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 232031
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
331 PM CDT Thu Apr 23 2015

.SHORT TERM...(This Evening through Friday)
Issued at 329 PM CDT THU APR 23 2015

A mid-level ridge was in place over the Central and Southern Plains
today with a mid-level trough progressing eastward across the
northeastern U.S. and another trough noted just north of the Baja
Peninsula.  At the surface, southeasterly winds prevailed as high
pressure remained stationed just east of the forecast area. This
southeasterly flow helped to keep a low/mid cloud deck in place
across much of the region today, limiting the daytime heating.
However, visible satellite imagery showed more breaks in the cloud
cover near the Kansas/Nebraska border, resulting in afternoon
temperatures reaching into the middle 60s. Further south in
locations with more persistent cloud cover afternoon temperatures
struggled to reach near 60 degrees.

The mid-level trough north of the Baja Peninsula will lift
northeastward toward the Four Corners region overnight and progress
north of the Texas/Oklahoma panhandles Friday afternoon, which will
help to provide ample mid-level support for thunderstorm activity
late Friday afternoon through Friday evening. Ahead of this
advancing trough, water vapor imagery this afternoon showed a few
weak embedded shortwaves developing along the lee-side of the trough
over Wyoming and Colorado with increased cloud cover noted over that
region. Models show these weak waves shifting eastward with the
eastward progression of the trough, moving into Nebraska and
northern Kansas overnight into Friday morning. In general,
short-range models have trended a bit weaker with the shower and
thunderstorm potential for late tonight through the morning hours as
the region should remain capped and forcing is limited. However,
with MUCAPE values upwards of 500-1000 J/kg and 45-55kts of 0-6km
shear, cannot rule out the potential for a few strong elevated
thunderstorms to develop in which some small hail will be possible.
Short-range models show that the precipitation should be pretty
isolated this evening and become more widely scattered through the
overnight hours before diminishing in coverage from west to east
during the mid to late morning hours. As a result, models suggest
that we could see a window of a few hours from mid/late morning
through early/mid afternoon in which locations are
precipitation-free and, according to some model soundings, may
potentially see some breaks in the cloud cover, especially closer
toward central Kansas. This diminish in cloud cover will allow for
more daytime heating in the afternoon hours to boost temperatures
into the low/mid 70s from central to east central Kansas, with
cooler temperatures in the mid/upper 60s across northeast Kansas
from the lingering cloud cover.  However, these high temperatures
will be very dependent upon how quickly the morning precipitation
dissipates and whether or not we are able to diminish the cloud
cover enough during the early/mid afternoon hours, so we will need
to continue to closely monitor these short-term conditions.


.LONG TERM...(Friday Night through Thursday)
Issued at 329 PM CDT THU APR 23 2015

Friday afternoon into the overnight presents a complicated weather
scenario with a likelihood of severe weather in the area, and a
potential for a few significant severe events.

The primary forecast questions through this period will be 1) How
far north does the warm sector surge? 2) How broad east/west will
the unstable warm sector be? 3) How much and how soon will the
boundary layer stabilize by mid to late evening?

In terms of the questions above, various model guidance are
generally in the same ballpark regarding how far north the warm
sector will surge, but the northern edge of the surface based
instability ranges from somewhere near the Nebraska border (GFS) to
a Council Grove to Lawrence line (NAM/NMM). The rest of guidance is
in between, and frankly the ECMWF rendition of the surface low and
warm front track looks to be quite reasonable, with the warm sector
coming as far north as Minneapolis to Manhattan to Holton line. Most
indications are also that the warm sector will not be particularly
broad east/west, and this *may* be able to limit the potential for
long track severe storms a bit as the individual storm forward speed
(40+ mph) should be faster than the system as a whole, and they
could move toward less unstable air with time. In terms of evening
stabilization, all indications point to an unstable warm sector
airmass through approximately 8-11 PM before becoming increasingly
stable. This is not entirely for sure as the surface low will track
directly across the area after midnight, but it does seem likely
that CINH will increase and the tornado threat decrease by late
evening.

The relative certainties in this forecast are following
1) Thunderstorms will develop and move across the forecast area. 2)
Wind shear parameters are very impressive and will support storm
organization. 3) The combination of steep lapse rates and strong
shear will support large hail (some very large) in storms both north
and south of the front.

The rest of the details are fuzzier but important.  Damaging wind
potential does not appear to be a huge threat given low LCL heights,
but the potential for some upscale growth by mid/late evening and
very strong ambient wind fields suggest that a wind threat could
develop. The potential for tornadoes is conditional, but very
present. A worst case scenario would be if cells can remain
semi-discrete or move east of the main cluster of convection as any
isolated supercell in the warm sector would have full access to 30+
kts of 0-1 km wind shear underneath a strong mid level steering flow
(60 kts at 500 hPa) and exhaust jet aloft (130 kts at 250 hPa).
There would seem to be a primary window of opportunity between 6 PM
and 10 PM for tornado potential, especially with any isolated
supercells, as the low level jet rapidly intensifies during this
period but inhibition is slow to increase. The take away message is
that the potential exists for all modes of severe weather, and while
there are complicating factors, it will be important to prepare for
a few significant severe storms.

Precipitation looks to exit the area by sunrise on Saturday. A cold
front will then move into the area from the north on Saturday
afternoon. There will be some weak instability across the area, but
most indications point to a slightly capped boundary layer with weak
forcing along the front so currently have a dry afternoon forecast.
However, if low level moisture is a bit deeper or if temperatures
ahead of the front warm up more than forecast, could possibly see a
storm or two develop.

The remainder of the forecast is rather uneventful. Have maintained
a slight chance for showers early next week as a slow moving closed
upper low drifts across the southern Plains, but for the most part
it looks like precip should remain south of the forecast area.
Temperatures look to be near or slightly below normal for much of
the long term period.

&&

.AVIATION...(For the 18Z TAFs through 18Z Friday Afternoon)
Issued at 1236 PM CDT THU APR 23 2015

Expectation is for VFR conditions to remain into the afternoon and
early evening. The lower confidence comes into the last half of
the TAF period with the mention of VCTS. The nature of this round
of showers and embedded thunderstorms does appear to be very
likely but scattered, so timing may need to be adjusted. Areas of
showers and thunderstorms could also bring CIG/VIS down to the IFR
category, but isn`t expected to remain after passage.


&&

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

$$

SHORT TERM...Hennecke
LONG TERM...Barjenbruch
AVIATION...Drake





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