Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 150519
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1119 PM CST Sat Jan 14 2017

.UPDATE...
Issued at 1119 PM CST Sat Jan 14 2017

I suspect based on obs that the surface ridge has been stronger
with low level dry air over northeast KS than the models have
accounted for. The last couple hours though show surface pressures
falling slightly across the forecast area and the 850 profiler for
DDC/GLD suggests the stronger low level flow is beginning to move
into southern KS. There is even some weak returns developing
northwest of DDC. I think this may be the beginning of the
freezing rain that moves further north into north central and
northeast KS that the HRRR has consistently shown. However
isentropic upglide from all of the models appears to be rather
disorganized until the synoptic forcing begins to affect northeast
KS Sunday night. Because of this I`m not sure there will be a
widespread shield of freezing rain, but more scattered showery
activity. So while the instinct is to maybe cut back on precipand
ice amounts, trends over the last couple hours suggest that all
the guidance pointing at precip moving in through the morning may
still be correct. This is the reasoning for not making wholesale
changes to the forecast at this point.

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.

&&

...Update to aviation forecast discussion...

.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 06Z TAFS through 06Z Monday)
Issued at 1119 PM CST Sat Jan 14 2017

The RAP has tended to do a better job with CIGS and precip this
evening than the model consensus or the NAM. So I`ve used the RAP
solution to craft the forecast, which keeps VFR conditions until
the stronger low level winds bring better moisture advection into
northeast KS, probably between 09Z and 12Z. Once the FZRA moves
in, MVFR conditions should develop. Models show a persistent low
level warm air advection pattern into Sunday evening. It is for
this reason I eventually bring some IFR CIGS into the forecast.
However confidence in timing the precip or lower CIGS is low and
adjustments will probably be needed.

&&

.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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