Scientific Forecaster Discussion
fxus63 kgid 290522
Area forecast discussion
National Weather Service Hastings NE
1222 am CDT Sat Apr 29 2017
issued at 645 PM CDT Fri Apr 28 2017
Updated the forecast to include some drizzle this evening.
Short term...(this evening through saturday)
issued at 352 PM CDT Fri Apr 28 2017
Aloft: a longwave trof was over the cntrl/wrn USA. A small mid-
lvl low was along the neb/Iowa border and it will cont to lift north
and weaken. The larger main upr circulation was over UT/co.
Upstream amplification was occurring and this was resulting in
additional height falls over the desert SW. The low will drop
into Arizona/nm tonight and head east across nm tomorrow. 12-hr height
falls (00z-12z) at fgz were 160 M.
Surface: a Continental polar air mass encompassed the nrn 2/3 of
the Continental U.S. With the polar front extending from new eng thru the
Gulf Coast states to Texas. Low pres was forming along the front over
nm. Another cold front extended from Michigan-IL-cntrl Kansas with a weak
low over ern Kansas. This low will dissipate and the associated front
will sink S and merge with the polar front...strengthening the
temp gradient along it...as the nm low ejects NE along it and
conts to organize thru tomorrow.
Precip conts to decrease north of I-80 and should be completely gone
by 23z. It took longer to dissipate than expected.
Tonight: cldy with steady temps and decreasing winds. A
significant area of rain should develop along the Kansas/OK border in
the rrq of a 120 kt upr-lvl jet streak. The nrn fringe of this
rain should lift to the Kansas-neb state line toward dawn. It cold be
cold enough for rain to mix with or change to snow over S-cntrl
neb. But most of S-cntrl neb should see little or no precip.
Sat: cldy...windy...and contd unseasonably cold. Periods of rain
gradually lifting/expanding north. If any snow is falling during the
am it will change to rain. Temps will struggle to get into the low
Long term...(saturday night through friday)
issued at 352 PM CDT Fri Apr 28 2017
Aloft: the upr low will be over ern nm at 00z/sun...embedded
within the based of the trof in a highly amplified longwave
pattern. The last 2 runs of deterministic models and GFS ensemble
members are tightly clustered on this low lifting NE across the Texas
Panhandle and wrn OK Sat night into sun and arriving over ern Kansas
by 00z/Mon. This is a very favorable track for substantial pcpn
over our fcst area (fa). The low is then fcst to cross mci into
Iowa by 12z/Mon and cont lifting north-northeast. Northwest flow will follow the rest
of next wk with progressive longwaves. There will be some
minor/weak shortwave trofs finished with a more substantial trof
passage Wed. Then significant height rises will occur as a high-
amplitude wrn USA ridge makes its way over the plns Fri.
Surface: deepening low pres will occlude over Kansas sun and then head
across Iowa to WI Mon. A weak low will dive southeast thru the forecast area Tue
followed by another missing just NE of the forecast area Thu. Then high pres
Temps: much colder than normal sun and Mon. Mon could be colder
than we're indicating if a swath of snow is pasted across part of
the forecast area. Then returning to near normal the rest of the week. Fri
currently looks decent...but could be cooler than we are currently
Precip: a very wet system will be in progress sun and end by dawn
Mon. There are a couple minor/light precip possibilities until Wed
eve...but they will pale in comparison to this weekend.
Periods of rain will cont Sat night thru Sun night...but there is
potential for rain to mix with and/or change to snow...possibly
moreso than we currently have depicted in the fcst. We are
especially concerned about Sun afternoon-eve when the deformation
zone pivots over the forecast area. Precip will probably fall as snow for
much of the day sun into Sun night. Accumulating snow late in the
Spring or fall (when leaves are on the trees) is bad news.
We saw how easy this cold air mass produced snow today...
especially along and north of I-80.
Watch reasoning: ordinarily we have warning or near-warning-lvl
snowfall in the fcst when we issue a winter stm watch (wsw). That
is 6" in 12 hrs or 8" in 24 hrs. Parts of this watch area do not
have 6". However...we have the discretion to issue based on
impacts (or potential impacts).
We remain very concerned about the potential for hvy wet snow
mainly Sun afternoon into sun eve. There are mitigating factors
(warm ground etc). If it difficult for snow to accumulation
during the day at this time of year. This very well may up mostly
melting on contact...but just in case it doesn't...we don't want
people to be caught unaware.
Svrl inches of hvy/wet snow with so much foliage on the trees
would be devastating with widespread damage and power outages
where trees exist. Much of the greatest tree coverage is in towns
It is entirely possible the accum does not pan out (which would
be very good) and that much of it will melt on contact. If this
occurs...this will not be a busted fcst. Wsw's only need 50%
confidence. We've issued this to highlight the "potential" for
what would be damaging accumulations of snow.
In the end...for this fcstr...the cost of not issuing and a major
snowfall occurring is just too great.
Please cont to closely monitor the fcst this weekend.
One other potential for hazard wx is high winds sun. Even if we
don't issue a winter wx advisory or a Winter Storm Warning...we
might need a High Wind Warning for part or most of the forecast area.
Aviation...(for the 06z kgri/kear tafs through 06z Saturday night)
issued at 1219 am CDT Sat Apr 29 2017
IFR condtions remain entrenched and ceilings/visbilities will
lower by midday as precipitation develops and spreads north into
the kear and kgri area. Precipition will continue nearly non-
stop through tonight and tomorrow. Best shot for snow comes just
after this forecast cycle. Well advertised storm will also bring
increasing northerly winds.
NE...Winter Storm Watch from this evening through late Sunday night
Kansas...Winter Storm Watch from this evening through late Sunday night