Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 191811
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
1211 PM CST Thu Jan 19 2017

.UPDATE...
Issued at 1200 PM CST Thu Jan 19 2017

Visibility has been gradually improving late this morning. A few
isolated locations may continue to see visibilities down to one
quarter of a mile early this afternoon...but the overall trend is
improving visibility and thus the dense fog advisory will be
allowed to expire at noon.

Temperatures will remain very steady today with the low stratus
and fog.

UPDATE Issued at 612 AM CST Thu Jan 19 2017

It`s been tempting to expand the ongoing Dense Fog Advisory east
to include the remainder of the eastern CWA, but based on trends
over the past 30-60 minutes, if anything, even some of the airport
sites in the central/west have come up "slightly" with a lot more
1/2 to 1 mile visibilities than 1/4 or less. So, while have no
intention of cancelling any ongoing Dense Fog Advisory counties at
this time, also have no immediate plans to expand eastward and
will continue handling these eastern counties with an SPS for
"near-advisory" conditions. Of course, if neighboring WFOs to our
east ultimate decide to issue Dense Fog Advisory then this becomes
another factor to consider if/when that occurs.

&&

.SHORT TERM...(Today and tonight)
Issued at 453 AM CST Thu Jan 19 2017

Hopefully folks were able to get out and enjoy the sunshine the
past few days, because we diving headfirst into a very gloomy/gray
and at times foggy and drizzly few days here. Despite this "bad
news", the comparably good news is that temperatures are expected
to remain at least slightly above freezing through these next 24
hours, thus squelching the risk of what would otherwise
undoubtedly be a more dire freezing drizzle/slick roads
situation if were were averaging 10 degrees colder.

Starting with the here and now as of 10z/4AM...kudos to several
short term visibility progs from models such as the SREF/HRRR/RAP
for really nailing the rapid onset of an advection fog event over
the past few hours. When the night began, most of the CWA was
mostly clear and fog-free. However, especially since 2 AM, a
nearly solid wall of low stratus and fog has rapidly expanded
north-northeast across the CWA on light-but-steady moist southerly
breezes. As with most fog events, one often never knows just how
far visibilities will tank given its fickle nature, but in this
case we`ve had enough automated sites drop into the 1/2 to 1/4
mile range along with webcam evidence to justify a Dense Fog
Advisory for all counties along/west of Highway 281. Ran the
advisory through Noon to hopefully capture the worst of it, but
this wouldn`t be the first nor the last time that at least a part
of the advisory area may need extended into the afternoon. Also,
cannot rule out dense fog creeping east of the current advisory,
and will be closely monitoring over the next few hours for a
possible extension to the east. If anything, steady southerly
breezes of generally 5-10 MPH seem to be keeping the truly dense
fog from being truly persistent, and thus many places will likely
keep fluctuating anywhere between generally 1/4 and 3/4 mile
visibility for several hours. In the bigger picture of the mid-
upper levels, water vapor satellite and short term model data
reveals a split flow pattern of sorts, with our main feature in
the Central Plains being a slow-moving, closed mid level low
almost directly overhead that is very moisture-starved in the
mid-levels. Meanwhile, a larger scale broader trough is noted over
the western CONUS, which will be more of a player in our weather
getting into Friday-Friday night. At the surface, a weak lee
trough to the west is promoting the aforementioned light-but-
steady south-southeast breezes and moisture advection, with
seasonably-moist dewpoints/seasonably-mild morning temps both
ranging from mid-upper 30s in most areas to low-mid 40s far south.

Now looking ahead forecast-wise over the next 24 hours...

Today:
Not an overly-"easy" forecast, but as already touched on, at least
the impacts of fog/non-freezing drizzle are a far cry from the
potential impacts if we were roughly 10 degrees colder. In the
mid-upper levels, the aforementioned slow-moving/mid-level
moisture starved closed low will lumber only very slowly east
across the heart of NE/KS as the day wears on, with its 500
millibar center only reaching northeast KS by sunset. Meanwhile,
surface and low-level features will remain remarkably unchanged,
with the High Plains lee trough to our west continuing to promote
moisture-laden south-southeast breezes up to around 10 MPH or
slightly higher. Low-level relative humidity progs from the
NAM/RAP are insistent that widespread low stratus (very low in
many areas) will hold firm over the entire CWA today, while just
to our west, a fairly sharp north-south dividing line between our
widespread overcast and considerably more sunshine will set up
along an axis from north central Neb into western KS. In other
words, you`d only have to drive a short distance west of our
western CWA to find some sun, but instead our entire 30-county
domain is expected to stay "socked in". The worst of the fog
should focus this morning, but we will have to closely monitor for
at least localized dense fog persisting into the afternoon. In
addition, if it hasn`t started already, saturation depth to
between 1/2 and 1 kilometer will prove plenty sufficient for
fairly widespread light drizzle at almost any given location
through the day, at least on an off-and-on basis. Drizzle is
always tricky to portray with a PoP as it is often non-measurable,
so in this case opted to gear the forecast toward a "high chance
of non-measurable drizzle" as opposed to what what might be
perceived as a "low chance of measurable drizzle" if a formal
20-30 PoP was included...different opinions may exist on this
though from later shifts. Temp-wise, certainly not looking at a
big boost from current readings, but the persistent southerly flow
should be good for roughly a 5-degree rise from current readings
and thus will aim most Neb zones into the low-40s and KS zones
more so the mid-40s. This is a few degrees below raw numerical
guidance on average.

This evening/tonight:
In many ways, very little change is anticipated from the daytime
hours, with the main exception being that limited nocturnal
cooling may again promote increasingly widespread "light" fog and
at least limited areas of potentially dense fog. In addition, the
continued low-level saturated layer will continue to foster areas
of light NON-freezing drizzle almost anywhere in the CWA, except
perhaps far western counties after midnight as forecast soundings
show the saturated layer decreasing to under 1/2 kilometer there.
Fog-wise, with the ongoing dense fog issues this morning opted not
to get too "cute" with fog for tonight yet, but blanketed a
generic "areas of fog with visibility 1 mile or less" everywhere.
In other words, another dense fog event is not out of the
question, but it`s too soon to pinpoint the most favored areas. In
the mid-upper levels, the slow-moving mid level system will
finally depart north-northeast toward the Sioux Falls region as
the night goes on, getting a "kick" from behind by the next
incoming disturbance. Temp-wise, assuming that we remain totally
socked in under low stratus as currently anticipated, most places
should struggle to drop more than 5-ish degrees from daytime
highs. Ended up keeping lows similar to previous forecast with a
range from low-30s northwest to upper 30s southeast, which is a
solid 4 degrees above raw MET/MAV guidance, which clearly looks a
little too cold given the setup.

.LONG TERM...(Friday daytime through Wednesday)
Issued at 453 AM CST Thu Jan 19 2017

We kick off Friday under the influence of a negatively tilted upper
level trough with drizzle and fog probable in the morning, continued
from the previous night, and drizzle could continue well into the
afternoon as a pronounced dry mid-layer is still forecast. By Friday
evening, we finally seem to saturate the elevated dry layer to have
the potential for rain, and then snow, once it gets cold enough.

By Saturday, we expect shortwave ridging behind the aforementioned
trough. For the rest of the weekend, a closed upper low spills into
the Plains far enough south to keep us in a zone of subsidence, so
it looks like a dry forecast into Monday as ridging ensues aloft.

Yet another broad synoptic trough punches into the Intermountain
West. With a positive tilt, this puts us within southwest flow aloft
with a leading piece of energy moving by Tuesday, and bringing
colder air with it, along with a good chance of precipitation. It
may take awhile for any precip to clear, as it appears to be a
rather elongated low that stretches back to the west. With the
overall synoptic trough influence, it appears that the cooler 30s
for highs might stick around beyond the limits of the extended
forecast. There is potential for a decent snowfall with this system,
but I am not the most confident with temperature profiles at this
point. Also, with the downward trend with Superblend POPs, this
places most of the CWA with POPs of 50 percent or lower. Both the
ECMWF and the GFS track the 700 mb low across the CWA, so at this
point, perhaps our northwest stands the best chance of some
accumulating snow. At this point, I would need a bit more
confidence, with the trends headed the other way for POPs, so this
will be left out of the HWO for now. We still have plenty of time
to get a better handle on this system next week.

&&

.AVIATION...(For the 18Z KGRI/KEAR TAFS through 18Z Friday afternoon)
Issued at 1200 PM CST Thu Jan 19 2017

The primary concern will certainly be focused on the possibility
of IFR and even LIFR ceilings and visibility through portions of
the TAF forecast period. We expect some improvement in visbility
and ceilings this afternoon, although we will likely remain IFR.
However, the visibility and ceilings are fairly likely to again
worsen as we head into the evening and overnight hours. Thankfully
temperatures are expected to remain above freezing wherever fog
and drizzle is present and thus no freezing fog or freezing
drizzle is expected. The wind will continue to be light and
generally from the south or southeast.

&&

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

$$

UPDATE...Wesely
SHORT TERM...Pfannkuch
LONG TERM...Heinlein
AVIATION...Wesely


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