Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 220352
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
952 PM CST Wed Feb 21 2018

.UPDATE...
Issued at 952 PM CST Wed Feb 21 2018

A few comments regarding our light snow/light freezing drizzle
producing system during the next 18-24 hours, including recent
expansion to Winter Weather Advisory:

1) Most importantly, have decided to expand the Winter Weather
Advisory to include the remainder of our western/northern zones on
the Nebraska side, effectively putting our entire coverage area
(CWA) in a formal Advisory from late tonight through 6 PM
Thursday. The most basic reason for this expansion was that
expected light snow/ice accumulations in the original Advisory
counties were simply too close to the forecasted amounts in the
previously non-Advisory counties to justify leaving them out. It
is possible that eastern portions of the CWA may need the Advisory
extended a bit beyond 6 PM Thursday, but will defer to later
shifts to make this call.

2) Although the mere presence of the CWA-wide Advisory implies
that travel caution is advised area-wide for slick road potential,
the details of "exactly" how this unfolds are still not clear-
cut. The low-mid level moisture profiles in various models
continue to suggest that BOTH light snow and light freezing
drizzle could occur at varying times and locations, depending on
whether or not sufficient mid-level saturation (cloud ice) is
present. Both of these precip types could waver back and forth,
and to further complicate things, some areas could go up to
several hours with no precipitation falling (in between areas of
forcing).

3) Although there could always be a small-scale/mesoscale
"surprise" lurking in there, our current expectations are for
light amounts of both snow and ice, with snow amounts mainly
around 1" or less, and ice amounts ranging from only a very thin
glaze, up to perhaps around one-tenth of an inch, with the higher
amounts of ice generally favoring counties near/east of Highway
281. If not for the POSSIBILITY of freezing drizzle, the snow
amounts alone would normally not justify formal Advisory issuance.
As for timing of accumulations, generally speaking, the MAJORITY
of the CWA should see the greatest coverage/accumulation of wintry
precip during the daylight hours Thursday, as forcing maximizes
itself in association with the incoming mid level shortwave trough
that is still centered well to our southwest over the southern
CA/AZ region at this time.

&&

.SHORT TERM...(This evening through Thursday Night)
Issued at 253 PM CST Wed Feb 21 2018

Radar is starting to show some light returns spreading across
north central Kansas this afternoon...indicating mid level clouds
are beginning to encroach on the local area. Expect isentropic
ascent to intensify across the local area as we go through the
evening and overnight hours...with precip developing across our
Kansas counties later this evening...and spreading north
thereafter.

There is still a bit of question on precip type and timing...but
confidence was high enough that we will see some impactful
weather over the next 24 hours to justify issuing a winter weather
advisory across much of the local area. At this time...precip
should start off as snow through the early morning hours for most
areas...before transitioning to light freezing drizzle from south
to north by daybreak. While only a glaze to a few hundredths of
freezing drizzle is anticipated through the morning hours...this
will be enough to impact surfaces across the region making for
difficult travel.

During the late morning through early afternoon hours
tomorrow...expect many areas to transition from light freezing
drizzle to light snow for at least a few hours...before a
transition back to light freezing drizzle is expected again during
the late afternoon hours. Because of this second transition late
in the day...opted to keep the winter weather advisory in effect
through 6 PM Thursday afternoon...despite there likely being
several hours in between when freezing drizzle will temporarily
come to an end. In fact...the expiration time of the advisory may
eventually need to be extended a few hours if we do transition
back to freezing drizzle late tomorrow afternoon...with some
models hanging onto this potential for much of the evening.

.LONG TERM...(Friday through Wednesday) Issued at 253 PM CST Wed
Feb 21 2018

As seasonably cool...but dry day...is expected to start the
extended periods as much of the local area will climb to near or
slightly above freezing...as the region remains situated between
tonights departing system and the next upper level system
forecast to impact the local area overnight Friday into Saturday.

As this next system...currently dropping south across the Pacific
northwest...organizes across the southwest on Friday...expect the
focus to turn to the potential impacts it will have across the
local area. At this time...the 700 MB low is expected to close off
as it crosses Nebraska on Saturday...bringing a chance for
rain...freezing rain...and accumulating snow to parts of the local
area. While ultimately the exact track of this system will play a
large role in the systems impacts...it is safe to say that this
bears watching...with significant snowfall totals possible across
our north...and accumulating amounts of freezing rain possible
across the southern half of the local area. Thankfully...this
system appears to be fairly progressive...with dry weather
returning for the latter half of the weekend. This should mark the
beginning of at least a few days of near to above normal
temperatures and dry conditions...as the main jet shifts south and
keeps the focus of subsequent systems away from the local area.

By midweek...we could seen another upper level system brush the
local area...but this system is not expected to be nearly as
significant as the Friday night through Saturday system...which
will have a significant impact on at least portions of the
plains.

&&

.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Thursday)
Issued at 612 PM CST Wed Feb 21 2018

General overview:
This is a somewhat low-confidence forecast, especially beyond the
first 12 hours, regarding various elements including
ceiling/visibility and precipitation type. In the most basic
sense, most of this first half should feature only gradually-
lowering mid-level clouds with perhaps a few flurries. Then, much
of the latter half of the period is expected to deteriorate to
MVFR/IFR ceiling and at least MVFR visibility, with a mix of light
snow and/or light freezing drizzle. Winds should not be a major
concern, with speeds averaging near-or-below 10kt the entire time,
and direction rather consistent from the east to east-southeast.
Read on for more details if interested...

Ceiling/visibility/precipitation type:
As outlined above, the latter half of the period becomes a lower-
confidence venture regarding ceiling/visibility and precipitation
type. As a best-guess, have introduced MVFR ceiling at 11Z and an
IFR at 15Z, but both of these could be off by up to a few hours
and also off by a category. Precipitation-wise, light flurries
could break out as early as late tonight, and have hinted at this
with a "VCSH" mention. However, any chance for steadier light
precipitation that might consistently lower visibility looks to
largely hold off until after 15Z and perhaps continue through much
of the remainder of the valid period. As for precip type, it would
seem to mainly favor light snow, but there is just enough of a
hint of occasional light freezing drizzle that both types are
currently mentioned. Total snow/ice accumulations are currently
expected to be fairly minimal, with less than 1 inch of snow, and
only a few hundredths of an inch of possible icing.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Winter Weather Advisory from 3 AM to 6 PM CST Thursday for
     NEZ039>041-046>049-060>064-072>077-082>087.

KS...Winter Weather Advisory until 6 PM CST Thursday for KSZ005>007-
     017>019.

&&

$$

UPDATE...Pfannkuch
SHORT TERM...Rossi
LONG TERM...Rossi
AVIATION...Pfannkuch



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