Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 131738
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1238 PM CDT Sun Apr 13 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Through Today
Issued at 430 AM CDT SUN APR 13 2014

...Today`s focus on severe thunderstorm potential with winter
weather possible overnight...

Sunday is yet another day with a complex weather setup across the
area and high impact weather expected. At 3 AM, A broad upper trough
was centered over the central Rockies while one of the main short
wave troughs was located over SE Colorado. Multiple weak short
wavelength impulses could be interpreted within the fringes of the
faster southwest flow aloft from NE New Mexico into Iowa. One of
these impulses was evident in low level wind profiles and even radar
imagery to some extent just west of Wichita. At the surface, a
trough axis extended from near Salina into the northeast corner of
KS and was impacting the surface wind fields with some enhanced low
level convergence along I-70. A cold front extended from roughly
Dodge City through Concordia and to Beatrice. The front had made
some gradual southward progression early this morning on the back
side of a short wave impulse that moved through southwest Iowa, but
with another impulse quickly following has essentially stalled with
a bit of a wavy shape coincident with those impulses.

Through Mid Morning:
Elevated instability of 1000 to 2000 J/kg was in place across the
area, especially in northern KS, but the elevated parcels appear to
be capped off by 100 to 200 J/kg of CINH evidenced by the RAP. The
local cap is evident in radar trends over south central KS, but is
likely weaker with northward extent and will weaken as the short
wave near ICT moves northeast into the area. Shear profiles are odd,
with a strong-weak-strong wind profile with height. Feel that the
cap may be overtaken as the short wave ascent overspreads the
stronger low level convergence and flow in the most unstable
isentropic layer impinges upon a sharper pressure gradient in the
vicinity of I-70. This should result in scattered t-storm activity
near/north of I-70 between 4 AM and 9 AM with the focus shifting
north with time. These storms would be elevated if they develop and
perhaps capable of severe hail if they can obtain some supercell
structure, although that is questionable based on the weakness in
the shear profile. Could see a brief period of subsidence behind the
wave as it passes, which could potentially keep the area on the
quiet side during the late morning hours.

This Afternoon:
By noon, expect the surface cold front to have progressed a bit
farther south on the heels of the morning activity, and seems likely
to be located in the vicinity of a Herington to Holton line.
Forcing from the main upper trough will begin to overspread the
frontal zone while low level convergence is forecast to increase
substantially within the frontal zone by mid day as well.
Temperatures will warm into the mid to upper 70s or warmer ahead of
the front with dewpoints in the 59 to 62 range. The capping
inversion will weaken a bit by mid day and all of these factors are
expected to lead to widespread convective development in the frontal
zone between roughly 11 AM and 2 PM. Wind shear seems pretty likely
to support supercell structures, but there remain occasional
weaknesses within the shear profile and the strength of forcing may
cause updraft interference and storm mergers into segments.
Individual storm motion will be toward the northeast but the slow
frontal progression at that time (perhaps even stalled front during
early afternoon) will probably allow cells to remain surface based
for a while without the boundary undercutting them. The greatest
severe weather threat appears to be large hail especially in any
embedded supercells, but if upscale growth into line segments moving
along the frontal boundary occurs, then a bit of a damaging wind
threat may materialize. LCL/LFC`s are quite low though which would
inhibit downdraft strength and wind potential to some extent. While
low level shear is not particularly strong, the very low LCL`s and
presence of ample low level instability may warrant at least some
potential for tornadoes although this too will be highly dependent
upon the presence of supercell structures with minimal interference
from nearby storms. As of now, it`s a small tornado potential but
non-zero. The cold front will gradually push southeast and out of
the forecast area by around 6 PM but elevated instability will
remain in place northwest of the front through the day and showers
with embedded thunderstorms are in the forecast for the entire area
through the day.

The other item of interest during the daytime hours will be strong
winds behind the cold front with gusts in the 45 to 50 mph range
seeming likely in north central KS this afternoon. Have issued a
wind advisory for the areas most likely to see the strong winds
during the day.

.LONG TERM...(Tonight through Saturday)
Issued at 430 AM CDT SUN APR 13 2014

By 00z Monday, models show the cold front situated just southeast of
the cwa. Showers and thunderstorms along and behind the front may
still be in place across far east central Kansas into the early
evening hours, but should exit to the southeast by mid evening.
Surface-based and MUCAPE should quickly diminish with the frontal
passage so the threat for strong to severe thunderstorms over east
central Kansas should diminish during the early evening hours.

The broad mid-level trough associated with this system will have two
embedded shortwaves that will impact the region. The first of these
shortwaves will help to push the cold front through the region and
will progress east of the area Sunday evening. Models show the
second shortwave over the central Rockies this evening and tracking
eastward over the region during the day on Monday. Between these two
waves, precipitation looks to diminish some late this evening into the
early overnight hours as the available lift decreases behind the
first exiting shortwave. However, model soundings still show a
shallow moisture layer present so could see some periods of
scattered light precipitation into the early overnight hours before
the second shortwave approaches the region. Models are in fairly
good agreement in having the moisture from this second shortwave
surge into central Kansas between 06z-09z and progress eastward
across the area between 09z-12z. However, there are still some model
discrepancies with regards to just how far south this shortwave will
dig into the Southern Plains, which has an impact on the exact
tracking of the moisture across the forecast area. Have trended more
with the deeper wave which would push precipitation out of north
central Kansas by Monday morning and keep precipitation over east
central Kansas into the late morning/early afternoon hours. Another
challenge with this forecast is with regards to precipitation type.
A tight pressure gradient is expected to develop behind the frontal
passage, resulting in very breezy northerly winds producing gusts
upwards of 35-45 mph tonight and only slightly diminishing overnight
into Monday with gusts of 25-35mph. This strong northerly flow will
bring a surge of cold air into the region with temperatures quickly
falling into the 30s tonight into Monday morning. Soundings show the
elevated warm nose quickly cooling this evening, but with the
shallow moisture layer in place by late this evening there is
uncertainty with how much (if any) saturation will be present in the
dendritic growth zone. Have slowed down the transition over to a
rain/snow mix some for this evening, but most locations could see
this mixed precipitation by Monday morning with much of north
central and far northeast Kansas likely transitioning entirely over
to light snow overnight. Any snow accumulations should be limited
due to the warmer ground temperatures, but cannot rule out light
accumulations of around one-half inch or less. This wave will
finally track east of the area Monday afternoon, ending the
precipitation. With the strong northerly flow persisting through the
day on Monday and cloud cover remaining in place for much of the
area, temperatures will be much cooler than normal with highs only
reaching into the 40s.

A more zonal mid-level flow will set up behind this exiting trough
with surface high pressure quickly building into the central conus
by Monday night. With this surface high centered over the region,
frigid conditions are expected with Tuesday morning low temperatures
likely plummeting into the middle 20s, so freeze headlines will
likely be needed. Temperatures should quickly rebound into the 50s
and 60s Tuesday and Wednesday as the surface high tracks east of the
area and winds shift to the southwest.

The next focus is on the mid-level trough models show developing across
the northern Rockies on Wednesday. The models are starting to come
into better agreement with the progression of this wave as it
advances into the central U.S., but there is still uncertainty with
the exact timing of this system and how far south the trough will
dig across the Central Plains on Thursday. Models show a frontal
passage on Thursday, so could see a spread in high temperatures from
northwest to southeast across the area. At this time, introduce PoPs
into north central Kansas Wednesday night with chance PoPs spreading
across the forecast area for Thursday and Thursday night. Most of
this precipitation should be in the form of rain, but there is a
chance for a rain/snow mix Thursday night across north central
Kansas as temperatures may drop back down into the low 30s.
Temperatures rebound once again into the 50s and 60s for Friday and
Saturday as surface high pressure moves in behind the exiting
system.

&&

.AVIATION...(For the 18Z TAFs through 18Z Monday Afternoon)
Issued at 1237 PM CDT SUN APR 13 2014

Limiting conditions should dominate the forecast with cold front
passing MHK recently and then TOP and FOE around 20Z. TSRA ongoing
and should persist for a few hours with instability waning with
time. Higher confidence in IFR cigs versus precipitation in the
bulk of the forecast. Will go ahead with -SN inclusion given for a
few hours given model trends. VFR should return by the end of this
forecast.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY until 9 PM CDT this evening FOR KSZ008>010-020-021-
034.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Hennecke
AVIATION...65





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