Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 120454
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1154 PM CDT Fri Apr 11 2014

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Saturday)
Issued at 341 PM CDT FRI APR 11 2014

19Z water vapor imagery shows a general northwest flow from the
central plains to the Pacific northwest. One shortwave was moving
southeast along the British Colombian coast while a closed low
slowly moved east towards southern CA. Surface obs show a weak ridge
from the MS river valley back northwest into northeastern KS and
southern NEB. Although this ridge has gradually been weakening and
higher dewpoints across OK and TX are just beginning to move back
north into central and eastern KS. About the only sensible weather
possible tonight is the development of a stratus deck over the
Ozarks and spreading north and west late tonight. Otherwise models
show the moisture return to be rather shallow and forecast soundings
maintain a capping inversion over the moisture. Because of this do
not expect any elevated convection to form. Confidence in the
stratus formation is marginal since the RAP13 fails to saturate the
boundary layer. However the RAP13 has shown a dry bias this
afternoon in the surface dewpoints. The pattern seems favorable for
good low level moisture return with a lee trough strengthening and
current obs show the moisture in place to move north. With this in
mind and southerly winds holding in the 5 to 10 mph range overnight,
think min temps will be mild with readings in the mid and upper 50s
due to warm air advection and limited radiational cooling.

Saturday is shaping up to be warm and breezy. The GFS appears to be
much cooler than other guidance as it hangs onto the stratus much of
the day. With the moisture return looking rather shallow, think the
low level moisture will eventually mix out and allow for some
sunshine and have relied on the NAM and ECMWF more for the forecast
Saturday. Models indicate a weak boundary will lay over across NEB
and into IA by the afternoon. But there remains a fair amount of
inhibition to deep moist convection through the day until the later
afternoon when daytime heating appears to weaken the cap.
Convergence along the dryline to the west should be minimal since
low level winds will parallel the boundary. So think if anything
forms it should be along the boundary in southern NEB and IA. There
does look like a weak perturbation moves overhead during the
afternoon which may also help storms develop along the front.
Initially 0-6km shear of around 35 kts suggest storms may have a
hard time organizing. 0-1 km shear parameters are even less
impressive in terms of tornado potential during the afternoon since
there is no real strong low level jet. Therefore think any storms
that form may tend to just set on the boundary and interact with one
another. However with the NAM showing instability on the order of
3000 J/kg, any storm that is able to remain discrete could pose a
greater risk. At this point, large hail looks to be the main
concern by the late afternoon. Have kept afternoon highs in the
upper 70s and lower 80s thinking the GFS is to pessimistic with
the low stratus.

.LONG TERM...(Saturday Night through Friday)
Issued at 341 PM CDT FRI APR 11 2014

Remainder of the weekend becomes much wetter and cooler. Main focus
Saturday evening should be in northern areas where front may dip in.
Increasing low level jet over the boundary should lead to elevated
thunderstorms becoming more numerous into the late evening hours,
but could also see early evening storms in far northeast sections
continue. Outflows from any early evening convection could shift the
main focus a bit south, and southern sections also see some
potential for convection in isentropic lift and increasing low level
moisture. Elevated CAPE values still in the 1000-3000 J/kg range
especially near the boundary with enough shear for some
organization, with these values decreasing and increasing with time,
respectively, through the night. Surface wind speeds look to die off
in the evening and should keep surface parcels from convecting, so
still looking at hail and perhaps wind as the main severe weather
concerns.

Cold front surges southeast across the remainder of the area by late
Sunday afternoon. Southern and eastern locations in more of the
concern for severe weather this day, though veered low level winds
and weaker instability should keep concerns similar to Saturday
night. Large scale forcing with the upper wave coming in from the
southwest brings a more widespread rain event for the entire area.
Still looking like heavy rains become possible with shear vectors
not far off the boundary and precipitable water values rise to near
1.25 inches, two standard deviations above April normals.  Strong
cold air advection will make temperatures a challenge, with early
day highs likely for the north and west.

The northern branch wave`s speed coming southeast through the Plains
remains a strong point of difference among the guidance, with the
slow NAM keeping the column rather saturated for more cold season
precip potential Sunday night into Monday. Have sided with the
faster GFS and ECMWF, thinking their handling of this wave`s
interaction with the Hudson Bay low will be superior. This should
bring some overnight clearing and may allow temps to fall to just
below freezing in the north and west. Monday night still looking to
be the coldest night of this forecast with the surface ridge passing
through and mainly clear skies after highs mainly in the 40s.

Moderation on tap for Tuesday and Wednesday, with the next upper
wave taking shape over the central CONUS around Thursday. The old
surface ridge will keep moisture return limited with this wave, and
timing differences are again present, so will keep precip chances
modest.

&&

.AVIATION...(For the 06Z TAFs through 06Z Saturday Night)
Issued at 1145 PM CDT FRI APR 11 2014

The nocturnal inversion has trapped some of the smoke from
burnings early in the day which is causing MVFR vis at some of the
taf sites. Do not expect much improvement to the vis overnight...in
fact they may decrease further as low level moisture spreads
northward across the area. Uncertain if the 1500 ft ceilings will
be scattered or broken since the models are disagreeing on the
moisture return.

&&

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

$$

SHORT TERM...Wolters
LONG TERM...65
AVIATION...Sanders





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