to insert some very low
probabilities of precipitation on one or more of these days. But
uncertainty is too great to determine when and where.
Mon: increasing clouds probable. Scattered thunderstorms will develop over the
High Plains late sun and the approaching trough and warm air
advection should could keep them going through the night with them
arriving here Monday.
Tue: possible cool frontal passage with a continuing chance of
Bottom line...looks dry the next several days with warmer than
normal temperatures. Very very nice weather. Ensemble means from multiple
agencies agree on the mean trough moving into the central USA and that
means temperatures will cool down for the start of Oct.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1219 PM CDT Tuesday Sep 23 2014
VFR conditions are forecast through the next 24 hours...but
conditions will need to be monitored for possible MVFR to IFR
conditions towards sunrise on Wednesday. Significant low level
wind shear will also be possible at both taf sites late tonight
into early Wednesday morning.
Cloud cover will likely be observed through much of the taf
period...although a general clearing trend is expected through
this afternoon. Tonight however...guidance does suggest cloud
cover could become more dense...and in fact one set of guidance
GOES as far as to suggest MVFR to periodic IFR conditions will be
observed at gri towards sunrise Wednesday. That said...confidence
is not overly high that MVFR-IFR conditions will be realized so
for now...will present a ceiling near 4000ft above ground level at gri...and go
with scattered clouds at 2000ft above ground level for both gri and ear and let
future shifts increase cloud density if need be. The development
of a jet streak just above the boundary layer may promote
significant low level wind shear at both taf sites tonight and at
this time...confidence is high enough to go ahead with such
conditions in the taf 04-13z. The aforementioned jet streak may
also promote shower and thunderstorm activity at both taf sites
tonight...but at this time the probability of shower and
thunderstorm activity actually being realized is too low to
include in the tafs. The surface wind will remain from the south
for the most part through the next 24 hours...sustained at 8-15kts
during the taf period and accompanied by higher gusts to around
21kts through this afternoon. Visibility restriction is not
currently expected at either taf site through the next 24 hours.