Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 181121
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
621 AM CDT Thu May 18 2017

...AVIATION UPDATE...

.SHORT TERM...(Today and tonight)
Issued at 342 AM CDT Thu May 18 2017

08Z water vapor imagery shows yesterdays shortwave lifting through
the upper Midwest while a closed upper low slowly drifts south over
UT. At the surface, a weak cold front has stalled out across east
central KS. An areas of low pressure was slowly deepening over
southeast CO and the OK Panhandle and surface obs indicated a very
moist airmass with dewpoints in the lower and mid 70s beginning to
advect north through central TX and southeast OK.

A potential severe weather outbreak today is front and center of the
forecast concerns. There appears to be good consensus in the various
model solutions with the overall setup. The main discrepancy is
timing when storms will move into the forecast area from the west.
The stalled cold front across east central KS is expected to lift
north to near the KS river valley by the late afternoon. South of
the boundary, a very unstable airmass with CAPE values between 3000
and 4000 J/kg is progged to develop as the moisture over the
southern plains advects north. Meanwhile a surface low is expected
to deepen over the TX panhandle with a dryline extending south and
the warm front draped across central KS. Models show little
inhibition to convection by peak heating. However there isn`t much
large scale forcing impacting the forecast area either, with the
closed upper low still over the central Rockies. So the expectation
is for storms to initiate along the dryline and near the triple
point in southwest KS where low level convergence will be maximized.
Then storms should move into the forecast area by the early evening.
Shear parameters are very favorable for tornadoes due to low level
winds remaining backed to the southeast with the surface low over
the panhandles. And 0-1KM shear is forecast to improve shortly after
00Z as the low level jet intensifies. So any discrete storms will be
capable of producing a tornado along with very large hail. The
latest HRRR/ARW/NMM suggest that by the time storms make it into the
forecast area that they will have formed into a linear MCS. This may
lower the tornado risk some but it still bares watching. Have
trended a little faster with the timing of thunderstorms based on
the convective allowing model progs. Again the biggest uncertainty
is timing when storms will move into the area. There may also be
some potential for storm development within the forecast area along
the warm front, but none of the models seem to weaken the cap enough
for this to occur. There is also an outside chance for an elevated
storm this morning along the frontal boundary. The NAM is showing
some reasonable isentropic lift into this boundary. But have held
off on including a POP in the forecast this morning since there is
not much support for this from the other solutions.

Highs today should range from the lower 70s north where cloud cover
and weak cold air advection limits heating. South of the warm front,
temps should warm to around 80. Lows tonight should fall into the
mid 50s and 60s.

.LONG TERM...(Friday through Wednesday)
Issued at 342 AM CDT Thu May 18 2017

The extended period consists of more severe thunderstorm potential,
followed by a brief dry period and then more thunderstorm chances.

Friday morning an MCS is expected to be ongoing across the eastern
periphery of the CWA. By this point in time, thunderstorms are
expected to be elevated. However, given the steep mid-level lapse
and effective bulk shear near 50 knots, large hail and damaging wind
threats should continue. The flooding threat cannot be discounted as
well, with good agreement amongst guidance with widespread 1-3" and
isolated higher amounts through Friday morning.

We then turn our attention to Friday afternoon and evening. Some
uncertainty still exists in severe potential Friday due to remnant
cloud cover and potential outflow boundaries from Friday morning`s
convection. However, good agreement amongst the synoptic and CAM
solutions in destabilization Friday afternoon. An upper level low
will begin ejecting across the central and northern plains
throughout the day on Friday. A warm front is forecast to surge to
near the KS/NE border by 00z Saturday. A sharpening dryline will
take shape across central and north central KS. Isolated
thunderstorms are expected to develop along the dryline by the late
afternoon. Given the buoyant atmosphere in place with upwards of
3000 J/KG of MLcape and effective shear of 50+ knots, supercellular
structures can be anticipated. Initially all modes of severe weather
appear possible. As the upper low begins to eject the warm front
will begin to progress southward as a cold front in 00z-03z
timeframe Saturday. Widespread convection is expected to evolve
along the boundary. Given the same instability and shear in place,
the threat for large hail and damaging will continue into the
evening hours. The flooding and flash-flooding will increase as
well. Another round of heavy rainfall with another 1-2" is expected
with the convection Friday evening/night. Any lingering showers or
storms are expected to exit the area by midday Saturday.

Surface high pressure will begin building in over the weekend. Rain
chances return early next week as a secondary upper level trough
traverses the central and northern plains. Shower and thunderstorm
chances increase late Sunday into Monday morning in response to an
embedded shortwave traversing the central plains. Its much too early
to get any specifics, however strong to severe thunderstorms appear
possible Monday afternoon and evening.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 620 AM CDT Thu May 18 2017

11Z satellite shows MVFR cigs moving south out of NEB and
developing along the stalled frontal boundary across east central
KS. The model consensus is for the terminals to generally remain
in between these lower CIGS through the morning and once the
boundary lifts north CIGS are progged to be above 3 KFT. So have a
VFR forecast going until the TS move in this evening. The HRRR has
shown very good consistency in timing and is supported by the GFS
and ECMWF. So have included a tempo group for when strong to
severe storms are most likely to impact the terminals. This
timing is based heavily on the HRRR.

&&

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

$$

SHORT TERM...Wolters
LONG TERM...Baerg
AVIATION...Wolters



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