847
FXUS63 KGID 251031
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
531 AM CDT Sat May 25 2024

.KEY MESSAGES...

- Mainly dry, mild and breezy to start the holiday weekend
  today, but chances for thunderstorms increase to 20-50% this
  evening, and 40-70% overnight.

- Some of this activity may be strong to severe with risk levels
  ranging from Marginal N of Hwy 91 (level 1 of 5) to Enhanced S
  of Hwy 24 (Level 3 of 5) per the SPC. Main concerns are large
  hail and damaging wind gusts, though isolated tornadoes and
  flash flooding are also possible.

- Isolated to scattered non-severe thunderstorms may redevelop
  Sunday afternoon, but Memorial Day itself looks to be dry,
  albeit a bit breezy out of the NW.

- Dry conditions continue through Tuesday daytime, but off and
  on thunderstorm chances return Tuesday night and continue
  through the end of the week.

&&

.DISCUSSION...
Issued at 515 AM CDT Sat May 25 2024

Today-Tonight:
Inserted some mention of sprinkles for various portions of the
forecast area this morning, but most areas will just experience
some incr mid to high clds and incr Srly flow. Gusts up to
around 30 MPH are possible throughout the day, which will make
the water on area lakes a bit choppy. Highs should range from
mid 70s N where clds will be thickest, to low to mid 80s far S,
closer to retreating warm front and thinner high clouds. Outside
of the rogue sprinkles, not seeing much for chances of rain
before 5pm - so not a bad start to the extended weekend.

Main focus of the short term period will be incr chcs for
thunderstorms this evening, beginning around 6-8PM W/SW of the
Tri-Cities, then 9PM-midnight for the rest of the forecast area.
So the majority of the severe threat will come around or after
sunset. Models remain in good agreement of the general pattern
evolution today, but vary with some key details that leads to
considerable uncertainty on exact coverage and severity. There`s
been little to no change in the convective outlook from the SPC
and risk levels still vary from Marginal Risk (level 1 of 5)
over the N, to Enhanced Risk (level 3 of 5) closer to Hwy 24 and
I-70. Still think the main risks will be large hail up to golf
ball size with initial and/or discrete activity, then
transitioning mainly damaging wind gusts of 60-70 MPH from
possible upscale growth into lines or clusters. Isolated
tornadoes will be possible with possible discrete supercells in
the Enhanced Risk area this evening, OR from QLCS spinups along
potential strengthening line of storms that moves W to E
overnight - somewhat similar to Thu night. Heavy rain is not a
widespread concern and most activity should move along at a
decent clip, but can`t rule out some isolated/minor flooding
over far SE zones. Most thunderstorm activity should end from W
to E around 06-09Z.

With the general forecast thoughts covered above, will attempt
to provide some greater details of possible scenarios on
convective evolution. Will start by saying that this still
appears to be a fairly late show for the local forecast area due
to timing of upper shortwave and delayed return of moisture. I
think our severe threat will emanate from some sort of
combination of two convective "regimes": 1) discrete or semi-
discrete supercells developing along retreating warm front over
central KS and 2) gradual upscale growth of convection that
originates over the High Plains of NE Colorado and Nebraska
Panhandle, then moves E/ESE along nose in intensifying LLJ and
theta-e advection.
1) This regime is probably the more uncertain of the two given
model discrepancies in the latitudinal placement of the
retreating warm front and associated timing of moisture return
as it relates to the arrival of incoming shortwave. Various CAMs
offer a relatively small window from around 00Z to 03Z for
possible iso supercell development somewhere in the area of I-70
to around Hwy 24. If a storm or two can develop, then
supercells are likely - possibly quite intense - given
combination of strong deep layer shear and moderate instability
on the nose of sfc dew points incr from the 50s into at least
low-mid 60s. Margin of error so to speak for this scenario is
small as sfc based parcels will probably remain capped beneath
H7 temps 9-11C if moisture return is delayed even slightly as
nocturnal BL cooling will set in especially beyond 03Z. However,
this time frame also offers a potentially significant
juxtaposition in time and space of enlarging low level
hodographs in response to backed sfc flow and developing LLJ,
lowering LCLs, and arrival of better upper ascent - all
potentially favorable for very large hail and a few tornadoes.
Gut feeling is that this regime will probably remain S/SE of the
forecast area, but it could be close if CAM solutions such as
00Z HRRR/ARW/NSSL are accurate. 2) This regime feels like a
safer bet as nearly all CAM guidance depicts at least scattered
high-based convection developing over the High Plains in
environment of very steep low to mid level lapse rates on the
nose of sfc thermal ridge and beneath increasingly favorable
upper forcing/height falls. Seems probable this activity would
then propagate to the E/ESE into central NE/KS during the late
eve and overnight within zone of strengthening theta-e
advection on nose of LLJ. Magnitude of severe potential from
this regime is a bit uncertain as MLCAPE will be limited
initially by poor moisture and the better instability doesn`t
arrive until around or after 03Z for mainly slightly elevated
parcels. However, if a significant cold pool develops, then an
organized MCS/QLCS could remain sfc based while tapping into the
greater theta-e with time and SEward extent, as well as
enlarging hodographs thanks to LLJ. Again, think some version of
this regime is most likely, with the main lingering questions
being the magnitude/coverage of mainly a wind threat. Similar to
Thu night, though, some QLCS tornadoes could also be possible
if the line remains sfc based given favorable 0-3km shear/CAPE.

Regardless, with it being an extended weekend at the beginning
of summer, there will likely be increased activities at area
lakes/campgrounds which can be especially vulnerable to
overnight intense convection with tornado/damaging wind risk.
The forecast will continue to update/refine throughout the day,
so those folks will want to remain informed to those updates.

Rest of the Holiday Weekend:
Most areas should wake up to dry conditions Sunday AM, but
redevelopment of at least iso to widely scattered thunderstorms
appears probable by early aftn as area remains under the
influence of upper shortwave and cool mid level temps will
support uncapped, tall/skinny CAPE. Deep layer wind shear is
quite weak, so not expecting anything more than heavy downpours.
Model guidance is mixed on coverage and W/SW extent of the
potential, but all areas will have at least a chc, and this
could again affect outdoor activities with a lightning threat.
Cloud and cool NW flow will keep temps in the 70s, perhaps even
60s if the aftn convection is more than isolated. Fortunately,
appears the rest of the holiday weekend will be dry and
seasonably mild, though a bit on the brzy side Memorial Day
Monday, which could affect those trying to get out on the lakes.

Rest of the Forecast:
Dry conditions continue through the daytime Tuesday with mild
temps in the 70s/80s. Warm air advection could kick off some
showers/elevated storms Tue night for areas particularly W/S of
the Tri-Cities. This looks to mark the beginning of another
modestly active period of off and on thunderstorm chances that
continue through the end of the week. However, any significant
return of instability looks to hold off until at least Thu - and
no particular time frame looks unusually ominous from a severe
weather perspective. A stronger trough/frontal passage is
progged in the ensemble guidance for sometime Fri-Sat, which
should provide the greatest chance/coverage for showers and
thunderstorms in the days 3-8 time frame.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1245 AM CDT Sat May 25 2024

VFR forecast through the period.

Clear skies currently will give way to incr high clds through
the morning. SCT to BKN high clouds expected much of the day.
Scattered showers and thunderstorms will become possible around
and esp. after sunset as a cold front moves in from the NW. Have
introduced VCSH around 03Z-04Z for now as coverage and exact
timing is still uncertain. Lgt Srly flow now will give way to
breezy conditions after sunrise. Sustained winds will incr to
15-20kt, with gusts up to around 30kt. Winds will back to the SE
and E Sat eve ahead of the cold front. Confidence: Medium.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...None.
KS...None.

&&

$$

DISCUSSION...Thies
AVIATION...Thies