Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 191804
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
104 PM CDT Sat Aug 19 2017

.SHORT TERM...(This afternoon through Sunday)
Issued at 400 AM CDT Sat Aug 19 2017

All was pretty quiet overnight over western and central Nebraska
as we remained under the influence of a ridge. However, this ridge
will transition eastward through the morning hours and into the
afternoon as a short wave tracks east of the Rockies this
afternoon. As a trough digs lee of the Rockies, we will see a
shift to southwesterly flow by late morning just off the surface.
In the boundary layer, winds will be more southerly with decent
moisture moving into the region via return flow setting up across
the area. Surface dew points this afternoon should range from the
mid 50s in southwest Nebraska to mid 60s in north central
Nebraska. This moisture combined with steepening lapse rates, as
high as 9 Deg C/km between the surface and 500mb, will favor some
stronger updrafts once the cap is eroded around mid to late
afternoon. Deep layer vertical wind shear appears favorable for
some more organized storms with values of 25-40 kts. It looks like
the ingredients should all come together near and just east of
the trough and where it will be positioned around mid afternoon,
which, at this time should be around a line from about Valentine
to Imperial with storms pushing eastward after about 4pm this
afternoon. Primary hazards with these storms will be wind and
possibly large hail, especially with any supercells that may form,
this could be very hazardous for those who are already positioned
themselves for the Eclipse and are camping.

.LONG TERM...(Sunday night through Friday)
Issued at 400 AM CDT Sat Aug 19 2017

The main forecast challenges over the next week are thunderstorm
chances and associated severe weather potential Sunday night and
clouds/precip during the eclipse Monday.

Zonal flow over Nebraska transitions to northwest flow aloft as a
Hudson Bay low digs a trough over the Great Lakes and a ridge builds
across the Western U.S. The ridge slowly moves onto the Plains
through the week, with the axis bisecting the forecast area by
Friday. At the surface, a lee side low develops over SE CO/NW KS as
a trough stretches north across the Neb. panhandle and Black Hills.
The system progresses to a warm front draped across the Sandhills
late Sunday, followed by a cool front late Monday. Midweek is
dominated by a high pressure over the northern Plains and Upper
Midwest. Another trough approaches the panhandle late Thursday into
Friday.

Sunday... Retained chc PoP for the Sandhills as the warm front will
provide sufficient lift and forcing for organized convection.
Soundings suggest decent moisture profiles from 700-300mb and steep
lapse rates from the sfc to 700mb. Low level moisture advection is
looking more impressive with dew points in the upper 60s across
central Neb. Primary concerns with the thunderstorms are gusty
winds, hail, and heavy rain. MUCAPE averages around 1500 j/kg Sunday
evening, with a good portion in the hail growth zone. However, with
limited deep layer shear and higher mid level humidity, precip
loading and melting will likely keep hail from becoming severe (for
long, anyway). Relatively drier low levels, sufficient DCAPE, and
heavy rain leading to downbursts increases the severe wind threat.
Storm motion and weak mid level winds suggest the potential for
localized minor flooding. Near normal PWAT values and storm motion
vectors nearly perpendicular to the front alleviate any major
flooding threat. Temperature wise, pushed highs up a degree or so
across the southwest, which is in the warm sector and also more
likely to stay dry.

Monday... A cirrus deck is looking likely with all models showing
high humidity in the 500-300mb layer. With a front in the vicinity
and in between rain makers, low and mid level clouds are also
possible. Soundings are more hopeful regarding the lack of
alto/strato decks though. SREF guidance hints at some stratus across
far northern Neb, but the chance is small. However, concerned about
fog potential in the morning. With preceding rainfall and light
southeast winds, conditions may be ripe. As the front begins
crossing the area at night, forcing increases for widespread precip,
especially east of Hwy 83. Soundings show abundant CAPE toward 2500
j/kg, and 0-6km shear increases to around 35kts by 06z Tue.
Hodographs also take on a strong veering profile before the fropa.

Tuesday and beyond... Precip chances doen`t appear as high Wed and
Thu with the sfc high trending farther west. This takes the best
moisture return toward the mountains, and the steering winds take
the convection almost due north toward the Black Hills. The best
chance of appreciable rainfall for western Nebraska comes with the
trough moving off the higher terrain on Friday. H85 temps remain
modest in the 15-20C range during the period, resulting in highs in
the 70s and lower 80s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1247 PM CDT Sat Aug 19 2017

Cumulus field starting to develop in southwest NEB. Otherwise,
some mid- and high-level clouds are moving east across western and
north central NEB. Attention turns to thunderstorm chances with
strong to severe storms possible. Current thinking is isolated-
scattered storms will develop between 4 and 6 PM central time
across southwest NEB into central-north central NEB with storms
congealing and moving east-south east out of the local forecast
area near mid evening. While confidence increased with respect to
TS affecting the KLBF terminal, less confidence with storms
affecting KVTN thus removed mention of thunderstorms in the TAF.
VFR conditions are expected to generally prevail through much of
the period, except due to effects of storms.

&&

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

$$

SHORT TERM...Allen
LONG TERM...Snively
AVIATION...ET



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