712
FXUS63 KLBF 221137
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
637 AM CDT Wed May 22 2024

.KEY MESSAGES...

- Outside of isolated morning showers across the Sandhills and
  north central Nebraska, dry conditions are expected for
  Wednesday.

- A strong cool front will dive into the area Thursday and bring
  the threat for severe thunderstorms, locally heavy rain, and
  strong winds behind it Thursday night into Friday.

- Recurring chances of rain and thunderstorms appear likely
  through the Memorial Day holiday as temperatures hover near to
  slightly below normal values for late May.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 419 AM CDT Wed May 22 2024

Early this morning, isolated to widely scattered showers were
tracking south and east across the Sandhills into portions of north
central Nebraska. These were largely driven by a shortwave
disturbance, evident on satellite and centered near the
Nebraska/South Dakota border east of the Pine Ridge as of 0800z (3am
CDT). Warm air advection with aided lift from the left-exit region
of a modest upper jet will continue to shift east by sunrise
allowing lift to wane and precipitation chances to end. The latest
short-term guidance, notably consecutive runs of the HRRR, suggest
low-end PoPs may linger across north central Nebraska through mid-
morning so have opted to keep a mention in through 9am CDT.
Expectations are for any QPF to remain light and generally less than
a tenth of an inch at any one location. Subsidence behind departing
shortwave should help lead to clearing skies and pleasant
conditions. Afternoon highs should settle to around 70 degrees with
a light southwesterly wind generally less than 20 mph.

Winds will continue to back to the south and eventually southeast by
late tonight. Increasing flow off the surface will help draw
increasing low-level moisture across the area. This occurs as modest
low-pressure begins to develop over far northeast Wyoming and
western South Dakota. This will hold overnight lows in check with
forecast values only reaching the middle 40s west to middle 50s east
as gusts range from 15 to 25 mph. These temperatures of roughly 5
degF above normal for the time of year.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 419 AM CDT Wed May 22 2024

Thursday into Friday...Overall pattern gradually shifts from zonal
mid-level flow to southwesterly flow across the central High Plains
as the next disturbance approaches from the west. Another shortwave
trough will approach the central Rockies by midday Thursday.
Increasing forcing for ascent as divergence aloft increases will
lead to lee-cyclogenesis in the vicinity of western South Dakota. A
sub-1000 hPa surface low will develop near KRAP, dragging a cool
front south into western Nebraska. Timing of this cool front through
the area favors early afternoon through late evening west to east.
With increasing southerly flow ahead of this feature, strong
moisture and warm air advection will drive daytime highs into the
upper 70s to middle 80s as surface dew points pool in the region.
Aloft, lapse rates will steepen as h7 to h5 mean flow advects a
stout EML across the region. While this will lead to strong capping
in the area depicted by SBCIN values exceeding 150 j/kg. As surface
convergence increases along the approaching surface boundary,
scattered to widespread rain and thunderstorms are expected to erupt
by late afternoon within an environment supportive of organized
severe weather: MLCAPE values ranging from 1500-3000 j/kg and 0-6km
BWD values of 40 to 50 knots. Increasing low-level forcing should
easily overcome the strong capping in place and coverage of
thunderstorms is expected to increase across central Nebraska where
PoPs increase to as high as 80-90% east of Highway 83. The Storm
Prediction Center (SPC) has recently upgraded the aforementioned
area to a Slight Risk (Level 2 of 5) and am in general agreement
with this. Forecast soundings depict clockwise curving hodographs
with ~100 j/kg and 150-200 j/kg of 0-1km and 0-3km SRH respectively.
This may lead to a modest tornado threat given low-LCLs in an
increasingly low-level sheared environment as the low-level jet
ramps up and 0-1km BWD increases to 15-20 knots. While lapse rates
appear quite favorable, weakening mid-level shear with southward
extent casts some doubt on favorable conditions for a significant
hail environment though 1-1.5" hail remains a concern. Meanwhile,
while afternoon DCAPE values climb to around 1000+ j/kg and 0-3km
delta theta-e values fall to around -30 degC, damaging winds appear
quite probable as storms grow upscale and enter central Nebraska in
the evening. With the progressive nature of the frontal boundary,
expect a swift end to precipitation prior to midnight as activity
tracks east and out of the area. While not overly significant, the
latest QPF for this event shows a bullseye of 0.75-1.25" across far
north central Nebraska. This on the heels of recent heavy rain may
present at least a localized flooding potential. As such, the
Weather Prediction Center has inserted a Marginal Risk of excessive
rainfall for central and eastern Nebraska. The majority of this
outlook remains outside the forecast area but I believe the threat
will present itself locally as well.

Meanwhile, behind the frontal boundary, attention quickly turns to
increasing signals for a strong synoptic wind event. Strong pressure
rises behind the passing front will approach 2-3 hPa per hour with
flow off the surface increasing to 35 to 50 knots. SREF
probabilities of sustained surface winds exceeding 25 knots approach
50%, which is a noteworthy signal in and of itself. Meanwhile, an h5
low will form and drift east across South Dakota during the
overnight hours. Mass response to the increasing height gradient
will propel flow at h85 and h5 to increase to 45 and 50 knots
respectively. While uncertainty is fairly high on magnitude of
winds reaching the surface during the overnight hours, boundary
layer mixing after sunrise Friday may be able to tap into the
stronger flow aloft. This will present a small window of
potentially seeing 50 knot (58 mph) gusts reaching the surface
as 0-2km flow becomes fairly unidirectional. Latest ECMWF
Extreme Forecast Index (EFI) shows values ranging from 0.5 to
0.8 suggesting modeled winds are on the higher end of the model
climatology and non-zero Shift of Tails suggests some individual
ensemble members exceed the 90th percentile of model
climatology. All this to say, a notable wind event is possible
Thursday night into Friday. Did opt to increase winds as a
result after collaboration with northern neighbors, but further
increases may be necessary with later forecasts. For now, the
forecast calls for gusts up to 45 mph. While temperatures will
fall into the 30s to 40s Friday morning, the strong winds may
preclude frost concerns so no headlines are expected at this
time even with lows nearing the mid 30s in the immediate shadow
of the Pine Ridge. Strong westerly winds will continue through
much of Friday. General cold air advection in the strong
northwesterly low-level flow will help keep afternoon highs on
the cooler side with forecast values in the 60s to low 70s, or 5
to 15 degrees below normal.

Saturday and beyond...overall confidence in a fairly active extended
period remains medium. Broad upper-troughing remains stagnant across
central Canada with multiple disturbances dropping south and east on
the backside of this feature through the weekend. This will promote
zonal to southwesterly flow as systems track through the area.
Overall agreement in deterministic models is fairly high regarding
timing and placement of a disturbance through the area by late
Saturday into early Sunday. NBM 50th percentile values show light
QPF across much of the region and inherited PoPs echo this idea with
Chance category (up to 40%) Saturday night into Sunday, increasing
to up to 50% by Sunday evening. NBM QPF probabilities exceed 60-70%
for > 0.01" over much of the region and approach 40% for exceeding
0.25" thus the potential for a wetting rain appears possible.
Thereafter and heading into the Memorial Day holiday, mid-level
ridging strengthens across the western CONUS with increasingly
amplified northwesterly flow locally as broad troughing stalls out
near the Great Lakes. This would suggest temperatures will quickly
moderate and even approach above normal values immediately upstream
by early week but arriving to western Nebraska by mid-week. NBM 50th
percentile temperature values begin to climb by Tuesday and reach
the lower 80s by late week. These types of patterns are generally
unfavorable for widespread precipitation chances and as a result,
a dry forecast is in place for Tuesday through the end of the
valid period.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 635 AM CDT Wed May 22 2024

Quiet aviation conditions are expected over the next day as upper
level ridging builds into the region. Ceilings remain above 6000
feet with no precipitation expected. The only concern will be some
stronger winds with gusts up to 20 knots as well as some low level
wind shear across north central Nebraska tonight. Otherwise, winds
remain under 10 knots.

&&

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

&&

$$

SHORT TERM...NMJ
LONG TERM...NMJ
AVIATION...Kulik