Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 230329
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
929 PM CST Thu Feb 22 2018

.UPDATE...
Issued at 928 PM CST Thu Feb 22 2018

Regarding the rest of tonight, including the Winter Weather
Advisory:

1) Needless to say, light freezing drizzle hung on several hours
longer than originally expected this evening across much of our
coverage area (CWA), but seems to finally be letting up in
coverage per the last few radar scans. Nonetheless, with automated
airport stations within much of the eastern half of the CWA still
reporting visibilities of 3 miles or less as of 9 PM (essentially
"ground truth" that light freezing drizzle is still falling),
along with actual ground truth reports of lingering light
freezing drizzle per social media and law enforcement sources,
opted to once again extend counties along/east of Highway 281
until Midnight. This may have been "overkill" for some areas, and
some of these Advisory counties could even end up being cancelled
early, but did not want to take any chances given the slower-
than-expected trends this evening. No matter, confidence is
actually pretty high that no Advisory will be needed past Midnight
as light westerly low-level flow continues bringing light freezing
drizzle to an end from west-to-east with time.

2) Once the Winter Weather Advisory is finally gone, about the
only concern the rest of the night is whether "impactful" fog
forms in parts of the CWA. While the prevailing light west-
northwest wind direction generally strongly discourages anything
more than "light fog" in our CWA, these winds are light enough
that some models (including HRRR/RAP) continue hitting especially
our southwestern counties fairly aggressively with fog
development, mainly during the 4AM-9AM time frame. Do not have
enough confidence to formally mention "dense fog" in forecast
products, but do have 1-mile-or-less visibility mentioned for
areas mainly southwest of a Kearney-Osborne KS line, where fog
development would seem most favored.


UPDATE Issued at 644 PM CST Thu Feb 22 2018

A few comments regarding our pesky ongoing combo of light snow
and/or light freezing drizzle:

1) First and foremost, have extended the Winter Weather Advisory
for our entire coverage area (CWA) 3 more hours until 9PM (except
for the 4 northeast counties that already went until 9PM), as the
slower-than-expected exit of at least light wintry precipitation
rendered the original 6PM expiration too early. As of this
writing, the majority of light measurable snow/freezing drizzle is
within our eastern counties, but radar and ground truth suggests
pesky very light freezing drizzle extending at least as far west
as Highway 183, and did not feel it was prudent to drop the
Advisory for these western counties yet either, as even very light
freezing drizzle could pose issues. It is possible that a few of
our far eastern counties may even need extended past 9PM, but this
will be addressed in a few hours. Per RAP13 relative humidity
progs in the lowest 1 kilometer, low level saturation deep enough
to promote freezing drizzle should gradually erode from west-to-
east over the next several hours, but again, a bit slower than
earlier expected.

2) This has been an interesting/pesky event, as much of the CWA
spent nearly all day in a "lull" with little to no precipitation
in the wake of mainly light snow overnight. However, as upper
forcing increased mid-late afternoon with the arrival of a fairly
decent shortwave trough into the Central Plains, areas of wintry
mix quickly increased in coverage, with several of especially our
southern counties reporting a light glaze of ice and slick
conditions, including at least a tenth of an inch of ice
accumulation in Burr Oak KS per a NWS coop observer.

&&

.SHORT TERM...(This evening through Friday)
Issued at 328 PM CST Thu Feb 22 2018

Winter Wx Advisory remains as posted earlier.

Hazardous Weather: A little snow or frzg drzl thru early this eve.

Aloft: RAP analyses/RAOB/sat imagery show a longwave trof in the W
with SW flow over the Plns. One weak shortwave trof lifted out of
the base of the Wrn trof this AM was over Wrn Neb and was continuing
to weaken. Another moved into the base this AM and was lifting into
NM/CO. This trof will cross the CWA this eve. Heights will rise a
bit in its wake as a shortwave ridge develops and crests over the
rgn tomorrow. There is a potent shortwave trof diving down the
backside of the Wrn trof today. As it reaches the trof base tomorrow
...the Wrn trof will begin moving E.

Surface: Strong high pres over Ont/Qb extended back into the rgn. It
will lose its influence as a weak sfc trof crosses the area tonight
in response to the forcing aloft. Then Nrn Rockies high pres will
begin building in and slide into the Nrn Plns tomorrow.

Winter Wx Advisory: believe it`s shaky on verifying. It`s mainly out
for frzg drzl potential and just don`t have a good feeling that it`s
going to materialize. We`ve had a couple reports thru the day here
and there...but predicting frzg drzl is usually difficult. Fear that
we may take a hit on this Advisory...but plan is to keep it going at
least to cover the evng commute "just in case" it materializes.
Confidence is low. What appears more likely is a bit of snow here
and there and it will come and go. Not looking at anything steady
for more than 1 hr.

Tonight: Cldy to start and if any precip is occurring after 00Z it
should be done by 03Z. Then decreasing clds and that presents a fog
problem

.LONG TERM...(Friday night through Thursday)
Issued at 328 PM CST Thu Feb 22 2018

Winter Storm Watch has posted for Fri night into Sat.

Potentially Hazardous Weather: Snow and frzg drzl Fri night-Sat.

Snow: we cont to monitor potential for a swath of svrl inches (6+)
across the NW 1/4 of the CWA. This is still 36 hrs out but our
current fcst offers:

6-8" from Lexington-Loup City-Greeley
3-5" Tri-Cities (and Cambridge-Columbus)
1-3" Phillipsburg-Geneva and York

These totals are subject to further revisions.

Believe we`ll see narrow band of snow develop and blossom in a
def zone from NE CO into SW Neb Fri eve. This band will move into
Wrn CWA after midnight and spread NE mainly thru S-cntrl Neb
during the day.

Preceding the snow (SE of the watch)...frzg drzl could be a
problem due to persistent lift in the absence of deep moisture
(dry DGZ). So things could ice up first before some snow moves in.

The EC/NAM are the most emphatic about lower precip amts over the
SE 1/2 of the CWA (basically S and E of the Tri-Cities).

As always there is model variability. There can always be a narrow
swath of locally higher snowfall amts than we are advertising.
Models rarely agree with each other (or with themselves run-to-
run) on where this occurs...especially beyond 24 hrs.

So cont to monitor future fcst updates.

As for the big picture: the mean trof will remain in the W thru
next Thu...but discontinuous retrogression downstream over the
Atlantic will eventually result in a trof also developing over the
E. This will force a longwave ridge to form over the Plns. While
this is a pattern change...things could still be interesting
in early Mar as the pattern heads toward being blocked.

Temps: below normal thru Sun...then near normal Mon-Tue. Possibly
returning to below normal Wed-Thu.

Precip: above normal due to Fri night-Sat event...then no
significant precip foreseen thru next Thu.

&&

.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Friday)
Issued at 644 PM CST Thu Feb 22 2018

General overview:
This is a low-confidence forecast, especially with regard to
ceiling/visibility trends during the first 16 hours or so, and
precipitation trends these first 2-3 hours. Beyond the first 16
hours though, confidence is overall-higher fairly high in VFR
ceiling/visibility (especially visibility), although there are
actually hints of an MVFR ceiling during this time as well.
Surface winds will not be a big issue, with speeds averaging under
10kt throughout, and direction trending from variable this
evening, to northwesterly late tonight/early Friday, then
eventually northeasterly Friday afternoon. Read on for more
element-specific details...

Precipitation:
Any lingering very light freezing drizzle and/or snow should end
within the next 2-3 hours, with dry conditions expected
thereafter. A very light glaze of additional ice is the main
issue.

Ceiling/visibility:
Especially through these first 6 hours (and possibly beyond), both
ceiling and visibility should fluctuate between IFR/MVFR
categories, with light fog lingering even after any light
precipitation ends. One of the big question marks is whether more
persistent fog (perhaps LIFR) could develop late tonight/Friday
morning, but confidence is low on how bad fog gets (especially
with light westerly breezes) so have kept prevailing visibility
no worse than IFR for now. Am currently indicating a return to VFR
(both ceiling and visibility) by late Friday morning, but some
models/guidance suggest that even if VFR ceiling returns for a
time, an MVFR ceiling could redevelop during the day so this will
need considered in later issuances.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Winter Weather Advisory until midnight CST tonight for NEZ040-
     041-047>049-062>064-075>077-085>087.

     Winter Storm Watch from late Friday night through Saturday
     afternoon for NEZ039>041-046-047-060-061-072-073-082.

KS...Winter Weather Advisory until midnight CST tonight for KSZ006-
     007-018-019.

&&

$$

UPDATES...Pfannkuch
SHORT TERM...Kelley
LONG TERM...Kelley
AVIATION...Pfannkuch



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