Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 201136
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
636 AM CDT Tue Jun 20 2017

.SHORT TERM...(Today and tonight)
Issued at 434 AM CDT Tue Jun 20 2017

Following roughly a 3-day respite from any severe thunderstorm
potential, today begins a multi-day stretch during which at least
some part of our county warning area (CWA) could be under the gun
for at least isolated severe. At least for today, the primary
concern should mainly focus within our southwest 1/4 to 1/3
(especially KS zones), with a relatively minimal concern for
severe near and especially north of most of our Highway 6/I-80
corridor counties. That being said, this kind of northwest-flow-
aloft regime is sometimes prone to at least mild surprises, and
would not be surprised if later SPC Day 1 outlooks make some
modest adjustments to the Marginal Risk area (as of this writing
the northeast edge of the Marginal is roughly an Elwood-Osborne
line). In other departments, today continues a gradual warm-up
that should peak in intensity tomorrow, with most of the CWA
expected to top out low-mid 90s (for most of our Nebraska
counties the first 90+ since Friday) with some upper 90s more
favored in our south- southwest.

Taking a look at the recent/ongoing weather scene as of
330 AM/0830Z: As expected, it`s been a quiet/dry and mostly clear
night, with only a smattering of mid level clouds noted streaming
over mainly our eastern zones. In the mid-upper levels, water
vapor satellite and short term model data confirm a pronounced
northwest flow regime, with the Central Plains residing within the
broad interface between a stout ridge anchored over the Desert
Southwest and a large-scale trough with various embedded
shortwaves centered over the Upper Great Lakes/southeast Canada.
At the surface, breezes are near-calm to generally light westerly
across the CWA, with a very weak pressure gradient in place, and
our area loosely positioned between two high pressure centers,
one centered to our south and the other over ND. Thanks to the
clear skies and very light winds, it`s a relatively efficient
cooling night, and lows in most areas should bottom out mid 50s-
low 60s, with typically-colder spots in the extreme north such as
Ord likely flirting with upper 40s again.

Now looking ahead forecast-wise through roughly sunrise
Wednesday...

Today/this evening (through roughly midnight):
As covered in part already above, the main concern is at least
isolated severe storm potential mainly in our southern/southwest
zones. In the mid-upper levels, forcing is actually looking fairly
meager, as there are no "significant" shortwave troughs noted in
the continued prevailing northwest flow, and if anything heights
rise very slightly as the southwest CONUS ridge nudges northeast
ever-so-slightly. Therefore, we need to look in the lower
levels/closer to the surface for our primary forcing, mainly
involving brute afternoon/diurnal heating and weak-modest
convergence in the vicinity of boundaries. Considering that
sustained wind speeds today are only expected to average 10-12 MPH
or less for the most part, the details of wind direction and
subsequent frontal position are actually a little tricky/murky. In
general, while westerly breezes will prevail this morning in most
areas, the afternoon will see more of a northerly-to-easterly wind
shift invade most Nebraska counties as a weak cold front is driven
southward by aforementioned high pressure over the Dakotas.
Meanwhile, more of s southerly/southeast wind direction should set
up near/south of the state line, setting up at least a weak west-
east frontal zone in this area. In addition, a subtle surface
trough will likely extend more north-south near the southwest
edges of the CWA by mid-late afternoon, as weak low pressure
extends eastward off the High Plains to the west. Given fairly
minimal capping/inhibition, the northward return of modest low
level moisture in the form of upper 50s-low 60s dewpoints,
combined with plentiful heating with highs ranging from low 90s
north to upper 90s south, along with the aforementioned
boundaries, should prove sufficient for at least isolated storm
development during the 3-5 PM time frame in/near our far
southwestern CWA. As usual, some models (NAM) seem a little
overdone with instability/CAPE, but even more conservative
solutions such as from the RAP suggest up to around 2500 J/kg
mixed-layer CAPE in the presence of seasonably-strong deep-layer
shear of 40-50kt. Leaning on a mix of the the 00Z high-res NAM,
NSSL WRF and latest HRRR for what appears to be the most favored
scenario, storms should fire up mainly near and especially south
of the state line with peak heating, possibly starting out as
briefly isolated supercells. However, a fairly quick transition to
more of a linear/cluster mode should transpire as activity
develops south- southeastward. Thanks to fairly large
temp/dewpoint depressions, these storms should be fairly high
based, largely mitigating a tornado threat. However, large hail to
around golf ball size and damaging winds are certainly on the
table. While the majority of afternoon/evening storm and severe
potential should focus near/south of the state line, at least
slight chance (20 percent) PoPs do extend about as far north as
I-80, mainly to cover the possibility of outflow from the initial
convection kicking off additional activity farther north, but this
is a lower-confidence scenario. Overall, with the pronounced
northwest flow aloft, storm motion should generally ride to the
south-southeast at 10-20 MPH, in theory also helping to confine
the main threat area to our southern/southwest counties. Timing-
wise, several models suggest that our main near-surface based
severe threat should end as early as 8-10 PM, but again, northwest
flow can by dicey so no guarantees.

Late tonight (mainly after midnight):
In theory (sometimes "famous last words"), the vast majority if
not all of our thunderstorm chances for the night should be over
with by this time, and opted to roll the dice a little and keep
these late night hours void of any formal mention. While a fairly
strong southwesterly low level jet will develop into and east of
the CWA as the night wears on, which would normally be a concern
for additional late night storms, there are 2 factors in play that
are expected to minimize these chances: 1) This low level jet
nose/convergence zone should largely focus to our east-northeast
post-midnight and 2) warmer air aloft will be building in from the
west, in theory limiting further development. In other
departments, late night breezes should become more southerly as
the effective frontal zone lifts back north as a warm front.
Despite what should be mostly clear skies, increased
dewpoints/moisture along with the southerly breezes should result
in low temps holding up 5-10 degrees milder than the current
night, ranging from low-mid 60s north to upper 60s/near-70 south.

.LONG TERM...(Wednesday daytime through Monday)
Issued at 434 AM CDT Tue Jun 20 2017

Northwest to near zonal flow is expected through the period. Along
with this will be several upper level waves that move through the
area and bring on and off thunderstorms through the period.

The first upper level wave will move into the area on Wednesday.
There is a surface low and cold front that is mainly to the
northwest of the forecast area through the day. By afternoon, the
wave has moved into the area and there will be a chance for
thunderstorms. The daytime hours will be quite warm with highs in
the mid to upper 90s. The mid levels are warm and there is a cap
across much of the area. Expect that once the cap breaks in the
afternoon, there will be some severe weather. The MUCAPE decreases
quite a bit toward midnight and although there will probably some
lingering thunderstorms, the severe threat will decrease.

There is another approaching upper wave on Thursday and the surface
front redevelops to the northwest, then moves into the forecast area
during the day Thursday. The front only gets into the northern part
of the area, but as the upper wave approaches, thunderstorms develop
in the north in the afternoon. As the front moves into Kansas during
the evening and overnight, the thunderstorm chances spread across
the forecast area. The thunderstorms continue across the area on
Friday as another wave works into the area. The front will move to
the south and the better chances for thunderstorms will be near the
front and as evening approaches the precipitation should come to an
end. With the front moving through the area, the temperatures will
be a little cooler than Wednesday.

For the period Friday night through Saturday night, the models have
some differences. There are at least a couple of upper level waves
coming through, but the models have some differences in the timing
and how much precipitation there may be. Have kept some mainly low
PoPs for at least parts of the area from late Friday night through
Saturday night. There is little to no CAPE, so have kept the
precipitation as showers. Will have to watch this period to see if
the models continue this trend. This period remains on the cooler
side.

Sunday and Monday are expected to be dry. There is a cool surface
high moving through the area. Skies should be mainly clear and
temperatures will remain cool.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Wednesday)
Issued at 636 AM CDT Tue Jun 20 2017

Confidence is actually pretty high in VFR conditions through the
period, with mainly just limited passing mid-high clouds. While
the chance of thunderstorms at both KGRI/KEAR is not truly "zero"
between the mid-afternoon and evening hours, the greater
potential still appears to reside at least 30-60 miles off to the
south. As a result, still cannot justify carrying even a basic
"vicinity" (VCTS) at this time, but this will need monitored very
closely because any storms that fire in the region could become
severe.

Surface wind speeds should not be much of an issue with sustained
values largely at/below 10kt (unless thunderstorm outflow comes
into play). However, wind direction will exhibit some variability,
initially starting out westerly this morning but eventually
becoming more northerly/easterly this afternoon/evening behind a
weak front and finally more southeasterly overnight. Some models
still suggest a possibility/likelihood of mentionable (30+kt shear
difference) low level wind shear (LLWS) after 06Z, but because
there are some conflicting signals in the details of this shear
and also because it is still 18+ hours away, will defer to the
next TAF issuance(s) to introduce if necessary.

&&

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

$$

SHORT TERM...Pfannkuch
LONG TERM...JCB
AVIATION...Pfannkuch


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