Severe Thunderstorm Warning 287, NWS Hastings, NE

Area Forecast Discussion
National Weather Service Hastings NE
640 AM CDT WED AUG 24 2016

.SHORT TERM...(Today and tonight)
Issued at 445 AM CDT Wed Aug 24 2016

The primary forecast challenge of these next 24 hours revolves
around trying to adjust/refine what will likely be a rather sharp
gradient between rather low/(almost non-existent for some)
measurable rain chances (PoPs) in our Nebraska zones, versus
better rain chances in KS zones (especially our southern- most 3
counties). If anything, this latest forecast package has toned
down PoPs a bit from the previous version, but quite honestly, if
a few of these very latest models including the 06z NAM and the
latest RAP13 run are onto anything, then we may very well still be
hitting the throttle too hard on PoPs for even much of the
southern CWA.

As for severe storm prospects, fully support SPC having removed
the Slight Risk area from our far southeast that had been on the
previous Day 2 outlook, and quite frankly, if some of the same
aforementioned latest model instability progs are onto anything,
would not be surprised to see later outlooks eat away at parts of
the existing Marginal Risk area as well. It's just getting harder
and harder to envision a legitimate severe weather threat even in
our far southern CWA, and this decreasing threat is also evident
in the last few runs of the SREF calibrated severe probs. All this
being said, at least for now, will maintain "national vs. local"
product consistency by hanging onto some low-end severe mention
in our southern zones in our Hazardous Weather Outlook (HWOGID).

Reviewing the past several hours and current situation as of 09z:
As was generally expected here 24 hours ago, it in fact turned out
that the vast majority of late afternoon/overnight severe
storms/heavy rainfall targeted areas slightly east of our CWA in
eastern NE/northeast KS, while we only had a few isolated cells
flare up before sunset including one briefly severe-warned in
York/Hamilton counties. After dark, other than a few brief showers
that clipped Polk County, the entire night has been quiet/dry
within our borders, with the main story being the steady invasion
of noticeably-drier air (50s dewpoints) advecting southward behind
a cold front that has now reached all but the far southeastern
CWA. Behind this boundary, most areas are seeing a steady light
north/northwest breeze of 5-12 MPH, although some far eastern
sites are reporting more of an easterly component likely due to
outflow emanating from eastern NE storms. Ahead of the front,
more humid dews well into the 60s are still holding on at sites
such as Hebron/Beloit. In the mid-upper levels, water vapor
imagery and short term model data also confirm expectations of 24
hours ago, as our CWA is experiencing a lull in forcing between
shortwave disturbances embedded within persistent southwesterly
flow aloft, with the departing wave now mainly over IA, and the
next upstream one approaching from the CO area, and responsible
for scattered showers near the CO/KS/NE border area. Well to our
north, a stronger closed "parent" low to the broad trough over the
central CONUS is churning east toward southern Manitoba. Back at
the surface, overnight lows appear on track to range from mid-
upper 50s far northwest, low 60s central, mid-upper 60s
southeast...although a gradual increase in mainly high level
clouds could hold up a few spots from reaching full cooling
potential.

Now looking ahead forecast-wise over these next 24...

Today (through roughly 00z/7 PM):
As mentioned in the opener, the overall thinking is that even our
slightly-toned down PoPs may still be too aggressive, as the
majority of the CWA is probably going to remain dry the majority
of the day. No matter what, the farther north one goes the higher
the odds of staying dry, while the farther south (especially south
of the state line) there are at least modest chances for passing
showers/a few thunderstorms. In the mid-upper levels, the next
batch of modest forcing and increasing high-level flow will
gradually move toward the CWA in the form of a pair of shortwave
troughs, one drifting mainly east out of CO and another tracking
more northeast out of the TX Panhandle area. However, with the
majority of appreciable convective instability now looking to
focus at least slightly to our south/east, it's probable that our
main feature today will be probably be no more than increasing
mid-high cloud cover, and although the worded forecast products
are largely "mostly cloudy" by this afternoon may not have hit sky
cover hard enough for parts of the area. As for PoPs/rainfall
chances, have confined nearly all morning chances (only 20-30
percent at that) to our KS zones. Then this afternoon, have these
very low chances extending about as far north as I-80, but have
confined the highest 50-60 PoPs to our far southeastern counties
mainly in KS. Although it's looking doubtful that mixed-layer CAPE
will average more than 500-1000 J/kg at best in our southeast,
increasing deep-layer shear to at least 30-40 kt could potentially
support a few strong/marginally severe storms with quarter size
hail the main threat. Have geared thunderstorm wording toward
"isolated" to try to downplay the overall-lightning coverage a
bit. North of roughly I-80, kept afternoon PoPs below mentionable
15+ percent thresholds, but did acknowledge a generic "slight
chance of sprinkles" just in case increasing mid-level saturation
is able to squeeze something out. Some models suggest parts of
western/north central NE could see a band of steady light showers
develop, but this activity appears to focus at least slightly
west/north of our CWA. In other departments, with the surface
front to our south and the pressure gradient largely dictated by a
strong high centered well off to our northwest over the Northern
Rockies, expect breezes to generally average 10-15 MPH from the
north- northeast (trending more easterly with time especially in
southern zones). High temps are a bit tricky as most
model/guidance sets look either a bit too warm/too cool depending
on how aggressively they are/are not accounting for the expected
increase in sky cover. Thus played a general middle ground will
resulted in fairly little change from previous, aiming for right
around 80 in the south/west and low 80s north/east where
clouds/precip are expected to be less of an issue. No matter what,
most areas expected to be anywhere from 6-14 degrees cooler than
yesterday. There will likely be a pretty decent dewpoint gradient
as well today, with mid-afternoon values aimed from very
comfortable upper 40s/low 50s north and northwest, to more
noticeably humid 60s in especially the southeast 1/4 of the CWA.

This evening/overnight:
Overall, the situation changes relatively little from the daytime
hours, with our best rain chances (currently no higher than 50-60
percent) focused within KS zones, and with lower mentionable
chances extending north up to around I-80, with very little if
any chance for anything more than maybe sprinkles north of there.
It's worth noting that the 06z NAM backed off from it's previous
run's particularly-aggressive coverage of rainfall tonight well
north of the state line and now looks more like most other models
with the best chances confined to our KS zones and especially
points just south from there in closest proximity to the
surface/near-surface frontal zone. Cannot completely dismiss
chances for potentially a few heavier showers/isolated storms
though in our south given a continued stream of low-amplitude
shortwave lift from the southwest. Could possibly see a marginal
severe threat flirt with our far south into the evening, but seems
pretty unlikely post-midnight. Breezes tonight expected to average
no more than 5-10 MPH northerly. As with highs today, lows are a
bit tricky as it will be a battle between seasonably low
dewpoints and what should be rather plentiful cloud cover. Because
of the expectation of appreciable sky cover, nudged up lows 1-2
degrees from previous (especially south), but still calling for
slightly below-average values ranging from mid-upper 50s
northwest, near-60 central to mid-60s far southeast.

.LONG TERM...(Thursday daytime through Tuesday)
Issued at 445 AM CDT Wed Aug 24 2016

The first part of the long term forecast is the main focus of
precipitation, after which short wave ridging increases behind a
passing trough and should bring warmer and drier weather to the
last half of the long term forecast period.

Continued low/small precipitation chances across the southern
forecast area...mainly Kansas...on Thursday with wave sliding
along/near stalled boundary. Frankly speaking this will likely be
adjusted as near term high resolution models start to latch onto the
system better. Below normal temperatures expected with thickening
cloud cover.

Friday and Saturday seem to bring the best shot for precipitation to
the area swinging its gate through the Central Plains, and some
southern stream energy working through Kansas. This pattern is dicey
though for giving high confidence rain chances as the models are far
from agreeable and have shown adjustments from run-to-run. While
the risk for rainfall is higher, severe weather risk does not appear
overly high with limited instability and shear not overly
impressive. Instability limited in part by increasing clouds and
temperatures below season normals...especially Friday and potentially
holding in the 70s.

Sunday brings the passage of the upper level trough to the north and
what should be a significantly lower risk for precipitation into the
middle of next week. Look for upper level short term ridging to move
into the NE/KS region. Inherited low risk precipitation chances from
WPC and while few changes were made, believe those chances are
likely overdone and the risk for precipitation Sunday afternoon
through Tuesday is pretty darn low. Temperatures will rebound to
above seasonal normals but remain in the 80s, a very typical range
for late August. Lows generally in the 60s...and winds in a nice
southerly 8 to 15 mph range for most of the period.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Thursday morning)
Issued at 640 AM CDT Wed Aug 24 2016

General overview:
Confidence is rather high in VFR ceiling/visibility throughout the
period...with only increasing mainly mid-high level clouds.
Although a passing shower/storm cannot be completely ruled out
mainly beyond these first 6 hours, the majority of the latest
models/guidance keep the bulk of the primary precipitation
potential at least 50 miles south of KGRI/KEAR so will leave out
any convective mention at this time. Surface winds are expected to
maintain some variety of northerly component through the period
(mainly ranging anywhere from 340-040 degrees). The overall
strongest speeds...with gust potential generally 15-20kt...should
focus during the heart of the day, generally 15-22z.

&&

.GID Watches/Warnings/Advisories...
NE...None.
KS...None.
&&

SHORT TERM...Pfannkuch
LONG TERM...Moritz
AVIATION...Pfannkuch

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