Area Forecast Discussion
Issued by NWS Topeka, KS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS63 KTOP 120931
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
431 AM CDT Sat Apr 12 2014

.SHORT TERM...(Today and Tonight)
Issued at 429 AM CDT SAT APR 12 2014

...Short term focus on late day convection and severe potential...

Early Saturday morning, a short wave trough was moving across the
northern Plains with a surface low moving along the NE/SD border and
a cold front extending southwest across western Nebraska. Meanwhile,
a broad trough was in place over the western CONUS with a closed low
drifting east across southern California. Surface low pressure was
deepening over southeast Colorado with a broad 40 to 60 kt low level
jet across the southern and central Plains. Moisture advection was
in full swing, with mid-50s surface dewpoints into the local area by
3 AM and low-60s dewpoints in central Oklahoma. By late afternoon
today, expect the surface low to deepen and drift east into SW
Kansas while a surface trough/cold front extends northeast into SE
Nebraska. A dryline seems likely to set up just west of a Salina to
Wichita line (although RAP/HRRR are intent in mixing the dry air
well into eastern KS...likely incorrectly) with increased definition
with southward extent. A low level thermal axis should point from
the surface low NE toward Salina where it will intersect the dryline
with slightly backed surface winds just south of this axis. High
temperatures today are likely to climb into the lower 80s with south
winds gusting to 30+ mph especially south of I-70.

Minor details of the pre-convective environment on Saturday are
going to have substantial impacts on overall convective coverage and
intensity. Perhaps the most important detail is the quality of low
level moisture and it`s impact on parcels ability to overcome the
late day cap. Surface Td greater than 60, or even 62, would
indicated a much better chance for late day initiation along the
dryline than Td`s in the 50s. Timing of a weak short wave will also
play a role in weakening the cap between 00Z and 03Z. If this weak
wave can adequately cool the capping inversion coincident with peak
instability, initiation would become much more likely than if it is
a bit slower to pass through. If storms are able to develop, the
strength of the mid level wind fields comes into question. A
weakness in the wind fields is forecast to be present between 21Z
and 01Z, and shear profiles may not be sufficient for supercell
development within this weakness, making storm persistence more
difficult. However, the wind profiles are forecast to rapidly
improve with stronger low and mid level winds by 03Z... favoring
supercell structures in any convection ongoing by that time.

The most likely scenario to play out seems to be a warm sector
characterized by 2000 to 3000 J/kg of MLCAPE with 0-6 shear in the
25 to 35 kt range by 00Z. One area of thunderstorm development will
be favored in SW Iowa late this afternoon, and will probably see
this activity develop southwest into NE Kansas along the cold
front/sfc trough, although this may not occur until evening as the
short wave trough moves over and the LLJ intensifies and impinges
upon the boundary. Large hail would be the primary threat with this
activity, especially if embedded supercell structure develops.
Damaging winds would also be possible especially near the front
through evening while the tornado threat will be near zero. Another
area of convection, much more conditional in terms of development,
may initiate near the intersection of the dryline and cold front/sfc
trough currently forecast to be in the general Salina area. This is
in an area of enhanced low level convergence and a relative minimum
in CINH. Any storms that develop in this area would seem likely to
take on supercell characteristics with a main threat being very
large hail with such steep mid level lapse rates and strong
instability. LCL`s are expected to be rather high and the tornado
threat appears very low, although these conditions would also pose
at least some potential for damaging winds. Some guidance suggests
these storms could move east and track south of I-70 for a while but
increasing inhibition would likely kill the storms before getting
too far into eastern KS.

As the night continues, expect the primary convective focus to be
across northern KS, north of the effective surface front and likely
shifting north with time as the front lifts north during the early
morning. Overall convective coverage in KS through the night is
questionable, but seem to at least have a persistent focus north of
I-70 where the LLJ impinges on the front amidst elevated instability
and favorable shear profiles for organized storms. May continue a
large hail threat...or even isolated damaging winds...through the
night but decreasing with time.


.LONG TERM...(Sunday through Friday)
Issued at 429 AM CDT SAT APR 12 2014

By Sunday morning, the broad mid-level trough will be stretched
across much of the northern and central U.S., with models in good
agreement that there will be two embedded shortwave troughs within
the large wave. At the surface, there are still some model
discrepancies with the speed of the cold front through the region on
Sunday. In general, the cold front looks to be stretched across
north central Kansas by 12z Sun and should exit southeast of the
area between 21z-00z. The exact timing of this frontal passage will
have an impact on temperatures as strong southerly winds ahead of
the front should keep temperatures mild across far east central
Kansas while winds should quickly back to the northwest and become
breezy behind the front, ushering much cooler air into the region.
As a result, north central Kansas will likely experience their high
temperatures first thing in the morning with falling temperatures
through the remainder of the day. At this time, have a large spread
for highs Sunday, ranging from the upper 50s to lower 70s.

The first of the embedded mid-level shortwaves looks to track
eastward across the region on Sunday, helping to provide additional
forcing. Should see some scattered showers and thunderstorms
developing along and behind the cold front during the morning,
becoming more organized and likely intensifying through the
afternoon as the front progresses eastward over the area. By Sunday
afternoon, models show mucape values reaching into the 1500-2000J/kg
range across east central Kansas with 0-6km bulk shear potentially
reaching upwards of 30-40kts. With these conditions in place near
the front, some of these storms could become strong to severe, with
large hail and strong winds being the primary threats. It`s worth
noting that the models show deep unidirectional shear along the
boundary, so could see these thunderstorms congeal into a line
during the afternoon and may also see periods of heavy rain,
especially across east central Kansas, with precipitable water
values potentially reaching near 1.20-1.30 inches. As the front
moves out of the area by around 00z, any lingering thunderstorms
across east central Kansas should become elevated.

By Sunday night, there is still some uncertainty with regards to how
much precipitation will still be in place across the area into
Monday morning. The second embedded mid-level shortwave will dive
southward across the High Plains Sunday night, but there are model
discrepancies with the location of the moisture associated with this
wave. The GFS/ECMWF show some drier air moving over the area with
the moisture from the shortwave remaining mostly to the south and
west, possibly clipping the far southern cwa. The NAM/GEM on the
other hand don`t push the moisture as far south and have it tracking
across much of the cwa Sunday night through Monday. Have trended
more toward the GFS/ECMWF and have trimmed PoPs back some Sunday night
into Monday morning with dry conditions by Monday afternoon.
However, with the cold air surging into the region behind the cold
front, low temperatures look to drop into the low/mid 30s so any
lingering light precipitation during this period could transition
over to a mix of rain and snow. In trending more with the drier
solutions, do not anticipate any snow accumulations at this time.

Surface high pressure moves in on Monday behind the exiting system,
with breezy northerly winds keeping conditions nearly 15-20F degrees
cooler than normal with highs only into the middle 40s. The center
of the surface high looks to track directly over the region Monday
night, resulting in very cold conditions with low temperatures
plunging below freezing into the mid/upper 20s. As a result, could
potentially be looking at freeze headlines for Tuesday morning.

As the mid-level trough advances toward the east coast,
fairly zonal flow sets up through mid week with conditions remaining
dry. With the surface high shifting east of the area and winds
shifting to the southwest, temperatures should quickly rebound back
into the 50s and 60s for Tuesday and Wednesday. Models show the next
mid-level trough developing over the northern Rockies on Wednesday,
however there are large model discrepancies with how this wave is
handled as to progresses into the central U.S., with the ECMWF
having the trough skim across the area Thu/Thu night while the GFS
has a closed low develop and tracks it directly over the cwa. With
these vast model differences, didn`t make too many adjustments to
the consensus blend for the end of the week except to trim PoPs back
to only slights for Thu/Thu night due to the model uncertainty.


&&

.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...Barjenbruch
LONG TERM...Hennecke
AVIATION...Sanders






USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.