Area Forecast Discussion
Issued by NWS Hastings, NE

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056
FXUS63 KGID 131121
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
521 AM CST Wed Dec 13 2017

.SHORT TERM...(Today and tonight)
Issued at 413 AM CST Wed Dec 13 2017

Upper level data showing little overall change in the northwesterly
flow in place across the Plains, set up between low pressure over
the northeastern CONUS and ridging along the West Coast. Satellite
showing an embedded vort max currently making its way southeast
through the western/central Dakotas. This system is also pushing
along a sfc cold front, roughly located along the SD/NE border.
Continued WNW winds and increased mixing ahead of the front and
brought temps up over the past couple hours, at 3 am most locations
are ranging from the upper 30s to mid 40s.

The main story through today will lie with this upper level
system/sfc cold front moving through. Models remain in good
agreement keeping this system on its southeastward track, taking it
across the eastern half of NE by midday today. With an overall lack
of moisture to work with, the forecast through the short term
remains a dry one. There is a swath of mid/upper level cloud cover
also sliding south with the system, part of which has already moved
into northern portions of the CWA. These clouds look to hang around
for at least a few hours, but by mid morning skies clear, and should
remain well into the afternoon. Cloud cover will be on the increase
this evening/overnight, as models show stratus building in from the
north.

The main impact the CWA will have from this system/front will be
with the winds. After a break from stronger winds yesterday, they
will return today. A tightened pressure gradient, CAA and increase
mixing potential into a corridor of stronger winds aloft will all
aid in increasing northwesterly winds through the day. Current
forecast sustained speeds top out around 30 MPH, and though the
winds aloft are shown by models to decrease through the afternoon
(when the better mixing would be), gusts around 45 MPH remain
possible. As with the past couple of windy days, a few locally
higher gusts wouldn`t be out of the question.

No notable changes were made to temperatures, with highs forecast to
reach near 50 in the north to the upper 50s in the south and lows
tonight in the mid/upper 20s. The dry airmass/dewpoints will remain,
resulting in RH values bottoming out in the 20-25% range across far
southern portions of the CWA. Current forecast doesn`t have any
county hitting critical fire wx conditions, so no headline planned,
but mention will remain in the HWO.

.LONG TERM...(Thursday daytime through Tuesday)
Issued at 413 AM CST Wed Dec 13 2017

General overview of this 6-day period:
Third night on the long term desk here, and honestly the big
picture forecast message remains quite consistent. The majority of
the time remains dry, but there are still 2 distinct chances for
what currently appears to be fairly light precipitation (probably
favoring snow over rain), but they both bear watching in case they
turn a bit more impactful. Temperature- wise, the entire time
frame continues our impressively-persistent stretch of above
normal, with 4 of these 6 days currently aimed at least 10 degrees
above the norm.

Likely/potential weather hazards:
At least for now, the Days 2-7 portion of our Hazardous Weather
Outlook (HWOGID) is "empty", as there are just no apparent issues
that carry high enough confidence/impact to highlight. That being
said, both of our modest snow chances (Thursday daytime and
Saturday night-ish) need watched closely to make sure they don`t
trend upward. See below for more details on this. As for fire
weather, at least for now there are no afternoons that appear to
be truly problematic/critical, thanks to higher relative humidity
and/or lighter wind speeds. Technically, a few of our far
western/southern counties do barely meet "near-critical"
thresholds for Friday, but given the current marginal nature and
the fact that it`s still 3 days out, opted against HWO inclusion.

Basic upper air/surface weather pattern:
The majority of this time frame features a continuation of our
current/recent northwest flow aloft pattern, directed between
large-scale ridging over the western CONUS and predominant
troughing over the east. The only exception is over the weekend
(Sat-Sun) when the primary/strongest short wave trough of these 6
days passes through, turning the flow aloft west-southwesterly for
a time. At the surface, winds will prevail either northerly or
westerly, depending on whether a cold front has just pushed
through or we are "in between" waves. For those tiring of our
recent rash of very windy days (such as today and a few days ago),
fortunately there is nothing that looks to be that strong.
However, surely Thursday, Saturday and Sunday will at least reach
"quite breezy" to "modestly windy" status, and are the clear
candidates for overall-windiest 3 days of these 6.

Basic precipitation overview:
As was the case yesterday, the official forecast features two
distinct opportunities for precipitation, and although chances
(PoPs) remain below "likely" levels, they both have trended upward
a bit. 1) Thursday daytime: See below for more details, but this
looks like an increasingly-good setup for at least scattered bands
of rain and/or snow showers, and am starting to think a few places
could see brief, modestly impactful visibility reductions in
passing snow bands despite minimal accumulation potential. 2)
Saturday night (possibly into Sunday daytime?): Being 4-5 days
out, confidence in how this plays out is admittedly pretty low,
but the more aggressive ECMWF solution suggests that a quick inch
of snow or so could occur, accompanied by quite-breezy northerly
winds. On the flip- side, the last few GFS runs are more sparse
with snow potential.

Basic temperature overview:
As mentioned above, the at-least-modestly above normal pattern
just keeps on rolling, with 4 of the 6 days currently featuring
highs mainly between the upper 40s-mid 50s (roughly 10+ degrees
above normal). Confidence remains pretty high that the outright
coldest days will be right away Thursday, and then Sunday, both
associated with noticeable cold frontal passages and possible
precip/plentiful clouds. Of all 6 days, the overall "biggest"
change was to Thursday, which was lowered 2-3 degrees from
previous. Even so, these colder two days with highs aimed between
the upper 30s-mid 40s would still be very slightly above normal.
As for overnight lows, most nights are aimed into the mid-upper
20s, except for prevailing low 20s Thursday night and perhaps
again Sunday night in some areas.

With the big picture story adequately covered, will finish with
some daily specifics, particularly focused on Thursday...

Thursday/Thursday night:
This currently looks to be the best candidate for "least pleasant"
day of these 6. Unfortunately, even with it only being 24-36 hours
away, confidence in exactly how precipitation will play out leaves
something to be desired. As mentioned, daytime high temps were
lowered a little, with much of the northern half of the CWA
probably failing to reach 40. Over the course of the day and into
at least the first part of the evening, a somewhat organized
shortwave trough will drop across our region from the northwest.
For sure, this wave will bring plentiful clouds and modestly
strong winds behind a reinforcing cold front (especially
afternoon), with sustained speeds generally 15-25 MPH/gusts at
least 20-30 MPH. The biggest issue is that even at this fairly
close time range, models vary on whether or not precipitation is
more spotty or more organized. If it were a little colder, this
would be a classic setup for scattered to numerous bands of quick-
hitting, passing snow showers (a little like the snow squall we
saw a week ago), with a few spots perhaps picking up legitimate
light accumulation and reduced visibility. While this could very
well happen in some places, the surface temperatures are very
marginal for robust snow, and some of this precip could at least
start as rain. Not surprisingly given the marginal temp profiles,
our in house snow accumulation tool officially depicts very little
if any actual snow accumulation (perhaps a dusting at most), and
mainly just in our northeast. What is a bit unsettling is that
some of the raw model snow output (yes, likely overdone to some
extent) suggests that at least a small part of mainly our eastern
CWA could see a quick-hitting 1+" of snow. While the majority of
precip should be done by sunset, lingered some small chances into
the evening hours mainly over our eastern third. The bottom line
is this: Keep up to date with the Thursday forecast, because we
may have to hit the "quick hitting snow shower" potential a little
harder, especially if surface temps end up a touch colder than
advertised.

Friday daytime-Saturday daytime:
These couple days look pretty much guaranteed dry and milder in
the wake of the Thursday system and just before the next one
arrives. Friday should actually be one of the more pleasant
upcoming days, with a return of sunshine, temps generally 7-10
degrees warmer than Thursday, and manageable west-southwest
breezes only 10-15 MPH/gusts to 20 MPH. While Saturday daytime
should remain fairly mild temp-wise, northerly breezes will be on
the increase as the next cold front moves in.

Saturday night-Sunday:
As mentioned in preceding paragraphs, this time frame carries a
lot of uncertainty precip-wise but bears watching, as the more
aggressive ECMWF suggests another quick-hitting snow event
(perhaps 1+") while the GFS is pretty sparse locally and keeps
most snow well west over the Front Range/Rockies. Timing- wise,
our official forecast currently confines all snow chances to
Saturday night, but later forecasts may need to linger this chance
into Sunday daytime per the latest ECMWF. At the very least,
Sunday looks breezy and chilly (highs low 40s most areas).

Sunday night-Tuesday daytime:
These last few days of the official forecast feature a return to
dry conditions and seasonably-mild temperatures (highs generally
around 50), as broad northwest flow returns to the Central Plains.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Thursday)
Issued at 514 AM CST Wed Dec 13 2017

The surface cold front accompanying the upper level disturbance
off to our northeast is knocking on the door of the terminal
sites, with upstream obs already reporting gusty NW winds.
Expecting the gusts to arrive in the next hour or so, and look to
continue through the day. LLWS will remain a concern over the next
couple of hours, until the surface speeds pick up more. Sustained
speeds of 25-30 MPH are expected later this morning/afternoon, and
gusts near 45 MPH will be possible. Speeds are expected to
diminish this evening/overnight. VFR conditions are forecast, with
plenty of sun for a good chunk of the day, once this swath of
mid/upper level clouds swings through. Tonight, lower level status
looks to build in. At this point, kept ceilings at 3500ft, but
upcoming shifts will have to take a look at whether or not lower
ceilings are needed.

&&

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

$$

SHORT TERM...ADP
LONG TERM...Pfannkuch
AVIATION...ADP



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