Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 220446
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1146 PM CDT Fri Apr 21 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Saturday)
Issued at 232 PM CDT Fri Apr 21 2017

Changes with this forecast package have revolved around precip
amounts and timing with most synoptic models previously over-
forecasting the areal extent of rainfall today. The 12Z TOP sounding
was quite telling with a 20 to 30 C Td depression at H800 with 25kts
of northeasterly flow helping to reinforce this dry wedge and erode
the precip shield to the south. The first wave of thermodynamically-
forced precipitation only managed to clip the SE portion of the CWA
with token rainfall amounts. The next wave of kinematically driven
rainfall looks to arrive later this afternoon and evening with the
approaching upper level low with isolated showers possible between
now and then in the southern CWA.

As with previous forecasts, have backed off on QPF amounts for this
afternoon and tonight given the latest CAM and radar trends. In
fact, there is a decent chance that the rain fails to reach the
KS/NE border as hinted by the NAM. Marginal values of MUCAPE
combined with the cellular nature of the precip taking shape towards
Wichita necessitated the mentioning slight chances of thunder
through the afternoon in the south. The main lobe of rainfall will
pass across the southern CWA overnight and exits tomorrow morning as
the H500 low pulls east. Tomorrow looks to see clearing skies with
afternoon CU under cool northerly flow as high pressure builds in
from the NW.

.LONG TERM...(Saturday night through Friday)
Issued at 232 PM CDT Fri Apr 21 2017

Sfc ridge builds into KS Sunday morning, with the axis bisecting
northeast Kansas at that time. Forecast lows vary somewhat between
mos guidance, however still maintain mid to upper 30s. Inserted
patchy frost along the KS river valley as low lying areas will
likely see frost develop with temps being towards the low 30s. Low
level southerly flow returns to the region by Monday with a lee
trough developing over the western high plains. Wind speeds are
forecast to strengthen from 20 to 25 mph sustained with gusts in
excess of 35 mph in the afternoon. Highs in turn warm into the
middle 70s Monday and Tuesday.

Broad troughing out west at the beginning of next week will
gradually shift eastward, ejecting embedded shortwave troughs
towards the CWA beginning on Tuesday. Model guidance is fairly
similar with the warm front being a focus area for showers and
isolated thunderstorms Tuesday and again on Wednesday evening. Many
differences remain however with the mid level lapse rates and
available instability for thunderstorms. Latest runs of the GFS for
example have 1500 J/KG of MUCape and 40 kts of bulk shear Tuesday
evening. On Thursday evening, the main upper trough axis becomes
negatively tilted on the GFS while the Ecmwf is a bit faster and
closely stacked over the eastern portion central plains. While still
a long ways out, there is the potential for a few severe storms with
the GFS scenario Thursday evening. Ample instability and bulk shear
in excess of 50 kts would certainly support rotating updrafts. More
details to come as the forecast will likely evolve.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1146 PM CDT Fri Apr 21 2017

Light rain and showers are expected to continue through the early
morning hours at terminals before pushing southeast of the area. VFR
conditions are expected even with this activity.  Skies will begin
to clear late tomorrow afternoon with light northerly winds expected
after 00Z.

&&

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

$$

SHORT TERM...Skow
LONG TERM...Prieto
AVIATION...Heller



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