Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 100459

1159 PM CDT Wed Jul 9 2014

...Update to short term forecast and aviation forecast

Issued at 1136 PM CDT WED JUL 9 2014

A short wave trough continues to move southeast out of north
central Nebraska. Convergence has been enhanced in advance of this
trough, and the 850 hPa flow continues to increase from the
southwest across central/western KS in advance of the wave. Early
thunderstorms have set up an effective outflow boundary from NW
into central KS as well. An MCV was evident in radar reflectivity
moving southeast from near Fairbury with enhanced precipitation
rates near and just ahead of it, likely exceeding 1" per hour.
Convective rates have been measured at 1.6 inches in 45 minutes
with the stronger storms as well already. This complex should move
across the entire area, with the duration dependent upon the
evolution of additional storms in central Nebraska and another
area in southwest KS.

At midnight, the instability axis with 1200 J/Kg or so of MUCAPE
was well west of the local forecast area, but most model guidance
suggests a gradual progression to the east...with 1300 J/kg of
MUCAPE along the highway 81 corridor by 09Z or so. This would
suggest that the focus for the western thunderstorm activity
currently ongoing around Great Bend should have a general shift
toward the east. Focused isentropic ascent should also impinge
upon the most unstable layer along this MUCAPE axis now through
12Z. The end result should be additional thunderstorm development
into areas that already received more than an inch of rain. There
are several ingredients in place for locally heavy rain rates
approaching 2" per hour at times (reference WPC mesoscale
precipitation discussion for more details). So, in addition to
some small potential for up to quarter size hail (effective shear
may not be sufficient for elevated supercell structures), this has
some potential to be a locally heavy rain event with perhaps some
local flash flooding.


.SHORT TERM...(This Evening through Thursday)
Issued at 328 PM CDT WED JUL 9 2014

12Z upper air data and recent water vapor imagery show moderate
northwest flow over the Central and Northern Plains. Rather strong
mid level baroclinic zone was along the Front Range with 700MB
temps of 15C at Riverton and Boulder, with 7C and 8C at Rapid City
and North Platte, respectively. Mid levels are rather moist
upstream as well with 0-4C dewpoints at 700MB. Low level ridging
remains across much of the Central Plains at 19Z per nearby radar
winds. Thunderstorms have repeatedly formed and dissipated in
north central Nebraska during the daytime hours.

Convection to the northwest may slowly intensify in the next few
hours as moisture slowly recovers on low level southerly winds and
the boundary destabilizes, but still believe convection will
struggle with its southeast track as it approaches Kansas. Greater
precipitation chances still on track for the late evening into
post-sunrise hours as warm air advection and deeper moisture combine
over central portions of Kansas and a weak upper wave passes through.
Instability remains rather marginal with high cloud bases and winds,
despite turning nicely with height, remain limited to keep severe
weather threats somewhat low but still present in steepening
mid-level lapse rates and area possibly downstream of possible MCS
from northeastern Colorado. NAM in particular has been rather
consistent with convection remaining nearly anchored over central
portions of the state through late night and early morning and given
what has been going on in north central Nebraska today, this cannot
be discarded. A minor concern for heavy rains would exist in this
scenario, but low level jet remains focused to the west and north
and precipitable water values will be recovering to near July
normals. Moderate mid level warm air advection continues Thursday
and may keep precip going for at least a few hours, but passing of
the upper wave should produce a drying trend through the day.
Dropped highs a bit with good cloud potential through at least the
late morning.

.LONG TERM...(Thursday Night through Wednesday)
Issued at 328 PM CDT WED JUL 9 2014

For Thursday night and Friday, mid level ridging is expected to
remain over the central Plains with the thermal ridge gradually
 laying over across the state. Models seem to be in good agreement
with the overall pattern and there remains a signal for elevated
storms along the nose of the low level jet Thursday night.
Isentropic surfaces show the better saturation and lift setting up
mainly over northwest MO. Because of this have kept only a chance
for thunderstorms across far northeastern KS. The bigger concern
may be the increasing heat for Friday as low level winds veer to
the south southwest and 850 temps warm to around 24C. The NAM
appears to be a warm outlier, but given the pattern and good
insolation through the day I didn`t want to completely rule it
out. Therefore have maintained highs in the mid and approaching
100 in central KS. With the winds veering, surface dewpoint temps
could mix out and have trended them down some. Because of this,
heat indices remain below heat advisory levels but will need to
watch this in the coming days.

The main focus of the extended forecast is the timing of a couple
cold fronts expected to move through this weekend and early next
week. Precip chances will likely be tied to when the fronts are in
the area. The models are in reasonable agreement in amplifying
the synoptic pattern with ridging along the west coast and into
the Canadian Rockies with a closed low digging south over the
Great Lakes. However the NAM seems to be a fast outlier in
bringing an initial front into the area on Saturday. The
GFS/ECMWF/GEM solutions want to keep the boundary mainly along the
NEB/KS state line through the day Saturday. Have opted to go with
the consensus solution and keep some small precip chances mainly
across the northern counties Friday night and Saturday with
warmer temps over east central KS than the NAM would show. A
stronger cold front is expected to have pushed through much of the
forecast area by Tuesday morning. Because of this, I`ve trended
POPs down for Tuesday night. Think the best chances for precip
should be Saturday night and again Monday night into Tuesday as
these fronts move across the area.

Other changes to the forecast were to trend temps cooler for
Tuesday and Wednesday as the ECMWF brings a stronger high pressure
system into the plains sooner than earlier runs. Have been a
little conservative on the cooler trend given the ECMWF prog of
850 temps around 8C suggesting highs in the 70s. Its hard to go
quite that cool for mid July just yet and the forecast has lows
around 60 with highs around 80.


.AVIATION...(For the 06Z TAFs through 06Z Thursday Night)
Issued at 1136 PM CDT WED JUL 9 2014

Still expeceting primarly VFR through the TAF period, even for the
majority of the period when showers and thunderstorms impact TAF
sites. There could be periods, especially at MHK, through 09Z or
so during which MVFR or brief IFR visibility is possible during
heavier precipitation. Uncertainty remains regarding how long TS
will remain in the area, with some potential for scattered storms
to impact TAF sites even after 12Z...but at this time the greatest
TS potential seems to be now through 12Z or so.




LONG TERM...Wolters
AVIATION...Barjenbruch is the U.S. government's official web portal to all federal, state and local government web resources and services.