Area Forecast Discussion
Issued by NWS Topeka, KS

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882
FXUS63 KTOP 290424
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1124 PM CDT Fri Apr 28 2017

...AVIATION UPDATE...

.SHORT TERM...(This evening through Saturday)
Issued at 300 PM CDT Fri Apr 28 2017

Well-advertised active and wet weather pattern continues to take
shape for tonight and this weekend across the Central Plains. Early
afternoon water vapor imagery clearly delineated an amplifying
trough across the Great Basin with an impressive 150 kt jet max
digging southeastward along the upstream flank of the trough. Over
the next 12 to 18 hours this trough will morph into a cutoff,
blocking low over New Mexico with SSW flow over eastern Kansas. An
effective warm front developed this morning along the I-70 corridor
with a mesolow tracking eastward along the front. As this low pull
eastward this afternoon and evening, the current 40-50 degree
temperatures over the far northern CWA will spill southeastward over
the rest of the area and set the tone for temperatures for Saturday.

This aforementioned boundary will be the focus for repeated rounds
of training showers and storms on Saturday along a potent ribbon of
950-850 mb Fg forcing and strong Gulf moisture convergence. The bulk
of the intense rain will be southeast of the CWA with precip over NE
Kansas driven more by strong isentropic upglide along the 290-300 K
surface fueled by deep moisture with 700-500 mb mixing ratios pushing
4-5 g/kg. The short-medium guidance suite has started shifting the
axis of heavier rain further north--with the NAM leading the way as
a distinct outlier--but this trend will bear watching over the next
12-24 hours given the amplified nature of the system. Thunderstorms
will be likely in the southeastern CWA on Saturday with MUCAPE
values possibly exceeding 1000 J/kg (especially in the northern NAM
solution).

.LONG TERM...(Saturday night through Friday)
Issued at 300 PM CDT Fri Apr 28 2017

The models are in decent agreement with the mass fields at 00z
Sunday, or the start of the long term forecast period. The
operational runs of the 12z GFS/NAM/ECMWF have the 500 mb low over
the Texas Panhandle. Over the next 24 hours, the low moves
northeast into eastern Kansas and continues to deepen. The GFS was
north of the ECMWF/NAM solutions with the placement of the upper
low. The GFS surface low is also further north and they are all
starting to occlude by that point. As we move through the night
Saturday and Sunday, the dry slot should move northeast across
eastern Kansas as the cyclone reaches maturity. A large
deformation zone precipitation shield will continue to produce
precipitation over the northwest and northern counties through
Sunday night. As the dry slot moves through the southeastern
counties, all but light precipitation should end on Sunday. Even
with the of subtle model differences, the overall progression of
the precipitation is somewhat consistent during this time period
with the NAM being more bullish with QPF. The official forecast
QPF for our area is 1.90 to almost 3 inches for the entire storm.

The thermodynamics get interesting in the deformation zone
Sunday and Sunday night, especially over our northwest. The NAM
was the coldest model and suggest some snow especially Sunday
night. The boundary temperatures remain just warm enough to keep
it a rain/snow mixed for some of the area Sunday night. This will
have to be watched closely for trends. The upper system is very
strong and it wouldn`t take much to change it over to wet snow in
our northwest which would result in accumulations.

After the storm moves into the Great Lakes on Sunday, a northwest
flow shortwave brings a chance of mainly light shower activity mid
week. By the end of the work week, we should see dry weather and a
warming trend as upper heights build over the rockies and
northwest flow prevails across the plains.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday)
Issued at 1124 PM CDT Fri Apr 28 2017

There is not a lot of change to the forecast as models continue to
show precip overspreading the area through mid morning. IFR and
LIFR CIGS have moved in and with the surface boundary remaining to
the south with northeast winds persisting at the terminals, see no
reason why CIGS would improve as a cool and saturated airmass
remains over the area. There may be a break in the precip sometime
in the afternoon. But trying to pick a window to remove mention of
RA is a fairly low skill exercise at this point.

&&

.HYDROLOGY...
Issued at 300 PM CDT Fri Apr 28 2017

Current rainfall forecasts of 2-3 inches over the next 48 hours have
pushed several river basin forecasts above action or even flood
stage, especially the Marais De Cygnes basin. Since these
forecasts are based heavily on QPF, they are subject to major
changes and thus only Hydrologic Outlooks have been issued at the
present time.

&&

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

$$

SHORT TERM...Skow
LONG TERM...Johnson
AVIATION...Wolters
HYDROLOGY...Skow



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