Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 302119
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
419 PM CDT SAT JUL 30 2016

.SHORT TERM...(This evening through Tuesday daytime)
Issued at 420 PM CDT Sat Jul 30 2016

Another day, another "battle" with below-average forecast
confidence especially regarding rain chances (PoPs) and the
potential for at least isolated/spotty severe storm/heavy rainfall
potential mainly through tonight. Beyond tonight, there are other
chances for thunderstorms through Tuesday, but at least "in
theory" the risk for severe storms seems lower. As is usually the
case, forecast confidence is overall-higher in temperature trends,
and the "big story" of the upcoming week remains a mainly 3-day
string of above-average summer heat and the potential for "near-
advisory" heat index values of 100+ degrees (official advisory
criteria is 105+). In the interest of time, and given plenty of
inherent uncertainty especially regarding storm chances, will keep
the deeper details to a minimum and focus on the basics.

A quick look at recent/current weather over the past several
hours:
This day shift started out very busy as a complex of strong to
marginally severe storms tracked from northwest-to-southeast
across the heart of the CWA this morning, centered along an
Ord/Loup City-Grand Island-Hebron axis. This activity, which
flared up very early this morning along the low level jet/leading
edge of mid level instability gradient was most noted for
producing widespread 2-4" rainfall in much of
Valley/Sherman/Howard counties, and although water has steadily
been receding today, one areal Flood Warning remains in effect for
at least a short time longer pending reports from county
officials. The remnants of these AM storms have departed all the
way into the KC area, and the CWA has enjoyed a dry and partly
cloudy/mostly sunny afternoon in its wake, with high temps likely
averaging in the 83-88 range most areas by day`s end. In the mid-
upper levels, we remain in northwest flow, and although there are
additional weak disturbances approaching, we appear to be in at
least a brief "break" in forcing behind the departing morning
disturbance. At the surface, dewpoints are on the rise (into the
mid 60s to near 70) as southeast breezes of generally 10-15 MPH
have kicked in on the backside of departing surface high pressure.

Now looking ahead forecast-wise over these next 72 hours or so:

Late this afternoon-tonight:
Uncertainty is definitely high in the details, and PoPs have been
kept below "likely" percentages, but there are two basic
possibilities for storm development. First of all, things continue
to destabilize in the wake of morning convection especially within
southern/western portions of the CWA where mixed-layer CAPE is
analyzed up to around 2500 J/kg in the presence of decent deep
layer shear of 30-40kt. However, with limited upper forcing and
slowly-warming mid level temps (700 millibar temps now up to
10-12C across roughly the southwest half of the CWA), it is
uncertain whether peak afternoon heating will allow storm
development. Should this occur, it would mainly focus in our
south/west near a remnant outflow boundary. Severe storms are
certainly possible should this occur. Whether or not the late
afternoon/early evening potential materializes, there is somewhat
higher confidence that an increasing nocturnal low level jet will
fire up at least isolated to scattered (potentially more
widespread?) storms later tonight along the edge of the mid level
cap. Based on the very latest model trends, this potential would
tend to better favor southern/eastern zones versus the north and
west, which is a good thing considering that our far northwest
zones need a break from heavy rain. Even late tonight, marginal
severe and localized flooding are possible. Tonight could be one
of those "all or nothing" convective scenarios for many areas.
Otherwise, low temps are aimed into the mid-60s to near 70 range.
Although confidence is low in development and severity, also
introduced a generic "patchy fog" mention for late tonight into
Sunday morning especially for areas where storms do not "mix up"
the atmosphere too much.

Sunday daytime:
With mid level temps/capping continuing to warm from west-to-east,
and also mid level riding/rising heights building in a bit,
confidence is fairly high that most of the CWA is dry most if not
the entire day, and have only left lingering small PoPs in far
eastern zones during the morning as a departure of the potential
overnight activity. Otherwise, similar to today with steady
southeast breezes but temps generally expected to be about 5
degrees warmer with upper 80s/low 90s in most NE zones and mid 90s
in KS. Could see some 100+ heat index values, but likely only
confined to KS zones.

Sunday night:
While some models keep the main thunderstorm potential north of
our CWA where mid level temps are a bit cooler, there is enough of
a hint (especially per the NAM) that especially the northern half
of our CWA could see some limited activity, and have re-introduced
slight chances to much of this area.

Monday daytime:
The heat continues to build, and high temps are again expected to
rise about 3-5 degrees versus the previous day with highs well
into the 90s all areas. Heat index not expected to breach advisory
criteria but could get close. Left most of the day dry, but
maintained some slight chances in western zones late in the
afternoon in case some storms can develop near a weak surface
trough and in the presence of hot temperatures/deeper mixing.

Monday night:
Blanketed the entire CWA with mainly 30 PoPS (especially from the
GFS), but uncertainty is high given limited forcing, warm mid
levels and inconsistent model signals as the latest ECMWF/NAM are
mainly dry.

Tuesday daytime:
Blanketed low-confidence slight chance 20 PoPS all areas, but
again could easily be dry for the vast majority of the CWA with
the main afternoon storm potential focusing more over the High
Plains to our west. At least for now, high temps appear very
similar to Monday (see above) with similar heat index values
near/slightly above 100 as well.

.LONG TERM...(Tuesday night through Saturday)
Issued at 420 PM CDT Sat Jul 30 2016

Looking at the mid week period into the next weekend, primary
concerns lie with hot temperatures on Wednesday, then precipitation
chances.

At the start of this time frame Tuesday night, the upper level flow
is generally zonal, as broad high pressure remains over the southern
CONUS and an area of low pressure spins over the Pac NW. At the
surface, a frontal boundary looks to be draped near the NE/KS state
line. Precipitation chances through the overnight hours aren`t
great, but with models showing the potential for a subtle
disturbance moving through the region and convergence along the nose
of a 30kt LLJ, can`t rule it out. At this point it looks to
primarily affect the southern half of the CWA.

Dry conditions are expected during the day on Wednesday, with some
low amplitude upper level ridging sliding through ahead of this
stronger disturbance now over the northern Rockies. Southerly winds
return across the region, ahead of cold front pushing its way into
western NE/SD through the day. Not a lot of change expected in
temperatures, with forecast highs in the mid/upper 90s CWA-wide.
Heat index values for most locations reach withing a few degrees
either side of 100 degrees, so will keep mention going in the HWO.

Better precip chances arrive Thurs/Thurs night and continue into
Saturday, with models in pretty good agreement showing that upper
level disturbance sliding east along the U.S./Canadian border into
the Hudson Bay by sunrise Friday. The accompanying surface is pushed
south into the CWA during the day Thursday, eventually stalling out
roughly near the NE/KS state line. This helps provide a focus for
precip through the end of the period, with Thursday evening/night
currently having the highest PoPs. Highs are forecast to range from
the mid 80s to mid 90s on Thursday to the lower/mid 80s by Saturday.

&&

.AVIATION...(For the 18Z KGRI/KEAR TAFS through 18Z Sunday afternoon)
Issued at 122 PM CDT Sat Jul 30 2016

Will state up front: beyond the first 6-9 hours, this is
definitely a lower-than-average confidence forecast with regard to
ceiling/visibility and thunderstorm chances, so stay tuned for
potentially significant changes/updates in later scheduled
issuances/amendments.

Confidence is in fact rather high in VFR and storm-free conditions
through at least these first 6 hours this afternoon with breezes
generally near/below 12kt from the southeast. Then later this
evening and especially overnight, there are increasing chances for
storms in the area, but because models are still rather
inconsistent on timing and location will only advertise a generic
"vicinity" (VCTS) from 03-09z, which would be the most favored
time frame. IF in fact storms do affect the terminals, severe
weather with hail, strong winds and heavy rain are possible.
Beyond the 09z time frame, the general model consensus is that
convection should be focusing more to the east of KGRI/KEAR. Then
the million dollar question is whether sub-VFR ceiling/visibility
might form in low stratus and/or fog. Despite model guidance
insisting on potentially IFR or worse conditions, am not willing
to go quite this pessimistic yet given that convection in the area
often "muddies" the potential for the lowest aviation categories,
especially if low-level breezes stay up a bit. For now will aim
for a "middle ground" approach with MVFR ceiling/visibility
focused from 09z-15z, but please note that conditions "could" be
worse than this if some of the current model guidance is in fact
onto something. Last but not least, there is also the potential
for a period of low level wind shear (LLWS) tonight mainly between
03z-09z, but with some of the latest guidance showing fairly
marginal shear values mainly just below 30kt, will keep this out
for now and let later issuances re-assess.

&&

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

$$

SHORT TERM...Pfannkuch
LONG TERM...ADP
AVIATION...Pfannkuch



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