Area Forecast Discussion
Issued by NWS North Platte, NE

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474
FXUS63 KLBF 270939
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
439 AM CDT Tue Jun 27 2017

.SHORT TERM...(Today and tonight)
Issued at 419 AM CDT Tue Jun 27 2017

The main forecast concerns begin mid afternoon and then carries
through tonight given the potential of strong and severe
thunderstorms. SPC Convective Outlook has the entire local forecast
area in a Slight Risk area with the main hazards large hail,
damaging winds, and brief heavy rainfall. Tornado threat cannot be
ruled out as well.

Latest water vapor imagery shows an upper level closed low in far
western British Columbia with a trough extending south into the
western half of the Intermountain West and a shortwave trough at the
base in western OR. A upper level ridge is downstream to the east
and extends from the Four Corners region into Saskatchewan. Latest
IR and NtMicro RGB satellite imagery shows stratus/fog developing in
south central NEB and southwest NEB.

The lead shortwave trough in western OR will move east across the
Northern Intermountain West early today and then across western WY
and Central Rockies by late afternoon. The upper level ridge will be
breaking down at that time and be displaced east across the eastern
Central and Northern Plains this afternoon. The aforementioned
shortwave trough should be over the NEB Panhandle by the early
evening. Meanwhile, surface trough will deepen in the Central High
Plains with a lee side low forming near northeast CO/NEB Panhandle
in the afternoon. Decent moisture transport will occur today.
Surface dew points are expected to rise into the upper 50s and lower
60s as moisture returns to western NEB. Precipitable water values
peak largely near 125% to 150% of normal outside the NEB Panhandle
by late afternoon. Moderate ML and MU CAPES across western NEB in
the afternoon/early evening given moisture with anticipated diabatic
heating and destabilization. Though convective inhibition will be
initially present and will erode towards late afternoon. A front
will be advancing eastward mid afternoon-evening. Will move east to
northeast CO/NEB Panhandle and then western NEB in the mid afternoon
and late afternoon/early evening, respectively, with initial
development expected to occur in the former. A potential fly in the
ointment is quantity and distribution of moisture given some
differences in guidance across western NEB this afternoon and
evening. Current thinking is the NAM appears overdone with this.
However, we will need to monitor the distribution of low-level
moisture and quantity of with respect to surface dew points over the
course of the day.

Anticipate initially isolated to scattered storms with respect to
coverage then thunderstorms congealing into one or two MCSs in
western NEB in the evening. Multicells/clusters and supercells are
expected for storm mode given parameter space (vertical wind shear
and SRH). Large hail and damaging winds appear as the primary
hazard, however, brief heavy rain is anticipated to be a threat as
well. Tornado threat cannot be ruled out also given SRH and low-
level vertical wind shear. Increased confidence with respect to
convective initiation and coverage given signal in SPC SSEO
spaghetti/paintball plots and neighborhood probabilities. Notable, a
signal is present in updraft helicity for rotating updrafts in their
simulated storms over the southern Panhandle and southwest NEB -
this support supercell storm mode too. Current thinking is
convection should be moving out of the local forecast area/ending
during the small hours.

.LONG TERM...(Wednesday through Monday)
Issued at 419 AM CDT Tue Jun 27 2017

Near zonal flow aloft and dry conditions are expected most of the
day Wednesday. By late afternoon however, surface winds shift east
to southeast, with some residual low level left in the wake of
Tuesday evening`s expected MCS. A shortwave trough approaching from
the northwest by late afternoon will combine with a weak and
somewhat moist upslope flow, with scattered thunderstorms expected
to develop across the panhandle. These should track east in an
isolated to scattered fashion Wednesday evening. At this time severe
potential looks low. Thursday the shortwave trough progresses east
across the Dakotas and Nebraska, driving a cold front southeast.
Appears that the front should be well southeast of the area by peak
heating, and this would be where the higher potential for stronger
storms would be. Agree with SPC shifting the severe threat mostly
south and east of the forecast area. There will still be some
potential for a strong storm in the post frontal environment due to
some decent elevated instability that lingers.

Flow turns northwest aloft Friday through Saturday, with drier mild
air filtering into the area behind Thursday`s cold front. Appears to
be another pleasant first half of the weekend with highs in the
upper 70s to lower 80s Saturday with light winds. The next shortwave
will dive southeast across the area Saturday night and Sunday. This
will strengthen a leeward trough Saturday night with a south to
southeast wind developing. This will rapidly draw warm moist air
back north into the area Sunday, with possible strong storms and an
MCS late Sunday afternoon and Sunday night.

Models diverge the first of next week, with the ECMWF slowing the
shortwave and attempting to close it off (not indicated in the
previous run) across the northern plains into eastern Nebraska. This
would bring cooler temperatures than currently forecasted along with
possible showers/thunder. The GFS ensemble mean actually has upper
level ridging building into the area during this same time and
supports highs in the 90s all areas. The going forecast forecast
sides toward the ensemble mean with highs in the 80s and mainly
dry.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1213 AM CDT Tue Jun 27 2017

Clearing skies over most of western Nebraska with high pressure
to the northeast. Southerly winds will bring stratus into
southwest Nebraska through the early morning hours between KIML
and KLBF. Low level wind shear will be possible over northern
Nebraska with a strong low level southerly jet at KVTN through the
early morning. Strong southerly flow with gusts over western
Nebraska ahead of a front heading towards the region. Have
included a prob group at both KVTN and KLBF timing of frontal
boundary somewhat in question in later periods. Good chance of
severe thunderstorms with large hail and strong thunderstorm
winds.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...ET
LONG TERM...Taylor
AVIATION...Power/ET



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