Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 251154
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
654 AM CDT THU AUG 25 2016

.SHORT TERM...(Today and tonight)
Issued at 435 AM CDT Thu Aug 25 2016

With any threat for legitimate severe storms seemingly extremely
low during these next 24 hours (and agree with SPC keeping the
Day 1 Marginal Risk area slightly south of our CWA), concerns
regarding ongoing and possible future hydro/flooding issues mainly
in our KS zones are starting to increase. For more detail/thoughts
on the hydro situation please refer to the separate "Hydrology"
section below. Otherwise, the majority of folks in the Nebraska
CWA are probably asking themselves "what flooding?", as much like
the past 24 hours the rain chances/amounts north of the state line
(and especially north of Highway 6) are looking very low if
existent at all.

Taking a look at the current scene as of 09z/4AM: Although we
certainly had at least "some" chance of rain in previous forecasts
for our southern zones, this forecaster has been admittedly
impressed with the fairly widespread overnight coverage south of
the state line, with most of our 6 KS counties likely to realize a
decent coverage of 0.25-0.50" amounts and locally 1+", despite
likely no more than a few hundred J/kg of elevated
instability/CAPE. Meanwhile, the vast majority of Neb zones have
remained dry, with only some light showers creeping over the state
line into the Red Cloud/Superior areas. This rather sharp
north/south cutoff to rain chances was very much expected and
well-handled by most models. In the mid-upper levels, water vapor
imagery and short term model data confirm a continued active
pattern aloft, as one low-amplitude shortwave disturbance after
another continues tracking through the Central Plains in
persistent southwesterly flow aloft, with meager instability being
offset somewhat by seasonably-strong lift in the entrance region
of enhanced upper level/jet stream flow centered to our northeast.
The main mid level wave responsible for the ongoing overnight
convection is currently tracking east-northeast out of eastern CO.
At the surface, the entire CWA remains north of a nearly quasi-
stationary front stretched from eastern KS into the TX Panhandle
area, with very light northerly to variable/near-calm breezes in
place thanks to the main influence of broad high pressure centered
to the northwest. Despite considerable cloud cover, temps have
managed to drop into the mid-upper 50s in most of the CWA, with
these values expected to hold pretty steady through sunrise for
overnight lows.

Now looking ahead forecast-wise through these next 24 hours...

Today (through roughly 7 PM):
In many ways, today will likely resemble yesterday (but hopefully
without additional isolated very heavy rain storms in our KS
zones). The basic upper level pattern stands firm, with continues
southwest flow aloft, but little to no appreciable convective
instability in most of the CWA, and perhaps only a few hundred
J/kg in KS zones. Models generally agree that the aforementioned
CO wave will continue tracking east-northeast over the CWA as the
day goes on, with forcing on it leading edge gradually departing
to our east. In theory, and as supported by models such as the
RAP13/HRRR/GFS, this means that steady rain/isolated thunder
chances across our south should gradually depart/diminish this
morning with mainly a dry afternoon. However, models such as the
NAM bring this into doubt a bit, as it somewhat-concerningly
generates a new round of convection over many of the same areas
seeing rain at this time. This seems like a bit of an outlier
though, and thus will confine any "likely" 60+ percent PoPs to
this morning with no higher than "chance" PoPs this afternoon.
Generally speaking, a line from roughly Geneva-Phillipsburg looks
to separate the much higher rain chances to the south from the
much-lower chances to the north, although will bring a token
slight PoP as far north as roughly the I-80 corridor. Heave
confined any formal thunder mention to areas mainly south of the
Geneva-Phillipsburg line. Otherwise, with weak pressure falls
occurring to the west, expect fairly light roughly 10 MPH breezes
today to gradually shift from a more northerly to more easterly
direction. Although especially northwest zones may see "some" sun
especially this afternoon, most of the CWA should remain pretty
darn cloudy. After busting high temps by 5-10 degrees too warm
yesterday, will hopefully have better results today as
models/guidance seem to be in better agreement in keeping things
cooler (but maybe still not cool enough?). At any rate, will aim
from low 70s south to upper 790s north, but if rain/thick clouds
really hang tough, some 60s are not out of the question in mainly
southern zones.

This evening/tonight:
According to a general consensus of most models, tonight could be
somewhat of a repeat of the current night, as persistent west-
southwest flow aloft and another in a series of mid level waves
tracks into the Plains, sparking increasing, mainly late-night
showers/non-severe storms into mainly our southern CWA to the
north of the surface front that remains to our south. This may
spell hydro concerns for our KS zones if this ends up being "round
3" of widespread rain in 36 hours, but this is no guarantee as
much of the heavier rain potential could also focus just to our
south/southeast. Due to inherent uncertainty in the details even
at this short time range, went with no higher than 50-60 PoPs far
southeast, while trailing off to only slight 20s near the I-80
corridor. Like the ongoing event, severe potential looks very low
with locally heavy rain the main threat. In other departments,
although confidence is fairly low, the light easterly breezes and
moist low levels could prove somewhat more conducive to at least
light/patchy fog development especially within the southwestern
half of the CWA, and went with a generic "patchy fog" mention
here. Of course, ongoing showers/storms could mitigate fog
formation altogether, and slightly lower boundary layer RH in the
northeast CWA looks to keep it largely at bay there. The bottom
line: not a sure thing. Temp-wise, aimed a few degrees above raw
guidance blend due to cloud cover expectations, and actually very
similar to the current night, with most areas bottoming out mid
50s to near 60.

.LONG TERM...(Friday through Wednesday)
Issued at 435 AM CDT Thu Aug 25 2016

Friday rain chances will be main concern. Starting at sunrise
Friday, our area will continue in a southwest flow pattern with a
broad mid/upper level trough situated to the west, with the axis
stretching from central Saskatchewan to southern Utah. Several
models note a lead shortwave moving northeast across Kansas and this
may bring us the first shot of rain during the daytime on Friday.
The flow should be progressive with main trough moving off the
Rockies and into the Plains Fri into Sat resulting in rain chances
continuing, with the best chance still looking to be Fri evening and
overnight. As far as a severe threat is concerned, the chances still
look to be fairly low across the Nebraska part of our CWA as most
models progging MU CAPE values to be marginal at best. Deep layer
shear not great but 0-6 km bulk shear could be around 30 kt which is
in that median range. Most of the CWA should remain cloudy much of
the day, limiting heating and instability, although the far south
could see a few breaks depending on how fast the early morning wave
moves through. SPC has included the southern edge of the CWA in a
marginal risk noting afternoon/evening convection could be
influenced by outflow boundaries generated earlier in the day. This
seams reasonable although at this point its tough to say just where
any boundaries would be located.

For the later part of the weekend and into next week it appears that
rain chances, while not zero, seem to decrease as mid level heights
build into the southern Plains and the main belt of westerlies
shifts north and over the northern Plains. Could still be a weak
wave or two that moves through the region but doesn`t appear likely
that we will be influenced by any surface boundaries as they will
remain north with the better upper level flow.

With modest riding building in, temps should rise a bit, with lower
to mid 80s expected for highs on Sat, and then mid to upper 80s for
the work week which is pretty close to the average for late August.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Friday morning)
Issued at 655 AM CDT Thu Aug 25 2016

General overview:
Confidence remains fairly in VFR ceiling/visibility throughout the
period...with mainly just plentiful mid-high level cloud cover
streaming overhead in persistent west-southwest flow aloft
especially during the first 12 hours. Visibility-wise, there
could be a shot at some light fog late tonight, but with
confidence low have only "hinted" at this possibility with a "6SM
BR" mention for now. Although a passing rain shower is not
completely out of the question especially tonight, the vast
majority of model guidance keeps steadier rain chances at least 50
miles south of KGRI/KEAR so will continue to leave out any mention
at this time. Surface winds should not be much of an issue...with
speeds averaging under 10kt. Direction will gradually shift from
more northerly to more easterly as time goes by...but at times
breezes will likely be light enough to exhibit some variable
direction as well.

&&

.HYDROLOGY...
Issued at 420 AM CDT Thu Aug 25 2016

The short version:
With a few rounds of moderate-to-heavy rain just in the past 18
hours, hydro concerns for especially our KS zones may be on the
increase over the next few days, with a lesser but non-zero
concern north of the state line as well.

Short term flooding concerns (generally the next 12 hours):
24 hours ago we were not necessarily expecting hydro issues to
crop up already this week, but thanks mainly to a slow-moving,
"heavy rainer" during the day Wednesday that dumped a fairly
narrow, southwest- to- northeast swath of generally 1.50-2.50" and
locally 3+" across the heart of our north central Kansas zones
(especially northern Osborne County), the South Fork Solomon River
gauge at Osborne has been above minor flood stage since Wednesday
evening. Over the past few hours, the trace seems to be cresting
somewhat, but unfortunately, a steady area of lighter rain with
embedded heavier downpours has moved right back over much of the
area of greatest concern over the past few hours. Per our latest
Flood Statement (FLSGID) product coordinated with the river
forecast from the MBRFC, expectations are (were?) for the Osborne
gauge to drop back below flood stage by late this morning.
However, this new rain event looks to generally drop a good
0.50-1.00" on average right over the basin, so do not be surprised
if at least a minor secondary rise occurs and keeps this point
Flood Warning in place at least well into the afternoon hours.
Elsewhere within our KS zones early this morning, not expecting
any truly impactful flooding issues as most areas outside the
flood warning area received well under 1" of rain on Wednesday and
should be able to handle another inch or so from this ongoing
round. We will be keeping an eye on radar/limited gauge data
though in case another concentrated zone of 2+" starts becoming
apparent.

Longer term flooding concerns (generally tonight through Friday
night):
Although confidence in amounts/placement is not there yet to
motivate a formal Flash Flood Watch mainly for our KS zones, the
next few shifts may want to at least consider the possibility,
especially if it keeps looking like Friday/Friday night could dump
another few inches of increasingly-unwelcome rain over the same
areas that have now been hit twice now with heavy rain just in the
past 18 hours. That`s not to say that our Nebraska zones are
completely out of the woods from at least minor flooding potential
over the next 2 days, but obviously with most areas north of the
state line having seen very little if any rain since Wednesday,
this area should be able to accept at least a few inches with
minimal issues. It`s worth noting that WPC has focused a Day 2
Slight Risk of excessive rainfall over most of our CWA (for
Friday/Friday night).

&&

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

$$

SHORT TERM...Pfannkuch
LONG TERM...Ewald
AVIATION...Pfannkuch
HYDROLOGY...Pfannkuch



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