496
FXUS63 KLBF 181710
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
1210 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Invading dry air behind frontal boundary will lead to elevated
  and critical fire weather conditions mainly for areas west of
  Highway 83 where Red Flag Warnings are now in effect.

- Low-end chance (< 20%) of showers and thunderstorms across far north
  central Nebraska Wednesday afternoon.

- Turning much cooler with a slow-moving system bringing a long duration
  rain event to the area early next week.

- More seasonable temperatures are forecast for next week with
  potential but confidence in precipitation potential remains
  low at this time.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 443 AM CDT Wed Sep 18 2024

Morning satellite analysis depicts a deepening low across eastern
Montana with a broad plume of moisture streaming north into this
feature. At the surface, a frontal boundary has stalled out across
our western zones but is expected to make a renewed push east
through the day as the disturbance to the northwest continues to
occlude and the dry conveyor grazes our western zones. This is
setting the stage for fire weather concerns with details found in
the following Fire Weather discussion.

The dynamic system northwest of the local area will continue to lift
north today, following the impressive upper-level dynamics to
include strong h5 height falls, upper-level divergence, and strong
DCVA. While surface features will continue to shuffle along with the
parent low across the Northern Plains, limited cross-front flow will
allow the stalled boundary to waver only slightly today. Steady
southwest flow ahead of this will maintain middle 60s dew points to
the east but a surge of dry air coincident with the dry conveyor
will infiltrate our western zones. This will promote a large
discrepancy from east to west where it`ll be warm and humid to warm
and dry respectively. Forecast highs remain largely unchanged with
afternoon peak values of the low to middle 80s. Winds will favor
behind the front (west), coincident with the strongest push of dry
air and deepest mixing. NWP forecast soundings show efficient
momentum transfer with gusts likely to reach 35 mph for the
northwest Sandhills. Further east, have introduced a limited chance
(< 20%) of showers and thunderstorms for far north central Nebraska.
This is in response to increasing low-level convergence in an area
of modest instability as MLCAPE values climb to the 1000-1500 j/kg
range. That said, capping looks to remain strong so it`s believed
that most location will remain dry. HREF ensemble max QPF shows
increased amounts early afternoon but signals are greatest across
eastern South Dakota. Should anything develop, severe weather is not
anticipated though a stronger core could still produce sub-severe
wind gusts and small hail. Any activity will quickly lift northeast
and return dry conditions to the area. The surface boundary will
make a small retreat late tonight but the lack of any upper-level
support will help maintain a dry forecast. Pooling moisture
following the recent rains main introduce some patchy fog to the
area but confidence is too low for inclusion at this time. Low
temperatures will fall into the 50s for most and low 60s east of
Highway 281.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 443 AM CDT Wed Sep 18 2024

Thursday and Friday...dry conditions are expected for each day.
Strengthening northwest flow will usher the frontal boundary to the
east and dry air will promote tranquil weather. Modest high pressure
will nose in front the east and track through the area by early
Friday morning. Temperatures will generally favor the 80s each
day which will remain on the warmer side of climatology.
Moisture return later Friday on the backside of departing high
pressure will be the first signs of an increasingly active
weekend and early week.

Saturday through Monday...Southern Plains ridging will retreat south
as the next disturbance digs through the Desert Southwest and begins
to approach the central Rockies. Southerly flow will increase
further on Saturday and bolster PWATs which are progged to surpass
1.00" yet again, or exceed the 90th percentile. The ECWMF Extreme
Forecast Index (EFI) has steadily increased its signal for Saturday
and Sunday and recently has begun showing values of 0.6-0.8 for
southern Nebraska with SoT values of 1-2. This is suggesting a
fairly anomalous precipitation event across at least the southern
half of Nebraska. Forcing increase late Saturday as height falls
overspread the area and the main h5 trough exits Colorado and moves
onto the Central Plains. Forcing for ascent maximizes Sunday which
closely matches the EFI peak signal. Ensemble solutions show mean
QPF values exceeding 1.00" across much of the area as a deepening
surface low lifts northeast from western Kansas into eastern
Nebraska. This puts western Nebraska squarely under the main
deformation band with a vertically stacked system largely remaining
immediately to the southeast. Barring any late shifts in forecast
track, this should also keep any dry air surge to the south. The
expected long duration rainfall event should bring beneficial
rainfall to portions of the area while also keeping temperatures on
the cooler side. Forecast highs fall to upper 60s to upper 70s
Saturday but will struggle to climb out of the low 60s Sunday.
Looking at lower NBM percentile values, a sharp gradient in expected
QPF is appearing likely and potential exists for northern Nebraska
to miss out on wetting rain potential. For now, will maintain
inherited PoPs which top out in the 40 to 75% range from north to
south. Total QPFs of 0.50-1.50" will be possible but refinements
with later forecasts will likely shift where the heaviest amounts
fall so folks should continue to monitor subsequent forecasts for
the latest. Precipitation will likely wane through the day Monday
with a return to dry conditions likely by the evening for most if
not all locations. Temperatures will moderate slightly as well, but
only into the upper 60s to lower 70s.

Tuesday and beyond...evolution of the upper-air pattern in the long
range remains somewhat in question. The overall idea is from
amplifying northwesterly flow towards the middle of next week. How
the departing trough evolves across the Great Lakes will be a big
thing to monitor with implications on the forecast going forward.
NWP guidance is hinting at another large scale trough moving onshore
the Pacific Northwest around the same time but progression and
placement Wednesday and beyond is uncertain at best. For now,
believe precipitation chances around the middle of next week will be
limited until we approach the following weekend when the system
encroaches on the Northern Plains. NBM probabilities for
precipitation don`t appear overly robust, however, so a continuation
of slightly above normal temperatures and dry conditions appears
probable.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1210 PM CDT Wed Sep 18 2024

A couple aviation weather concerns exist for western and north
central Nebraska terminals, mainly low stratus clouds and gusty
winds. A stubborn stratus deck is resulting in MVFR ceilings
from near LBF to ONL midday, and will gradually lift through the
afternoon. Across northwest Nebraska are gusty southwest winds,
which will overspread more northern terminals (VTN) today. After
a period of light/variable winds overnight, westerly winds
restrengthen tomorrow midday.

&&

.FIRE WEATHER...
Issued at 443 AM CDT Wed Sep 18 2024

Red Flag Warnings have been issued for Fire Weather Zones 204 and
208 due to the combination of strong winds and low afternoon
humidity values.

A surface boundary will creep east today before settling across the
central and eastern Sandhills this afternoon. This will be the
dividing line between much richer low-level moisture characterized
by middle 60s dew points to the east and much drier air to the west.
This drier air will be enhanced by strong downslope flow as the dry
conveyor belt from a deep surface low to the northwest grazes the
area. This will lead to a large discrepancy across the area with
afternoon humidity minimums ranging from around 50% in north central
Nebraska to near 15% in the far northwest. This will be most
apparent in Zone 208 where forecast humidity minimums will range
from ~15% at Nenzel to 48% at Butte. Opted to include 208 in the Red
Flag Warning given favorable overlap of the lowest humidity and
strongest winds west of Highway 83.

Forecast soundings show deep mixing, nearing 3km in the vicinity of
the Pine Ridge, which will support efficient momentum transfer of
winds through unidirectional flow to the surface. This will likely
tap into stronger flow around h7-h6 layer where speeds will approach
30-35 knots. This should be more than enough to yield 35 mph gusts
at the surface with a few gusts nearing 40 mph on the higher terrain
around the Pine Ridge possible. MRMS observations show a few select
locations managed to see 0.10-0.20" of rainfall Tuesday night, but
with plentiful sunshine and the strong winds forecast for today this
is likely to not be overly beneficial. Recent hi-res guidance,
notably the HRRR/RAP and deterministic NAM, all depict sub 20%
humidities in these areas. NBM deterministic guidance shows fairly
widespread 15-20% afternoon humidity minimums. Using a blend of all
guidance, went with the 50th Percentile value which produced sub 20%
humidity northwest of a Chappell to Valentine line and ~15% humidity
northwest of a Nenzel to Alliance line. Winds should peak around
early afternoon and remain strong into the early evening before the
boundary layer begins to stabilize with the loss of insolation.

Was on the fence about including Fire Weather Zone 206 but felt the
limited spatial coverage north of Highway 2 and west of the timezone
line was not enough to warrant inclusion. Areas near and just south
of McKelvie should also exercise caution as conditions would warrant
the rapid spread and erratic behavior of any fire starts. Elsewhere,
humidity above 30% and winds below 20 mph limit overall concern.
Wetting rains appear likely early next week beginning as early as
Saturday but confidence in precise QPF is low. For now, greatest
potential of seeing wetting rains is generally south of Highway 92
with decreasing confidence with northward extent.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM CDT /8 PM MDT/ this evening for
NEZ204-208.

&&

$$

SHORT TERM...NMJ
LONG TERM...NMJ
AVIATION...Snively
FIRE WEATHER...NMJ