Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 231002

Area Forecast Discussion
National Weather Service Hastings NE
502 AM CDT TUE AUG 23 2016

.SHORT TERM...(Today and tonight)
Issued at 502 AM CDT Tue Aug 23 2016

Clearly the paramount concerns during these next 24 hours
involves trying to pin down the likelihood/placement/severity of
thunderstorm chances. In short, we appear to be looking at two
fairly distinct rounds of storm potential: the ongoing one this
morning and then a separate, probably more vigorous/potentially
severe round late this afternoon/evening that would primarily
favor northern/eastern zones versus the south/west.

We note that the SPC Day 1 outlook changed fairly little from the
previous Day 2, focusing the official Slight Risk mainly within
the eastern half of the CWA (and points east from there) with the
Marginal extending back to the west and including all but our
extreme western counties. That being said, compared to 24 hours
ago, confidence in the overall coverage (and to some degree
likelihood) of severe storms within our CWA borders has probably
waned a bit if anything. Models seem to be more of "split mind"
regarding severe chances, with some such as the GFS and previous
00z NAM showing a bit greater potential/coverage, while others
such as the 00z ECMWF, latest RAP13 runs and latest 06z NAM
somewhere between barely developing potentially severe storms in
our far northern/eastern fringes, or essentially missing our CWA
altogether and instead focusing a higher-confidence severe threat
within the OAX CWA of eastern Neb. So, even at this fairly close
range, the question still remains: will our GID CWA deal with at
least "some" severe weather threat this afternoon/evening or not?

Turning to a quick look at the current scene as of 09z/4 AM:
Despite not even having an official mention of late night/early
morning storms in our fcst 24 hours ago due to lack of confidence
in development, parts of the western/southwestern CWA have in
fact experienced a fairly concentrated- but-potent little storm
cluster/mesoscale convective system (MCS) that continues shifting
slowly north-northeast at this hour through the LXN/EAR/HDE areas.
Although radar has occasionally shown signs of small hail/gusty
winds, thus far this cluster has remained pretty solidly sub-
severe and is only dumping some quick, decent rains of generally
0.50" to perhaps 1.50". This storm cluster is basically being
driven by a combination of elevated moisture advection within a
fairly stout southwesterly 40-50 knot low level jet evident at 850
millibars, along with halfway decent mid-upper forcing provided by
a low-amplitude, positive tilt shortwave trough gradually working
itself through the heart of the local area in modest southwesterly
flow aloft, well to the south of a more potent/powerful closed low
churning into southern Saskatchewan. At the surface, steady south
breezes persist across the local area in the pressure gradient
between an expansive eastern CONUS high and a High Plains lee
trough to the west. Low temps appear on track to bottom out mid-
upper 60s most areas.

Now looking ahead forecast-wise...

Early this morning (through around sunrise/7 AM):
Pretty much just extrapolating radar trends and following trends
from the latest NAM, HRRR etc., it`s evident that the
aforementioned little storm complex will continue riding to the
north-northeast into areas mainly west and north of Grand Island.
While it appears most areas southeast of the current rain shield
may stay dry, would not be surprised to see some east-
southeastward expansion into more of the area.

Daytime/evening hours:
Very basically, we are following the premise that there will
probably be a fairly evident lull between the departing morning
activity and possible (key word "possible") redevelopment of late
afternoon activity. Starting with this morning, despite perhaps to
around 1000 J/kg elevated CAPE to work with, and am not currently
expecting severe storms to evolve from the current complex as it
continues tracking northeast and likely exiting northern/eastern
portions of the CWA by mid-late morning in association with the
leading portion of the aforementioned mid- level wave. Storm
intensity will need watched though, as sometimes these morning
complexes can throw a few "surprises" especially along their
southern/western flanks. Otherwise for the early-mid part of the
day, southerly breezes will again be on the increase, and although
not as windy overall as yesterday, expect several hours of
sustained 15-20 MPH with higher gusts in most areas, except
lighter in the far west near an approaching pre-frontal trough.
Sky cover is a bit uncertain for the day as a whole, but generally
expect mostly sunny/partly cloudy central and west behind the
departing storms with eastern areas more prone to hang onto more
clouds. Overall though, made very little change to high temps,
aiming from mid-upper 80s central and east to low-mid 90s
south/southwest. Then this afternoon, once again, will there be
potentially severe storm re- development? Although
dewpoints/instability will be on the rise, raw NAM CAPE values
appear a bit overdone but following the more conservative GFS
still yields a healthy 2000-3000 J/kg mixed-layer CAPE across much
of the CWA by peak heating. However, deep-layer shear is
relatively weak at largely under 25 kt, so storm mode would tend
to favor multicells more than supercells, although brief supercell
structures certainly possible. One possible limiting factor for
our CWA is that a good chunk of the forcing provided by the mid
level wave will largely be departing east by mid-late afternoon,
possibly putting our area in a zone of weak subsidence and instead
focusing most of the "action" to the east. That being said, with
potent CAPE and low-level forcing starting to increase into our
far north along the incoming cold front, cannot discount at least
limited coverage of strong/severe development especially within
our north and east counties, with higher-res solutions such as the
00z NSSL WRF particularly aggressive showing a possible MCS
tracking south-southeast through much of the CWA especially in the
7 PM-Midnight time frame. With so much model disparity, we can
just no longer justify "likely" PoPs in any given area, and thus
have a range from higher chance northeast to only slights
southwest. Main threats with any severe storms generally hail to
around ping pong size and winds to around 60 MPH. Probably cannot
completely rule out a brief spin-up near our eastern edge, but
probability appears very low and unworthy of a formal HWO

Late tonight (generally post-midnight):
Although some models such as the NSSL WRF and GFS show that our
far eastern/southeast edges could still see some limited and
potentially strong activity into the late night hours, the overall
model consensus is that most of the CWA becomes dry/storm-free.
This is generally because low level flow veers ahead of the
invading cold front and our CWA continues to get farther behind
the main push of forcing from the departing wave, while forcing
from the next upstream wave still remains to our west through the
night. That being said, with decent elevated CAPE lingering even
behind the incoming cold front, wanted to at least acknowledge a
low chance for a late night shower/storm within mainly the
southeast 2/3 of the CWA. Otherwise, north-northeast surface
breezes will gradually build into more and more of the CWA as the
night goes on behind the front, ushering in drier air as well. By
night`s end, there could be a decent temp and dewpoint gradient in
place, with overnight lows aimed from upper 50s northwest (with
40s/low 50s dews) to upper 60s southeast (with mid 60s dews).

.LONG TERM...(Wednesday through Monday)
Issued at 502 AM CDT Tue Aug 23 2016

West southwesterly flow upper level flow is expected to prevail
across the local area through the extended periods with multiple
disturbances bringing a chance for showers or thunderstorms to at
least parts of the local area each day.

Starting off Wednesday...expect northerly flow behind a passing cold
front to overtake the region with high temperatures struggling to
climb much above 80 degrees across the majority of the local area.
As this weak cold front stalls near the NE/KS state lines early
Wednesday morning...expect it to become the focus for shower and
thunderstorm activity through the day. WHile there will be a chance
for showers and thunderstorms across nearly the entire forecast area
throughout the day...the best chance for any strong/severe
thunderstorms will lie generally south of interstate 80...where the
better instability and shear will reside. Not surprisingly, much of
this area is either highlighted in a marginal or slight risk for
severe thunderstorms Wednesday afternoon/evening. While severe
parameters are fairly marginal...quarter size hail and 60 mph wind
gusts will both be possible.

As this frontal boundary shifts further south on Thursday...
thunderstorm chances will be mainly limited to portions of north
central Kansas...ahead of the next upper level disturbance
anticipated to eject out of the Rockies on Friday. With global
models in fair agreement on the timing and location of this next
disturbance...model blends introduced likely chances for
thunderstorms Friday night and opted to maintain this potential in
the morning forecast. In addition...with more significant
instability and shear...this could also become another opportunity
for some severe thunderstorms...but did not highlight the potential
in the HWO just yet as there is plenty of time for this to change.

Otherwise...additional chances for daily thunderstorms extend into
the early portions of the new work week as multiple upper level
waves traverse the plains. These chances for thunderstorms will be
accompanied by near seasonal temperatures...with high temperatures
over the weekend and early next week generally running in the lower
to the middle 80s each day.


.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Tuesday night)
Issued at 131 AM CDT Tue Aug 23 2016

General overview:
Although VFR ceiling/visibility is officially advertised
throughout the period and is likely to prevail the vast majority
of the time, there are a few caveats involving mainly the
possibility of brief sub-VFR conditions in either thunderstorm
activity, brief lower stratus, or both of these. In addition, low
level wind shear (LLWS) remains an issue right away this morning
during these first 6 hours or so. Moving on to more specifics...

Have maintained VFR for now for both elements, but some
models/guidance suggests that a period of MVFR stratus could
develop and potentially persist for several hours mainly during
the daylight hours. Confidence just isn`t there to "bite" on this
yet though. Otherwise, obviously any passing thunderstorm
activity could at least briefly reduce especially visibility below
VFR, and this will need monitored especially late this

Thunderstorm potential:
There appear to be two windows of opportunity for possible
thunderstorm activity, and both of these have been covered with a
basic/generic "vicinity" (VCTS) mention for now, as confidence in
likelihood/occurrence is still too low/too far out in time for
anything more. The first of these windows appears to be 09z-14z
this morning as scattered activity spreads through the area within
a low level jet axis (KEAR likely being slightly more favored),
while the second window is focused from 22z-04z late this
afternoon/tonight. This latter possibility could feature a severe
storm risk as well with hail/strong winds possible.

Winds/low level wind shear:
Right off the bat early this morning through 12z will maintain a
mention of LLWS as roughly a 30kt shear difference exists between
the surface and roughly 1200 ft, where southerly speeds are
topping out around 45kt. Otherwise, surface winds will remain
fairly breezy from the south through much of the period, with gust
potential over 20kt for much of the day. However, confidence in
wind trends decreases by evening. Although an eventual switch to
prevailing northerly breezes should occur behind a passing front,
the presence or lack of thunderstorm activity could easily alter
the local wind fields noticeably from current expectations,
including the possibility of brief very strong gusts.


.GID Watches/Warnings/Advisories...


SHORT TERM...Pfannkuch
AVIATION...Pfannkuch is the U.S. government's official web portal to all federal, state and local government web resources and services.