Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 191203
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
703 AM CDT Wed Apr 19 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 309 AM CDT Wed Apr 19 2017

An upper level trough across the central Rockies this morning will
lift northeast out into the central plains Today and then into the
Midwest Tonight.

Early this morning a nearly stationary front extended from Delphos,
east to Topeka, then northeast across northwest MO. North of this
boundary scattered showers and thunderstorms were moving to the east
northeast across northeast KS. A few isolated thunderstorms extended
southwest to west of MHK. Most of the thunderstorms this morning
should remain north of I-70 as the weak boundary will begin to shift
northward to the NE border by noon. The elevated thunderstorms will
not be severe though the stronger storms may produce some small hail
through the mid morning hours across northeast KS.

We`ll see a break in the showers and thunderstorms through the late
morning and into the early and mid afternoon hours as a strong EML
overspreads eastern KS ahead of the H5 trough lifting northeast into
the central high plains.

As the H5 trough lifts northeast into NE a surface low will deepen
across northeast NE and northwest IA. A surface cold front will push
southeast across central NE and western KS through the early and mid
afternoon hours. The front should reach the northwest counties of
the CWA after 21Z. Surface convergence along the cold front combined
with large scale ascent ahead of the H5 trough will weaken the CAP
for discrete surface based thunderstorms to develop along and just
ahead of the surface cold front. Most models show MLCAPE values
increasing to 1500 to 2500 J/KG ahead of the front. Also the 0-6 KM
effective shear will be 40 to 50 KTS. The combinations of vertical
windshear, instability and steep mid-level lapse rates allow for
strong to severe thunderstorms to develop. Initially these surface
based storms will be discrete and the environment will be favorable
for supercells. The primary hazard would he large hail and damaging
outflow wind gusts. The low-level winds will be slightly veered to
the south-southwest but most numerical models show 0-1 KM SRH to be
in the 100-200 J/KG range ahead of the cold front. This would be
sufficient for any discrete supercell to produce isolated tornadoes
from the late afternoon into the early evening hours across north
central KS. Eventually the scattered supercells will congeal into a
line of thunderstorms. The LLJ will increase this evening, so there
could be a few meso vortices developing within the line of storms.
Although, the primary hazard with a line of thunderstorms would be
damaging winds and perhaps large hail if any of the updrafts
embedded within the line develop mid-level rotation. The best chance
for supercell thunderstorms will be across north central and
northeast Kansas during the late afternoon and evening hours. The
line of storms should form before the entering east central KS
during the mid and late evening hours. Eventually the line of
thunderstorms will push east and southeast of the CWA during the
early morning hours of Thursday. There may be heavy rainfall across
east central KS as the line of storms develops and slowly shifts
east-southeast across east central KS. Some areas could see near an
inch of rainfall across east central KS. Any heavy rainfall may
result in flooding of low-lying areas and along rivers.

Highs Today will reach the lower to mid 80s within the warm sector.
Lows tonight behind the front will drop into the upper 40s across
north central KS and mid 50s across the southeast counties of the
CWA.

.LONG TERM...(Thursday through Tuesday)
Issued at 309 AM CDT Wed Apr 19 2017

Thursday through Tuesday ...

By Thursday morning, the mid-level trough over the Northern Plains
will be advancing toward the Great Lakes region. The surface cold
front contributing to the storms late today will be exiting
southeast of the CWA early Thursday morning, with surface high
pressure advancing into KS behind this boundary.  There is the
potential for some lingering post-frontal showers and isolated
thunderstorms into Thursday morning across east central KS before
advancing high pressure shifts this precipitation east of the area
with dry conditions prevailing late morning through the remainder of
the day. Northerly winds ahead of the high pressure will support CAA
on Thursday, however the "cool-down" in temperatures will actually
drop highs back to the seasonal normals in the mid/upper 60s with
overnight lows in the 40s.

By the end of the week, focus shifts to the mid-level trough over
the Pacific Northwest which will quickly dig southeastward across
the Rockies and into the High Plains on Friday.  Models are coming
into better agreement with the timing and tracking of both the mid-
level and surface lows, with the mid-level low expected to skim
across southern KS Friday into Saturday. The tracking of the surface
low has trended a bit further south, with models keeping the center
of the low generally tracking eastward across OK and AR and barely
skimming the KS/OK and AR/MO borders. Models are also trending a bit
faster with the eastward advancement of this system across the CWA,
so have continued to diminish PoPs during the weekend. As the
surface low lifts northeastward toward KS, showers are expected to
lift into central and east central KS early Friday morning and
gradually expand northward across the CWA through the day as the
surface low skims just south of the CWA and the mid-level trough
advances into western/central KS.  As a result, expect areas of
likely PoPs across the entire CWA Friday and Friday night with
periods of moderate rainfall possible. If areas of moderate to
locally heavy rainfall occur, then there may be a chance for some
flooding on creeks and rivers.  The combination of northeast winds
and overcast skies from rain showers and isolated thunderstorms
Friday into Saturday will result in cool temperatures with highs
dropping into the 50s/near 60 degrees and lows plunging into the
low/mid 40s.  With the faster eastward progression of this system
from the models, these showers look to diminish from northwest to
southeast across the CWA during the day on Saturday with dry
conditions across the entire CWA by Saturday evening.

A mid-level ridge will build into the central U.S. behind this
exiting system, with prevailing surface high pressure supporting dry
conditions through the remainder of the weekend into early next
week.  With the center of the surface high tracking east of the CWA,
the prevailing southwesterly winds will support WAA with high
temperatures rising back into the 60s on Sunday and into the 70s
through the first half of next week.  The next potential for
precipitation may be Wednesday into Thursday as another mid-level
trough dives into the Central Plains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 656 AM CDT Wed Apr 19 2017

South winds will increase through the mid morning hours to 20 to
30 KTS with some gusts up to 40 KTS through the afternoon hours.
A line of broken thunderstorms should move southeast across the
terminals through the evening hours. Winds will switch to the west
and northwest behind the front. Visibilities and ceilings may
drop down to MVFR criteria during thunderstorms this evening.

&&

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

$$

SHORT TERM...Gargan
LONG TERM...Hennecke
AVIATION...Gargan



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