Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 202312
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
612 PM CDT Wed May 20 2015

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Thursday)
Issued at 334 PM CDT WED MAY 20 2015

Persistent and deep low cloud with shallow but cool airmass keeping
temperatures rather cold today. Record lowest maximum temp for May 20
at Topeka is 56 and appears in jeopardy, with Concordia so far tying
its record lowest max of 52. Still seeing reports of  drizzle from the
trapped low level moisture in the early afternoon. Upper wave that
helped bring last night`s precip was entering the Great Lakes early
this afternoon with some cirrus spreading northeast into the
Colorado ahead of the next southern branch next upper wave.

Main concern through Thursday is fog potential tonight. Latest IR
and visible satellite suggesting some breaks in the stratus
especially into southern Nebraska with clearing into southern South
Dakota. Surface high over northwestern Nebraska sinks southeast
tonight into Friday with some north wind continuing but slowly
drying the lower levels. Will continue with steady clearing of the
lower cloud occurring through the early morning, but expect some
increase in high cloud from the west late in the night. Obviously
the ground conditions are rather wet from the recent rains and
overnight clearing leads to a good setup for radiational fog, but
given uncertainty on clouds both exiting and entering the area,
still some mixing of the boundary layer, and the rather short
nighttime period of late May, have kept fog mention out.
Significantly warmer though still cool highs on track for Thursday,
mainly in the mid 60s.

.LONG TERM...(Thursday Night through Wednesday)
Issued at 334 PM CDT WED MAY 20 2015

Thursday Night through Saturday:

For Thursday night, models show the Canadian ridge of high pressure
still influencing the region as a shortwave ridge axis propagates
overhead. Since there is no real moisture advection or obvious
forcing, have continued with a dry forecast. Lows will depend on
whether mid and high clouds are able to stream in overnight from the
southwest. Think with skies initially mostly clear that lows will
have the opportunity to drop into the mid 40s. If cloud cover moves
in sooner, The forecast could be on the cool side.

Models indicate better moisture advection for Friday through
Saturday as mid level southwest flow develops ahead of the next
closed low moving into the southwestern U.S. This should cause
surface pressures to fall in the lee of the Rockies with surface
winds becoming more southerly. The uncertainty in the forecast is
tied to the transient vort maximum the models show. It appears as
though a lead wave will lift northeast across the area Friday
morning. Curiously most of the model solutions split the precip into
an area moving east along the KS/OK border and and area lifting
north through central NEB. I suspect that since the forcing is
relatively weak that the models are developing a local area of
subsidence causing the precip to dissipate. Have kept a POP going
through the day Friday but also trended chances down based on the
apparent consensus in the models. The next potential shortwave is
progged to lift out on Saturday. So after continued chance POPs
Friday night, have maintained some likely POPs for Saturday as
moisture should be in place if a wave does lift out. Overall models
show instability remaining close to nil for Friday and Friday night.
Although fairly modest CAPE is progged to develop through Saturday.
As it does, 0-6 KM shear appears to weaken to around 30KTS. So while
there may be enough shear and instability for an isolated strong
storm, an organized severe weather risks appears to be limited at
this time. Increased cloud cover Friday and Friday night is expected
to keep temps in a narrow range with highs in the mid 60s and lows
in the lower 50s. There should be a slight warming trend for highs
Saturday as low level warm air advection increases. Have highs in
the mid 60s to around 70, but this could be to cool if some sunshine
breaks out.

Saturday night through Wednesday:

Saturday night into Sunday the upper-level trough will deepen and an
embedded shortwave will begin to move NE bringing a chance for rain
and thunderstorms all day Sunday.  Any thunderstorms Sunday
afternoon should stay below severe criteria due to lower 0-6km shear
values, even though at least a moderate amount of instability will
be present.  Monday, this trough will lift NE over the area allowing
for more thunderstorm development Monday afternoon into the
evening.  For these storms, better shear values are seen although
the better instability shifts south of the CWA.  From here, models
diverge with the exiting of this system.  The GFS is quicker and
moves precipitation out by Tuesday morning, leaving only slight
chances for the rest of the period.  The new ECMWF is much slower
and has this upper-level trough deepening Tuesday before departing
Wednesday morning.  The previous ECMWF model run and ensembles hint
that this solution may be suspect; however, if this new ECMWF
solution is correct, a round of strong thunderstorms is possible
Tuesday night. Basically, confidence towards the overall pattern
mid-week becomes lower and will need to be monitored. A warming
trend is seen throughout the period with temperatures reaching the
upper-70s/lower-80s by the beginning of next week.  Lows will stay
slightly above climatology for this time of year in the lower 60s.

&&

.AVIATION...(For the 00Z TAFs through 00Z Thursday Evening)
Issued at 606 PM CDT WED MAY 20 2015

Multiple cloud layers present at 00Z with the lowest near 1800 ft,
next at 2700 feet, and also near 4000 ft. Believe that cloud
layers will scatter and eventually erode completely...first at the
lowest levels and working their way up. This will bring an end to
MVFR conditions soon at MHK, and at TOP/FOE by 03Z or so...
although with some uncertainty regarding exact timing. MHK has the
best chance for clear skies and light winds before sunrise, which
would cause vis restrictions if these conditions develop. TOP/FOE
could see the same, but believe clear skies will wait until after
sunrise with no fog.

&&

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

$$

SHORT TERM...65
LONG TERM...Heller/Wolters
AVIATION...Barjenbruch






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