Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 150340
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
940 PM CST Sat Jan 14 2017

.UPDATE...
Issued at 940 PM CST Sat Jan 14 2017

The radar at 02Z showed 925MB winds were still from the north
indicating there may be some low level dry air advection still
occurring. With the 00Z NAM and 18Z GFS showing little in the way
of strong synoptic forcing until late in the day Sunday, have been
looking at isentropic surfaces for signs of lift and forcing for
precip. However profiler data shows the stronger more southerly
winds remain well to the south over TX while 850 winds over
eastern KS are still veered to the southwest. This doesn`t seem
very favorable for moisture advection into northeast KS. Because
of this have trended POPs down 10 to 20 percent and precip
overnight may be more the isolated/scattered nature than a
widespread shield of precip. Will continue to monitor trends and
adjust.

UPDATE Issued at 537 PM CST Sat Jan 14 2017

Light north winds at the surface suggest there may still be some
weak dry air advection going on and the light freezing rain
appears to be fighting this dry air based on recent radar trends.
00Z sounding that just went up shows this dry layer to be about
2500 feet deep. because of this have delayed the onset of precip a
few hours. Latest RAP shows low level isentropic lift increasing
on the 295K surface around 03Z and this is when the freezing
precip is anticipated to spread further north into the forecast
area.

&&

.SHORT TERM...(This evening through Monday)
Issued at 331 PM CST Sat Jan 14 2017

Current satellite shows holes in the cloud cover that have caused
temperatures to warm into the mid 30s. Slight isentropic downglide
has contributed to this drying, but has kept the dew points much
lower. The only exception would be far eastern KS where the moisture
is deeper and closer to the ground. An area of showers and rain is
moving towards the area from the southwest. It should arrive in the
forecast area late this afternoon and evening. Wet bulbing due to
evaporation should cool the temperatures into the 20s and lower
30s therefore resulting in freezing rain once the column saturates.
Some of the high res models are struggling with this batch of
precipitation by either eroding it away or tracking it south of the
forecast area. They are also underestimating the precipitation
coverage. There is a chance that once freezing rain begins it might
not stick to objects or pavement until conditions cool this evening
and wet bulbing is finished. Also, temperatures are forecasted to
cool a few degrees in general due to the loss of daytime heating.
Most of the area stays below freezing before sunrise tomorrow
morning.

Larger scale models bring a continuous stream of precipitation in
the form of freezing rain tonight for areas mainly along and south
of I-70. This is when these areas could receive significant icing.
While more isolated to scattered freezing rain showers are possible
north of I-70. By late tomorrow morning the freezing rain may still
be ongoing for most of the area. Temperatures will begin to warm to
near freezing starting in east central KS, and that trend will
gradually move northward into the afternoon. This will be the tricky
part of the forecast since a few degrees separates freezing rain vs
rain. The models are now hinting that the majority of the
precipitation tomorrow late morning and onward will fall along and
north of I-70. This is when these areas could receive significant
icing. Light precipitation is still possible elsewhere during this
time frame. There could be a lull Sunday evening and overnight
before the main upper level low pressure lifts out over the plains.
By this time a majority of the area may be above freezing. Although
there is a chance that portions of north central KS could stay in
the freezing rain. This would cause higher amounts in these areas by
Monday morning. Generally ice amounts will be between 0.25" and
0.75" with locally higher amounts possible.

A closer look of at the 12z GFS ensembles show that the operational
GFS has some of the warmest temperatures by a few degrees out of all
of the members. Also, the 18Z NAM keeps wet bulb temperatures below
freezing for a majority of the area through tomorrow evening. The
operational GFS has the some of the lowest total QPF compared to all
of the ensemble members for northeast KS through tomorrow night. The
orientation of the 850 mb low level jet would suggest strong
moisture convergence to support this widespread precipitation
through tomorrow. Neutral advection at the surface under light
northeast winds would suggest steady temperatures. Not sure how much
mixing of the warm nose down to the surface will actually take
place. All of these factors would lead me to believe that widespread
significant ice accumulations as still possible.

.LONG TERM...(Monday night through Saturday)
Issued at 331 PM CST Sat Jan 14 2017

On Monday night the upper level low will move overhead and off to
the northeast. There is some potential for moisture to wrap around
the low, interact with an incoming weak vort max from the
northwest, and possibly support a last shot of a light wintery
mix of precipitation. Chances for anything to accumulate are low
and additional impacts are unlikely but will continue to monitor
this period in case deep moisture and lift are a bit stronger or
if the upper low moves out a bit slower.

Temperatures warm markedly for the rest of the forecast period.
Several negatively tilted upper level short wave troughs will
cross the Plains between Thursday and next weekend, but moisture
return will be limited prior to the weekend. For now have only
very small pops for the system on Friday, which looks to have warm
temperatures to support rain if precip develops. The late weekend
storm system looks potentially more interesting but the current
storm track looks to have the local area on the warm side of
things.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday)
Issued at 537 PM CST Sat Jan 14 2017

Don`t have a great deal of confidence in IFR VSBY and CIGS just
yet as precip continues to battle the dry air at the surface.
Since the RAP and HRRR tend to favor an MVFR forecast, have leaned
that way for a little longer into the evening. However continued
moisture advection over the surface high should lead to IFR CIGS
so have followed the bulk of the guidance and bring CIGS down.
Timing the freezing rain and when CIGS worsen is the biggest
uncertainty and have generally followed the consensus. However
amendments are likely as the event unfolds.


&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Ice Storm Warning until midnight CST Sunday night for KSZ024-026-
035>040-054-055.

Ice Storm Warning until noon CST Monday for KSZ008>012-020>023-
034.

Ice Storm Warning until noon CST Sunday for KSZ056-058-059.

&&

$$

UPDATE...Wolters
SHORT TERM...Sanders
LONG TERM...Barjenbruch
AVIATION...Wolters



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