Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 180840
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
340 AM CDT Fri Aug 18 2017

.SYNOPSIS...
Issued at 338 AM CDT Fri Aug 18 2017

H5 analysis tonight had high pressure anchored off the coast of
nrn California. A secondary high was located over the northern
Gulf of Mexico. Further north, a strong shortwave was located over
the UP of Michigan with another low located over northern
Manitoba. Across the intermountain west and Central Plains,
northwesterly flow aloft extended from Nebraska and the Dakotas
into western Canada, where a ridge extended from northern
Washington into northern Alberta. A shortwave trough was located
across northwestern Nebraska and western South Dakota. Current
radar imagery has the shortwave over northeastern Nebraska.
Showers and thunderstorms extended from central into northeastern
Nebraska and had cleared the forecast area as of 3 AM CDT. Skies
were mostly clear in the west and northwestern cwa while mostly
cloudy skies prevailed in the east, and southern forecast area.
Readings as of 2 AM CDT ranged from 59 at Thedford to 66 at North
Platte.

&&

.SHORT TERM...(Today and tonight)
Issued at 338 AM CDT Fri Aug 18 2017

Over the next 24 hours, H5 heights will increase across the
central and northern rockies, as well as the northern plains. This
will lead to clearing skies and increased warm air advection
today. H85 temps will increase to 20 to 26C this afternoon. This
will equate to mid 80s in the northeast to the lower 90s in the
far southwest. With the MET and MAV guidance in good agreement,
utilized a blend of the two for highs today. For lows tonight,
went ahead and undercut guidance a couple of degrees to better
reflect drier boundary layer air as indicated in the latest NAM12
soln. Sfc dew points in this model are in the lower 50s by 12z
Saturday.

.LONG TERM...(Saturday through Thursday)
Issued at 338 AM CDT Fri Aug 18 2017

The main forecast challenges over the next week are thunderstorm
chances and associated severe weather potential Saturday and Sunday
nights and the overall weather conditions during the eclipse Monday.

Nebraska starts the weekend in zonal flow aloft as an H5 high is
centered over the southern Plains. Toward the middle of next week, a
Hudson Bay low begins digging a trough over the Great Lakes as a
ridge amplifies over the Rockies. That setup places western Nebraska
into northwest flow. At the surface, a low develops off the higher
terrain, dragging a boundary onto the High Plains. The front stalls
across western Nebraska late Saturday into Sunday, then drops
southeast as a cold front on Monday. Midweek is dominated by a high
pressure over the northern Plains and Upper Midwest, placing
Nebraska in a return flow regime. The next front approaches the
panhandle late Thursday.

Precipitation for Saturday and Sunday... Expanded schc TSRA a bit
Saturday afternoon as the GFS is more aggressive with the mid level
moisture and low level lift. Still a lot of dry air remains sub H7
per forecast soundings, so any convection will likely be isolated
and concentrated invof trough, generally the eastern panhandle and
western Sandhills. The models are showing a slightly stronger LLJ
than previous runs, up to 35 kts across the Sandhills. CAPE and mid
level lapse rates support stronger storms, however the severe threat
appears marginal due to deep layer shear staying under 30 kts. Low-
mid level veering wind profile would suggest some organized
convection, probably upscale growth to clusters. Storm motion is
progged at 10-15 kts, so cannot rule out a locally heavy rain
threat. Depending on precip coverage and how much the winds lighten
overnight, some fog may develop in the Platte River Valley and parts
of central Nebraska. For Sunday, the setup is fairly similar to
Saturday with late afternoon/early evening initiation across the
west and late evening evolution across the east. Deep layer shear
still limits the greater severe potential. The nose of the LLJ digs
into the Dakotas, placing the best forcing and convergence north of
the forecast area.

Monday... Fairly significant differences between the GFS and ECM in
terms of fropa timing, with the ECM placing the front just east of
the area by 00z Tue while GFS has it bisecting the Sandhills and SW
Neb. Nevertheless, precip chances remain negligible as Sunday
night`s convection will be well east of the area or having already
dissipated and the next round of convection is yet to initiate.
Clouds, on the other hand, will play a more significant role. The
evening model suite is consistent with a band of cirrus over the
area at 18z, and the ECM also hints at some mid-level clouds across
SW Neb. The transparency of the clouds is yet to be determined.

Precipitation for Monday night through Thursday... The surface high
over the Great Lakes pulls a steady stream of moisture onto the
Plains, which coincides with upslope flow at H85. Rounds of
convection off the CO/WY mtn ranges will likely spread downstream
into Nebraska.

Temperatures... Raised max temps Saturday a degree or two, trending
toward the warmer guidance. H85 temps near 28C, decent southerly
flow, and early sunshine should contribute to sfc temps in the lower
to mid 90s. Maintained upper 80s to lower 90s for Sunday for the
same reasons. Substantial CAA behind the front Tue-Wed as H85 temps
struggle to reach 20C.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 152 PM CDT Thu Aug 17 2017

VFR conditions overnight and on Friday at the KLBF and KVTN
terminals. Northwest winds behind a weak front will be light at up
to 10 kts on Friday. Some scattered to broken high cloudiness
overnight, then clear skies to only a few high clouds Friday and
Friday evening.


&&

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

$$

SYNOPSIS...Buttler
SHORT TERM...Buttler
LONG TERM...Snively
AVIATION...Roberg


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