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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

                            
000
FXUS63 KTOP 151146
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
546 AM CST Fri Feb 15 2019

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 350 AM CST Fri Feb 15 2019

A fast-moving winter weather system is still on track to affect 
the region today.

A low-amplitude, negatively-tilted shortwave trough embedded in a
predominantly zonal 160-kt sub-tropical jet was noted in water 
vapor imagery along the Front Range in Wyoming and Colorado early 
this morning. This subtle PV lobe will be the main driver for our 
well- advertised winter weather event today. MRMS composite 
reflectivity plots show the atmosphere over western Kansas and 
southwest Nebraska has been steadily saturating over the last few 
hours--and this trend will continue and spread eastward with the 
attendant broad isentropic upglide and increasing H700-500 
moisture transport regime. The deeper tropospheric cold front that
pushed through yesterday afternoon has all but cleared the 
forecast area this morning, with the H850 warm nose pushing south 
of the forecast area between now and 12-15Z. Therefore, expect 
that snow will be the singular p-type at the onset of the event.

CAM guidance solutions have backed off slightly on the onset of 
the snow--not surprising given the lower-tropospheric dry wedge 
noted in the 00Z TOP RAOB and RAP/NAM profiles. Still expect that 
snow should saturate to the surface around 12-13Z in the western 
forecast area and possibly closer to 13-14Z east of the Flint 
Hills. The question then becomes how much snow will fall. Did 
lower snowfall amounts to the 2-4 inch range with a few areas 
exceeding 4 inches along and north of I-70. This was done for 
several reasons, most notably due to the fast forward motion of 
the system and the lack of any coherent, quasi-stationary 
mesoscale forcing. The vast majority of the lift will be deep 
kinematic and isentropic in nature, which will certainly produce a
wide swath of moderate snowfall. 

However, the window for the heaviest accumulating snowfall looks 
to be about 6 to 8 hours at most, and it is very difficult for 
synoptic-scale lift to sustain 1"+/hr rates without some 
mesoscale forcing component. That being said, both the RAP and NAM
do hint at a region of sloped Fn vector convergence and a very 
subtle layer of MUCAPE above the DGZ being realized between 15-21Z
in their omega fields; however, its residence time over a 
particular area is only a couple of hours at most. There is also 
some concern that the DGZ is not overly thick, maybe 2-3 kft at 
most, which could keep snow ratios from getting much above 12:1 to
14:1. The going snowfall forecast lines up well with 
RAP/SREF/GEFS/GFS/HRRR forecast solutions and plumes. The 
NAM/NAMNEST, not surprisingly, is the high outlier of the model 
suite and still brings 6+ inches with its mesoscale band and 2-4 
inches elsewhere. The lack of run-to-run continuity with the 
placement of the band over the last four runs of the NAM (which 
has varied from the northern to southern forecast area) does not 
instill confidence in the value of its forecast.

The deep kinematic support wanes between 21-00Z as the heights 
begin to rise behind the departing (and decaying) H300-500 wave. 
Some profiles do lose their mid-level cloud ice late in the event,
but this ice loss is tied very closely to the loss of the main 
synoptic lift and with boundary-layer temperatures of -10 to -12 
C, expect that ice-nucleation could still take place with any 
residual lower-level moisture. Surface ridging, which has been 
building into the region all night, lingers through the day and 
tonight. While RAP/NAM soundings indicate that low clouds may 
linger overnight, any areas that clear out over the fresh snowpack
have the potential to see low temperatures crash quickly. Did 
trend slightly cooler than the previous forecast and kept lows in 
the single digits to low teens and wind chill values in the single
digits below zero.

.LONG TERM...(Saturday through Thursday)
Issued at 350 AM CST Fri Feb 15 2019

There is about a 12-hour gap between systems with the next 
shortwave trough quickly following on Saturday. Introduced a 
slight chance for snow and light freezing drizzle towards north 
central Kansas aft 12Z where decent warm advection commences 
within a saturated layer near 850 mb. Temps are in upwards of -10C
through this saturated layer so may see more light snow than the 
freezing drizzle. By 18Z Saturday, guidance is aligned with upper 
trough axis centered over central Nebraska where the bulk of the 
precipitation occurs from the afternoon through early evening. 
Latest NAM and GFS runs are continuing to show decent QG forcing 
in addition to some mid level frontogenesis with the system in our
area, enhancing qpf amounts for a short period. The deterrent for
accumulating snowfall remains to be the drier mid level air 
coming in from the south aft 18Z, transitioning snowfall to a mix 
of snow and light freezing drizzle. Precip is expected to end by 
sunrise Sunday with total snow accumulations around 1 inch or less
in addition to a light glaze of ice. 

Departing upper trough shears out towards the Great Lakes region 
with only a slight chance for snow showers towards north central 
Kansas on Monday as a weak lead wave quickly passes through. A 
much stronger upper trough is progged to deepen southward over the
western CONUS before lifting out towards the central plains on 
Tuesday evening. With the exception of minor inconsistencies, the 
track of the upper wave is fairly consistent between guidance with
qpf maximizing Tuesday night and Wednesday. Have increased pops 
in the area to categorical with snow being the predominant type. 
There remains the potential for accumulating snowfall so stay 
tuned to forecast updates. 

Forecast remains somewhat active for the remainder of the 
extended given the broad mid level troughing pattern in place 
towards the west. The next system and chances for snow arrive as 
early as Friday, but forecast certainty is too low at this point. 
Temps throughout the week remain below normal with highs warming 
to the middle 30s on Wednesday and Thursday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 537 AM CST Fri Feb 15 2019

Ceilings will deteriorate to MVFR, possibly IFR, levels this 
morning as snow develops over the area. Visibilities will likewise
fall to a mile or less in the heavier snow through the day. The 
snow comes to an end from west to east between 21 and 00Z and 
ceilings lifting later in the evening. Winds will be from the 
northeast at 5 to 10 kts through the period.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM CST this evening for 
KSZ008>012-020>024-026-034>040-054>056-058-059.

&&

$$

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