619
FXUS63 KLBF 170905
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
405 AM CDT Mon Jun 17 2024

.KEY MESSAGES...

-  Lingering thunderstorms early this morning will continue to
   pose a risk of locally heavy rainfall across northern
   Nebraska, before exiting the area by early this afternoon.

-  Redevelopment of isolated to widely scattered thunderstorms
   this afternoon and evening will lead to an increasing threat
   for severe weather, with very large hail, damaging winds, and
   a tornado or two possible.

-  Additional rounds of thunderstorms will persist nearly each
   day into the end of the week, with at least some continuing
   threat for strong to severe thunderstorms by late week.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 404 AM CDT Mon Jun 17 2024

Currently, an ongoing MCS continues to move eastward across the
Sandhills, with an east to west oriented band of robust convection
centered north of HWY 20 in Keya Paha and Boyd counties. This line
has primarily brought locally heavy rainfall, though multiple gusts
of around 40 to 45 miles per hour have been observed as well. This
line and its associated outflow will continue to push east early
this morning, gradually exiting the area through late morning.

Attention then turns to a round of potentially significant severe
weather across much of western and north central Nebraska tonight.
For the near term, will have to watch the progression of the outflow
from this morning`s storms, currently stretching from central into
portions of southwest Nebraska. It is uncertain how this outflow
will impact the progression of a frontal boundary, currently
bisecting from just south of IML to south of OFK, as it slowly lifts
northward this afternoon. Given the strong warm advection across the
broad warm sector in south central Nebraska, not anticipating the
outflow boundary to dampen the front southward much this morning.
Aloft, southwesterly flow prevails, with an upper low anticipated to
slide eastward into the Intermountain West into this evening. Broad
positive vorticity advection and subtle height falls are expected to
overspread the area through the day today, with nothing in the way
of a more obvious shortwave trough upstream. At the surface, broad
lee cyclogenesis will persist into this afternoon across eastern
Colorado, with the surface low expected to gradually eject
northeastward towards southwest Nebraska by tonight. In response to
the increasing surface cyclogenesis, increasing pressure falls and
strengthening warm advection should promote the aforementioned
surface front to lift northward towards the Sandhills into this
afternoon. Along and south of this boundary, dewpoints in the upper
60s to low 70s are anticipated, along with surface temperatures
warming into the 90s. Lapse rates aloft will remain very steep, and
in combination with the warm/moist boundary layer will send MLCAPE
to above 3000J/kg for areas south of the frontal boundary. Moisture
pooling near the boundary may promote locally higher instability as
well. As temperatures warm this afternoon, residual CIN should erode
across the warm sector, with forecast soundings suggesting a largely
uncapped environment by late afternoon. That said, a lack of
upper level support should limit robust convective initiation to
locally enhanced convergence near the aforementioned surface
boundary and potentially just ahead of the northeastward lifting
surface low in northeast Colorado. This should lead to isolated
to widely scattered convective development this afternoon
across the area, in closer proximity to surface boundaries.
Initial updrafts may struggle owing to dry LFC-LCL relative
humidity, and multiple CI attempts may be needed for an updraft
to eventually become dominant. This will be aided where TCU can
cluster and better shield themselves from dry air entrainment,
likely again near the surface front. Mesoscale details will
drive the zone of greatest threat today, and boundary
progression will need to be monitored closely. Guidance
currently paints this front reaching somewhere near the HWY 2 to
HWY 20 corridor by late this evening, though will at least
mention there has been a gradual southward trend noted in high-
res solutions over the last 12-24hrs.

Should robust CI occur, the environment will be very supportive of
rotating updrafts, with forecast soundings indicating near pure
streamwise vorticity in the lowest few kilometers of the hodograph
by late evening. This is in combination of strengthening backed
southeasterly surface winds (primarily in closer proximity to the
surface front) and an increasing southerly LLJ (H85 flow ~40-50kts)
that should overspread the Plains near/after sunset. Broad
southwesterly flow in the mid-levels will lengthen hodographs with
height somewhat as well. This points towards a corridor of all
severe hazards should robust supercellular development occur
this evening near the surface front. Any tornado threat will
likely be maximized near the front should a storm anchor
nearby. Instances of large to very large hail will likely
accompany any dominant supercells as well, both near the front
and further south. It appears there will be at least a short
window where storms could maximize a robust severe weather
threat, before CIN quickly increases again near/after sunset and
updrafts will have a propensity to become quickly elevated. The
better signal for CI with respect to high-res guidance is
across far northeast CO ahead of the surface low, with this
convection lifting into southwest NE and presenting a threat for
large hail/damaging winds. Lesser signal exists for robust CI
along the surface front. With largely subtle forcing, mesoscale
details will drive the difference between just some cumulus
development and robust supercells. This will continue to be
monitored very closely, and future forecasts should be monitored
closely.

Convection will largely weaken after sunset and exit the area prior
to midnight. The deepening surface low will keep warm advection
strong, and should promote increasing synoptic winds overnight. In
fact, some high-res guidance paints southerly wind gusts even as
high as greater than 50mph overnight. This is largely due to enough
mechanical mixing to tap into the stronger southerlies aloft, and
confidence in this is not high for now. Still, a period of 40-45mph
gusts look likely tonight, and should keep temperatures very mild in
the upper 60s to low 70s across southwest and central Nebraska,
where they will remain south of the front. The boundary layer looks
to decouple to the north of the front, leading to lows in the upper
50s to low 60s across northwest Nebraska.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 404 AM CDT Mon Jun 17 2024

A surface cold front will quickly push through the area Tuesday
morning, as the aforementioned surface low ejects into eastern
SD. This front largely looks to clear the area by tomorrow
afternoon, keeping a renewed round of convection off to the
east. Still, at least some guidance suggests the frontal
boundary is slow enough to keep a threat for scattered
thunderstorms across far eastern Custer and Wheeler counties.
Confidence in the timing of the front remains lowered, and will
be monitored going forward. A threat for large hail and damaging
winds would accompany storms should the front slow enough to
impact more of central and north central Nebraska.

The area remains post-frontal on Wednesday and increased cold
advection behind the boundary should lead to a day of well below
average temperatures in the upper 60s to low 70s. The return of
southerly flow late this week should bring with it returning
thunderstorm chances, and these look to persist into the weekend.
Aloft west-southwest flow late week gives way to northwesterly flow
this weekend, and at least some threat for severe weather should
return into this weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1250 AM CDT Mon Jun 17 2024

Near term TAFs will be dealing with timing of ongoing
thunderstorms as they move eastward across central Nebraska
early tonight. With upstream convection expected to train over
KVTN will go with IFR VSBY and prevailing +TSRA for the next 2
hours, and will also mention some hail. However there has not
been much in the way of any indication for convective winds
upstream from KVTN so will not mention any enhanced winds there
but will monitor closely. Much of the convection moving east of
Highway 61 is expected to slide just north of KLBF but will use
a one hour TEMPO for thunder there to handle the passage.

With a front draped across the region, expect IFR CIGs will
remain just north of KLBF once the thunderstorms push off to the
east, but KVTN will see IFR conditions with any improvement to
MVFR holding off until after daybreak. Expect VFR at KLBF and
MVFR at KVTN will persist thorugh the day on Monday until
another round of thunderstorms develops toward 00Z and persist
through the end of the valid period.

Outside of convection, winds will generally be 10kt or less with
low level wind shear persisting until a bit after daybreak. KLBF
will see some gusts at or above 25kt develop later Monday
morning as the low level wind field starts to enhance with an
increase to at or above 30kt toward the latter portion of the
valid period. KVTN will be on the cool side of the front,
keeping gusts in check through low level wind shear will be
prevalent.

&&

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

&&

$$

SHORT TERM...Brown
LONG TERM...Brown
AVIATION...MBS