635
FXUS63 KGID 291155
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
655 AM CDT Mon Apr 29 2024

...Aviation and Short Term Update...

.KEY MESSAGES...

- Although cannot 100% guarantee that a few rogue/high-based
  showers won`t try drifting over mainly our KS zones late this
  afternoon-evening, this is UNDOUBTEDLY a dry, tranquil and
  mostly sunny/clear 24 hours for the vast majority of our
  coverage area (CWA), with the only mentionable hazard (for
  plants that is) being the ongoing Frost Advisory through 9 AM
  for much of the western 2/3 of our coverage area (CWA).

- Take advantage of today`s quiet/pleasant weather, because
  especially the Tues afternoon-Thurs AM time frame already
  brings back two separate chances for thunderstorms (some
  potentially severe), although plenty of question marks abound
  with both of these.

- Beyond Wed night-early Thurs AM, there are no "obvious" severe
  storm threats through the rest of the 7-day, but the upper
  level pattern remains active with various disturbances passing
  by and keeping our forecast "littered" with several/low-
  confidence chances for showers and/or storms.

- Temperature-wise: a fairly typical regime for late-
  April/early-May prevails, with highs on most days in the
  60s/70s and lows on most nights in the 40s/50s. The best
  chance for 80s looks to be Tues mainly south and southeast of
  the Tri Cities.

&&

.UPDATE...
Issued at 655 AM CDT Mon Apr 29 2024

- Update regarding recent fog development mainly in our western
  Platte valley area:

Although not expecting to be long-lasting (only a few hours at
most), airport obs and NDOT cams clearly reveal that varying
degrees of fog have materialized in the Platte River valley/I-80
corridor of primarily Dawson and western Buffalo counties (this
is usually one of our most favored areas for "sneaky fog" when
99% of our CWA remains fog-free.

Anyway, have added a stripe of basic "patchy fog" to the
official forecast for the next few hours, but again this issue
should be too fleeting in both areal coverage and duration to
warrant a formal Advisory.

&&

.DISCUSSION...
Issued at 525 AM CDT Mon Apr 29 2024

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

- If anything, added quite a bit of attempted detail to our
  Hazardous Weather Outlook (HWOGID) for the separate
  storm/possible severe weather chances between Tues afternoon-
  Thurs AM, but even so there are still plenty of unresolved
  details to sort through (see below for more).

- Temperature-wise: although not monumental changes by any means:
1) highs for Wed have trended down around 5 degrees especially
in our south (a possible sign that the main warm front might
struggle to lift north?)

2) Highs Friday have trended up around 5-degrees area wide, with
less of a cool push evident in the wake of Thursday`s cold
front.


-- 7-DAY FORECAST DETAILS (heavily focused on the first 3 days):

- CURRENT/RECENT WEATHER SCENE AS OF 4 AM:
Ahhh, what a refreshing break to catch a brief respite from
ongoing or imminent thunderstorms/severe weather threats (as
brief as it may be). Speaking of refreshing, confidence is
fairly high that this ongoing night/early morning will be the
overall-chilliest one of the next week (except for perhaps Thurs
night?). Thanks to a relatively ideal radiational cooling setup
(skies now almost totally clear and light breezes ranging from
near-calm to 7 MPH most places, morning lows are on track to
bottom out between 33-39 degrees across most of our CWA. While
at least brief/patchy frost cannot be ruled out within pockets
of many counties (especially sheltered/low-lying places), our
official Frost Advisory remains in effect for most counties
along/west of Highway 281, which are overall most favored to see
somewhat more widespread frost development particularly between
5-7 AM. In the mid-upper levels, water vapor satellite imagery
and short term model data clearly show our latest departing
upper low churning across the SD/MN border, while we are in the
midst of likely at least 30+ hours of overall-quiet weather
before our next disturbance visible off the Pacific Northwest
coast comes into play.

- TODAY-THIS EVENING:
Other than perhaps some scattered "fair weather" cumulus
expected this afternoon, this will surely be no worse than a
mostly sunny and seasonably-pleasant day CWA-wide, and also
likely a dry day- evening for the vast majority of our area as
transient/low amplitude upper ridging prevails aloft. The only
possible "catch" is that some models (especially NAM-based)
generate a few spotty, high based sprinkles/showers late this
afternoon- evening primarily in our KS zones where some
weak/elevated instability interacts with an initially weak low
level jet. Next few shifts will need to keep an eye on this in
case some small rain chances (PoPs) need introduced/re-
introduced to our forecast in this area, but consensus of most
models either develop nothing or keep it slightly south of our
CWA. But other than this one possible "catch", it`s hard to beat
today`s weather, as winds will be seasonably-light from mainly
a west- southwesterly direction, with speeds (including gusts)
mainly just 5-15 MPH. Afternoon high temps most areas will
improve by a good 10-15 degrees versus yesterday, and made
little change with most areas aimed a few degrees either side of
70, and mid to even a few upper 70s most prevalent in KS zones.


- OVERNIGHT:
Like late this afternoon-evening, cannot completely rule out a
few rogue sprinkles/light showers passing through here or there,
but not enough of a signal to "ruin" our going dry forecast as
slightly better chances for this should reside to our east and
also northwest. However, we will likely at least see a gradual
increase in mid-high level clouds, which combined with slowly-
increasing southerly winds (10-15 MPH toward sunrise) will keep
temperatures tonight considerably milder, with lows aimed
between 48-54 most places.


- TUESDAY DAYTIME-NIGHT:
Most importantly, a POSSIBLE severe storm risk returns, but
latest trends point toward a more brief-in-nature threat within
our particular CWA (especially our far east), with a more
concerning/higher-confidence risk instead taking aim mainly on
the OAX/TOP/ICT counties just to our east-through-southeast.
Will break this period down by topic:

1) Basic upper air/surface situation (including temps):
Aloft, a fairly vigorous, open shortwave trough will quickly zip
from the Central Rockies to the Great Lakes over these 24 hours,
sending a fairly quick-moving surface cold front across our CWA
from west-to-east over the course of the day. The day will start
with breezy south winds area-wide (gusts easily 20-30 MPH), but
as the day wears on this front will pass through from west-to-
east, turning breezes westerly and northwesterly (also breezy
behind the front with gusts 25-35 MPH likely). Ahead of the
front, the southerly winds will draw low level moisture back
northward with dewpoints of at least mid 50s likely (maybe upper
50s?) Among more pressing issues, high temps will depend on the
exact timing of the frontal passage, but have highs aimed from
mid-upper 70s in roughly our northwest half (including Tri
Cities) to low-mid 80s especially in our southeast 1/3.

2) Rain/Severe storm prospects:
Already in the morning, small chances for showers/weak storms
return to mainly our north-northeast zones. However, the million
dollar question is clearly: where exactly do vigorous storms
fire along the front in the afternoon? Consensus of several
models (including a few new 06Z runs) are honing in on a little
faster frontal passage, which would be nothing but good news for
our CWA. Latest higher-res HRRR/NAMNest fire up storms very near
our eastern fringes (Hwy 81 corridor) around 3-5 PM, but then
quickly move them off to the east-southeast of our CWA by very
late afternoon-evening. That being said, a few 00Z model runs
reflected a slower front with storms perhaps initiating as far
west as around Highway 281...putting more of our CWA under the
gun. Given these timing uncertainties, can`t blame SPC for
keeping the west edge of the Day 2 Marginal Risk near Hwy 281,
but if latest trends/consensus holds this could be trimmed east
a bit in later updates. No matter whether storms affect our CWA
or not, they could become severe quickly with healthy mixed-
layer CAPE of 1500-2500 J/kg in the presence of around 40KT of
deep layer shear. There could initially be a few discrete
supercells capable of producing mainly large hail (at least golf
ball size), and if storms congeal quickly into at least a
broken line then damaging winds are on the table too. Given the
relatively high-based nature of storms, the tornado threat could
be muted somewhat, but it`s non-zero for us and a little higher
just to our east. IN SUMMARY: we have a highly conditional
threat for severe storms Tues afternoon in especially our
eastern zones, but it`s also possible we could be spared
completely.

Once storms move out of our area by early evening (if they ever
affect our CWA to begin with), MOST of our CWA will probably
remain dry through the late nigh hours. That being said,
especially southern zones could see some some mainly elevated
convection (perhaps strong?) toward sunrise Wed as a low level
jet kick in.


- WEDNESDAY DAYTIME-OVERNIGHT (through Thurs AM):
At least "on paper" this time frame looks to carry higher-
confidence chances for more widepsread storms and quite possibly
a severe threat...particularly near/south of the KS border
where SPC has focused it`s Day 3 Slight Risk.

Aloft, the next in our parade of upper systems gradually
approaches from the Rockies during the day, with a strong
surface low developing in southwest KS. One of the biggest
question marks for our CWA is whether we see mainly elevated
instability north of the main warm front which is currently
projected to reside near or even south of our far southern CWA
through late afternoon, or whether this front can lift north
closer to the NE border during the evening, making surface-based
storms and even a possible tornado threat more realistic. For
what it`s worth, we just had a situation a few days ago
(Saturday), when the main surface front failed to surge north
into our CWA and muted a higher-end severe threat, but no
guarantees that this happens this time around. There are a lot
of things to sort out before this threat arrives over 48 hours
from now, but particularly the NAM paints a concerning picture
into especially our KS zones with perhaps 3000 J/kg mixed-layer
CAPE, around 40KT of deep layer shear and increasing low
level shear as the evening wears on and the low level jet cranks
up. Whether or not we have a tornado threat depends so much on
how far north this front can make, but with healthy elevated
instability extending well north into our Nebraska counties, at
least a large hail threat could extend up through most of our
CWA. Not to be overlooked, this could also be a good setup for a
decent swath of heavy rains, and although not reflected this
high in our official QPF forecasts (which often under-do
localized convective maximums), parts of our CWA could easily
see 1-3". Fortunately, most areas will have had several days to
dry out by then, but at least localized flooding/flash flooding
is on the table (WPC Excessive Rain Outlook has a good chunk of
our CWA in a Slight Risk).

On less-impactful notes, much of Wednesday daytime could simply
be cool and perhaps even drizzly to the north of the front in
KS in an easterly wind regime, and we could end up with more of
of north-south temperature gradient than currently reflected
(currently aimed from 60s most of Nebraska zones to 70s in KS.


-THURSDAY-FRIDAY:
In the wake of the passing cold front Wed night, Thursday should
be a mainly dry and moderately-windy day with northwesterly
gusts likely 30+ MPH. Latest deterministic models suggest Friday
should be mainly dry as winds flip back around to southerly, but
timing uncertainty at that range keeps low-confidence chances
for rain in our forecast.


-SATURDAY-SUNDAY:
Needless to say, confidence in details are VERY low by this time
frame, as general model consensus is for active west-
southwesterly flow to continue, bringing almost impossible-to-
time shower/thunderstorm chances through our area (and it could
easily remain mostly dry too for some or most places). We have a
lot of days to get through before we can fine tune our going
low-confidence weekend PoPs with any degree of certainty.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 655 AM CDT Mon Apr 29 2024

- General overview for KGRI/KEAR:
For the most part, this will be a "quiet"/uneventful aviation
period, with the main exception being the development of
moderately-strong low level wind shear (LLWS) very late in the
period (early Tues AM). There is high confidence in VFR
ceiling/visibility throughout, and for the majority of the
period winds will be seasonably-light (even gusts mainly under
15KT).

- Wind details:
- Surface winds:
Breezes today will average sustained around 10KT/gusts up to
around 15KT out of a mainly westerly direction. This evening,
there will be a brief period of variable direction/very light
speeds before a gradual increase out of the south- southeast
occurs mainly post-midnight (gusts perhaps 15+KT very late in
the period).

- Low level wind shear (LLWS):
Introduced a formal LLWS group very late in this period
(starting 09Z) as a strong south-southwesterly low level jet
cranks up, with winds in the 1-2K ft. AGL layer accelerating to
at least 40-45KT. This results in at least 30-35KT of shear
magnitude between the surface and this level...plenty worthy of
TAF inclusion.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Frost Advisory until 9 AM CDT this morning for NEZ039-040-046-
     047-060>062-072>075-082>085.
KS...Frost Advisory until 9 AM CDT this morning for KSZ005-006.

&&

$$

UPDATE...Pfannkuch
DISCUSSION...Pfannkuch
AVIATION...Pfannkuch