Area Forecast Discussion
Issued by NWS Topeka, KS
FXUS63 KTOP 271125
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
525 AM CST Fri Feb 27 2015
...Update to aviation forecast discussion...
.SHORT TERM...(Today and Tonight)
Issued at 332 AM CST FRI FEB 27 2015
A series of shortwave troughs were noted on water vapor imagery
this morning over the western CONUS. Closest disturbance is
currently across Colorado and the Panhandles, expected to impact
areas south and west of the CWA later today. Meanwhile the main
trough axis was sliding southward over the Pacific Northwest,
expected to bring periods of snow and sleet to northeast Kansas
beginning this weekend.
In the meantime, surface ridge in control of the region today as
light northerly winds veer to the east and southeast by tonight.
Passing disturbance to the south increases high clouds throughout
the day with highs similar to yesterday in the lower 20s.
First vorticity maxima ejects eastward this evening, with chances
for snow increasing from the panhandles into portions of central
Kansas overnight tonight. The CWA remains dry through 6 AM as
saturation in the mid and lower levels increases, creating some
insulation for lows a tad warmer in the lower to middle teens.
.LONG TERM...(Saturday through Thursday)
Issued at 332 AM CST FRI FEB 27 2015
Saturday through Sunday, winter weather and accumulating snowfall
remain likely for the whole forecast area. Models have shown
reasonable continuity with the overall synoptic pattern while
mesoscale differences show some variability in where the higher
snow amounts may occur. The latest trends from the 00Z guidance
have been to shift the higher QPF to the south slightly,
pinpointing areas along I-70 and south as the most likely area for
higher snow amounts. As a closed low digs south along the CA
coast, low level flow is progged to return from the south
increasing moisture advection and isentropic lift into the cold
air already in place. However the weather does not look to be
strongly forced as models indicate only weak perturbations within
the southwesterly flow aloft. There continue to be indications of
some mid level frontogenesis as well. But except for the GFS which
wants to amplify the pattern a little bit more than the model
consensus, the frontogenesis and associated vertical motion
appears to be modest. Because the forcing in general leaves
something to be desired, the forecast anticipates snow amounts to
generally be in the advisory range. Ensemble plumes seem to
cluster in the 2 to 5 inch range across the forecast area which
would fit in with the expected forcing. However there are a couple
factors that have led me to maintain a forecast of 3 to 6 inches.
The first being relatively high snow to liquid ratios initially
with the system. The GFS and NAM show ratios between 15:1 and 20:1
as the snow is forecast to begin during the day Saturday. By
Sunday morning, these ratios are likely to decrease to around 10:1
as warm air continues to advect north. The second consideration is
models showing some decent conditional symmetric instability and
possibly weak upright instability Saturday night into Sunday
morning which could enhance the snowfall rates. So for these two
reasons have kept the forecast on the high side of the ensemble
clusters. Think that the probability for widespread accumulations
above 6 inches is relatively small at this time and would expect
any amounts of this magnitude to be localized. Considered issuing
a winter weather advisory for Saturday through Sunday morning.
However given the small scale variability, felt it was reasonable
for the day shift to take a look at the 12Z guidance first.
Forecast soundings from the NAM and GFS suggest the precip type
should be all snow Saturday night. By Sunday morning, persistent warm
air advection could cause a warm nose to move into east central KS
causing sleet to mix in with the snow. Although the models may not
be as far north with the warm nose. This will be something to
watch in later runs. With cloud cover and precip Saturday and
Saturday night, temps should not show much of a diurnal change from
highs to lows and have lows Sunday morning about 5 degrees cooler
that the highs Saturday. Models show some dry air working into the
area Sunday afternoon and Sunday night with little if any vertical
motion. Because of this think there will be a break in precip.
allowing temps to warm above freezing Sunday afternoon.
For Monday through Wednesday, the upper trough to the west if
expected to propagate east through the plains. There are some
timing differences between the GFS and ECMWF which impact the
forecast for Tuesday and Wednesday. At this time, have leaned
slightly towards the slower ECMWF solution which would lift a lead
shortwave through the plains Tuesday night, about 12 hours slower
than the GFS. It would also keep warmer temps in place across the
forecast area Monday and Tuesday with any precip likely in liquid
form. Low level warm air advection is progged to persist through
the night Monday night so there is the chance we could see slowly
rising temps through the night. The warm air advection should be
warm enough that parts of eastern KS warms into the lower 50s for
Tuesday. For now the better chances for precip are Monday night
and Tuesday, but if models continue to trend slower the POPs may
need to be shifted later in time to Tuesday night.
Things should dry out by Wednesday evening as northwest flow aloft
returns and another ridge of high pressure moves south through the
plains bringing dry air and cooler temps to the plains.
.AVIATION...(For the 12Z TAFs through 12Z Saturday Morning)
Issued at 522 AM CST FRI FEB 27 2015
VFR prevails at terminals as exiting high pressure veers light
north winds to the east and southeast below 10 kts. Lower cloud
cover increases from the southwest as next disturbance and
deteriorating conditions approach after 12Z Saturday.