Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 012328
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
628 PM CDT Fri May 1 2015

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Saturday)
Issued at 333 PM CDT FRI MAY 1 2015

19Z water vapor shows a ridge of high pressure centered over the
AZ/NM boarder and broad cyclonic flow from the Gulf of AK through
western Canada. Within this pattern, much of the shortwave activity
remains along the Canadian boarder. At the surface, a ridge of high
pressure was centered over the MS river valley with a weak trough of
low pressure from northwest KS through the eastern Dakotas.

For tonight and Saturday, there are two potential sources for
forcing convection, and confidence in either one is rather so-so.
The first is a possible convectively induced vort max (MCV) coming
off the central Rockies this evening. The HRRR and RAP have been the
most aggressive in holding precip together off the higher terrain
and into the forecast area through Saturday morning. These solutions
have also be to aggressive in this afternoon`s precip so I don`t
know how much weight to put behind this idea with the GFS and NAM
shearing out any MCV. At this point, have taken a wait and see
approach for the convection to fully move off the mountains.
Additionally the MCV may tend to move just southwest of the forecast
area. The second forcing mechanism may end up being the low level
jet and isentropic lift. The NAM and GFS seem to favor this,
developing precip along the NEB state line and moving it into
northern MO Saturday morning. However it appears the low level jet
will be veered to the southwest with the stronger wind speeds to the
west of the area. Additionally theta-e advection is marginal with
the low level jet. The models do prog some isentropic lift over far
northeast KS which may explain why they are generating precip.
Nevertheless confidence is marginal since the low level jet itself
is not very strong and moisture advection with the jet is weak. Have
kept some 20 to 30 percent POPs across the northern counties through
the morning to account for the possibility.

Lows tonight should be a little more mild with readings remaining in
the lower and mid 50s. Think skies are likely to remain at least
partly cloudy with debris clouds spreading in from the west, helping
to keep lows in the 50s. Models show some warm air advection through
the day Saturday as low level flow remains southerly. Based on 850
temp progs and some insolation through the afternoon, think highs
should warm into the upper 70s and lower 80s.

.LONG TERM...(Saturday Night through Friday)
Issued at 333 PM CDT FRI MAY 1 2015

Increasing low level southerly flow Saturday night into Sunday
brings precipitable water values further above normal to near 1.5
inches. Main forcing mechanism approaches Sunday afternoon as a
northern branch wave allows a cold front to settle south into the
area, likely from southwest Kansas into east central Nebraska. Could
see far western locations impacted late in the afternoon as storms
could fire not far to the west-southwest, but chances increase in
the next few periods as the front ceases its southerly push, between
the northern and southern branches. Just how far the front gets is
obviously a challenge and will be key for precip potential. Have
tapered southern end of precip chances for Sunday night, with best
mean chances Monday into Monday night until southerly flow returns
to the northwest, ushering the front back north. At this point Tuesday
appears the most likely dry period, with a modest but increasingly
powerful upper wave rotating northeast into the Plains Tuesday night
into Wednesday night. Expect most locations to see at least some
precip in this period with deep south flow keeping available
moisture plentiful. Beyond this, model differences increase for a
much less confidence forecast, though a modified front could be in
play with a still warm and moist airmass to support chance PoPs. As
for thunderstorm severity, mid/upper flow is not very strong being
between the northern and southern branches. With greater confidence
in the late weekend/early next week boundary in the vicinity, there
will of course be low level shear potential here with ML CAPE likely
around 1500 J/kg. In the latter wetter period, the wave`s
orientation keeps winds somewhat unidirectional with height, and
instability specifics anything but certain with periodic precip
quite likely. Expect highs to be generally near to a few degrees
above normal, with main problem being depth/persistence of
cloud/precip north of the front Monday to bring possibly cooler
temps possible in northern areas.

&&

.AVIATION...(For the 00Z TAFs through 00Z Saturday Evening)
Issued at 624 PM CDT FRI MAY 1 2015

VFR conditions expected through a majority of the period. There
is a slight chance that an isolated shower or thunderstorm could
form or pass near and north of the taf sites tomorrow morning
around sunrise. Otherwise occasional gusty winds under 20 kts
expected through tomorrow afternoon.

&&

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

$$

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





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