Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 171752
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
1252 PM CDT Thu Aug 17 2017

.SYNOPSIS...
Issued at 357 AM CDT Thu Aug 17 2017

H5 analysis from earlier this evening had a strong shortwave over
eastern South Dakota with a trough extending south into eastern
Nebraska and southern Kansas. High pressure was located off the
west coast of the CONUS with a second high pressure located over
southern Georgia. Strong subsidence west of the eastern South
Dakota trough along with surface high pressure, has led to clear
skies and light winds across the forecast area overnight.
Temperatures as of 2 AM CDT ranged from 50 at Gordon to 60 at
O`Neill.

&&

.SHORT TERM...(Today and tonight)
Issued at 357 AM CDT Thu Aug 17 2017

Thunderstorm chances later today into this evening will be the
main forecast concern over the next 24 hours. For today: High
pressure will track east this morning, allowing a lee side trough
of low pressure to deepen across eastern Wyoming and northern
Colorado. Southerly winds will increase low level warm air and
moisture advection across the panhandle and western sandhills this
afternoon. With surface heating and the approach of a mid level
shortwave, lapse rates will steepen across the panhandle.
Approaching 21z, the latest NAM12 soln is indicating a nice thetae
ridge in the panhandle, along with a swath of 1500 j/kg CAPE into
the eastern Nebraska panhandle. As the shortwave trough
approaches late this afternoon, thunderstorms should initiate
along the Cheyenne ridge and track east into the sern Nebraska
panhandle. By early evening as the wave encounters deeper low
level moisture in the western sandhills, thunderstorms may become
more numerous along and east of highway 61. Storms will track east
tonight as favorable mid level warm air advection tracks across
South Dakota and northern Nebraska. As for the severe threat: The
best chances for strong to low end severe storms will be in the
southwestern and west central forecast area. This is mainly a
timing issue, ie. timing when mid level lapse rates are steepest
and surface temps are warmest. Deep layer shear is on the order of
25 to 35 KTS this afternoon, which would be favorable for some
rotating storms with large hail and wind gusts being the main
threats. As for highs this afternoon, the latest MAV and MET
guidance has trended warmer and will be boosting highs 1 to 3
degrees over the pvs fcst.

.LONG TERM...(Friday through Wednesday)
Issued at 357 AM CDT Thu Aug 17 2017

Synopsis: As an H5 ridge builds across the Intermountain West and
progresses east onto the Plains, western Nebraska transitions from
northwest flow to zonal flow over the weekend. A secondary high
develops over the Desert Southwest toward midweek, which turns the
flow northwesterly again. At the surface, one cool front passes
through the forecast area early Friday. Another boundary stalls
across the panhandle over the weekend as a lee side low develops,
then crosses the Sandhills as a cold front on Monday. A high
pressure settles into the Upper Miss Valley late week, setting up
return flow over the Plains and an upslope regime.

Precipitation: Pushed the PoPs east of the panhandle Saturday
evening as the best moisture lies across central Neb. Dew points
really drop off west of Hwy 83 behind the sfc trough, whereas values
reach the lower 60s from BBW to ANW and points east. Convection will
likely begin as isolated storms in the western Sandhills, then grow
upscale into clusters or a line Saturday night as the mean wind is
nearly parallel to the boundary. Sufficient elevated CAPE exists
after sunset to support continued strong storms, but with deep layer
shear only around 25 kts, the overall severe potential looks
limited. Near normal PWAT and storm motion around 15 kts suggest
some heavy rain potential but limited flash flood threat. Sunday
evening, moisture appears in better supply farther west with 60+ dew
points from IML to VTN. Forcing also increases with a shortwave
traversing the northern Plains and the LLJ strengthens. The GFS and
ECM show 35-40 kts over the Sandhills, but the nose is over the
Dakotas. The severe threat also looks a little better as 0-6km shear
approaches 30 kts and MUCAPE remains around 1000+ j/kg. Kept schc
TSRA over north central Neb Monday evening as the cold front is
still pushing across the region. Chc PoPs return late Wed to account
for downstream convection from the CO/WY ranges as upslope flow
begins.

Temperatures: Trended warmer Fri-Sun as persistent southerly flow at
H85 pushes temps above 25C and expect decent mixing during the
afternoon. Warmest temps will likely be across the panhandle in the
drier regime. The H85 thermal ridge bisects the area Monday, which
should still allow upper 80s to lower 90s for highs, especially
southwest Neb. The GFS and ECM diverge Tuesday and beyond. The ECM
places the high over the northern Plains, keeping H85 temps around
15C Thu and Fri. The GFS quickly sets up WAA post fropa with H85
temps around 20C Thu and Fri.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1236 PM CDT Thu Aug 17 2017

Latest satellite imagery shows mostly clear skies overhead.
Elsewhere, convection has started with enhanced cumulus apparent
in extreme southeast WY and modest radar returns seen on the
regional radar display. Mostly clear skies will prevail until
mid-late afternoon when high and convective clouds develop or move
into western NEB. Current thinking is isolated thunderstorms will
move off the higher terrain with some development across western
NEB with storms moving east to southeast direction this evening
into late tonight. Continued the use of VCTS at this time for
both KVTN and KLBF terminals given forecast areal coverage of
storms. Meanwhile, southerly winds this afternoon will shift and
become northwesterly over night. A gradually clearing can be
expected tonight beginning mid evening from west to east.

&&

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

$$

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



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