Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 271219
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
619 AM CST Mon Feb 27 2017

.SHORT TERM...(Today and tonight)
Issued at 440 AM CST Mon Feb 27 2017

Although not a truly simple forecast by any means given modest
uncertainty especially regarding sky cover, temps and whether or
not any fog/perhaps a few spits of drizzle might develop, these
24 hours actually do look a little more straightforward than they
did over the weekend when chances had looked a little better for a
possible wintry mix in northern zones (since removed from fcst).
Barring the unexpected, nearly the entire CWA is expected to
breach 50 degrees today (first time in 5 days for the Tri Cities),
although a steady south breeze will keep a coolish feel going for
sure.

Taking a look at the current/recent weather scene as of 4AM/10z:
As anticipated, it`s been a very quiet night, with essentially
clear skies and very light southerly/easterly breezes averaging
from near-calm to no more than 7 MPH. In the mid-upper levels, the
Central Plains lies under broad west-southwest flow aloft,
downstream from a larger-scale shortwave trough making its way
through the northern CA/OR/NV area. At the surface, a weak ridge
axis remains the dominant influence for a short time longer,
extending all the way westward from a strong high pressure system
centered off the Mid-Atlantic seaboard. Given the rather favorable
radiational cooling regime, overnight lows appear on track to
bottom out 15-23 for most of the CWA, with a few limited
colder/warmer exceptions. Despite the light winds, a continued dry
airmass locally is "saving" us from the expanding fog situation
affecting parts of eastern KS and perhaps extreme southeast NE yet
this morning.

Now looking ahead forecast-wise through today/tonight...

Today:
A dry forecast continues CWA-wide, and while confidence stands
pretty high in this for the vast majority of the area, there are
subtle hints in a few models that a few spits of drizzle could
brush the extreme southeast edges this afternoon as an area of
lower stratus expands northward out of the Southern Plains.
However, with most models keeping this very small/light precip
chance at least 50-100 miles east-southeast of our domain, will
stand behind the going dry forecast. Stepping back and looking at
the bigger picture aloft, subtle lift and strengthening flow aloft
will start overspreading our area as the western trough edges
closer, although any mid-level saturation should remain very
minimal. At the surface, the pressure gradient will steadily
tighten as pressures fall to the west, which along with vertical
mixing into strengthening southerly low-level flow will result in
a fairly breezy day, especially during the typical mid-morning to
mid-afternoon time frame, when most areas can expect southerly
sustained speeds 10-20 MPH with occasional gusts as high as 25 to
perhaps even near-30 MPH. For sure, sky cover and resultant temps
as the main challenges today. As for sky, obviously we will be
starting off quite clear. However, at the very least, there should
be gradually increasing high level cloud cover from the west as
the day goes on, at least pushing things into partly cloudy
territory. The bigger concern though is how lower-level stratus
will evolve. Most models suggest that at the very least, a
transient area of lower clouds will rapidly expand northward
across at least southern/eastern portions of the CWA as the day
goes in, in response to moisture advection in the southerly flow.
The million dollar question is, is this more of a brief glancing
blow of low clouds or does a decent chunk of real estate become
"socked in" for several hours? Given the inherent uncertainty
here, did not get too cute with sky grids, but generally have
"mostly cloudy" percentages for at least part of the day mainly
focused within the eastern half of the CWA. Temp-wise, will be
counting on steady south breezes and fairly decent low-level warm
air advection to be the dominant factors versus increasing clouds.
As a result, actually nudged up highs 2-3 degrees from previous
fcst most areas, calling for a range from upper 40s far north,
mid-50s central and upper-50s/a few 60s in KS zones. Fortunately
for possible fire weather issues, although the afternoon will be
breezy, slowly-increasing dewpoints into the 30s most areas should
keep relative humidity (RH) well-above even "near-critical"
thresholds, although this could be a somewhat close call in far
southwestern counties such as Furnas/Rooks where dews may hold
down in the 20s. Even here though, have RH staying slightly above
25% and thus will not be mentioning fire weather in the Hazardous
Weather Outlook (HWOGID).

This evening/tonight:
As mentioned in the opener, we are not as concerned about
possible wintry mix or possible dense fog issues as we were 24-48
hours ago, and in fact the official forecast remains dry at this
time, despite what could be a continued "non-zero" chance of some
brief drizzle especially in far north/northeast zones. Generally
speaking in the mid-upper levels, our local area is "split"
between better forcing/lift focusing both to our northwest
(associated with a disturbance ahead of the main western trough
tracking mainly from WY into SD) and also to our southeast
(associated with better moisture advection/isentropic lift), which
could even develop some thunderstorms as close to us as the KC
metro area. Meanwhile, things should remain quiet precip-wise in
our area, with again the possible caveat of a few spits of drizzle
mainly far north. At the surface, a fairly strong surface low of
around 999 millibars will start the evening in northeast CO but
quickly push east into Nebraska while weakening very slightly,
reaching our western CWA by around midnight and then the NE/IA
border by 12z/6AM. This passing low will have a noticeable impact
on surface winds, with light southeast breezes early in the
evening picking up and becoming more southerly 10-15 MPH by
midnight, followed by an invasion of steady westerly and even
northwesterly breezes late in the night as a trailing weak cold
front passes by. Thanks to these decent overnight wind speeds and
resultant low-level mixing, fog chances do not look quite as good
as before. That being said, with fairly small temp/dewpoint
depressions, there could be at least patchy/light fog development
mainly within the northeast half of the CWA for part of the night,
before the switch to more westerly winds late scours out much of
the limited fog potential by night`s end. So, for now have
maintained a generic "patchy fog" mention for mainly the northeast
half, but confidence in fog becoming widespread/dense is not high,
and thus no mention in the HWO for now. Lastly, low temps are a
bit tricky as well, as this has the looks of a night that could
easily end up milder than expected given what should be at least
partly-mostly cloudy skies and decent breezes. Ended up nudging up
lows 1-2 degrees from previous, aiming for a range from near-30
far north and west-central, low-mid 30s central and upper
30s/near-40 far southeast. This means that even if a little
drizzle does happen to develop, the odds of it actually freezing
and causing possible issues is quite low for the vast majority of
the CWA.

.LONG TERM...(Tuesday through Sunday)
Issued at 501 AM CST Mon Feb 27 2017

Main forecast challenge will be determining chance and type of
precipitation Tuesday afternoon/evening. Also, fire weather concerns
on Wednesday.

We start out Tuesday with upper level southwest flow as a trough
approaches from the western slope of the Rocky Mountains. There
could be a bit of fog in our northeast, but signals do not look all
that strong. By the late afternoon, the axis of this wave will be
just about at the front range, and will be close enough to give us
some vort maxes strong enough to kick up some rain, mainly in our
south/southeast where there is a better moisture profile. At this
point, any precipitation should be rain as highs will reach the 40s
and 50s across the entire CWA.

There will be a very limited time for precipitation potential as
this wave will be moving at a good clip. This has been a well
advertised wave by most numerical models and there seems to be good
agreement with general timing and placement of this feature. There
are different forecasts for how far north precip might make it, with
the NAM being the farthest north among most others. If we do manage
to get any snow, it will be either flurries or light snow showers
for maybe a couple of hours when it finally gets cold enough for
snow in the evening, but the trough axis should be through most of
the CWA by midnight to 3 am, so not looking for snow accumulation
with this wave.

We turn to fire weather potential for Wednesday as we will still
have enough of a pressure gradient at the surface on the northeast
side of a surface high that will be sliding southeast into the
southern Plains. Good subsidence behind the mid-upper wave will also
help boost temperatures warmer while dewpoints will drop.
Particularly the southwest CWA is of greatest potential for fire
weather issues, and we could be at least dealing with near-critical
fire weather issues there. Farther north/northeast, temperatures
will be a bit cooler and dewpoints a bit higher, so we are not quite
as concerned for theses areas, mainly southwest of a line from
Elwood to Superior, Nebraska.

The rest of the forecast is dry as west northwest flow becomes
zonal, and rather mild weather will ensue, with 60s for highs over
the weekend.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Tuesday morning)
Issued at 618 AM CST Mon Feb 27 2017

General overview:
Other than high confidence in continued VFR conditions under
mainly clear skies through these first 6 hours or so, much of the
remainder of the period features lower-than-average confidence
with regard to both ceiling/visibility trends, as some
models/guidance are insistent on at least MVFR/IFR conditions
prevailing much of the latter half while others keep primarily
VFR. On top of all of this uncertainty, low level wind shear
(LLWS) looks to be an issue too mainly during the latter half of
the period tonight. Will now take a closer look per element...

Ceiling/visibility:
Compared to previous TAF cycle, unfortunately confidence has
dipped even more regarding ceiling/visibility trends. Right away
late this morning/early afternoon, there are increasing hints of
at least a few hours of MVFR ceiling as a batch of low stratus
builds in from the south. Was not willing to fully "bite" on this
yet, but did introduce a low-end VFR ceiling through the
afternoon. In theory, if any sub-VFR ceiling does materialize this
afternoon, it could go away for a time this evening before
returning this evening/overnight, at least for a few hours before
an increasing westerly wind component tries to scour it out.
Again, models/guidance are ALL over the place with how this plays
out, so for now will stay the course with an MVFR ceiling and
low-end VFR visibility in light fog from 04-09z, but as usual
this is very subject to fine-tuning.

Winds (including low level winds shear (LLWS concerns):
While breezes are starting out very light/southerly early this
morning, southerly speeds will pick up by late-morning and have
increased speeds versus previous TAFs with gust potential into the
20-25kt range especially 18-23z, before easing back a little from
the southeast during the evening. Then during the night, a passing
cold front will turn breezes more westerly with time, and will
rely on later issuances to give this overnight shifting direction
more detail. Last but not least, there are two windows of
opportunity for LLWS. First off, some marginal LLWS (shear
magnitude mainly just under 30kt) could occur right away this
morning through about 15-16z before surface winds increase, but
have held off on a formal mention. There are seemingly better
chances for stronger LLWS tonight mainly after 04z as winds start
shifting behind the passing front, but with so much uncertainty in
other elements of the forecast between now and then and given that
this is beyond the first 12 hours, have opted to defer the
introduction of any LLWS to the next forecast(s).

&&

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

$$

SHORT TERM...Pfannkuch
LONG TERM...Heinlein
AVIATION...Pfannkuch



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