Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 211134
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
534 AM CST Wed Feb 21 2018

...AVIATION UPDATE...

.SHORT TERM...(Today and tonight)
Issued at 408 AM CST Wed Feb 21 2018

09Z water vapor imagery shows the long wave trough over the western
half of the country. There appears to be a low amplitude shortwave
rotating through the base of the upper trough over AZ, while a
better defined shortwave digs down the backside of the upper trough
just off shore of the CA coast. At the surface, high pressure was
centered over NEB with an obvious cold front stretching from central
TX through central AR. Satellite imagery shows decent convection
occurring along the front in central TX.

For today, the weather should be quiet as dry air within the surface
ridge limits precip chances. Models suggest there could continue to
be some weak low level cold air advection into the afternoon. And
high clouds from the convection to the south may inhibit insolation.
So high are forecast to remain at or below freezing.

However things change by this evening. Models show warm air
advection increasing over the surface high late in the afternoon. It
appears this is in response to the subtle wave in the base of the
upper trough. There are also indications of some convectively induced
vort maximum lifting across the area from the central TX convection
this evening. As the warm air advection increases this evening so
does the isentropic upglide. So there appears to be reasonable
forcing affecting the forecast area this evening. Models also show a
slug of moisture advecting north with the warm air advection. So
precipitation is likely to overspread the area tonight. The question
is in what form that precipitation will be. Initially models show
mid level saturation such that there should be ice in the cloud.
However this saturation in the dendritic growth zone is progged to
dry out towards 12Z. On top of that, the warm air advection is
likely to bring a warm nose back over east central KS which looks
warm enough melt any snow that may fall for areas south of
Interstate 70. But if one looks at forecast soundings, the cold air
near the surface is in the -5C to -8C range for much of the area
which should promote refreezing of any hydrometeor. With this
complicated thermodynamic profile, a wintry mix of precipitation is
expected to impact the entire forecast area. With this in mind, will
expand the winter weather advisory to include the entire forecast
area. North central KS, or areas along a line from Minneapolis to
Marysville, should see more snow initially with a change to freezing
rain and sleet late in the night. Snow amounts around 1 inch are
possible with ice accumulations expected to be less than a tenth of
an inch. The I-70 corridor could see mainly a mix of precipitation
with snow, sleet and freezing rain for much of the night. Snow and
sleet amounts up to an inch with around a tenth of an inch of ice is
forecast for these areas. Areas along the I-35 corridor, where the
warm nose if expected to move over, may end up seeing more ice and
sleet and snow. There could be 1 to 2 tenths of new ice
accumulations with snow amounts generally less than a half inch.
Deciding on when to end the advisory is not trivial as the forecast
keeps some chance for freezing drizzle into Thursday evening. Opted
to take the advisory until Noon Thursday as temp trends should begin
to warm above freezing for parts of the area. This will also give
later shifts time to evaluate how the forecast is shaping up after
Thursday morning.

.LONG TERM...(Thursday through Tuesday)
Issued at 408 AM CST Wed Feb 21 2018

Transitioning into the extended period, drizzle and/or freezing
drizzle looks to come to an end Thursday night. The active weather
pattern looks to continue into the weekend with precipitation
returning late Friday into Saturday.

Thursday: After 12Z Thursday morning precipitation is expected to
transition from a wintry mix to freezing drizzle as the depth of
moisture decreases across the CWA. Although model solutions
greatly vary on magnitude, all solutions decrease isentropic
upglide by midday, which could decrease precipition coverage.
Transitioning into Thursday afternoon, a secondary shortwave
impulse will eject from the base of the longwave trough into the
central Plains. Good agreement exists amongst guidance with
traversing the shortwave trough across the CWA between 18 and 21Z
Thursday. The right entrance of an H3 speed max looks to
overspread the CWA Thursday afternoon as well, which could also
assist with large scale ascent and increasing precipitation
coverage once again. Low-level WAA is progged to overspread the
region during the afternoon hours Thursday as the stubborn surface
ridge ejects into the northeastern CONUS. Therefore, have
precipitation transitioning to all liquid from southeast to
northwest during the afternoon and evening hours. Additional ice
accumulations between 12Z Thursday and 00Z Friday look to remain
on the order of a few hundredths. Precipitation looks to end from
west to east Thursday night.

A brief 18 to 30 hour lull in precipitation is expected Friday before
the next chance of rain and wintry weather returns to northeast KS
Friday night. Model solutions continue to converge on the evolution
of the main longwave trough with a neutral to slightly negatively
tilted trough traversing the area Saturday afternoon. Lee
cyclogenesis is progged to take place across northeast NM late
Friday night. In response a strengthening LLJ on the order of 30-
40kts should increase BL moisture. Increasing isentropic ascent
between 280-295K may yield rain showers or drizzle during the pre-
dawn hours Saturday morning. ECMWF and NAM thermal profiles suggest
the potential for freezing rain or freezing drizzle for areas north
of I-70, while the GFS maintains all liquid precipitation across the
entire CWA. The exact track of the surface low will play a huge role
in precipitation types across the area. At this point, ice
accumulations look to remain minor -- less than 0.10" -- stay tuned
for forecast refinements. Aside from the wintry concerns, a few
thunderstorms are possible given mid-level lapse rates near 6 C/km.
Model solutions suggest upwards of a couple hundred J/kg of MUcape
may exist during the predawn hours Saturday -- especially southeast
of the KS Turnpike (I-35,335,70). Regardless of the surface low
track, precipitation chances look to come to an end Saturday evening.

A secondary shortwave trough looks to dig into the western CONUS
over the weekend and eject into the central Plains early next week.
However, little moisture return will limit precipitation coverage. At
the moment have a dry forecast for Monday. As for temperatures
through the extended period, a slow warming trend is expected with
widespread 50s by Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 534 AM CST Wed Feb 21 2018

There is good agreement among the models for another overrunning
event tonight. Precip type will be a challenge with warm air
overrunning the shallow cold air and the likely drying out of the
dendritic growth zone. Until the precip starts around 03Z, VFR
conditions are likely with dry air in the surface ridge precluding
any clouds or precip. Once the warm air advection begins to
saturate low levels, deteriorating conditions should follow with a
wintry mix and IFR conditions anticipated late in the forecast
period.

&&

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

&&

$$

SHORT TERM...Wolters
LONG TERM...Baerg
AVIATION...Wolters


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