Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 272110

Area Forecast Discussion
National Weather Service Topeka KS
310 PM CST Mon Feb 27 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 310 PM CST Mon Feb 27 2017

Recent water vapor imagery and RAP analysis showing upper flow
becoming more southwest ahead of a shortwave moving into
California. Another upper wave was moving east across southern
Canada. Surface pressures were falling quickly over central Kansas
with warmer air surging north. Deeper moisture with this warmer
air keeping skies partly sunny, with enough depth of the moisture
for a few showers to form in eastern areas.

Several items of interest in tonight into Tuesday afternoon. May
see a few showers into the early evening in the east where
isentropic upglide remains, with weak instability keeping a
thunder mention going. Southerly winds will persist overnight and
attempt to saturate a shallow layer near the surface. Some models
are squeezing out some drizzle, but given the shallow moisture
depth and little vertical motion, have not included it.

With the aid of the northern wave, initial surface low moves
across southern Nebraska tonight with next low moving across
northern Kansas during the day. Cold air advection slowly works
in, but should have enough clearing of the low cloud for a warmer
day for most locations ahead of a cold front. Have increased temps
most areas with this in mind but confidence in high temps is not
high. Mixed- layer CAPE increases to at least a few hundred J/kg
in east central Kansas. Wind fields will be quite strong and could
support a some updrafts capable of producing small hail. To the
northwest, frontogenesis aloft will bring at least a small chance
for rain to develop late in the afternoon.

.LONG TERM...(Tuesday night through Monday)
Issued at 310 PM CST Mon Feb 27 2017

The bulk of the interesting weather will be in the first 9-12
hours of the period on Tuesday night/early Wednesday morning. A
modest meridional shortwave trough axis will be centered along the
High Plains at 00Z Wed with a 140 kt jet steak rounding the base
of the trough and lifting northeast. Increasing 925-700mb
frontogenetical forcing, modest ageostrophic ascent in the left-
front region of the jet, and a combination of slantwise/upright
instability will set the stage for a narrow band of precipitation
bisecting the CWA. The major questions revolve around the location
of the banded precip and attendant BL thermodynamic properties.
The 12Z GFS was a northern outlier in the placement of the band in
comparison to the NAM/ECMWF/SREF/NMM/ARW, so leaned towards the
southern solution of the placement of the precip.

The thermodynamic setup was a bit trickier to resolve. Sounding
profiles in the mid levels depict a classic heavy snow setup with
omega maximized through the center of the DGZ. MUCAPE values
above the Fg zone will range from 100-200 J/kg, more than ample to
enhance precip rates during the 3-6 hour period. To account for
the intense snowfall rates and mesoscale nature of the band,
adjusted the Tuesday night forecast as follows:

1) Raised snow ratios to a mixture of the NAM/GFS, substantially
higher than SuperBlend.

2) Raised the rain/snow thresholds in the ForecastBuilder to
36/40, 2 to 3 degrees higher than the default.

3) Adjusted hourly temperatures down to near the wet bulb shortly
after the onset of precip.

Even with all these tweaks, surface Tw values of +2 to +4 C may
still be too much to overcome to generate any meaningful snowfall
amounts. The updated forecast snowfall amounts only tops out at
0.5" near Manhattan, but even a couple degree difference in the
wet bulb could result in several inches of snow along a very
narrow corridor. The strength of the ongoing CAA and whether or
not the precip can modulate the BL fast enough will play a key
role in precipitation type and snowfall accumulations. Stay tuned
for further updates!

Strong longwave ridging will build over the western CONUS after
this system pushes out by 12Z Wednesday. The result will be
steadily warming temperatures and mostly clear skies for the
latter half of the work week and the weekend with the next wave of
any consequence not arriving till early next week. H850 temps
will rebound to 0 C Thursday, +5 C Friday, +9 C Saturday, and
then +14 C Sunday. Thus, look for the return of well above
average temps in the 60s and 70s by this weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1130 AM CST Mon Feb 27 2017

Difficult TAF period as stratus moves towards the TAF sites. Have
opted to keep sites VFR as the deck looks to be right on the edge
of MVFR conditions. So these are are possible this afternoon.
After 00Z, MVFR looks more likely with even IFR ceilings possible
through the morning at TOP/FOE. LLWS still looks to be marginal
and have held off on including it in this TAF issuance.




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