Forecast Discussion
NWS Omaha/Valley, NE

000
FXUS63 KOAX 271125
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
625 AM CDT Sat Apr 27 2024

.KEY MESSAGES...

- Severe thunderstorms Today and Saturday with strong tornadoes,
  large hail, damaging winds, and flash flooding possible.
  Greater flooding threat on Saturday night.

- Severe thunderstorms possible on Sunday as well, to a lesser
  degree, but brief tornadoes possible as well.

- Storms wrap up by Monday, with warmer temperatures arriving
  next week. Another round of showers and storms possible
  Tuesday/Wednesday.

&&

.DISCUSSION...
Issued at 330 AM CDT Sat Apr 27 2024

Multiple violent tornadoes scoured the forecast area yesterday,
leaving a path of devastation in their wake. Multiple damage surveys
will be conducted on Saturday in order to begin to determine the
number and intensity of each tornado. Overall, the surveys will
take days, if not weeks to complete, with 2 more days of severe
weather waiting on our doorstep. Our hearts go out to all of
the people that have been impacted by Friday`s event.

We had a brief reprieve tonight to catch our breath, as storms
festered off to our east, over eastern Iowa. A frontal boundary
attendant to the surface low associated with Friday`s storms has
sagged south of forecast area overnight. Quiet weather will prevail
through the morning hours on Saturday, before that boundary lifts
north as a warm front ahead of the next low pressure system. Round
two of severe weather will ignite along this boundary near the
Nebraska/Kansas border by around 2 PM Saturday. Large hail,
strong winds, and potentially a strong tornado or two will be
possible once again as these storms lift north toward I-80. The
boundary will eventually stall, bisecting our CWA Saturday
evening. With PWATs approaching the 90th percentile,
climatologically, we could see some heavy rain production from
these storms. While it depends on how far north the front is
able to travel, should these storms overlap the location of
Friday`s heavy rainfall/saturated ground, areas of flooding
would be likely. Therefore, we opted to hoist a Flood Watch from
locations along and north of a line from Seward, NE to Red Oak
IA for Saturday afternoon through Sunday morning.

Our third and final round of severe storms (at least for the
immediate future) arrives Sunday as the surface low passes over
eastern Nebraska and southwest Iowa. The greatest chance for severe
storms looks to be just to the south east of our forecast area
SUnday afternoon and evening. however large hail, strong winds, and
perhaps a few tornadoes could clip far southeast Nebraska and
southwest Iowa.

Storms finally exit the region late Sunday night/early Monday
morning. Overall, our pattern remains active through next week, with
multiple chances for showers and storms beginning Tuesday and
persisting through next weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 625 AM CDT Sat Apr 27 2024

Main forecast concern will be thunderstorm timing this afternoon
into this evening as a line develops near the NE/KS border and
pushes north. Currently included -TSRA mention at all sites for
2 hours based on timing showed by latest guidance, though exact
timing could very easily change. In addition, some additional on
and off storms could persist into the evening and overnight
hours, though kept those as -SHRA mention for now. Once again,
storms could be severe, with large hail, damaging winds, and
tornadoes all possible. Otherwise, expect some SCT to BKN MVFR
clouds at OFK through the period. Widespread MVFR to IFR
ceilings then look to move in by this evening. Winds will start
out of the northwest and gradually become northeast, outside of
any storms. Speeds will generally be in the 6 to 12 kt range,
with some gusts of 20 to 25 kts at OFK later in the period.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Flood Watch from 4 PM CDT this afternoon through Sunday
     afternoon for NEZ015>018-030>034-042>045-050>053-065>067.
IA...Flood Watch from 4 PM CDT this afternoon through Sunday
     afternoon for IAZ043-055-056-069-079-080.

&&

$$

DISCUSSION...KG
AVIATION...CA

NWS OAX Office Area Forecast Discussion

Forecast Discussion
NWS Hastings, NE

000
FXUS63 KGID 271038
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
538 AM CDT Sat Apr 27 2024

.KEY MESSAGES...

- Although a completely different "animal" than Friday`s event
  (warm front driven instead of tightly-wound upper/surface low),
  here we go again on a severe weather threat, with
  particularly counties along/south of the NE/KS border of
  greatest concern this time around where an Enhanced Risk
  (level 3 of 5) exists from SPC.

- While all severe weather hazards are possible today-tonight
  within our coverage area (CWA), the main threat for very large
  hail and possible tornadoes should focus within the Enhanced
  Risk area in our south (a strong tornado even possible if a
  supercell tracks along the frontal zone).

- Although opted against a formal Flood Watch (see reasoning
  below in separate HYDROLOGY section), spotty
  flooding/possible flash flooding issues cannot be ruled out in
  addition to the severe storm threat.

- Although isolated/scattered storms will occur Sunday daytime,
  fortunately the track of the primary surface low pressure
  center (and attendant cold front) looks to track more south-
  through-east of our CWA (as opposed to directly across it such
  as Friday). As a result, we currently look to largely be
  spared a severe storm...although a few storms with up to
  around 1" size hail cannot be ruled out.

- Following a break in thunderstorm chances mainly Sun night-Mon
  night, various low-confidence chances return especially Tues-
  Thurs as an overall-active pattern continues.

- Temperature-wise: nothing unusual for late-April/early-May
  whatsoever, with highs most days somewhere in the 60s/70s and
  lows most nights in the 40s/50s. However, one of the chillier
  nights with more widespread mid-upper 30s is Sun night-Mon AM,
  and some frost cannot be ruled out especially north/west of
  the Tri Cities.

&&

.DISCUSSION...
Issued at 537 AM CDT Sat Apr 27 2024

-- PRIMARY CHANGES TO THIS 7-DAY FORECAST VERSUS THE PREVIOUS
 ONE (issued Fri afternoon):

- Nothing major to speak of. The overall severe weather threat
  for this afternoon-tonight remains largely on track/unchanged,
  and (fortunately) it still looks like we`ll be spared from
  much of a severe storm risk Sunday daytime (albeit possibly a
  close call mainly in our far east).

- In the less impactful department, models continue to offer
  varying ideas regarding temperatures both in the shorter and
  longer term, and based on a blend of various models/guidance,
  highs were actually lowered at least 2-6 degrees in most areas
  today and Sunday, and also for Thurs-Fri.


-- 7-DAY FORECAST OVERVIEW/HIGHLIGHTS (heavily focused on the
 first 24-36 hours):

- CURRENT/RECENT WEATHER SCENE AS OF 430 AM:
As was expected, it`s fortunately been a quiet/dry night in the
wake of an active Friday that started with early-AM heavy rain
and ended up with early-afternoon tornadoes generally along a
path between the Ravenna-Elba-Wolbach areas in our northern CWA
(please note that damage surveys will continue today and we ask
your patience in awaiting these results). In the big picture of
the mid-upper levels, water vapor satellite imagery and short
term model data confirm that we are in a brief lull between
strong upper level low pressure systems, with yesterday`s
disturbance now departed to over MN, while the next one is
currently churning over the Four Corners region. At the surface,
weak high pressure has settled in, which has allowed breezes to
really lighten up and shift more northerly overnight (most
places under 10 MPH at this time). Under mostly clear skies
overall, low temps are on track to bottom out mainly upper
40s-low 50s.

- TODAY-TONIGHT:
Needless to say, spent the vast majority of "mental attention"
on these first 24 hours. In the big picture, the heart of the
incoming upper low will reach eastern CO by early evening, while
a surface low deepens to around 996-999 millibars over southwest
KS. A potentially very sharp, west-southwest to east-northeast
oriented warm front/stationary front will extend out from this
low, initially remaining just south of our CWA in central KS
much of today, before probably lifting north into mainly our KS
zones late this afternoon-evening, and providing a focus for
severe thunderstorm development.

One of various uncertainties with the ultimate magnitude of our
severe threat is just HOW FAR this frontal boundary can push
into our southern CWA. If it happens to remain closer to I-70,
our threat for truly surface-based and potentially tornadic
storms would be muted, but if it happens to lift closer to the
NE border then the currently-advertised Enhanced Risk area
(including 10% "hatched" tornado probs) could stand a better
chance of coming to fruition. Leaning heavily on higher
res/short term model solutions (namely HRRR), the general
expectations for today tonight are as follows:

1) This morning-early afternoon (including temperatures):
The vast majority of our CWA likely remains dry/storm-free,
although a few spotty storms cannot be ruled out mainly in our
KS zones along the far northern fringes of an initially mainly
elevated instability axis. If anything models have trended a bit
cloudier/cooler north of the main frontal boundary today, as we
currently have large areas of lower stratus approaching both our
northern and southern CWA, and expect fairly extensive to
gradually take over north of the front as the day wears on. As a
result, high temps were lowered 2-5 degrees most areas, and we
are now calling for a only low 60s north/west, mid 60s central,
but then a higher likelihood for 70s to even near-80 in our KS
zones in closer proximity to the warm front:

2) Mid-late afternoon into this evening:
Between 2-6 PM, we expect initial (and probably very rapid)
development of storms along/just north of the frontal zone in
our KS zones, which could quickly become severe as either
transient supercells or possibly congeal into a line fairly
quickly. Either way, assuming forecast mixed-layer
CAPE/instability of around 2000 J/kg is realized in the presence
of strong deep layer wind shear of at least 50-60KT, things
could get active very quickly. At least initially, the most
intense storms should remain tied to the frontal zone, while
weaker activity (but still with mainly a hail threat) will lift
farther north into our Neb zones on the cool side of the front.
Please note that most areas especially north of I-80 could
easily make it through most of the afternoon with little to no
activity whatsoever.

Following the initial round of afternoon storms, there MIGHT be
a relative lull before another uptick in convective
intensity/coverage occurs mainly 7-10 PM as the low level jet
increases and the surface boundary tries to lift a little
farther north, but still likely remaining in our KS zones.
During this time frame, an increase in low level shear/helicity
MIGHT lead to an increasing tornado threat, with the HRRR
suggesting that an intense supercell (or two) could try
"latching onto" the front and tracking across mainly parts of
our KS zones. Meanwhile, farther north, more widespread (and not
as intense storms) will overspread much of our Nebraska zones,
with the primary threat likely being hail and localized heavy
rain. If anything, am starting to think that counties especially
north of I-80 may not see much of a severe storm threat at all,
as even elevated instability should largely remain under 1000
J/kg (would not be surprised if SPC perhaps pushes the various
severe risk areas a bit farther south in later Day 1 outlooks?).

3) Late evening-overnight:
Although mainly a transient hail/heavy rain situation could
persist well past midnight, expect the MAIN sever threat to wane
by around midnight, as the aforementioned frontal zone starts
sinking southward again as the main surface low shifts to
in/very near our extreme southern CWA by sunrise Sunday.

- SUNDAY DAYTIME:
Barring a major change in model trends, we are fortunately NOT
expecting a repeat of Friday`s tornadic activity in our CWA, as
instead of the main surface low pressure center tracking
directly over us (and putting much of our area in the warm
sector), it is instead expected to track just barely along and
mainly east of our far southern and eastern CWA borders
(generally from central KS to the NE/IA border over the course
of the day. This should keep the vast majority of our CWA in the
cool sector of this system, in a regime marked by breezy west to
northwest winds. That being said, this surface low track will
need monitored VERY closely, as any westward shift could at
least briefly bring southerly winds into especially our far
southeast CWA around mid-day, which could perhaps result in a
sneaky/brief tornadic threat. However, the more likely scenario
is that we see mainly just isolated/scattered mainly weak
storms, but with perhaps just enough elevated instability to pop
a few stronger updrafts with nickel-quarter sized hail
potential. The bottom line: we agree with SPC keeping the main
severe risk categories just east our CWA. Temperature wise,
highs were lowered a good 3-6 degrees most areas given the
overall-cooler expectations, ranging from only mid 50s northwest
to upper 50s/low 60s central, and mid-upper 60s far southeast.

- SUNDAY NIGHT:
Assuming skies clear efficiently, light winds will result in
what will likely be the chilliest night of the next week, with
widespread mid-upper 30s and MAYBE some frost development
especially north/west of the Tri Cities.

- MONDAY-MON NIGHT:
Although not "guarantee dry", our forecast remains dry as we
reside in between systems.

- TUESDAY-FRIDAY:
Spent VERY little time looking at these periods, but the bottom
line is that periodic chances for rain/thunderstorms remain in
the forecast as various upper level disturbances (of overall
weaker intensity than these last few) pass through the region. A
few surface frontal passages are also evident mainly around the
Tues/Thurs time frames. One or more rounds of strong to at least
marginally-severe storms probably cannot be ruled out between
Tues-Thurs, but details are very murky at this time.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 120 AM CDT Sat Apr 27 2024

- General overview for KGRI/KEAR:
While there is high confidence in VFR conditions through at
least the first 6-9 hours and high confidence in
dry/thunderstorm-free weather through at least the first 15
hours, anything beyond those respective timeframes brings
increasing chances for sub-VFR ceiling and thunderstorms. Aside
from any possible thunderstorm-related enhancement, winds will
not be a major issue, although gusts around 25+KT will be likely
by later in the period especially Saturday evening.

- Ceiling/visibility/precipitation (thunderstorm) details:
Starting with ceiling trends: confidence in VFR and mainly clear
skies these first 6-9 hours, although a few fleeting patches of
MVFR stratus could pass through. However, already by as early as
15-18Z chances for MVFR ceiling will start to increase, but just
how quickly this happens is still in question. For now, have
introduced scattered MVFR mention at 15Z, but do not indicate
prevailing MVFR ceiling until 18Z (stay tuned for adjustments
here). Once showers/thunderstorms arrive by late afternoon/early
evening, in theory there should be a better chance of a more
solid/persistent MVFR ceiling (perhaps even IFR at some point?),
but have seen many instances where convective activity keeps the
low levels "mixy" and at least delays onset of sub-VFR ceiling.
Again something to watch.

Shower/thunderstorm trends (strong storm potential):
Other than maybe a few rogue sprinkles before then, confidence
is high in dry/thunderstorm-free weather until roughly 23Z. From
that point onward, fairly widespread thunderstorm activity could
persist at least in the general area through the remainder of
the period. Although KGRI/KEAR will reside slightly north of the
main severe storm threat area, storms could easily pose at least
a small hail/gusty wind threat. Given these convective chances
are still well beyond the first 12 hours, have simply maintained
a generic "vicinity thunderstorm" VCTS for now.

- Wind details (not accounting for possible convective
  influences late in the period):
The very lightest winds of the period (mainly at-or-below 10KT)
will occur these first 6-9 hours as direction shifts from
westerly to northerly. Then, during the daytime hours and into
the evening, winds will gradually increase out of the
northeast, with sustained speeds commonly 15-20KT/gusts around
25+KT.

&&

.HYDROLOGY...
Issued at 537 AM CDT Sat Apr 27 2024

- REGARDING HEAVY RAIN POTENTIAL THIS AFTERNOON-OVERNIGHT:

Fortunately from a flooding hazard perspective, the vast
majority of our coverage area (CWA) picked up less than 0.50"
of rain during the daytime-early evening Friday. As a result,
for those various parts of our CWA that picked up anywhere from
2-5" of rain during the Thursday evening-Fri AM event, they will
end up having roughly 24 hours (or more) of "drying time"
before the next round(s) of storms moves in this afternoon into
tonight.

As is typically the case with convective rains, confidence is
is not overly-high regarding how this next round will unfold
amount-wise, as much will depend on how efficient rain might be
able to develop and possibly "train" to some degree along and
especially north of the frontal zone that will likely remain
quasi-stationary near and south of the NE/KS border. However,
consensus of various models and a look at HREF probability
products (including 24-hour PMM) suggest that MOST of our CWA
will probably not exceed 1-2" (although localized higher
exceptions are certainly possible).

Considered joining OAX on a Flood Watch mainly for those parts
of our Nebraska CWA that received the most widespread 3-5"
amounts Thurs night-Fri AM, but at least "in theory" expect the
heaviest rain this afternoon-tonight should fall at least
slightly south of the main swath from the other
night...hopefully over a handful of counties especially south
of I-80 that really "missed out" over the last few days. Also, as
we saw the other night most areas that had only 1-3" had few
flooding impacts. For these reasons, along with national
guidance from WPC keeping their initial Day 1 Excessive
Rainfall Outlook category as only Marginal for our CWA, have
opted against a formal Flood Watch for this afternoon-overnight,
but while acknowledging in our Hazardous Weather Outlook
(HWOGID) that at least localized flooding/flash flooding cannot
be ruled out.

&&

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

&&

$$

DISCUSSION...Pfannkuch
AVIATION...Pfannkuch
HYDROLOGY...Pfannkuch

NWS GID Office Area Forecast Discussion