Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 260505
AFDTOP

AREA FORECAST DISCUSSION
National Weather Service Topeka KS
1205 AM CDT TUE APR 26 2016

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Tuesday)
Issued at 332 PM CDT MON APR 25 2016

This afternoon an intense upper level trough was located across the
western US and will lift northeast across the plains Tuesday
afternoon and night.

Tonight, the upper trough will dig southeast into the four corners
region. As a 60 KT H5 jet max begins to lift over the central
Rockies, which will result in a lee surface trough deepening across
the high plains, causing the low-level flow across the southern and
central plains to increase from the south. The LLJ will increase
from the south-southwest and advect deep gulf moisture northward
across OK into central KS. Most numerical models show MUCAPE
increasing to 2000 to 3500 J/KG towards 9Z with 30 to 50 KTS of 0-6
KM effective shear. The forecast soundings show that the moisture
will be deep enough along with isentropic lift at the 300 theta
level for parcels to reach their LFC and produce elevated
thunderstorms. Some of these elevated storms may develop mid-level
rotating updrafts after 6Z, which could result in large hail
developing. It looks like the best area for severe thunderstorms
will be north of I-70 between 9Z and 16Z TUE. Most models are
showing that these storms will lift northeast out of the CWA by 18Z
TUE. The ECMWF and ARW-WRF show the storms continuing through most
of the afternoon across the eastern counties of the CWA.

Tuesday and Tuesday night. Once again the model solutions vary on the
position of the dryline and the strength of the EML. The NAM model is
the slowest with the H5 trough lifting northeast across eastern CO
into western NE. The NAM shows the mid and upper level flow backing
through the afternoon hours into the evening hours and has the
dryline as far west as an Osborn to Russell to Great Bend to
Pratt line, with a strong enough CAP to prevent deep moist
convection within the warm sector. The GFS and ECMWF has the
dryline farther east across the western counties of the CWA by 00Z
WED with a weaker CAP and deep moist convection developing along
the dryline after 21Z TUE across the western counties of the CWA.

If the morning thunderstorms shift northeast of the CWA during the
mid and late morning hours, and we see some insolation across the
western counties, than MLCAPES will increase to 3000 to 5000 J/KG
during the late afternoon hours. Thunderstorms that develop during
the late afternoon and early evening hours may be discrete
supercells capable of producing, large hail, damaging winds and
tornadoes. At this time I`m not certain about the development of
strong or long track tornadoes given the vertical wind shear
profiles. Most models show a nearly meridional flow from 850mb to
300 MB and only slight turning in the low-levels. If the instability
is high enough then we may not need strong vertical windshear for
discrete cells to produce tornadoes but for strong tornadoes the
surface and 850mb winds will need to back more to produce stronger
low-level vertical windshear.

As the upper trough begins to lift northeast across western KS
during the evening hours the isolated to scattered supercells should
develop into line segments of storms. These storms will produce
large hail and strong winds, as long as the instability remains
moderate to high through the remainder of the evening hours. The line
segments of severe storms or squall line of storms should gradually
push east across the CWA with individual updrafts moving northward
within the line. If storms train across one area for a period of 2
to 3 hours than there may be a flash flooding potential, especially
areas that received heavy rainfall last evening. The storms should
slowly push east cross the eastern counties of the CWA through the
early morning hours. The intensity of the storms may decrease a bit
after midnight as instability decreases through the early morning
hours of WED.

My thoughts are that the best chance for widespread severe weather
will be across the western half of the CWA during the late afternoon
and evening hours with perhaps some discrete supercells that may
produce tornadoes especially across the northwest counties of the
CWA where 850mb and surface winds will be backed closer to the warm
front in southern NE. As the upper trough lift northeast across the
high plains, then the stronger ascent should cause numerous storms
to develop down the dryline across eastern OK. these storms will move
north as the dryline slowly move east across the CWA. Any stronger
updrafts embedded in the squall line as it moves east across the CWA
could produce large hail along with damaging outflow winds and heavy
rain. Even if line segments develop there could be enough storm
relative shear for meso vortices to develop and produced weak short
lived tornadoes through the night across the remainder of the CWA.
Also, training storms may cause flash flooding.

The dryline/pacific front will shift east of the CWA by 12Z which
should bring an end to the widespread thunderstorms from west to
east across the CWA during the early morning hours of Wednesday.

.LONG TERM...(Tuesday Night through Monday)
Issued at 332 PM CDT MON APR 25 2016

Wednesday Morning Through Monday...

Negatively tilted upper level trough will continue to move northeast
across the central plains. Shower and thunderstorm activity is
expected to move northeast out of the area throughout the day. Model
disagreement amongst the placement of the dryline and warm front
Wednesday afternoon. ECMWF and GFS continue to place the dryline
east of the KS/MO border, and the warm front in the KS/NE/IA border
region. The NAM is the slowest with the dryline progression and
places it just west of the KS/MO border. At this point, current
thinking is the potential for any severe weather on Wednesday will
be east the CWA. Upper level ridging builds in quickly Wednesday
night and Thursday ahead of the next southwestern trough.

The next upper level cutoff low is expected to eject across the
central plains Friday into Saturday. Multiple shortwaves will rotate
around the main low, generating multiple rounds of rainfall Friday
through the weekend. ECMWF and GFS diverge with surface features,
with that said, severe weather may be possible Saturday afternoon.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday Night)
Issued at 1205 AM CDT TUE APR 26 2016

Confidence is low on the specifics but concern for limitations remains
with light winds and lower levels moistening. Short range models
struggling with the near term but pointing to more active
convection firing toward 09Z and likely persisting for a few
hours. Expect increasing mixing to bring VFR for the late periods.

&&

.TOP Watches/Warnings/Advisories...
NONE.
&&

$$

SHORT TERM...Gargan
LONG TERM...Barjenbruch/Baerg
AVIATION...65



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