821
FXUS63 KLBF 171122
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
622 AM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Storm Prediction Center has upped the latest outlook for
  Tuesday to an Enhanced Risk (level 3 of 5) for areas west of
  Highway 61 with a Slight Risk (level 2 of 5) extending east to
  Highway 83.

- Strong to severe thunderstorms are expected to roll off the
  higher terrain late today with the primary threat being
  damaging wind gusts of up to 80 mph and secondary threats of
  isolated large hail and heavy rain.

- Strong southerly winds gusting up to 50 mph across much of the region
  Tuesday will promote elevated fire weather conditions.

- Active weather looks to return this weekend into early next
  week as another trough within southwest flow aloft tracks
  across the area. At this time, precise details remain somewhat
  uncertain.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 424 AM CDT Tue Sep 17 2024

Morning satellite analysis shows broad southwesterly flow across the
central CONUS as deep troughing continues to track through the Great
Basin. Further east, broad southeast ridging remains in place with
an occluding system situated across the Carolina`s impeding its
influence for a select few stretching from the Smokey Mountains
through the Delmarva Peninsula.

Through today, the deep h5 cutoff centered over Nevada will begin to
quickly lift east-northeast through western Wyoming and onto the
Northern Plains by late tonight. Upper-level dynamics remain strong
with this feature: strong 6-10 decameter (dam)/12 hours height falls
will increase to 8-14 dam by late tonight as the main PV anomaly
tracks across the Bighorns of northern Wyoming and into the western
Dakotas by early Wednesday. Rapid cyclogenesis under strong upper-
level divergence will foster a sub-995 hPa low tracking southwest to
northeast out of the central Rockies and through the Nebraska
Panhandle. NAEFS advertises this as below the 5th percentile and
generally only something the area sees once every 2 to 5 years
according to the Return Interval values. With deepening low pressure
in the lee of the Rockies but broad surface high pressure further
east, a constricting pressure gradient will yield strong southerly
winds through the day. Already this morning we`ve seen gusts nearing
40 mph with VWP data indicating strong flow off the surface that
will likely continue through the day. An area of concentrated WAA is
helping drive some isolated rain showers over central Nebraska.
Various short-term guidance is in good agreement that this activity
will be quick to depart to the north and east, leaving the bulk of
the daytime dry. With the strong southerly flow, moderate mixing
should be ample to tap into anomalous h85 and h7 temperatures with
nearly a 3-5 degC climb from Monday`s values. This lends confidence
in boosting today`s forecast highs into the upper 80s to middle 90s
across the area. With the strong winds and afternoon temperatures
nearing 15 to 20 degF above normal, am concerned about a fire
weather threat today. While afternoon humidity values look to remain
above critical levels, dried fuels and strong winds would be
favorable for the erratic and rapid spread of any fire starts so
extreme caution is advised. Utilized a blend of NBM90 for wind gust
speeds which closely lined up with momentum transfer of multiple NWP
solutions. Forecast soundings all generally highlight mixing up to
h7, or about 2km up, with an increasing LLJ late in the day
supporting gusts in the 40 to 50 mph range for late afternoon and
early evening.

While strong kinematics and the subsequent mixing will work against
the moisture influx, afternoon dew points will still range from the
middle 50s west to middle 60s east. As steep lapse rates remain in
place aloft, ample instability will build during the day with MLCAPE
values progged to reach at or near 1500 j/kg along a narrow corridor
in our western zones. H5 flow will ramp up at the same time, with an
increasingly sheared environment through the latter half of the day.
Inhibiting this somewhat will be the negative tilt of the h5 trough
and the highly meridional flow aloft. All this is moot however as a
cold front quickly dives east from Wyoming and into west central
Nebraska by this evening. Shear will be maximized in close proximity
to this boundary and will be more than enough for organized
convection. While the increasing LLJ will lead to long/looping
hodographs in the 0-3km layer the modestly high LCLs, lack of line
normal 0-3km shear vectors, and expected rapid progression of cold
pools limits confidence in seeing a tornado threat materialize. As
mentioned though, merging cold pools along and ahead of strong
frontal forcing should lead to a primarily damaging wind threat.
This is the main driver for SPC`s upgrade to an Enhanced Risk,
primarily west of Highway 61. Dry sub-cloud layers with inverted-v
profiles will be a good environment for merging cold pools and the
damaging wind gusts. Can`t rule out a few gusts nearing 80 mph and
indeed, various CAM solutions depict just this for the Nebraska
Panhandle and far northeast Colorado. While hail is always a
possibility with any severe storm, maximum expected hail size will
be more limited owing to expected storm mode, marginal instability
aloft, and lack of greater shear aloft. Still, can`t rule out a few
storms capable of Ping Pong ball size hail but most reports will
likely remain at 1.00" in diameter or less.

Storms will be fairly progressive, with anticipated storm motions of
20-30 mph. This should limit the heavy rain threat overall even
though PWAT values remain anomalously high. Forecast values
surpassing 1.00" are nearing +2-3 sigma in NAEFS climatology. Higher
NBM percentile values suggest some areas could surpass 1.00" and the
HREF generally agrees with this potential remaining on an isolated
basis. While ensemble mean values are generally limited to a half
inch or less, Probability Matched Mean values show more coverage of
0.75-1.25" amounts for the late evening. All this to say, flooding
is not expected to be a major issue but a few instances of low-land
flooding cannot be ruled out. Behind the sweeping cold front, drier
air will settle into our western zones while low-level mugginess
lingers overnight into early Wednesday for our east. As lapse rates
aloft diminish, convection should wane by Midnight and through
Wednesday morning. HREF guidance echoes these thoughts well with
decreasing probabilities of > 40 dbz towards sunrise. The going
forecast has a dry forecast by sunrise Wednesday for areas west of
Highway 183 but maintains < 20% PoPs for areas east into the
daytime. With the expected clouds and breezy conditions, lows will
struggle to fall much with forecast values ranging from the middle
60s east to low 50s in the far west.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 424 AM CDT Tue Sep 17 2024

Wednesday...A dry forecast for most is expected though lingering low-
level moisture  ahead of the stalled frontal boundary will be a
feature to watch for central and eastern Nebraska. Deep surface low
will drift across eastern Montana and into North Dakota with and
occluded front stretching north and east and the cold front
splitting the Plains near the Highway 281 corridor by late in the
day. To the west of this, dew points will fall into the upper 30s to
low 50s. This will likely preclude any instability concerns as high
temperatures only climb into the middle 80s. Further east, MUCAPE
values appear likely to reach the 1500-2500 j/kg range but upper-
level dynamics having pushed further north will limit overall shear
and forcing in the area. Some CAM guidance does manage to convect
during the afternoon but all advertise east of our forecast area so
will maintain a dry forecast for most if not all locations.

Thursday and beyond...a brief period of dry weather returns to
the forecast. Southern Plains ridging will build north into the
area and subsequent height rises will quell activity across the
region. PoPs increase slightly late Friday into Saturday as
moisture recovers on the backside of departing weak high
pressure. NBM 90th percentile values paint QPF across much of
the area Friday night into early Saturday. This is as the next
mid-level disturbance tracks out of the southwest and onto the
Central and Southern Plains. Variability in NWP guidance casts
some doubts on precise evolution of this system, especially as
the preceding system looks to linger across the Canadian
Prairies if not the far Northern Plains. Ensemble guidance
varies considerably by early next week as well, further casting
doubt on expected outcome. General thoughts are for a
progressive h5 low to track out of the central Rockies and into
the local area beginning Saturday. Progression may be stymied by
downstream disturbance so track may be slow through the area.
This supports a long duration rain event across at least
portions of the area along with cooler temperatures. NBM
probabilities of > 0.25" in a 24 hour period increase quickly
through the day Saturday and peak Sunday morning (for the
previous 24 hours) across eastern Nebraska where a bullseye of
70% centers over the LNK/OMA corridor. While values decrease
with westward extent, probabilities still exceed 50/50 for much
of the local area so potential remains nominal locally. NBM
probabilities decrease through the day Monday and this coincides
with the greatest h5 negative anomalies quickly filling in
across the central CONUS. Thereafter, uncertainties exist with
evolution of upper-level pattern. Generalizing, ensemble
solutions suggest ridging in the west and troughing into the
east with trends strengthening each respective feature. This
favors above normal temperatures and drier than normal
conditions, which closely matches the latest Climate Prediction
Center 6-10 and 8-14 day outlooks.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 618 AM CDT Tue Sep 17 2024

VFR conditions prevail across western and north central Nebraska
through a bulk of the period. Southerly winds will remain strong
through the period with widespread gusts of 25 to 35 kts, with
locally higher gusts up to 40 kts this evening and tonight,
impacting KVTN. Anticipate scattered to widespread TSRA will
track west to east across western and north central Nebraska
between ~00Z to 09Z Wednesday. Localized MVFR/IFR cigs/vsbys
with strong, erratic wind gusts likely near/in any
thunderstorms. Confidence remains meager on exact timing and
specific site impacts, including strength of thunderstorm
winds. As confidence increases with subsequent forecasts,
inclusions and amendments will likely occur.

&&

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

&&

$$

SHORT TERM...NMJ
LONG TERM...NMJ
AVIATION...Viken