Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 251751
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1151 AM CST Wed Feb 25 2015

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 321 AM CST WED FEB 25 2015

Water vapor imagery this morning depicts two shortwave troughs
impacting the region...one was shifting eastward across Texas and
Oklahoma while a trailing disturbance enters northern Montana. This
wave is expected to impact northeast Kansas later this afternoon
into the early evening hours.

Surface and h85 trough currently over western Nebraska tracks
southeast towards the area today as southwesterly winds increase
just after sunrise. Main change to the forecast was temperatures and
precip chances based on guidance trending with a slower track of the
cold front and incoming trough. Strong westerly warm air advection
through 850 mb quickly boosts temps to the middle and upper 50s for
highs. Wind speeds and gusts gradually increase to near 10 to 20 mph
with gusts to 25 mph sustained as the cold front approaches. With
the warmer temps in place, any precipitation that occurs through 6
PM will mainly be in the form of light rain. Best chances for precip
are mainly focused north of Interstate 70.

Chances for precip increase to likely after 6 PM this evening as the
upper support with the wave enters southern Iowa. Initial precip type
is a rain and snow mix as the sub-freezing temps lag a few hours
behind the boundary. Latest models are still fairly consistent with
the track of the upper trough impacting the edge of far northeast
and east central areas of the CWA...with heavier amounts further
east. Weak signals of low and mid level frontogenesis in cross
sections and coinciding lift in the dendritic zone suggest light
amounts from a dusting up to an inch of snowfall, focused along and
east of highway 75. Warmer ground temps however should restrict any
accumulation to grassy and elevated surfaces.

Next concern for this evening is the strong winds and very cold
temps to arrive early Thursday morning. Tightening pressure gradient
behind the cold front result in sfc northwest winds increasing
between 15 and 25 mph with gusts up to 35 mph beginning late this
afternoon through the evening. Not confident in winds reaching
advisory criteria and will hold off on issuing for this forecast.
Main impacts from the winds and snowfall are visibilities which may
occasionally be reduced especially in the early evening period.
Stout cold air advection in the boundary`s wake will quickly drop
temps into the low teens and single digits for lows. Wind chills
fall as well between 3 and 12 degrees below zero Fahrenheit.

.LONG TERM...(Thursday through Tuesday)
Issued at 321 AM CST WED FEB 25 2015

Thursday - Friday night:
The post-frontal airmass is very cold and will move quickly
straight out of Canada and into the local area. A reinforcing shot
of cold air will follow shortly behind the main front, so even
with a good chance for some sunshine on Thursday the temperatures
will be 25 to 30 degrees below normal. 850 temperatures as cold as
-20C will hang around into Friday morning, and the low will prove
tricky to forecast as high pressure builds in from the north and
mid level clouds attempt to build in from the south. It seems
likely that despite the proximity of the surface high, surface
winds will remain at 5 kts or better for much of the night so
temperatures will stay a bit warmer...and any incoming clouds
would keep things a bit warmer too. The exception may be far
northern KS where winds could go calm with clear skies.
Regardless, the wind chill is likely to range from -5 to -15
early Friday morning. Clouds should increase through Friday, so
while there will be weak warm advection it will not matter much
and highs will remain very cold. Dry conditions are expected
through this period.

Saturday - Saturday Night:

Saturday into Saturday night appears to be the more straight-
forward part of this period of precip. All model guidance has
been remarkably consistent in ejecting a very weak short wave
trough across the area very early Saturday followed by a stronger
short wave trough during the afternoon/evening with additional
weak vort maxima overnight. The first weak wave will help
saturate a somewhat dry airmass. The second wave will bring with
it deep, persistent isentropic ascent along with additional
moisture and upper level support. The additional short waves
overnight may also work with increased low level convergence and
developing frontogenesis as winds in the northern part of the area
gain a more northerly component and the temperature gradient
tightens. The GFS and GEM are slower with the onset of precip and
generally have less precip but see no reason for this to be the
case as both exhibit the persistent lift and only minimal dry air
to overcome on Saturday. GFSens even suggests very high
probability of earlier onset and equal or greater precip amounts
than the operational GFS. With this in mind have increased PoPs
through the evening and expect precip to start from SW to NE
between late morning and early afternoon. There may be a few
breaks in the wake of the short waves during which the dendritic
layer dries a bit but isentropic ascent continues...and this could
result in periods of light freezing drizzle. Snow will probably
not be particularly heavy, although there are indications of CSI
or weak convective instability at times directly associated with
the passage of the strongest vort maxima. However, even without
heavy snow, the duration of the event will likely allow several
inches to accumulate in much of the area by sunrise on Sunday.

Sunday - Sunday night:
This period is perhaps the most uncertain of the long term. Will
need to closely watch a northern stream short wave trough forecast
to dive south out of Canada during the period as the interaction
of this wave with the flow pattern overhead will drive our local
forecast. The northern stream remains active and meridional over
northern Canada throughout this period so it seems that any strong
wave diving south will have momentum on its side for a deep
southerly push. Current run of the operational GFS is weaker with
this system and absorbs some energy into the southwestern CONUS
trough. It is also influenced by timing of another somewhat
stronger short wave ejecting from the southwest into the Plains.
This results in less southward push of the surface high and allows
precipitation, at times heavy, to continue over the area in the
form of snow or a mix. Meanwhile, the ECMWF and GEM (especially
the ECMWF) are quite strong with the northern trough and blast dry
high pressure into the area. This would act to give us a break
from the precip into early Monday and would result in several
inches less snowfall than otherwise. It will certainly be
interesting to watch how this interaction plays out.

Monday - Tuesday:
At some point in this period, a rather strong, neutral to
negatively tilted trough is likely to move across the Plains. A
significant portion of model guidance tracks the max 500 hPa
height falls over northeast KS although this is obviously subject
to change. This current consistency in track is interesting as
ECMWF and GFS operational runs handle the evolution of this trough
(and interaction with other pieces of energy) very differently.
This suggests that the current model consistency should perhaps be
taken with less confidence than it would lend at first glance.
Regardless of where it tracks, given the general flow pattern
surrounding the energy this system appears likely to take on the
shape of a rather classic mid-latitude cyclone. The northern
sector of the system has potential to produce a rather impressive
snow storm while the southern part of the storm will see plenty of
precip in the form of rain. This storm system is certainly worth
watching as the potential impacts are on the high end...but very
dependent upon the eventual evolution and storm track.

&&

.AVIATION...(For the 18Z TAFs through 18Z Thursday Afternoon)
Issued at 1151 AM CST WED FEB 25 2015

VFR conditions are expected until frontal passage in the 21Z to
00Z time period at the terminals. Light precipitation is expected
with vsbys remaining above mvfr in the form of rain and snow. MVFR
cigs are expected behind the front along with winds shifting to
the northwest and north around 20kts with gusts to around 30kts
through 13Z then decrease to around 15 kts by 16Z Thursday.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Bowen
LONG TERM...Barjenbruch
AVIATION...53






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