Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 170604

Area Forecast Discussion
National Weather Service Hastings NE
104 AM CDT Mon Oct 17 2016

Issued at 104 AM CDT Mon Oct 17 2016

Time will tell whether or not it is truly "justified", but a few
hours ago decided along with the previous evening shift forecaster
to pull the trigger on a Dense Fog Advisory for the majority of
the eastern half of the CWA through 11 AM, based largely on a
consistent signal for at least "some" dense fog development in
the last few runs of the HRRR/RAP13 visibility products. So far
any dense fog has been very "fleeting"/hit and miss based on
airport obs/webcams, but in theory, a more concentrated corridor
of dense fog could/should become established by sunrise
especially along a northeast-southwest corridor where winds are
lightest along the path of a passing surface low, and just to the
southeast of a weak cold front.


.SHORT TERM...(This evening through Monday)
Issued at 326 PM CDT Sun Oct 16 2016

Stratus and some patchy fog continues to plague the northeast
portion of the forecast area this afternoon. South and western areas
have some sunshine. With the clouds, the temperature has been kept

Main concern for the period will be if there will be fog again
tonight. There is a surface low to the north of the forecast area
with light winds expected through the night and into Monday morning.
All of the models have some low level moisture across the area. The
HRRR has lower visibilities for late tonight and the SREF has a
pretty good chance for visibilities lower than a mile. The best
location for this would be in the eastern part of the area and
toward the central part of the forecast area. That area could have
visibility around a mile or less again in the morning. Some of the
models show it starting after 03Z tonight and moving out better than
it did this morning, having it gone by 15Z. If it does move out on
time, the highs tomorrow should be in the 80s, but if the clouds
linger too long will have to watch the temperatures in the
northeast/eastern part of the area.

.LONG TERM...(Monday night through Sunday)
Issued at 326 PM CDT Sun Oct 16 2016

The surface low over the area on Monday will sink to the south and
bring a cold front through the area mainly late Monday afternoon and
evening. It will be late enough in the day to have little impact on
temperatures Monday and even Monday night will be similar to lows
tonight. The colder air will finally be felt on Tuesday with
temperatures cooling off. Tuesday and Tuesday night will continue
dry with a weak surface high that moves through the area. An upper
level wave moves into the area Wednesday and there is a chance of
showers mainly across the Nebraska portion of the area during the
day Wednesday and into the evening hours before they move out.
Temperatures cool off Wednesday night and Thursday remains on the
cool side as a surface high moves through the area. An upper level
ridge builds to the west of the area and there will be some warm
advection Thursday night through Saturday. Temperatures will show a
little warming trend through this period. Another cold front moves
through the Sunday and cools temperatures off a bit.


.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Monday night)
Issued at 104 AM CDT Mon Oct 17 2016

General overview:
While confidence is high that VFR ceiling/visibility will prevail
through at least the latter half of the period at KGRI and quite
possibly the entire time at KEAR, there is considerable
uncertainty especially with regard to these first 9-10 hours, as
especially KGRI could rapidly "tank" to VLIFR conditions in dense
fog, potentially for several hours. Confidence is high in dry
/rain-free conditions at both sites. See below for more element-
specific details...

As already mentioned, these first 9-10 hours will be rather
challenging, as there is a good chance that especially KGRI will
see initially MVFR visibility deteriorate to at least IFR and
possibly VLIFR in dense fog. Meanwhile, KEAR is JUST far enough
west that "in theory" it should narrowly avoid fog issues, but
this is no guarantee either. The bottom line is, that while
confidence was not high enough to go prevailing VLIFR at KGRI, it
was introduced as a TEMPO from 08z-12z and could very well extend
a bit later, while KEAR has been left as VFR but will "hint" at
light fog potential with a "6SM BR" mention. Once boundary layer
winds turn more westerly by mid-late morning, confidence is high
in VFR from then on.

Surface winds/possible low level wind shear (LLWS):
The majority of these first 12 hours will feature a
light/variable surface wind, thanks to the presence of a weak
trough axis working across the area. Then, by late morning/early
afternoon, a pronounced increase in westerly breezes will take
place behind a passing front, with sustained speeds increasing to
at least 15-20kt and gust potential 20-25kt. These westerly
breezes will then let up somewhat during the evening. As for LLWS,
some models suggest there could be a period of "TAF-worthy"
LLWS centered around 03z tonight but given this is so far out
toward the end of the period and with plenty to deal with before
then, will hold off on including this for now.


NE...Dense Fog Advisory until 11 AM CDT this morning for NEZ041-

KS...Dense Fog Advisory until 11 AM CDT this morning for KSZ006-007.



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