Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 210500
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1200 AM CDT Mon Aug 21 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Monday)
Issued at 324 PM CDT Sun Aug 20 2017

As of 20z Sunday afternoon a cluster of LLJ driven showers and
thunderstorms across northern MO. An OFB has pushed south and west
of the storms and extends from near Hiawatha southeastward
towards the Kansas City metro. The lack of any agitated Cu field
along the boundary and surface based inhibition ~100 J/KG suggests
convection is unlikely. With the sultry BL in place, heat
indicies have risen into the low 100s across much of the area. A
stationary surface boundary was positioned from southwestern NE
into northwestern IA. A subtle shortwave coupled with a 30-40kt
LLJ should contribute to thunderstorm development well after
midnight in central and eastern NE. Steering vectors towards the
ESE should keep activity north and east of the outlook area.

Well the day that many have been anticipating has nearly
arrived...TOTAL SOLAR ECLIPSE DAY!! Unfortunately, model guidance
continues to point towards an expansive cirrus shield across much
the area. A cutoff upper trough off the CA coast continues to usher
Baja moisture into the Four Corners region. As stated previously,
all guidance ejects a shortwave across the central Plains, greatly
increasing cloud cover Monday afternoon. That being said, very
subtle differences in timing could play a huge role in the
visibility of the eclipse. There`s basically two fronts in regards
to the cloud cover forecast...The first is the GFS/EC...These 2
models eject the shortwave slightly faster, resulting in 300-500mb
RH values near 100 percent (mostly cloudy skies) by 18z area wide.
The second front is the NAM/Canadian...These 2 models are slightly
slower with their progression, with ultimately the same result but
between 18 and 21z rather than by 18z. As if cloud cover wasn`t
enough of a concern, scattered showers are possible area wide during
the late morning and early afternoon hours. Isentropic lift near
310K will overspread the area Monday morning and continue through
the afternoon. HERE`S TO HOPING THE CLOUD COVER AND RAIN SHOWERS
HOLD OFF UNTIL MID AFTERNOON!!

Later that afternoon the threat of for strong to severe
thunderstorms increase along the aforementioned cold front and upper
level trough. By late tomorrow afternoon isolated to scattered
thunderstorms are expected to develop across north central KS and
southern NE. Steep mid level lapse rates (~6-7 C/KM) will contribute
to MLcape near 3000 J/KG. Sufficient deep layer shear near 30 knots
suggests organized convection. Large hail and damaging winds the
primary concerns. Upscale growth into an MCS looks to take shape
after 00z. High temperatures will top out near 90 Monday afternoon.
With the moist BL remaining in place, another day of heat indicies
near 100 is expected. Stay Safe!

.LONG TERM...(Monday night through Sunday)
Issued at 324 PM CDT Sun Aug 20 2017

For Monday night and Tuesday, models are in good agreement for
showers and thunderstorms to overspread the area with decent
vorticity advection and frontogenesis moving across the forecast
area. Initially there could be some severe weather with the
storms as models prog deep layer shear of 30 to 40 KT with
reasonable instability. But the bigger concern will be for very
heavy rainfall. Precipitable water values are forecast to
increase to between 2 and 2.5 inches Monday evening which is near
or above the climatological max for this time of year.
Additionally models suggest that mid level flow may parallel the
surface boundary for a period of time Monday night before the
front finally pushes through Tuesday morning. With some of the
ensemble plumes showing rainfall amounts ranging between 1 and 3
inches Monday night, felt like there is a strong enough signal for
flash flooding and have issued a flash flood watch as a result.
Confidence in heavy rainfall is high. The main uncertainty is
where the heaviest rainfall axis may set up and how long the
rainfall may stay over a single location. If it turns out the
front is much more progressive and moves right through, the
duration of heavy rain may be cut down enough to limit the flash
flooding risk since flash flood guidance is around 2 inches in an
hour for much of the forecast area.

For Tuesday evening through Friday, models continue to depict
surface ridging with northwest flow aloft. The drier and cooler
air associated with the surface ridge and a lack of obvious
forcing within the northwest flow leads to a dry forecast with
below normal temps. Lows are forecast to be in the upper 50s and
lower 60s while highs only reach the lower and middle 80s.

By Friday night and into Sunday, the upper ridge to the west
breaks down and the surface ridge weakens as it moves east. This
is expected to allow a return flow to develop. There is also some
indications from the medium range solutions for a low amplitude
wave to move through the central plains Saturday. Because of this
there are some small POPs in the forecast for Late Friday night
and through Saturday night. Models do not really show any strong
warm air advection within the pattern so temps are expected to
remain relatively cool for the end of August.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1157 PM CDT Sun Aug 20 2017

Expect VFR conditions through 20Z...then thunderstorm chances
will increase. Several numerical models show the potential for
widespread thunderstorms Monday night. There may be some moderate
vertical windshear across the terminals through the early morning
hours of Monday as the a southwesterly low-level jet increases to
30 to 35 KTS.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Monday evening through Tuesday morning
for KSZ011-012-022>024-026-036>040-054>056-058-059.

&&

$$

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


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