Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 200205

Area Forecast Discussion
National Weather Service Hastings NE
905 PM CDT Tue Sep 19 2017

Issued at 904 PM CDT Tue Sep 19 2017

A couple items to mention since the routine afternoon issuance:

1) Overnight convection not a truly "zero" chance:
While the probability of thunderstorms within our coverage area
(CWA) overnight is still deemed below forecast-mentionable
thresholds (15+ percent), want to emphasize that it is not truly
"zero" either. Within the past half hour, a storm has quickly
developed/gone severe in extreme north central NE along the
invading cold front, and short-term higher res models such as the
HRRR/RAP13 suggest that convection could try building far enough
south down along the invading front to at least result in a
somewhat "close call" for mainly our extreme northern/east-central
counties at some point mainly during the 03-07Z/10PM-2AM time
frame. While this will obviously be monitored very closely, at
least for now, have kept PoPs no higher than 5-10 percent, and
will continue counting on residual capping/weaker forcing versus
farther north to keep our overnight frontal passage a dry /storm-
free one.

2) Fire weather:
Allowed the Red Flag Warning for Furnas/Phillips/Rooks counties to
expire on schedule at 8 PM. Although relative humidity (RH) values
remain below the critical 20-percent threshold even as of this
writing, wind gusts have dropped below 25 MPH and the RH will
continue to rise through the night as temps cool. Although
vegetation/fuel greenness might have been a touch marginal for
truly "explosive" fire growth potential today, at least the
meteorological combo of low RH/gusty winds easily met Warning
criteria. Looking forward, we now get a one-day respite from fire
weather issues before at least near-critical levels likely return
for Thursday afternoon, again primarily in our southwest.


.SHORT TERM...(Today and tonight)
Issued at 321 PM CDT Tue Sep 19 2017

The main issue in the short term is the passage of a cold front
tonight across the area.

Prior to the front, the Red Flag Warning will continue through
8 pm for the far southwest forecast area. While nearby counties
could also touch extremely low RH values, likely won`t be for
long enough period to be included with the current warning area.

The models have been supportive/similar/solid in frontal timing,
roughly through the Tri-Cities around 05-06Z and through the
forecast area by morning. The main sensible weather change will be
much drier air for Wednesday/Wednesday night. Frankly, Wednesday is
a heck of a nice day with sunshine, low dewpoints, mild but warm
temperatures and north winds becoming light as they shift to the
south in the afternoon.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 225 PM CDT Tue Sep 19 2017

The long term is highlighted by a very warm start as the week
ends, followed by what appears to be widespread rain event the
latter half of the weekend and early next week.

As for the warm weather, that will be thanks to strong southerly
flow ahead of western U.S. trough. Low level moisture surges north
during the day Thursday a somewhat humid feel, though not quite a
humid as today. Moisture continues to increase through Saturday.
Warm temperatures highlight the pre-rainfall period with highs in
the 80s and lower 90s Thursday/Friday before showing signs of
cooling Saturday, especially in the northwest forecast area. May be
warm enough Friday for a calendar day "record high minimum
temperature". Current record warm minimum temperatures for Grand
Island and Hastings are 67 and 65 degrees, and we may not drop below
70 for the day Friday.

While we do have shower/storm chances in the forecast as early
as Friday afternoon in the northwest, the honest truth is this
rain event will likely be delayed/later than advertised for most
areas due to the blocky/stagnate high pressure to our east. However
the current forecast does reflect the greatest rainfall potential
from Saturday night through Monday, and that does look reasonable.
GFS/EC remain focused on a feed of moisture from the southwest ahead
of the Great Plains trough. GEFS depicts 2X above normal
precipitable water flowing up into the are by Sunday. That moisture
rides along the slowing frontal boundary and several rounds of
showers and some thunderstorms roll southwest to northeast across
the area. Through Sunday, the heavier amounts look to be west of
U.S. Highway 281 after which the eastern areas start seeing
increased rain chances Sunday night and moisture. It is certainly
possible some 3+ inch type rain amounts will fall, maybe not
everywhere, but its certainly on the table. It appears even if
everyone doesn`t see heavy rain almost everyone will see some rain.
Mid level lapse by Sunday are pretty marginal for thunderstorms,
though there is some CAPE to work with. This looks more like a rain
and isolated thunderstorm event and have focused more on the rain
showers Monday and Tuesday. As one would expect, temperatures will
cool was the rain and front move into the area with highs in the 60s
and lower 70s by Monday.


.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Wednesday)
Issued at 700 PM CDT Tue Sep 19 2017

General overview:
Confidence is high in VFR ceiling/visibility with only passing
high clouds, and confidence is also high in precipitation
/thunderstorm-free conditions, leaving winds as the paramount
issue, especially during these first 6-8 hours. Will address this
in more detail...

Winds (including low level wind shear (LLWS) likelihood):
All eyes are on a fairly strong cold front approaching from the
west. During the first several hours this evening, winds will
remain steady from the south with gust potential at least 18-23kt
(a bit higher at KGRI versus KEAR). Ahead of the approaching
boundary, there will be a 2-3 hour period of mentionable (30+kt)
LLWS, as south-southwest winds ramp up to 45-50kt within roughly
the lowest 1500 ft above the surface. The aforementioned surface
cold front is slated to arrive at KEAR around 04Z and KGRI around
05Z, abruptly switching winds to northwesterly and likely somewhat
gusty for at least a few hours with gust potential to around 25kt.
There could also be a brief period of marginal LLWS behind the
surface front, but at least for now this does not appear strong
enough to merit formal mention. As the night wears on the post-
frontal winds will subside with speeds averaging at-or-below 10kt
through the rest of the period, and in fact becoming outright
light/variable during the last several hours Wednesday afternoon.




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