Area Forecast Discussion
Issued by NWS Hastings, NE

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906
FXUS63 KGID 090510
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
1110 PM CST Sun Dec 8 2019

...Aviation Update...

.UPDATE...
Issued at 958 PM CST Sun Dec 8 2019

Forecast appears largely on track for tonight, but did make a few
minor changes.

First off, believe the 00Z NAM is overdone with pcpn along and
just behind the front late tonight into early Mon AM. Low level
FGEN (H9-H85) is quite strong, but deep layer moisture is lacking,
as evidenced by the 00Z LBF sounding. Think RAP/HRRR trends are
more realistic in keeping most pcpn N of I-80. RAP/HRRR forecast
soundings indicate some steep lapse rates/minor CAPE above H7
near ODX, and this may be just enough extra ascent to allow some
lt pcpn to make it through the low level dry air. Again, though,
think the low level dry air will limit QPF to few hundredths or
less.

As far as PTYPE, simplified types a bit and removed sprinkles and
flurries. Also think freezing rain is quite unlikely as forecast
soundings indicate sfc temps and mid level wet bulb temps fall
below 0C basically simultaneously. If anything, a very brief
period of sleet could be possible. Regardless, should be so lt
that exact types have very low impact to sensible wx.

Main issue will be strong NW wind gusts 40-45mph immediately
behind front through the AM hours.

&&

.Discussion...
Issued at 219 PM CST Sun Dec 8 2019

Upper air analysis from this morning along with GOES water vapor
imagery showing Alberta clipper dropping SE in Montana with a
separate wave digging south along the CA coast, and the remainder
of the CONUS in zonal flow at this time. At the surface, high
pressure is dominating most of NE/KS with a lee trough to the west.
With weak pressure gradient over the area, winds are much lighter
than yesterday at this time.

Quiet weather continues into the evening before the clipper system
noted above, pushes a cold front south across the region overnight.
Northwest winds will increase dramatically behind the cold front and
it should remain windy for most of the daytime Monday. In addition,
a blast of arctic air currently in NDak, is poised to slide in
behind the front with high temperatures Monday expected to be some
20-25 degrees cooler than what we are experiencing today.

While strong winds and colder temperatures are pretty much a lock
later tonight and Monday, a few more questions remain as to how much
precipitation we will get and what form will it take as it falls?
Overall it still appears QPF will be low, with a tenth of an inch
being on the generous side. Onset of any precip should mainly be
after midnight, beginning in the northwest CWA, and then ending by 9
am CST in the east.  Precip type the most difficult variable.
Looking at forecast soundings, it is going to take a few hours for
the bottom of the layer to saturate. There will also be a brief
period of time with a warm layer aloft so ice pellets and or
freezing rain/drizzle is possible before the cold air rushes in and
everything falls as snow before ending. So the forecast plan is to
start with drizzle, then a mix, and finally all snow. This being
said, because of the limited QPF anticipated, and the better mid
level forcing off to our northeast, am not expecting any major
impacts from the precip.

Less active but cooler weather is then expected into the middle of
the work week, with a bit of a warming trend Thu and Fri. A Hudson
Bay low is slated to remain stationary early, keeping us in cooler
NW flow. By Wed, this low should push northeast with the CONUS flow
again becoming zonal. Operational runs of the GFS and EC do bring a
few shortwaves across the CONUS from Thursday into the weekend, but
at this time it appears the better forcing will remain north and
south of us as the waves ride the northern and southern jet streams.
This should hold true for Thu and Fri. The weekend might play out
differently as we could see at least a chance for some rain/snow
with a disturbance or two moving across the Central Plains.

&&

.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Monday)
Issued at 1101 PM CST Sun Dec 8 2019

A strong cold front is advancing south tonight and located in
northwest Neb attm. The front will reach the terminals around 09Z
bringing a northerly wind shift and noticeably windy conditions
with gusts 35-40kts. Low clouds at LIFR levels are advecting south
behind the boundary and while models are not overly robust with
the low cigs, cannot discount the current conditions and have
included a tempo group with LIFR clouds. Post frontal winds will
remain strong til around mid aftn for the terminals and cloud
heights will gradually raise and clouds will also begin to scour.
Some light precip is also possible tonight/early Monday and radar
is already picking up on some light returns. Precip type should
initially be liquid but as cold air deepens a wintry mix is
possible before ending. Little if any accumulation is expected.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...None.
KS...None.

&&

$$

UPDATE...Thies
DISCUSSION...Ewald
AVIATION...Fay



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