Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 281059
AFDGID

AREA FORECAST DISCUSSION
National Weather Service Hastings NE
559 AM CDT Wed Jul 28 2021

.DISCUSSION...(Today through Tuesday)
Issued at 558 AM CDT Wed Jul 28 2021

Before breaking things down into separate Short Term (first 48
hours) and Long Term (Days 3-7) sections one last time this week,
here is an overview of the main stories of the entire 7-day:

1) Continued (and overall-worse) heat and humidity:
A coverage area (CWA) wide Heat Advisory continues into this
evening, and several more counties were added into the existing
Advisory for Thursday afternoon-evening (now including all but our
extreme northern-most 3 counties). Admittedly, Thursday carries a
little uncertainty on justifying "true" Advisory criteria for some
counties mainly north of I-80 depending on frontal timing and
possible cloud cover/spotty convection, but based on our current
official forecast the areal expansion was necessary. Interestingly
(and unfortunately), there are now signs that our going forecast
for Friday may be at least slightly too cool (latest models
trending warmer), so although Friday is currently void of any
official Advisory, this day bears close watching as if warming
trends continue then at least parts of our south may eventually
require an Advisory for the FOURTH straight day. Fortunately,
confidence is still fairly high that legitimately cooler (not as
hot) air will arrive CWA-wide by Saturday.

2) A multitude of rain/thunderstorm chances Thursday-Tuesday:
Admittedly, we focused more on heat concerns with this forecast
package than convective details, but these will require increasing
attention over the next few days. While some spotty/weak activity
cannot be ruled out as soon as very late tonight/Thursday AM in
our north, there is a non-zero chance for at least isolated
strong/perhaps severe storms as early as Thursday afternoon-
evening, with perhaps an even better chance of at least limited
strong/severe activity during the late Friday afternoon-Saturday
night time frame. Officially, SPC has not assigned a formal severe
outlook to our CWA for Thursday (Day 2), but has assigned dual
Marginal areas within roughly 100 miles both to our west-
northwest and east-southeast, so would not be totally surprised to
see a Marginal creep into our area in later outlooks. Meanwhile,
our extreme northeast is officially clipped by a Marginal category
on the latest Day 3 outlook for Friday.

- SHORT TERM...through Thursday night:

Overview:
The paramount focus continues to be Advisory-worthy heat, as
Thursday temperatures/heat index have crept up enough that we are
now expecting very similar max heat index values (mainly 100-109
degrees) on both days for the vast majority of the CWA. As
mentioned above, Thursday carries a little more uncertainty north,
but looks like pretty much a "slam dunk" Advisory situation for at
least our southern half to 2/3rds. A secondary concern (but one
that will require more attention over the next 24 hours) is at
least an outside chance of spotty but potentially strong
/marginally-severe storms as soon as Thursday afternoon-evening.

A look at the current/recent weather scene as of 5 AM:
As expected, an absolutely uneventful, humid and seasonably-warm
night, with low temperatures on track to bottom out no lower than
70-72 degrees most areas. Aside from any thin smoky haze aloft,
skies are pristinely clear overhead, as water vapor imagery and
short term model data clearly confirm that a stout/sprawling dome
of upper level high pressure is anchored almost directly over us
(500 millibar height a respectable 596 decameters). At the
surface, light mainly southerly breezes prevail (speeds mainly
near-to-below 7 MPH)

Today-this evening:
Obviously heat/humidity is really the big/only story, as the
entire CWA is all but guaranteed dry through at least midnight.
Both actual high temperatures and heat index values are expected
to climb at least a few degrees higher than yesterday (although
like yesterday, would not be surprised if we are aiming just
barely too high on actual temps most areas). Officially though,
we`re still calling for actual highs 95-100 degrees (far southwest
zones most favored to actually hit 100), with continued high
dewpoint values (upper 60s-low 70 most areas) yielding max heat
index 100-109 most areas. One slight difference than yesterday
(and likely a welcome one) is that there will be little bit more
afternoon breeze, with southerly sustained speeds generally 10-20
MPH (highest in our west-northwest) and a few gusts as high as
20-25 MPH. Officially, the Heat Advisory is currently slated to
expire at 9 PM for Valley/Greeley/Nance in our far north, but
continues for (at least) one more day everywhere else.

Late tonight:
Almost left the previously dry forecast intact (and actually did
for the majority of the CWA). However, various 00Z/0Z models
suggested just enough of a hint of spotty, weak, high-based
showers and/or thunderstorms at least flirting with our northern
zones between midnight and sunrise that opted to introduce a low-
confidence slight chance (20 percent PoP) to counties north of
I-80. This low probability scenario would be supported by weak
elevated instability within a west-east oriented zone of mid-level
saturation that could try developing along the far northern
periphery of the big upper ridge as it starts drifting/departing
slightly to our south. Again though, the vast majority of the CWA
is simply in for another dry and warm/muggy night with light
southerly breezes (becoming more variable in direction toward
sunrise ahead of the approaching weak cold front), with low temps
aimed 72-74 most areas.

Thursday daytime-evening:
Heat remains a concern, but at least isolated/spotty thunderstorm
chances enter the picture as well. In the mid-upper levels, the
center of the big ridge continues sliding slightly off to our
south-south over KS, allowing weak ripples of energy along its
northern edge to increasingly become a factor over our CWA. But
the real challenge, especially for the "exact" magnitude of heat
is trying to pinpoint the timing/position of a weak cold front
slipping southward through roughly the northern half of the CWA
over the course of the day before it stalls out, with most models
generally stalling it somewhere between I-80 and the KS border.
Behind this front, breezes will switch to northerly-then-easterly,
while ahead (south) of it, southerly breezes should become more
southeasterly by/during the afternoon. As for actual temperatures,
our southern zones (especially KS) will likely be just as hot if
not very slightly hotter than today (highs 99-101), while the
central CWA should be about the same as today (mid 90s), but our
far north a good 4-7 degrees cooler (low 90s). However, as is
often the case behind weak fronts as flow turns easterly, even our
slightly cooler north will likely see oppressive humidity
(dewpoints low-mid 70s). So, the net result is actual heat index
values topping out fairly uniformly across the vast majority of
the CWA (mainly 100-109), with only the aforementioned far
northern 3 counties falling a bit short of justifying Heat
Advisory issuance. That being said, there is a little big of
uncertainty especially with the Advisory counties between I-80 and
Highway 92, as it is possible that spotty weak morning
showers/storms (low probability) and maybe isolated afternoon
storms near the front (maybe a bit better chance) could generate
at least localized pockets of enhanced clouds/rain-cooled air to
hold down the heat a bit. But really, the main concern is that IF
any storms do manage to form along/near the front in the heating
of the afternoon, they could be strong to perhaps marginally-
severe, with RAP13 indicating a west-east corridor of healthy
mixed-layer CAPE at least 2-3000 J/KG, but fairly marginal deep
layer shear only around 25KT (this , along with weak capping being
the limiting factors to a more obvious severe threat). Per various
models, at least spotty convection is at least a small possibility
near/just north of the surface boundary mainly 4-8 PM, and thus
have expanded slight PoPs southward to roughly the Hwy 6 corridor
through the afternoon-evening to account for this scenario.

Thursday later evening/overnight:
There are really two main convective possibilities here. One is
that any isolated storms that develop during peak heating
gradually fade by/near nightfall and the rest of the night stays
storm-free with any late night development focusing at least
slightly off to our north-northeast. However, other models suggest
that especially our far northeast zones could at least be clipped
by late night storms developing in a low-mid level warm air
advection regime, with perhaps another threat of strong to
marginally-severe activity should this occur. With this still
being nearly 48 hours away, no point in getting too detailed, so
for now our late night forecast confined any thunderstorm chances
to areas mainly north of a Loup City-Geneva line, but highest
chances along/north of an Ord-Osceola line. Meanwhile, our
southern/southwest CWA stands a high probability of a dry/storm-
free night. In the presence of light easterly breezes, low temps
should be slightly cooler than tonight (most areas aimed 68-72).


- LONG TERM...Friday daytime through Tuesday:

Friday...

Main concerns for Friday lie with potential for lingering heat
issues and thunderstorm chances.

At 12Z Friday, models remain in pretty good agreement with the
continued northwesterly flow aloft, as the main upper high is
stretched roughly from the eastern CO/WY border southeast into the
Gulf Coast region. At the sfc, there is expected to be a stalled
boundary near the NE/KS state line. Models showing there is the
potential for some lingering showers/storms across far northern
portions of the CWA along a more elevated boundary/convergence, so
some small PoPs remain, mainly north of a Loup City/Geneva line.

Through the daytime hours, models are showing the potential for that
sfc boundary to push north as a warm front, thanks to a deepening
sfc low roughly over eastern CO. Just how far north it gets is
uncertain. If the trend of the front pushing up into northern
portions of the CWA holds, afternoon high temperatures across
central areas are likely too low. Doubt there`d be much change in
highs across north central KS from what is currently in the
forecast. Current highs range from the mid 80s north, to either side
of 90s through central areas, to near 100 across our far
south...with little change in dewpoints from the previous days. Have
time to see how things trend with this being a couple days out yet,
but may have to consider extending the heat headline into Friday for
portions of the area.

As far as precipitation chances go, PoPs linger throughout the day
and into the evening/overnight hours. Main focus during the day is
near that sfc boundary with any subtle upper shortwave moving
through, with lift aided into the evening/overnight hours by
increasing convergence along the nose of a southerly LLJ. The best
chances look to be focused across the north-northeastern half of the
CWA (current PoPs may be too generous area-wise), with models
showing at least modest amounts of MUCAPE and better deeper layer
shear than previous days...leading to some concern of strong/severe
storms. SPC Day 3 outlook currently clips the far northeastern
corner of the CWA.

Weekend into early next week...

Thunderstorm chances linger into Saturday, but models show the main
focus shifting south with time. Between outflow from any northern
activity and a better push from a stronger upper low moving south
toward the Great Lakes and troughing digging further south into the
Midwest/eastern CONUS, the main sfc boundary looks to push south of
the CWA Saturday night. Sunday daytime-evening is currently dry,
with the better forcing/PoPs south of the CWA and NNWrly flow in
place aloft. The hotter airmass is also pushed out of the area, with
highs Saturday in the 80s for most/low 90s south and low-mid 80s for
Sunday.

For Monday and Tuesday, models not in bad agreement showing the
continued more amplified NNW flow aloft across the region. NBM
inserted some low PoPs both days, mainly western areas for Monday,
more of the CWA for Tuesday. Can`t rule that out with the potential
for weak shortwave disturbances and low confidence in timing/track.
Highs generally in the mid 80s look to continue into both days.

&&

.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Thursday)
Issued at 1250 AM CDT Wed Jul 28 2021

Ceiling/visibility/precipitation:
High confidence in VFR ceiling/visibility and continued rain
/storm-free conditions, with only some very limited high-level
cirrus and/or smoke.

Winds:
Although not a big concern by any means, southerly breezes will be
a touch stronger during the daytime hours, with sustained speeds
commonly 10-13KT and gusts up to around 18KT (especially 17-23Z).
Lighter southerly speeds mainly under 10KT will prevail both early
this morning and again late in the period this evening.


&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Heat Advisory until 9 PM CDT Thursday for NEZ046>049-060>064-
     072>077-082>087.

     Heat Advisory until 9 PM CDT this evening for NEZ039>041.

KS...Heat Advisory until 9 PM CDT Thursday for KSZ005>007-017>019.

&&

$$

DISCUSSION...Pfannkuch/ADP
AVIATION...Pfannkuch


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