Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 210850

Area Forecast Discussion
National Weather Service North Platte NE
350 AM CDT WED SEP 21 2016

.SHORT TERM...(Today and tonight)
Issued at 331 AM CDT Wed Sep 21 2016

The main challenges in the short term period were temperatures and
shower/thunderstorm chances. A marginal risk remains that some
storms could be strong to severe, and the expected time period for
this is in the late afternoon-evening. Said area of marginal risk
is along and east of a line from Callaway to Rose to Naper.

The 500 hPa ridge over the Southern Plains and Lower Mississippi
Valley into the Ohio Valley continues to break down over the next 24
hours as a trough amplifies over the western CONUS. A modest
southwest flow aloft will continue over the forecast area, by and
large 20-35 kts. Deterministic guidance remains in good agreement
with a short wave trough moving SW-NE from CO across western and
north central NE in the afternoon that will help provide large scale
ascent. Meanwhile, sub-tropical moisture will continue to be
transported into central NE at low-levels with greatest moisture
more so being transported into eastern NE. Wherein across western NE
and north central NE precipitable water values will rise to about 1
to 1.25-1.5 by the evening, respectively.

A surface low is expected to be near center of the CWA at the start
of the period with a warm front extending northeast from the low
across far north central NE to extreme southeast SD. The
aforementioned low will move northeastward and weaken into a trough
as a new lee low develops near the Front Range by the afternoon.
Wherein a front is expected to be positioned across roughly the
southeast-half of the forecast area, although guidance shows some
differences in the location. Moisture should be decent given the set
up with surface dew points in the mid 50s to low 60s across north
central NE by mid afternoon, upper 40s to mid 50s across western NE.
While cloud cover will affect daytime heating and insolation, high
temperatures are expected to read largely from mid 70s to low 80s
across the northwest half to low 80s to near 90 across the southeast
half. Overnight, clouds and elevated winds will keep temperatures
warm with low temperatures expected to be 10-15 degrees above normal
(or warmer in portions of north central NE). Lows tonight are
forecast in the 50s to low 60s.

Current thinking is that there will be increased chances for
thunderstorms, primarily isolated, near/along the frontal boundary
beginning late afternoon. Then chances increase as we get into the
mid-late evening across southeast quad of the forecast area. A low-
level jet is expected to develop across western KS into southwest
NE-central NE in the evening with the nose near the southeast
quad of the forecast area and a speed max approaching 50 kts. As
such, possible thunderstorms will then continue over the eastern
portion of the forecast area into and just after midnight with
storms then moving east out of the area. Instability is expected
to be decent (greater than 1000 J/kg) and, given the deep layer
shear, convective storm mode is anticipated to be mainly
multicells, with 0-6 km bulk shear generally around 23-33 kts.
Latest cycle of the SPC SSEO at 00Z seems shows storms developing
late afternoon, with a strong signal near midnight based on
simulated reflectivity. This increases confidence for increasing
chances in the evening. Given the SPC SSEO output and other hi-res
guidance, best chances for a strong or severe thunderstorm appears
to be more so in north central NE. This is consistent with the SPC
Day 1 Outlook where a marginal risk area is present across roughly
the eastern-third of the forecast.

.LONG TERM...(Thursday through Tuesday)
Issued at 331 AM CDT Wed Sep 21 2016

A difficult temperature forecast is shaping up for Thursday in the
wake of a shortwave trough that will have ejected northeast across
the area from the deepening upper low across the western Conus. A
cold front will stall somewhere in or near the forecast area, with a
low cloud stratus deck behind the front due to pooling low level
moisture. The NAM is the farthest south with the front, and has it
extending from McCook up toward Norfolk. GFS and ECMWF are farther
north, as well are the other Hi-Res models such as the NMM and ARW.
Will not change previous forecast highs much, as am uncertain where
the front will end up. Thinking though that the NAM may be a bit far
south, and it has actually trended a bit north from the previous
run. Will run with highs near 80 degrees from Ewing to Broken Bow to
North Platte to Grant, where front seems most likely to be located.
As mentioned the pooling of moisture, low clouds, and northeast
upslope winds will tend to really hold temperatures down for
locations to the north of the front. Have lowered highs a few
degrees into the mid 60s across north central Nebraska well north of
the front.

Southerly low level jet will develop Thursday night as the stalled
cold front begins to move north as a warm front. Mid level
instability increases after dark, and with increasing upper level
support from the approaching upper level low, will maintain the low
chance pops for thunder across northern Nebraska.

The warm front will lift north of the area Friday, with a very warm
day on tap as highs climb into the mid to upper 80s. The upper low
will continue to progress east through the central Rockies, with
deepening surface low pressure across eastern Colorado. An inverted
surface trough will extend north-northeast from the surface low
across western Nebraska, and will likely be the focus for
thunderstorm develoment late Friday afternoon and evening.

Models have come into agreement tonight regarding the upper low
crossing the region this weekend. The consensus is for a more
progressive low which will track from the central Rockies to North
Dakota by Saturday. This will drive a cold front through the area
Saturday. It appears that the front will come through more quickly
then previous model runs were indicating, and the better chances for
thunder/rainfall with the front may focus east of the area. Post
frontal on Sunday into the first of next week looks dry and cool,
with highs trending down into the mid and upper 60s.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1216 AM CDT Wed Sep 21 2016

Increasing high clouds across western Nebraska overnight with light
and variable winds. Low level wind shear is expected at KLBF into
early morning with a southerly jet and light winds at the
surface. Surface boundary across South Dakota will drift into
northern Nebraska towards sunrise and there is the possibility of
some stratus on the north side of the front. Have left out of KVTN
taf for now with uncertainty of front shifting south. Have
included prob group for tsra late at KLBF taf site.


.LBF Watches/Warnings/Advisories...


LONG TERM...Taylor
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