Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 252345

Area Forecast Discussion
National Weather Service Topeka KS
545 PM CST Sat Feb 25 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Sunday)
Issued at 232 PM CST Sat Feb 25 2017

Recent water vapor imagery and morning sounding data showing
upper flow becoming more zonal between upper wave moving northeast
across the Great Lakes and multiple waves coming southeast into
the western CONUS. Weak surface ridge was over eastern sections of
Kansas with gusty southwest winds returning to central and
southwestern portions of the state. Visible satellite showing snow
cover from Friday over northern areas nearly all melted by 19Z.

Upper waves to the northwest become more separate over tonight, but
will bring some features of note into the area. Upper jet core
shifts north into southern Missouri while deep and increasing
southwesterly flow develops below, leading to isentropic ascent
through much of mid and lower troposphere, focused across east
central Kansas late tonight into early Sunday. The ability for the
upper divergence with the jet to be collocated with this upglide and
eventual saturation of the mid levels are the main questions for
precip chances. Will keep chances fairly close to previous numbers
with opportunities for measurable amounts limited, keeping highest
pops in the Chance range. Progressive nature of pattern brings
precip chances to an end by early Sunday afternoon with some
clearing taking place in the north and west. Despite overnight warm
air advection, temps above the surface support snow formation, but
increased cloud cover and winds near the surface ramping up with
time should keep temps steady or perhaps rise a bit late in the
night. Precip type should slowly change to rain Sunday with
continued warm air advection.

.LONG TERM...(Sunday night through Saturday)
Issued at 232 PM CST Sat Feb 25 2017

There are a couple chances for precip next week. The first chance
appears to be Monday afternoon and evening as models are in
reasonable agreement with some warm air advection. There is a
narrow band of mid level moisture that advects through eastern KS
into northern MO. Forecast soundings show some modest instability
(CAPE up to 100 J/kg) occurring as mid levels saturate for a
brief time. Overall forcing for precip looks unorganized with no
obvious shortwave in the mean flow aloft. And since the moisture
return could be better within the warm air advection pattern,
precip chances appear to be slight at this time. Also the
instability prog is not strong enough to introduce a mention of
thunder yet.

The more likely precip event will be late Tuesday through
Wednesday morning. Models show some cyclogenesis occurring in the
lee of the Rockies on Tuesday and then move a surface low east
across the region through Wednesday morning. There is pretty good
agreement in some mid level frontogenesis developing across north
central and northeast KS Tuesday night in addition to the upper
trough moving across with some PVA. Because of this have likely
POPs in the forecast. The GFS is a little quicker in saturating
the column Tuesday afternoon and developing precip along the
frontogenetical band. However the model consensus shows the best
forcing and saturation occurring through the night Tuesday and
into Wednesday morning. The less certain aspect of the system is
the temp profile and track of the surface low. The GFS is the
deepest and furthest north with the surface low while the ensemble
mean has varied from a more southern track like the ECMWF/GEM to
a northern track like the operational GFS. On top of that, there
simply does not appear to be a very cold airmass moving in behind
the system as models originate the surface high from the Pacific.
Because of this think there could be a good portion of the precip
event where temps are warm enough for mainly rain. In any case,
snow ratios could be rather low since the airmass is a modified
Pacific airmass rather than an arctic high. Nevertheless temps
across north central and northeast KS should drop below freezing
after midnight for the potential to see some snow accumulations by
Wednesday morning. Have based the temp forecast and resulting
precip type on the model blend. This would suggest snow
accumulations around an inch could occur across the northwest
third of the forecast area, or along a line from Concordia to
Marysville. Also looked into the potential for some convection
within the warm sector Tuesday evening. The GFS brings some
surface based instability into southeast KS and western MO just to
the southeast of Anderson CO. Since it is the deepest and
furthest north with the surface low, opted to not include thunder
across parts of east central KS until there is better agreement in
how far north the warm sector might get.

For Wednesday night through Saturday, the forecast is for dry
weather to occur as models show northwesterly flow aloft
transitioning to upper ridging by the weekend. A weak clipper
system system Thursday night looks to remain northeast of the
forecast area. This clipper system is expected to keep highs in
the 50s and lows in the 20s for Friday. Mid level ridging and low
level warm air advection should cause temps to warm into the 60s
for Saturday, and have been a little conservative in the warmup
with mid 60s. The ECMWF prog of 925MB temps would support highs
nearing 70 on Saturday for north central KS.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 538 PM CST Sat Feb 25 2017

VFR conditions for the 00Z TAF period at this point. Chances
remain over all terminals for perhaps light snow heading into the
12Z time frame. But, such small chance is too low at this time to
include. Cloud coverage increases overnight, but lowest CIGS still
expected to be above MVFR criteria. Winds will generally be from
the SSW through most of the period. Thinking is LLWS potential
will not come into play as most model guidance in agreement that
winds don`t fully decouple overnight.




LONG TERM...Wolters
AVIATION...Drake is the U.S. government's official web portal to all federal, state and local government web resources and services.