Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 201151
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
651 AM CDT WED JUL 20 2016

.SHORT TERM...(Today and tonight)
Issued at 430 AM CDT Wed Jul 20 2016

By far the main concern through these next 24 hours is the "true"
start of the well-publicized heat that will persist through the
rest of the work-week. All warning/advisory headlines remain
unchanged for now, although want to state up front that if current
heat index trends for Saturday persist, then a possible extension
of headlines may eventually be needed for at least portions of the
CWA into Saturday. However, given inherent uncertainty in the
details at the day 4 range, we feel there is still plenty of time
for an extension-in-time if one becomes warranted (see below for
more headline info).

On a more minor note, while the vast majority of the CWA will
undoubtedly remain dry, a somewhat unexpected flare-up of rogue
showers brushing the extreme west-central/northern CWA overnight
reminds us that even when the large-scale weather pattern is
dominated by upper ridging, it`s often not truly a "zero" chance
of rain especially along the northern periphery of the ridge core
and especially at night when a decent low level jet comes into
play. Along these lines, although we continue to advertise a "dry"
forecast for the daytime and Wednesday night hours (meaning
chances less than 15 percent), it is not out of the question that
especially these same western/northern zones could again see a
small threat of showers /non-severe storms mainly this
evening/tonight. For those following closely, our precip chances
(PoPs) in this part of the CWA are closer to 10 percent than
outright-zero.

Some comments/background info about the Heat Warning vs. Heat
Advisory breakdown:
Based on our latest heat index forecast, want to be brutally
honest that the delineation between the Warning counties and the
Advisory counties has gotten a bit "fuzzier". In other words,
there will likely be quite a bit of uniformity in the actual peak
heat index values expected across the CWA, and in some cases the
heat index will likely be just as "bad" in parts of the Advisory
area than they are in the Warning. Admittedly, this is not an
ideal situation given that in a "perfect world" a Warning headline
implies a more serious situation than an Advisory. However, many
variables go into deciding where to draw these sometimes
seemingly arbitrary lines, not the least of which are the
complexities of multi-office collaboration/individual forecaster
preferences at the time they are issued. Although it could be
reasonably argued that the entire CWA be under a Warning given the
overall heat index uniformity, given that the current
Advisory/Warning breakdown was just established 12 hours ago and
that the "big picture" portrayed by us and our neighboring offices
still looks reasonable, it has been decided to not make any
changes at this time. On a separate-but related-note, want to make
it clear that the Warning was issued not for the expectation of
widespread 110+ heat index (normally needed to trigger a one-day
Warning situation), but instead because of the
prolonged/cumulative impacts caused by at least 3 days of
generally 100-108 heat index. For anybody still bearing with me
on this paragraph (especially media partners), the take-away
message is this: please don`t make a "big deal" out of the Warning
vs. Advisory breakdown across our CWA in this particular case, as
overall it will be almost equally hot/miserable in both areas.

With all the basics/background info out in the open, let`s get
back on track to the current/upcoming weather scene...

Current/recent trends:
As mentioned above, other than a small flare-up of rogue showers
that barely brushed our far northern/west-central zones and are
now largely departing into north central/northeast NE, the night
has played out very much as expected with dry conditions over the
vast majority of the CWA, along with mainly clear skies and balmy
temperatures, held up in part by fairly steady southerly breezes
generally around 10 MPH. Actual low temps are on track to bottom
out 70-75 most areas. Water vapor satellite and short term model
data clearly show the center of the roughly 597 decameter ridge
(at 500 millibars) anchored over the OK area. Despite very limited
forcing, given that our CWA resides along the northern periphery
of the ridge center, some very subtle disturbances are barely
skirting the northwestern CWA as they ride clockwise around the
edge of the ridge from eastern CO toward SD, one of which teamed
with a fairly strong low level jet to promote the aforementioned
batch of weak convection.

Today (including early this morning):
Very little change in the large-scale scene, as various models
keep the ridge-center anchored over the OK area. In turn, not much
change from yesterday at the surface either, with a modest
pressure gradient between a High Plains lee trough and an eastern
CONUS ridge promoting another day of steady/borderline breezy
southerly winds with afternoon sustained speeds generally 15-20
MPH with some higher gusts in most areas (although likely a bit
weaker in far western zones nearer the trough axis). Precip-wise,
will linger a slight chance for a rogue shower in our far north
for a few more hours, but by sunrise expect that this small chance
should be over with, paving the way for a dry and overall-mostly
sunny day despite some passing high clouds. Due in part to the
slightly warmer start versus yesterday and also a slight warming
of the low-level airmass, there is little doubt that high temps
most areas should average at least 2-3+ degrees hotter than
yesterday. Have most NE zones aimed into a 96-101 range, with more
so 99-102 in KS. Peak heat index values generally 100-108.

This evening/tonight:
As already mentioned in the opener, the forecast continues as
officially "dry" all areas, but with the same aforementioned
large-scale features remaining remarkably similar to the present
situation (including the position of the ridge axis center, the
development of another strong low level jet, and hints of more
subtle mid level waves barely brushing the northwestern CWA), it
is just not out of the question that a few rogue showers/weak
storms might again try getting into counties such as
Dawson/Valley/Greeley again. For now though, will simply advertise
"silent" 10 percent PoPs given that the odds of staying dry are
much higher than getting measurable rain. Once again, southerly
breezes should average roughly 10 MPH through the night. Given the
warmer day in store and all other factors being almost equal, it
follows reason that lows tonight should also hold up slightly
milder than the current night (especially in eastern/southern
zones), and am going with low 70s north and west, to more of a
74-77 range in the east/south.

.LONG TERM...(Thursday daytime through Tuesday)
Issued at 430 AM CDT Wed Jul 20 2016

Small chances for thunderstorms exist across the region through the
entire extended periods with unseasonably hot temperatures expected
for Thursday and Friday...before the upper level ridge breaks down
and more seasonable weather returns to the local area late in the
weekend.

Aloft...expect a large area of pressure across the center of the
country to extend into the central and northern plains...allowing
for another hot afternoon across the local area both Thursday and
Friday. With 850mb temps near 30C and good mixing expected across
the region both days...temperatures as well as heat index values
will soar by mid afternoon...with heat index values over 100
expected across the entire forecast area...and nearing 110 in the
warmest spots. As a result...do not expect any changes to the
excessive heat warning that is currently in effect for both days. In
fact...with temperatures being slow to fall behind a passing pacific
front on Saturday...could eventually need to extend the headline for
parts of the local area into Saturday. Thereafter...more zonal flow
returns aloft and a cooler...yet seasonably airmass...will settle in
across the plains...allowing for high temperatures to return closer
to 90 degrees across the local area late in the weekend through
early next week.

While widespread thunderstorm coverage is not anticipated with the
previously described pattern...there is definitely not a zero chance
for thunderstorms either. In fact...expect multiple weak
disturbances to pass across the region...with the first of these
expected late Thursday afternoon/evening. While forcing with this
wave is rather minimal...the CAP is expected to erode during the
afternoon hours Thursday...allowing for at least isolated
thunderstorm development. Despite the relative minimal instability
and shear values apparent in model soundings over this time
period...there is little inhibition and a deep inverted V
soundings...indicative that if some storms do develop...strong to
severe wind gusts are not out of the question. Not
surprisingly...the storm prediction center has areas primarily north
of interstate 80 highlighted for a marginal risk of severe weather
Thursday afternoon/evening.

Thereafter...a better wave is expected to make its way across the
region on Friday...as the aforementioned Pacific disturbance tracks
across the northern plains. This passage will mark the shift to a
more zonal flow pattern across the northern half of the country for
several days...with multiple off and on again chances for
thunderstorms dependent on embedded upper level disturbances
traversing the region through the extended periods.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Thursday morning)
Issued at 651 AM CDT Wed Jul 20 2016

Confidence in VFR ceiling/visibility is extremely high, with only
limited high level cloud cover drifting overhead at times. Confidence
is also fairly high that it will remain dry/storm-free, although
there are some subtle hints that a bit of rogue activity could at
least come "somewhat" close to especially KEAR at some point
during the period and this will need monitored.

The main issue looks to be low level wind shear (LLWS), both right
away this morning for a brief time and then again later this
evening/overnight. In both cases, at least 30kt of shear
difference is anticipated between the surface (where breezes will
average around 10-12kt from the south) and roughly 1000 ft (where
winds will average around 40kt from the south-southwest). During
most of the daylight hours, LLWS should not be an issue as surface
breezes gradually increase with gust potential 20+ kt especially
17z-00z.

&&

.CLIMATE...
Issued at 430 AM CDT Wed Jul 20 2016

Although it`s far from guaranteed to happen (each site could
easily fall slightly short on at least 1 of the 3 days), it`s at
least possible that both of our primary climate records sites
could reach 100 degrees on consecutive days this week. Should this
occur, it would mark the first time in almost exactly 4 years...

- Grand Island airport:
Last time 100+ degrees on consecutive days: July 21-24, 2012...a
4 day stretch! Prior to 2012, the previous time was Aug 11-12, 2010.

- Hastings airport:
Last time 100+ degrees on consecutive days: July 22-24, 2012...a
3-day stretch. Prior to 2012, the previous time was Jul 31-Aug 1,
2006.

&&

.GID Watches/Warnings/Advisories...
NE...Excessive Heat Warning from noon today to 7 PM CDT Friday for
     NEZ039>041-046>049-060>064-072>077-084>087.

     Heat Advisory until noon CDT today for NEZ039>041-046>049-
     060>064-072>077-084>087.

     Heat Advisory until 7 PM CDT Friday for NEZ082-083.

KS...Heat Advisory until 7 PM CDT Friday for KSZ005>007-017>019.

&&

$$

SHORT TERM...Pfannkuch
LONG TERM...Rossi
AVIATION...Pfannkuch
CLIMATE...Pfannkuch


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