Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 211645

Area Forecast Discussion
National Weather Service Hastings NE
1145 AM CDT Wed Jun 21 2017

Issued at 1145 AM CDT Wed Jun 21 2017

In the last fcst update recently published...high temps were
lowered from Valley County ESE to Polk County. An outflow boundary
from tstms far to the NE has cooled these areas a bit.

Winds are backed at ODX/BVN/OLU. If this cooler air mass doesn`t
mix out and winds remain backed...we will have to watch for
supercell/tornado potential if a tstm develops up there this

UPDATE Issued at 943 AM CDT Wed Jun 21 2017

After looking over and analyzing the 12Z data...the CWA remains in
the warm sector with a sfc trof extending S from Wrn SD thru Wrn
Neb and down the CO/KS border. Sfc dwpts were in the low-mid 60s E
of this trof. The 700 mb temp increased from 13-15C at LBF from
00Z-12Z and from 12-14C at DDC.

Nonetheless...expect deep mixing W of the trof will erode the cap
and tstms will initiate over SW Neb and NW KS near and just W of
the trof in the 2-4 PM time frame. This trof may shift a little E
but should remain just NW of the CWA.

It is interesting to note that svrl members of the NCAR CAM
ensemble delay initiation until 7-9 PM.

Efficient cold pool generation will then trigger additional tstms
sufficient to form one or more tstm clusters that will propagate
E into the CWA where sfc temp/dwpt depressions will still be
sizable (30F)...but considerably higher instability will exist due
to the higher dwpts.

Environment: MLCAPE 2000-3500 J/kg. Winds fields are not strong.
So shear in the cloud-bearing layer will average around 20 kts
over the NW 1/2 of the CWA. Better SE 1/2 with 30-35 kts.

Will keep hail size of golf balls in the HWO (and not go larger)
given the weak shear where tstms initially develop. By the time
they get into the higher instability...believe wind will be the
greater issue.


.SHORT TERM...(Today and tonight)
Issued at 439 AM CDT Wed Jun 21 2017

Clearly the paramount focus of these next 24 hours is on the
potential for at least isolated/scattered severe storms late this
afternoon into this evening, with this being the second in a
three-day string with severe storm potential within our county
warning area (CWA). Unlike the isolated severe storms
yesterday/last night, which were pretty clearly focused near/south
of the state line, the potential for severe storms today is more
prone to affecting a greater percentage of the CWA, and agree with
the initial SPC Day 1 convective outlook in pretty much broad-
brushing our entire domain in a Slight Risk. In other
departments, and has been well-advertised for days now, today
still looks to be the overall-hottest day of the week, and heat
index values in some spots could come rather close to meeting our
official Advisory criteria of 105 degrees (some spots may see
actual high temps pretty similar to yesterday, but with a slightly
higher heat index). If it were earlier in the warm season/if we
weren`t as accustomed to heat, we would have given stronger
consideration to a formal Advisory, but the plan for now is to
just continue emphasizing in the Hazardous Weather Outlook (HWO),
social media graphics etc.

Taking a look at the recent/ongoing weather scene as of
330AM/830Z: Overall the past 12 hours have played out pretty much
as expected. We did in fact see isolated severe storms affect
parts of our southern CWA Tuesday afternoon/evening near the
developing warm front, "highlighted" by a lone storm that tracked
east-southeast through parts of Harlan-Franklin-Webster-
Nuckolls-Jewell counties, and yielding reports of fairly
substantial wind damage in Superior. Once this storm exited, the
late night/early AM hours have been quiet/storm-free within our
CWA, although a small cluster of non-severe storms is presently
tracking southeastward through northwest KS, but is currently
expected to miss our area just to the southwest. Meanwhile, well
off to the northeast, storms are finally starting to flare up
within the low level jet exit region in northwest IA. In the big
picture of the mid-upper levels, things are changing a little from
24 hours ago, as our mean flow is becoming more zonal (west-east)
with time as opposed to northwesterly, although we still remain
solidly northeast of the center of the notable/hot ridge centered
over AZ/NM. At the surface, the front that stalled/developed near
the state line yesterday is lifting northward across the CWA in
response to weak low pressure along the western NE/SD border,
marked by a switch to predominant southerly breezes of 5-15 MPH.
Low temps are on track to hold up in the 65-70 range most areas, a
solid 5-10 degrees warmer than yesterday.

Now looking ahead forecast-wise through roughly sunrise

Today/this evening (through roughly midnight):
Again, severe storm potential is the main focus, and support the
nearly CWA-wide Slight Risk from SPC. Compared to yesterday`s
warm-front-focused isolated severe in our south, today is a
"different animal" in several ways, including the fact that the
main focusing low-level boundary will instead be a north-northeast
to south-southwest oriented trough axis that will be nudging into
our far western/northwest zones by peak heating of late
afternoon/early evening. In addition, today features solidly
warmer mid level temps (evidenced by 700 millibar values of
generally 12-15C). As a result, overall capping/convective
inhibition is stronger and will require more to be overcome. While
nearly all models break this cap on at least an isolated basis
resulting in rapid/explosive severe storm development, it may be
closer to 6-7 PM before the "main event" really starts getting
going (similar to last Tuesday`s event). Backing up a little, will
continue to carry an officially dry/storm-free forecast through
the morning and early-mid afternoon hours, as capping/lack of
forcing should rule pretty solidly. That being said, we will need
to keep an eye on the very light showers/few weak storms currently
roaming through both northwest KS and northern NE to make sure
none of this sneaks into our edges before dissipating and/or
moving away later this morning. Certainly for much of the day, the
main story will be the heat, although fortunately accompanied by a
fairly steady southerly breeze especially in our eastern half.
Temp-wise, it seems we have been under-shooting highs by at least
2-3 degrees all week, so in response have nudged up today`s values
a few degrees higher than maybe would have otherwise. This puts
most of the CWA 96-100, with some far southwestern counties such
as Furnas perhaps even making a run toward 105. With dewpoints in
most areas (except for perhaps far west) expected to hold up
solidly in the 60s even through peak heating, this would make for
near-Advisory heat index values of 100-103 most areas. Wind-wise,
sustained south-southwest speeds in much of the east should
average 15-25 MPH/gusts up to around 30 MPH while western areas
will experience lighter and even somewhat variable winds closer to
the approaching trough axis. By mid-late afternoon into early this
evening (ranging mainly 5-7 PM depending on the model), the strong
heating and modest convergence along the surface trough is
expected to breach the cap, pretty much anywhere along the length
of the western half of the CWA in proximity to the boundary.
Taking a model average, considerable mixed-layer CAPE of 3000-3500
J/kg should lead to fairly rapid severe storm development. While
deep-layer shear on the order of 30kt is not overly-strong, expect
at least a brief window of opportunity for slow-moving supercell
structures, but as cold pools are generated expect a mainly
cluster/linear mode to unfold and track east-southeast into parts
(perhaps much of?) of the central/eastern CWA. Because it`s nearly
impossible to tell at this point whether activity will remain more
isolated or become more widespread (perhaps evolving into an MCS),
rain chances (PoPs) have been capped at no-higher-than 50 percent
for now at any given location. Threat-wise, initial activity could
easily produce hail to golf ball size despite the fairly warm mid-
levels, but with time expect more of a damaging wind threat
(perhaps 70+ MPH) to evolve. While one can never completely rule
out a rogue tornado given such high instability, the high-based
nature of this convection (evidenced by 30-ish degree
temp/dewpoint depressions) along with relatively weak low- level
shear should help to mitigate this aspect. Brief heavy rain of
perhaps 1+" is certainly possible as well, but the fairly
progressive nature of expected convection is not expected to yield
a widespread/organized flooding threat. Given the largely daytime-
heating-driven nature of this convection, expect the vast
majority of the CWA to be storm-free by midnight as activity
departs east.

Late tonight (mainly post-midnight):
As just stated, am again expecting another rather quiet late
night/early morning period for the vast majority of the CWA in the
wake of evening storms, but did linger some slight chance PoPs in
extreme southeast zones for a couple of hours post-midnight just
in case. Unless convective outflow really disrupts the ambient
flow field, late-night breezes should average 5-15 MPH from the
south. Low temps will again be quite mild and similar to those of
the ongoing night, with most places bottoming out 65-70.

.LONG TERM...(Thursday daytime through Tuesday)
Issued at 439 AM CDT Wed Jun 21 2017

A cold front moves through the Nebraska portion of the forecast area
during the day on Thursday. An upper level wave moves into the area
during the afternoon. Expect the morning to be dry, then during the
afternoon there could be some thunderstorms developing along and
north of the cold front. During the evening, the area of
thunderstorms should move to the south as the front continues to
move south of the area during the evening and into the overnight
hours. Expect some of these storms to become severe mainly late in
the afternoon and evening.

By Friday, the front is in central Kansas. There could be a few
lingering showers and isolated thunderstorms Friday morning, but
they should diminish during the day. Friday will be quite a bit

For Friday night through Sunday there is northwest flow and a few,
mainly week, upper level waves that move through the area. There is
also a surface high at the surface. The models bring some
precipitation to at least parts of the area at different times. Not
sure how much precipitation there will actually be. After the front
moves through, winds are from the north and the dew points fall into
the 40s. There are the upper level waves, but there is little
moisture to work with and little instability. Have kept the low PoPs
and have just showers. Temperatures remain on the cool side.

Sunday night into Tuesday the surface high gets a little stronger
and the upper level waves are much weaker. Have kept this period dry
and with the sunshine, temperatures should start warming back up.


.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Thursday)
Issued at 647 AM CDT Wed Jun 21 2017

General overview:
Unless thunderstorms happen to get into the mix this
afternoon/evening and cause issues, confidence is pretty high in
VFR ceiling/visibility persisting through the period with only
limited mid-high clouds. Will now go into more element-specific
detail for those interested...

Precipitation/thunderstorm potential:
While confidence is fairly high that the first 9-11 hours of the
period should remain storm-free, have maintained a generic
"vicinity" (VCTS) mention to cover the late afternoon-evening
hours to account for the possibility of vigorous severe
thunderstorm development in the general area (obviously later
issuances can refine details). Should any storms directly impact
the terminals, severe weather in the form of 50+kt gusts and/or
large hail are possible.

Surface winds and low level wind shear (LLWS):
Obviously not accounting for any impossible-to-resolve-yet
thunderstorm outflow effects, the vast majority of the period is
expected to feature a southerly wind component. The overall-
strongest speeds should slightly favor KGRI versus KEAR, and
should occur between 16-23Z with gust potential of 20+kt. Thanks
to a lingering, fairly strong strengthening low level jet early
this morning, have maintained a formal mention of LLWS at KGRI for
just the first hour before LLWS weakens below mentionable




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.