Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
259 PM CDT Friday Aug 22 2014
Short term...(this evening through saturday)
issued at 256 PM CDT Friday Aug 22 2014
The highlight of the short term period will be the fact we are
expecting thunderstorms today/tonight. There still remains some
uncertainty for timing...location...and overall development. The
warm front to our south...which is situated over the Kansas/Nebraska
border...is just moseying along today and has caused some concern
for development. Showers/storms should develop along this boundary as
the cap begins to break. Storm Prediction Center mesoscale analysis Page has US capped
until about 22-23z. At this time we should see the beginning stages
for initiation along the boundary to our south. Ample instability of
about 2000-3000 j/kg will be in place...due to a very moist boundary
layer and steep lapse rates. Most unstable cape could be as high as
4000 j/kg. Bulk shear 0-6 km will be around 30 knots...and this is
not too impressive...however...low level shear will be strong.
Earlier runs of the rap indicated helicity values near 400 m2/s2
would be in place around 23z for western portions of the County Warning Area. These
values will push eastward through the afternoon/evening. These low
level helicity values coupled with ample instability will pose a
threat for some isolated tornadoes...mainly for our Nebraska
counties. With that being...wind and hail will be the main threats
today/tonight. The best chance for severe weather will be across our
Nebraska counties...but the entire County Warning Area does have the potential for
severe weather. I do expect thunderstorm activity to kick into gear
around 6 or 7 PM...if not sooner.
Long term...(saturday night through friday)
issued at 256 PM CDT Friday Aug 22 2014
Various rain chances for the weekend and in to the first part of
the work week...along with fluxuating temperatures are primary
forecast concerns. Will keep this fairly short due to possible
severe weather developing later this afternoon.
Starting out Saturday evening in the middle/upper levels...there should
be a large amplitude trough over The Rockies with a closed low
situated over east central Montana...as the current system over the
Pacific northwest drifts eastward. This will push the ridge a little east
as well with the axis stretching from Lake Superior to Mississippi.
The better forcing with the low pressure should be to the west and
north of our region. At the surface...expect a frontal boundary to
be to our north across sdak at 00z and then this will push into our
area by Sunday morning. Has not taken a whole lot to set off
convection the past week and while the set up for Saturday night is
not as good as tonight...will probably still need some low probability of precipitation
given approaching weak cold front and anticipated low level jet.
Previous shift afd noted some heat burst potential with any
thunderstorms that do develop and this scenario still in place.
Weather should be quiet during the daytime on Sunday behind the cold
front and possible Sunday night although both the NAM and GFS are
developing some precipitation to the west and moving across the County Warning Area
Sunday night. GFS appears to overdevelop a vorticity maximum although
difficult to totally discount with cyclonic flow in upper levels.
Monday night and Tuesday night still look active as yet another high
amplitude trough is slated to move across The Rockies and into the
plains. At the surface an east/west stationary front will hang out
over the area resulting in the classic nighttime rain events. By
Wednesday the extended models in decent agreement in pushing the
trough east through the County Warning Area with riding then moving in to quiet
things down for the remainder of the week.
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1240 PM CDT Friday Aug 22 2014
Conditions at both terminal sites are expected to remain in VFR
throughout most of the taf period. The only concern will be
thunderstorm activity later this afternoon/evening. At this
time...conditions will likely deteriorate. However...confidence is
not extremely high at this point for timing...location...and
overall likelihood of development. At this point...we do expect
showers/storms to develop along the warm front which is just
south of both terminal sites...near the Nebraska/Kansas border.
This front will mosey north throughout the day and once the cap
breaks showers should begin. The best chance for thunderstorm
activity is indicated in the taf to begin around 01z. The taf will
be amended for any changes once initiation becomes clearer. Cloud
cover will increase throughout the day and overnight as well...but
ceilings should remain VFR through the valid taf periods.