Area Forecast Discussion
Issued by NWS Hastings, NE

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683
FXUS63 KGID 281045
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
445 AM CST Tue Feb 28 2017

.SHORT TERM...(Today and tonight)
Issued at 445 AM CST Tue Feb 28 2017

Although we are not expecting any truly "high impact" weather,
it`s another tricky 24 hours of forecasting, especially with
regard to whether or not the currently-modest precip chances
advertised mainly during the afternoon/evening hours amount to
much. While the current forecast products look pretty "innocent"
and imply less-than-likely chances of rain showers possibly
transitioning to snow showers late in the day (with essentially
zero accumulation), it is a bit Gun-nerving to see some models
developing what could be 1 or 2 narrow bands of at least moderate
precipitation, which in a "worst-case scenario" might end up being
a surprise inch-or-so of wet/slushy snow especially in the
southeast CWA. Although will openly admit here that this scenario
is in the realm of possibility, model consistency is just not
high enough to gear the official forecast products this direction,
meaning that next few shifts will have to watch precip trends
closely just in case. Otherwise, as a secondary and much-shorter
term issue, at least patchy, transient pockets of fog will still
be possible for a few more hours this morning mainly within the
northeast 1/4 of the CWA where a west/northwest wind component has
not become established yet.

Taking a look at current/recent conditions as of 4AM/10z:
Other than some seemingly very localized/patchy fog (GRI airport
briefly reported 1/4 mile although local webcams showed nothing
close to that), it`s been another rather quiet night under
variable cloud cover ranging currently ranging from mostly clear
southwest to downright overcast under an area of low stratus that
has recently developed over our far north including Ord. In the
mid-upper levels, we are under broad southwest flow aloft ahead of
a fairly low-amplitude but large-scale trough migrating across the
Intermountain West. As expected to be the most likely scenario, it
has in fact been a dry night locally, with any limited shower/thunderstorm
activity in a more unstable airmass to our east remaining over
IA/MO. At the surface, a respectable low pressure center (around
1002 millibars) is now centered near the far eastern edge of our
Neb counties, after starting off last evening in northeast CO. As
this low has passed by, east-southeast breezes from earlier in the
night have steadily switched to out of the west-northwest and we
have even some some gusts of 20+ MPH. As mentioned, some very
localized fog did develop in the brief light-wind regime near the
passing low, but as suspected 24 hours ago any fog has certainly
been quite limited and not much of a story at all. The variable
cloud cover and changing wind speeds have made hourly temp trends
a little challenging, but as expected this will end up being the
overall-mildest night of the next several, with actual lows
holding up somewhere in the 29-38 range for the vast majority of
the CWA by night`s end.

Now turning to the forecast details through today/tonight...

Early this morning (through around 8 AM): Will maintain a generic
mention of patchy fog in northeast zones for a few more hours, but
not expecting widespread visibility issues with the
surface/boundary layer wind turning westerly.

Today (through around sunset):
As mentioned in the opener, even at this very close time range,
the jury is still out regarding whether only a small fraction of
the CWA sees precipitation (mainly this afternoon), or perhaps a
decent chunk of the area. You can pretty much find examples of
both deterministic and higher-res short term models that support
both scenarios. As a result of this inherent uncertainty, have
kept precip chances (PoPs) in less-than-likely percentages, but
have at least "some" chance mentioned almost everywhere, mainly
for this afternoon. In the mid-upper levels, forcing gradually
increases as the day wears on, as the large-scale trough axis
reaches the NE/CO/KS border area by late afternoon. Out ahead of
it, hints of a coupled upper jet structure will help to induce a
respectable southwest- northeast oriented swath of mid-level
frontogenesis/saturation. There are even hints of some meager,
mainly non-upright instability, thus making some of the model-
generated depictions of narrow banding not surprising. At any
rate, for the morning hours will only mention a slight chance of
sprinkles in Neb zones as forcing still remains pretty meager.
Then this afternoon, there is the potential for rapid development
of rain showers, perhaps changing to wet, slushy snow showers,
especially in a swath across the heart of the CWA, maybe including
the Tri Cities. In theory, rain showers should be the predominant
precip type, thanks to high temperatures (raised very slightly
from previous fcst) expected to range from near-40 far north, mid-
upper 40s central and mid-upper 50s far south. However, wet bulb
cooling effects could lead to a sneaky transition to at least a
snow mix. Again, this is all assuming that legitimate precip
actually develops within the frontogenetic forcing, as some
especially higher-res models show little more than sprinkle
potential through sunset. In other departments, except for perhaps
some partial sunshine in some areas this morning, the majority of
the day should average mostly cloudy-cloudy. Although not as windy
as tomorrow will be, steady northwest winds of generally 10-15 MPH
with some gusts to around 20 MPH should be the norm.

This evening/tonight:
Pretty much the same main forecast issue as the afternoon hours,
regarding how much precipitation will affect our CWA during the
evening (or whether most of it might miss us just off to the
south and/or east). No matter what happens, confidence is pretty
high that our entire CWA is precip-free by midnight at the latest
as the aforementioned large-scale rough axis passes by. Getting
back to before midnight though, chances for potentially narrow-
banded rain showers and/or snow showers will continue across
mainly the southeast 1/2 to 1/3 of the CWA, with precip chances
ending earlier in far western/northwest zones. Given the onset of
nocturnal cooling, there may be a better chance for one of these
"sneaky" little slushy snow bands to set up somewhere in our
southeast zones before precip ends altogether. For the umpteenth-
time though, there is just not enough forecast confidence to try
pinpointing this likely small-scale feature in a snow
accumulation grid or even going higher than 50 PoPs at this time.
Like during the afternoon, there are hints of weak instability,
with perhaps even some meager elevated convective instability
brushing into our far southern KS zones in the evening. Have not
put a formal mention of thunder in the forecast, although the risk
for a rogue rumble or two is probably non-zero (far better
thunderstorm chances will focus 100-200 miles east-southeast of
the CWA). At the surface, breezes will remain elevated, averaging
10-20 MPH from the northwest in the evening (strongest in southern
areas), then turning more westerly late in the night with
sustained speeds easing back a bit to 10-15 MPH. Despite what will
likely be rapidly clearing skies in most areas post-midnight
(except for the possibility of some low clouds filling back into
our northeast zones), the breezes should keep temps from really
tanking. Made very little change to lows, aiming for low 20s far
northwest, mid 20s central and near-30 far southeast.

.LONG TERM...(Wednesday daytime through Monday)
Issued at 445 AM CST Tue Feb 28 2017

Main forecast challenge will be fire weather on Wednesday.

As a longwave trough exits the central Plains, we will be in an area
of subsidence as a surface high pressure will build into the
southern Plains and give us some stout surface pressure gradients
in central Plains.

This is a case where Superblend is underdoing wind speeds by a solid
3 kts and is underestimating the amount of mixing that could occur.
CONSMOS is probably much more accurate in this case. Trends continue
for drier conditions by Wednesday afternoon, especially
south/southwest with the anticipated mixing up to near 725 mb in our
southwest, which will likely bring down some wind gusts of 30 mph or
so to the surface. Relative humidity continues to trend lower in our
southwestern CWA, and we currently have some areas getting into
critical fire weather range. We are still far enough out to not
issue a fire weather watch, especially as this is only on the cusp
of being a critical fire weather, rather than a slam dunk. We`re
still zeroing in on the area south a line from Elwood to Superior
Nebraska, which includes all of north central Kansas. We actually
have some spotty areas of our north central Kansas CWA that are
forecast to reach critical fire weather conditions. For now, this is
a bit dicey, as a lot hinges on how warm it can get for highs. We
pretty much know that the wind speeds will be sufficient for critical
fire danger, but dewpoints will also depend on how high we mix.
Currently, it looks like it could be a close call.

For temperatures, since I am expecting abundant sunshine, and at
least average highs and lack of appreciable cold air advection, I
decided to go closer to CONSMOS for highs and bump temps up a couple
degrees from Superblend as we get some decent mixing and downslope
flow with wind generally from the northwest. I am expecting highs in
the mid 40s to around 50.

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. Looks like a pattern where we will have to pay
attention to fire weather issues. Monday looks especially troubling
next week, but of course, that is a long way off. The only major
outlier to the completely dry forecast is that the Canadian model is
forecast some qpf over parts of our northern CWA for Sunday
afternoon/evening, but the area is so small, and the model is such
an outlier, that this has been ignored as of now, unless we can get
better agreement, or consensus, or consistency.

&&

.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Tuesday night)
Issued at 1247 AM CST Tue Feb 28 2017

General overview:
As evidenced by the fairly stark changes in ceiling/visibility
expectations in the last few TAF issuances, models/guidance
continue to struggle considerably especially regarding how these
first 12-18 hours play out. Fortunately, no matter what transpires
during this uncertain time frame, confidence in VFR conditions is
relatively high again by the final part of the period Tuesday
evening. Will take a closer look now at various elements...

Ceiling/visibility/precipitation:
Given how things have played out so far tonight, it`s quite
obvious that earlier model runs were hitting fog and sub-VFR
ceiling potential too hard, or at least too soon. Although am not
overly-confident in this, have geared this TAF issuance in a more
optimistic light with only very light VFR fog potential early this
morning and have also removed an MVFR ceiling, although have at
least maintained a scattered-MVFR cloud mention to hint at the
possibility. Should at least MVFR materialize and require further
adjustments, it would probably focus during the 12-00z time frame.
Precipitation-wise, models also continue to struggle at whether or
not at least a brief window of opportunity for steady
precipitation (rain and/or snow) might develop, but if this were
to occur it would most likely favor 18-00z, and have introduced a
generic "vicinity shower" (VCSH) to hint at the possibility.

Winds:
Right away early this morning, a surface low is migrating directly
across the area, which will be resulting in fairly light breezes
shifting from southeasterly to northwesterly. Previous TAF
issuances had indicated a period of respectable low level wind
shear (LLWS) early this morning, but with the very latest trends
focusing mentionable thresholds (30+kt of low level shear) at
least slightly south of both KGRI/KEAR, have somewhat reluctantly
removed this formal LLWS mention. Getting back to surface winds, a
relatively consistent northwesterly direction with speeds topping
out around 10-12kt is anticipated through most of the period, with
direction trending a little more westerly toward the end of the
period.

&&

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

$$

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



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