Severe Thunderstorm Warning 307, NWS North Platte, NE

Area Forecast Discussion
National Weather Service North Platte NE
627 PM CDT Tue Aug 8 2023

...Aviation discussion...

.SYNOPSIS...
Issued at 251 PM CDT Tue Aug 8 2023

Key Messages:

- Severe storms tonight move from west to east across the area

- Below average temperatures remain across the area Wednesday, warming
  back to near average for this time of year Thursday and Friday
  and through the weekend

- Low confidence for widespread precipitation, after tonight,
  through the end of the week

&&

.SHORT TERM...(This evening through Wednesday night)
Issued at 251 PM CDT Tue Aug 8 2023

A potent upper PV anomaly, associated with a slowly eastward
propagating upper low, will move into the High Plains this
evening. This will create a strengthening band of westerlies in
the resultant nearly zonal flow. This will develop a surface
trough in the lee of the Rockies with increasing south to
southeast flow developing in response to the lowering pressures.
This will further result in an increase in low level moisture
across especially the western portion of NE, and therefore a
narrow reservoir of CAPE from western KS/northeast CO into the
Panhandle of NE and far southeast WY. The upslope southeasterly
flow will initially focus convective initiation near the favored
terrain features of northern CO and southeast WY, with the motion
of storms a little quicker than previous events due to the better
flow aloft. The combination of stronger mid level westerly flow
and southeast low level winds will result in favorable deep layer
shear for storm organization as storms mature further east
eventually. The balance of good deep layer shear with favorable
CAPE will set the stage for a few supercells, and with long
hodographs as a result of the good shear, large to very large hail
will certainly be a possibility. The favored location for this
juxtaposition will be mainly south of a line from Oshkosh in
Garden county, to Stockville in Frontier county. In addition, some
weak mid level backing ahead of the aforementioned strong short
wave should also support splitting supercells early, and 12Z CAMs
do suggest this potential with both right and left members having
some potential to sustain. As would be expected, right movers in
the presence of low level backed flow, and hence better 0-1km
shear would have the better chance to produce an isolated tornado,
however drier low levels would initially favor better downdraft
CAPE and therefore outflow dominated storms. As always however,
storm scale interactions make this a difficult challenge and
therefore some confidence in a tornado threat does exist for late
afternoon and evening especially along boundaries. Confidence is
highest in supercell development early in the event with a
transition toward a more linear system or linear segments by late
evening and an eastward propagating MCS overnight.

In the wake of the MCS and exiting short wave aloft, northerly flow
will dominate the low levels Wednesday as upper heights rise
ahead of a building upper ridge across the western CONUS. This
will promote a drying trend through the first half of the day
Thursday with temperatures remaining cooler than average for this
time of year Wednesday.

.LONG TERM...(Thursday through Tuesday)
Issued at 251 PM CDT Tue Aug 8 2023

The long term upper pattern by Thursday continues to be a fairly
high wave number around 5, which suggests a continuation of the
progressive pattern across the northern hemisphere. As upper
ridging intensifies across the southern Plains by the end of the
week, the stronger band of westerlies will retreat northward to
the northern Plains which will suggest a lesser coverage of strong
to severe storms by the end of the week for western and north
central NE. The flow will transition to northwesterly aloft across
the northern High Plains by the first of next week as anomalous
troughing develops across the northern Plains into the upper
Midwest.

More specifically, the aforementioned transitory upper ridging
across the Rockies will shift east Thursday while being knocked down
by the next potent upper PV anomaly to move into the northern
Plains. At the same time, the strengthening westerlies in the mid
levels will transition into the central High Plains and will emerge
into western NE by late in the day. This will develop a dryline
across the western Sandhills into north central NE, which may serve
as an initiating source for thunderstorm development late in the day
Thursday. However, low level return flow will be weak and mostly
directed into central and far north central NE. So although shear
will be sufficient for storms, the best low level instability
will be east of the mentioned boundary. Therefore coverage will
likely be isolated, though storms that do develop and are able to
maximize the availability of CAPE could be strong to severe.
However, confidence in more widespread severe weather Thursday is
low at the moment.

For Friday into Saturday, the combination of the cold frontal
passage late Thursday with increasing downslope flow across the
western portion of the area will leave a slightly drier airmass in
place. The previously mentioned transition to northwesterly flow
aloft will then start to take shape by late in the weekend beginning
with a cold frontal passage on Sunday. This will be a weak front
and will quickly be replaced by return flow ahead of the next,
more potent system dropping out of Canada which helps to reinforce
the upper trough over the Midwest. Accompanying this trough will
be a fairly stout cold front which will move through the area late
Tuesday. This front will have decent temperature changes with it,
in addition to a much drier airmass behind it, which is beyond
this forecast period. Probabilities for the warmest temperatures
are highest Monday and especially Tuesday as compressional warming
ahead of the strong cold front will push temps above average.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 627 PM CDT Tue Aug 8 2023

Thunderstorms across the eastern panhandle and sandhills will
continue to grow upscale as they move eastward. Expect these
storms will arrive at both KVTN and KLBF at approximately the same
time between 02Z-04Z, so will indicate this timing and include
gusts of 40kt to 45kt in the TAFs and amend as necessary. There is
a signal for a bloom of some fog/stratus toward daybreak and will
indicate a period of MVFR conditions for a few hours past
daybreak Wednesday. Expect a return to VFR across the region by
mid Wednesday morning with good flight conditions persisting
through the end of the valid period.

Strong erratic wind gusts can be expected in the vicinity of any
thunderstorms. Outside of thunderstorms, winds will generally be
light at or below 10kt.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

SYNOPSIS...Stoppkotte
SHORT TERM...Stoppkotte
LONG TERM...Stoppkotte
AVIATION...MBS

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